Rheological measurement system

Abstract

A system 100 for rheological measurement of a viscoelastic sample 103, comprises; a sample holder 130, an indenter tip 111, a force sensor 104, an indentation mechanism 10, a position sensor 106 for m

Description

Field of the invention This invention relates to a system, method, and apparatus etc. for determining one or more rheological properties of viscoelastic materials. Background to the invention Understanding the physical properties of materials, particularly the rheological properties of viscoelastic materials, can play a crucial role in advancing product development. The rheological properties of a material influences its behaviour under different conditions and how it will fare through various production processes. For instance, viscoelasticity grants plastic the capacity to absorb energy, flex, and rebound without cracking. In the realm of food materials, rheological properties significantly impact processing approaches and sensory attributes. Present challenges in traditional methods of rheological characterisation include the time-consuming nature of both the experimental measurements and the complexity associated with interpreting outcomes using existing rheological experimental and analytical tools. Rotational, oscillatory, and capillary rheometry are time consuming, especially when exploring the materials’ low frequency (or low shear rate) viscoelastic properties, with corresponding rigs being significantly expensive (e.g. over £60,000). Rotational rheometry has been found by the Applicant to overestimate viscosity measurements compared to capillary rheometry when used on solid soap and soap-like materials. The Applicant seeks to address at least some of the above issues and aims to provide an improved technique for determining the rheological properties of viscoelastic materials. Summary of the invention According to a first aspect of the present invention there is provided a system for rheological measurement of a viscoelastic sample, the system comprising: a sample holder; an indenter tip; a force sensor configured to measure a force at the indenter tip; an indentation mechanism arranged to urge the indenter tip toward the sample holder; a position sensor for measuring an indentation depth of the indenter tip; and a processing system; wherein the system is configured to: receive a viscoelastic sample into the sample holder; use the indentation mechanism to urge the indenter tip toward the sample holder to indent the viscoelastic sample; acquire force data using the force sensor and acquire indentation depth data using the position sensor during indentation of the viscoelastic sample; send the force data and indentation depth data to the processing system; wherein the processing system is configured to determine, from the force data and the indentation depth data, one or more rheological properties of the viscoelastic sample. According to a second aspect of the present invention there is provided a method for rheological measurement of a viscoelastic sample, wherein the method comprises using a measurement apparatus, the measurement apparatus comprising: a sample holder; an indenter tip; a force sensor configured to measure a force at the indenter tip; an indentation mechanism arranged to urge the indenter tip toward the sample holder; and a position sensor for measuring an indentation depth of the indenter tip, the method comprising: receiving a viscoelastic sample into the sample holder; using the indentation mechanism to urge the indenter tip toward the sample holder to indent the viscoelastic sample; acquiring force data using the force sensor and acquiring indentation depth data using the position sensor during indentation of the viscoelastic sample; sending the force data and indentation depth data to a processing system; and using the processing system to determine, from the force data and the indentation depth data, one or more rheological properties of the viscoelastic sample. According to a third aspect of the present invention there is provided a computer implemented method for rheological measurement of a viscoelastic sample comprising: receiving force data from a measurement apparatus, wherein the force data relates to a force measured by a force sensor of the measurement apparatus during indentation of a viscoelastic sample; receiving indentation depth data from the measurement apparatus, wherein the indentation depth data relates to an indentation depth of an indenter tip determined by a position sensor of the measurement apparatus during indentation of the viscoelastic sample; and determining from the force data and the indentation depth data, one or more rheological properties of the viscoelastic sample. According to a fourth aspect of the present invention there is provided a software tool for rheological measurement of a viscoelastic sample, the software tool comprising instructions for: receiving force data from a measurement apparatus, wherein the force data relates to a force measured at an indenter tip by a force sensor of the measurement apparatus during indentation of a viscoelastic sample; receiving indentation depth data from the measurement apparatus, wherein the indentation depth data relates to an indentation depth of the indenter tip determined by a position sensor of the measurement apparatus during indentation of the viscoelastic sample; and determining from the force data and the indentation depth data, one or more rheological properties of the viscoelastic sample. According to a fifth aspect of the present invention, there is provided a processing system for rheological measurement of a viscoelastic sample, the processing system comprising a memory and one or more processors, wherein the memory stores software which, when executed by the one or more processors causes the processing system to: receive force data from a measurement apparatus, wherein the force data relates to a force measured at an indenter tip by a force sensor of the measurement apparatus during indentation of a viscoelastic sample; receive indentation depth data from the measurement apparatus, wherein the indentation depth data relates to an indentation depth of the indenter tip determined by a position sensor of the measurement apparatus during indentation of the viscoelastic sample; and determine from the force data and the indentation depth data, one or more rheological properties of the viscoelastic sample. According to a sixth aspect of the present invention there is provided a (e.g. non-transitory) computer-readable storage medium comprising instructions for: receiving force data from a measurement apparatus, wherein the force data relates to the force measured at an indenter tip by a force sensor of the measurement apparatus during indentation of a viscoelastic sample; receiving indentation depth data from the measurement apparatus, wherein the indentation depth data relates to an indentation depth of the indenter tip determined by a position sensor of the measurement apparatus during indentation of the viscoelastic sample; and determining from the force data and the indentation depth data, one or more rheological properties of the viscoelastic sample. The present invention, thus, provides for determination of one or more rheological properties of a viscoelastic sample. Embodiments of the invention determine (e.g. calculate) one or more rheological properties of a viscoelastic sample using data obtained from a force sensor and a position sensor. The indenter tip indents the viscoelastic sample and indentation depth data and force data are acquired by the system which are used to determine the one or more rheological properties of the viscoelastic sample. Furthermore, the experimental method is simplified as a single indentation can provide the same or better experimental data compared to other techniques which require time-consuming angular frequency sweeps, in the case of rotational methods, or shear rate sweeps, in the case of capillary rheometry. Embodiments of the invention may be able to determine broadband rheological properties across a wider and continuous range of frequencies than are available using existing techniques. For example, embodiments of the invention provide access to lower frequency broadband rheological properties than is possible using capillary rheometry. Furthermore, using indentation instead of capillary rheometry advantageously requires much lower sample sizes - i.e. viscoelastic samples for indentation measurements typically weigh a few milligrams (e.g., less than 0.2 grams), whereas viscoelastic samples for capillary rheometry measurements require kilograms of material to explore the same range of shear rates / frequencies. Typically, rheological measurements of viscoelastic samples are performed using rotational, oscillatory, or capillary rheometers. The Applicant has found that none of these existing techniques are able to obtain broadband rheological information as efficiently as the present invention. Indentation devices can also be significantly less expensive compared to existing apparatus for rheological measurements. The Applicant has found that embodiments of the invention may allow rheological characterisations of viscoelastic materials to be performed at least 25 times faster than existing techniques. The invention may thus provide improved efficiency and cost-effectiveness of rheological measurement. The rheological measurements may be used to characterise the rheology of a product (e.g. solid soap or solid syndet) and may be used for material studies and product development. The efficiency of determining one or more rheological properties of the viscoelastic sample has been tested for samples made of different materials. For example, for embodiments where the viscoelastic sample is a solid soap or soaplike material, it has been found by the Applicant that it takes approximately 100 seconds to determine rheological properties of the sample over a range of frequencies spanning from approximately 0.01 Hz to approximately 100 Hz, whereas a conventional apparatus such as a capillary rheometer would take approximately 43 minutes to do the same. For embodiments where the viscoelastic sample is a rubber-like material such as styrene-butadiene rubber (i.e. tyre rubber), embodiments of the invention can determine rheological properties of the sample in 1 day where such measurements would typically take 30 days using known techniques, when the same range of frequencies is explored. The sample holder; the indenter tip; the force sensor; the indentation mechanism; and the position sensor may form (e.g. at least part of) a measurement apparatus. The measurement apparatus may comprise an ‘indenter’. The measurement apparatus may comprise the processing system or comprise one or more wired or wireless connection(s) to the processing system. The steps of receiving force data and receiving indentation depth data may comprise storing the force data and storing the indentation depth data in a or the memory (e.g. of a processing system). In a set of embodiments, the one or more rheological properties of the viscoelastic sample comprises: a frequency dependent complex shear modulus, G*(w), of the viscoelastic sample. For example, the one or more rheological properties of the viscoelastic sample may comprise a frequency dependent elastic modulus, G’(w), and a frequency dependent loss modulus, G”(w), of the viscoelastic sample. In a set of embodiments, the one or more rheological properties comprises a frequency dependent complex viscosity, of the viscoelastic sample. For example, the one or more rheological properties may comprise a real part of the frequency dependent complex viscosity, ^'(rv), and an imaginary part of the frequency dependent complex viscosity, if (cd). The one or more rheological properties may be provided as one or more output(s), e.g. to a user. The one or more rheological properties may be provided as one or more output(s) of the system and / or the processing system. The one or more rheological properties may be one or more frequency dependent (e.g. rheological) functions. The one or more rheological properties may be visually output via a display device, e.g. as one or more graphs. In a set of embodiments, the processing system outputs one or more of the list comprising: (i) a frequency dependent elastic modulus, G’(w), of the viscoelastic sample; (ii) a frequency dependent loss modulus, G”(w), of the viscoelastic sample; (iii) a frequency dependent complex shear modulus, G*(w), of the viscoelastic sample; (iv) a frequency dependent complex viscosity, ^(cd), of the viscoelastic sample (v) a real part of the frequency dependent complex viscosity, and (vi) an imaginary part of the frequency dependent complex viscosity, In a set of embodiments, each of the force data and the indentation depth data is measured at a plurality of time points. For example, a force function, F(t), and / or an indentation depth function, d(t), may be generated from the force data and / or the indentation depth data. Preferably, the force data and indentation depth data are time series data. In a set of embodiments, the processing system generates (e.g. calculates) a deformation function of the viscoelastic sample using the indentation depth data (e.g., the indentation depth function) and information relating to a shape (i.e. geometry) of the indenter tip. The deformation function may be used to determine the one or more rheological properties of the viscoelastic sample. A Poisson’s ratio for the viscoelastic sample may also be used in the calculation of the deformation function. Therefore, determining one or more rheological properties of the viscoelastic sample may comprise determining a deformation (e.g. a deformation function, A(t)) of the viscoelastic sample. The deformation may be a deformation over time. In a set of embodiments, the processing system calculates a Fourier Transform of the force function, i.e. F(m), and a Fourier Transform of the deformation function, i.e. Z(<»). This may be calculated for use in determining the one or more rheological properties of the viscoelastic sample. In a set of embodiments, the processing system calculates a frequency dependent complex shear modulus of the viscoelastic sample by dividing the Fourier Transform of the force function, i.e. F("), by the Fourier Transform of the deformation function, i.e. A(a)), of the viscoelastic sample (i.e. = The indenter tip may be connected to the force sensor. The force sensor may be a load cell. The indenter tip may be a transducer of the force sensor. The force sensor may measure the normal force applied to the indenter tip. The indentation mechanism may be a mechanism for causing vertical movement of the indenter tip. The indentation mechanism may comprise any suitable means for causing movement of the indenter tip. For example, the indentation mechanism may comprise a motorized micrometric screw or a linear actuator. The indenter tip may be made of any suitable material. Preferably, the indenter tip has a compliance much smaller (e.g. at least ten thousand times smaller) than the compliance of the viscoelastic sample under investigation. In a preferred set of embodiments, the indenter tip is made of metal. Preferably, the indenter tip comprises a truncated cone shape (i.e. geometry) comprising an indentation base (i.e. the indentation base being the truncated end of the truncated cone) and a (e.g. slanted or sloped) side surface bounding the indentation base. The side surface may be oriented at an inclination angle 0 relative to the indentation base (i.e. the angle 9 being relative to the plane of the indentation base). Preferably, the indentation base is the part of the indenter tip that first contacts the viscoelastic sample during indentation. Preferably, the indentation base is a circular indentation base. The indentation base may have a maximum dimension (e.g. diameter) of at least 0.5 mm, e.g. at least 1 mm. The inclination angle 9 is preferably less than 90 degrees - e.g. between 40 and 80 degrees - e.g. approximately 60 degrees. The viscoelastic sample preferably has a maximum dimension (e.g. width, e.g. diameter) at least 5 times greater than the radius of the indentation base. The indenter tip may have a diameter that gradually decreases toward the indentation base. The indenter tip may have its smallest diameter at the indentation base. The indenter tip may be arranged so that the diameter of the indenter tip decreases in a direction toward the sample holder. The Applicant has found the truncated cone shape to be particularly effective for determining the rheological properties of viscoelastic samples. For an indenter tip comprising a truncated cone shape, the Applicant has provided an equation for calculating a deformation function. In a set of embodiments, the processing system calculates a deformation function of the viscoelastic sample using the following equation: b.n = ‘^»(f + by * {cos-* + sin [cos-1 f-----------+ (-----------Y[; L \(d*tan(0)+b) / J \(d*tan(fl)+b) / J wherein represents the instantaneous deformation (i.e. describing the deformation of the viscoelastic sample) as a function of d, b and 9, d is the indentation depth of the truncated cone indenter tip, b is a radius of an indentation base of the indenter tip, 9 is an inclination angle of the truncated cone and v is Poisson’s ratio of the viscoelastic sample. In a set of embodiments, the processing system: performs interpolation on the force data and / or indentation depth data; and performs virtual oversampling on interpolated functions derived from the force data and / or indentation depth data. The interpolation used may be a spline interpolation method. The interpolation may be performed on the force data and the indentation depth data. The processing system may perform virtual oversampling on the interpolated data (e.g. using an oversampling factor in the range of 10 to 1000). The oversampling may be performed on an interpolated force function and either an interpolated indentation depth function or an interpolated deformation function. The processing system may be configured to determine, from the force data and the indentation depth data, one or more rheological properties of the viscoelastic sample. The processing system may comprise a rheological property determination module configured to determine, from the force data and the indentation depth data, one or more rheological properties of the viscoelastic sample. The processing system may be provided separate to or as part of the indenter. The processing system may comprise a computer running software comprising instructions which cause the processing system to determine the one or more rheological properties of the viscoelastic sample. The processing system may comprise a memory and one or more processors, wherein the memory stores software which, when executed by the one or more processors causes the processing system to determine the one or more rheological properties of the viscoelastic sample (e.g. from the force data and the indentation depth data). The processing system may optionally comprise a wired or wireless connection for controlling the measurement apparatus, e.g. the processing system may control the indentation mechanism and thus, the movement of the indenter tip. The viscoelastic behaviour of a material may be described by the degree to which its viscosity, is dependent on shear rate, y. As will be appreciated by the skilled person, the ‘power-law region’ of a graph of log(viscosity) against log(shear rate) can be used to ascertain viscoelastic behaviour of a material. Taking the logarithm of both sides of the equation for viscosity as a function of shear rate - rj(y) = k|y|(n~V - it follows that in the power-law region of a log(viscosity) against log(shear rate) graph the gradient is n-1 which allows a power-law index, n, to be determined. For materials exhibiting Newtonian behaviour the power-law index, n, is equal to 1. For materials exhibiting viscoelastic behaviour the power-law index, n, is lower. Thus, the system may be a system for rheological measurement of a viscoelastic sample having a power-law index from 0.1 to 0.4, e.g. from 0.2 to 0.3, in the power-law region of a graph of log(viscosity) against log(shear rate). In a set of embodiments, the viscoelastic sample has (e.g. gives rise to) a powerlaw index from 0.1 to 0.4, e.g. from 0.2 to 0.3, in the power-law region of a graph of log(viscosity) against log(shear rate). Though the invention may be suitable for a range of viscoelastic materials, the Applicant has found particularly good agreement with the results of capillary rheometry when the invention is used on viscoelastic samples having a powerlaw index similar to solid soap - e.g. between 0.1 to 0.4, e.g. approximately 0.2. The viscoelastic sample may be a viscoelastic solid sample. In a set of embodiments, therefore, the system is a system for rheological measurement of a viscoelastic solid sample. The Applicant has found embodiments of the invention to be particularly suitable for determining the rheological properties of soap and syndet. In a set of embodiments, the system is a system for rheological measurement of (e.g. solid) soap and / or (e.g. solid) syndet and the viscoelastic sample is a (e.g. solid) soap and / or (e.g. solid) syndet sample. The viscoelastic sample preferably comprises a flat (e.g. top) surface for the indenter tip to indent. The indenter tip preferably is urged toward the sample holder normal to the flat surface of the viscoelastic sample. For some viscoelastic materials, the flat surface may be obtained by warming the viscoelastic sample and / or mechanical pressing (e.g. using a hydraulic press). In a set of embodiments, the indenter tip indents the viscoelastic sample to a maximum indentation depth equal to or less than half of a height of the viscoelastic sample. The indentation mechanism may urge the indenter tip toward (e.g. a free surface of) the viscoelastic sample and stop when the force sensor acquires a low threshold value (e.g. 0.1N). Therefore, the indenter tip may stop moving when contact is made with the viscoelastic sample. The position sensor may be set to a zero value before the indenter tip is moved by the indentation mechanism to indent the sample. Therefore, the position sensor may measure the indentation depth relative to the position at which the indenter tip first contacts the viscoelastic sample (i.e. where the force sensor acquires a low threshold value). In a set of embodiments, the indenter tip is moved into the viscoelastic sample by a predetermined distance (e.g. the maximum indentation depth) at a constant speed and, when the predetermined distance is reached, the indenter tip is static for at least 100 seconds. The indentation mechanism may stop urging the indenter tip toward the viscoelastic sample when the predetermined distance is reached. The predetermined distance may be programmed into the processing system (e.g. control unit) for controlling the indentation mechanism. This allows force data to be collected as the viscoelastic sample gradually deforms postindentation. It will be appreciated by the skilled person that a measurement time of approximately 100 seconds is significantly faster than the measurement time required using existing techniques. The sample holder may be a temperature-controlled sample holder. In a set of embodiments, the temperature-controlled sample holder comprises: a platform; a heating plate for controlling the temperature of the viscoelastic sample; a rotatable sample tray comprising a plurality of recesses, wherein each recess is for holding the viscoelastic sample; and a sample cover arranged to expose a single recess of the plurality of recesses at a time. The platform may allow the sample holder to be secured to a surface (e.g. a measurement apparatus or a table top) and may also help to elevate the viscoelastic sample to an appropriate testing height. The platform may have a flared base defining a plurality of apertures. The plurality of apertures may allow the sample holder to be bolted to existing holes in the indenter base, fixing the position of the sample holder while elevating the other components of the sample holder. As will be appreciated by the skilled person, viscoelastic materials may exhibit different behaviour at different temperatures. Therefore, the heating plate may allow the temperature to be controlled, e.g. to be constant during indentation, so that variations in ambient temperature do not affect the rheological measurements. In a set of such embodiments, the platform, the heating plate, the rotatable sample tray and the sample cover each comprise a respective aperture for receiving a common elongate member (e.g. a screw or bolt) therethrough. The platform, the heating plate, the rotatable sample tray and the sample cover may be secured together via the common elongate member and a fastener. The rotatable sample tray may, therefore, hold a plurality of viscoelastic samples at once. Each recess may be cylindrical. Each recess may have a shape of a short cylinder. For example, each recess may have a diameter of at least 5 mm and a height of 1 to 4 mm. The viscoelastic samples may be moulded to shape of the recesses before indentation testing is done. For example, the viscoelastic samples may be heated to become more fluid and are moulded to the shape of the recess. Alternatively or additionally, the viscoelastic sample can be flattened by a hydraulic press. This preparation may allow the samples to have consistent dimensions and a flat exposed sample surface. The plurality of recesses may be spaced uniformly around a central aperture of the sample tray. In a set of embodiments, between each recess of the rotatable sample tray there is an opening for inserting a temperature sensor (e.g. a thermometer or temperature probe).The openings may be positioned uniformly around an edge surface of the rotatable sample tray (e.g. a surface of the rotatable sample tray bounding a surface comprising the plurality of recesses). The openings may be elongate cavities extending radially inwards, e.g. toward the centre of the sample tray. One or more temperature sensor(s) may be inserted through the side of the rotatable sample tray. This may, thus, allow for better monitoring of the temperature of the viscoelastic samples during measurement. The sample cover may define a cutout (i.e. gap) sized and shaped to expose a single recess at a time. A different recess may be exposed by rotation of the rotatable sample tray. The sample cover may provide insulation to the viscoelastic samples which are not undergoing testing, helping to control the temperature and other conditions such as humidity. In a set of embodiments, the sample holder for receiving the viscoelastic sample comprises at least one cylindrical recess having a diameter greater than its height. In a set of embodiments, a maximum indentation depth is equal to or less than half of the height of the recess. Such arrangements described above may be beneficial in that they are able to perform multiple rheological measurements of viscoelastic samples more efficiently while controlling the temperature of the viscoelastic samples and are considered novel and inventive in their own right. Thus, when viewed from a further aspect of the invention there is provided a measurement apparatus for rheological measurement of a viscoelastic sample, the measurement apparatus comprising: an indenter tip; force sensor configured to measure a force at the indenter tip; an indentation mechanism arranged to urge the indenter tip toward the sample holder; a position sensor for measuring an indentation depth of the indenter tip; and a temperature-controlled sample holder comprising: a platform; a heating plate for controlling the temperature of the viscoelastic sample; a rotatable sample tray comprising a plurality of recesses, wherein each recess is for holding the viscoelastic sample; and a sample cover arranged to expose a single recess of the plurality of recesses at a time. Similarly, the temperature-controlled sample holder is considered novel and inventive in its own right. Thus, when viewed from a further aspect of the invention there is provided a temperature-controlled sample holder for an apparatus for rheological measurement of a viscoelastic sample, comprising: a platform; a heating plate for controlling the temperature of the viscoelastic sample; a rotatable sample tray comprising a plurality of recesses, wherein each recess is for holding the viscoelastic sample; and a sample cover arranged to expose a single recess of the plurality of recesses at a time. It has been found by the Applicant that for a viscoelastic sample having an unknown Poisson’s ratio, the invention may also be used to determine a Poisson’s ratio of a viscoelastic sample, e.g., using the equation already described: A(d,b,e)=^.(d. tan(S) + by . {coS~i + Sin [cOS-1 (-----y----)1 + (-----y----)1. L \(d*tan(0)+b) / J \(chtan(0)+b)7 J An unknown Poisson’s ratio may be determined by determining the one or more rheological properties of the viscoelastic sample (e.g. using an indentation method (i.e. a first rheometric technique) in accordance with the invention) and obtaining rheometric data representative of the one or more rheological properties of a viscoelastic sample using a second (i.e. further) rheometric technique, e.g. capillary rheometry, for a similar viscoelastic sample and performing a comparison between the results. For example, the similar viscoelastic sample may consist of the same type of viscoelastic material as the viscoelastic sample. However, given the difference in sample weight required by capillary rheometry and the indentation method described herein, the sample size may differ. The value of the Poisson’s ratio may be determined by varying the value of the Poisson’s ratio until the one or more rheological properties determined by the method according to the present invention and the rheometric data from the second technique agree. Agreement with the rheometric data of the second technique may be defined in any suitable way, e.g. agreement may be reached when a threshold level of overlap (e.g. above 60%, e.g. above 70%, e.g. above 80%) between results is achieved, taking into account uncertainties. The one or more rheological properties may be a real part of a complex viscosity -e.g. which may be plotted on a graph of log(viscosity) against log(shear rate). Therefore, in a set of embodiments, a Poisson’s ratio of the viscoelastic sample is determined by: determining from the force data and the indentation depth data, using an initial test value of a Poisson’s ratio of the viscoelastic sample, one or more rheological properties of the viscoelastic sample; receiving rheometric data representative of one or more rheological properties of a similar viscoelastic sample, the data being obtained using a second rheometric technique; and changing (e.g. varying) the value of the Poisson’s ratio until the one or more rheological properties agrees with the rheometric data. Such a method may be beneficial in that it allows an unknown Poisson’s ratio of a viscoelastic sample to be determined and is considered novel and inventive in its own right. Thus, when viewed from a further aspect of the invention there is provided a computer-implemented method of determining a Poisson’s ratio of a viscoelastic sample comprising: receiving force data from a measurement apparatus, wherein the force data relates to a force measured by a force sensor of the measurement apparatus during indentation of a viscoelastic sample; receiving indentation depth data from the measurement apparatus, wherein the indentation depth data relates to an indentation depth of an indenter tip determined by a position sensor of the measurement apparatus during indentation of the viscoelastic sample; and determining from the force data and the indentation depth data, using an initial test value of a Poisson’s ratio of the viscoelastic sample, one or more rheological properties of the viscoelastic sample; receiving rheometric data representative of one or more rheological properties of a similar viscoelastic sample, the data being obtained using a second rheometric technique; and changing (e.g. varying) the value of the Poisson’s ratio until the one or more rheological properties agrees with the rheometric data. For example, the initial test value of the Poisson’s ratio may be 0.5 and the value of the Poisson’s ratio may be gradually reduced from 0.5 until the correct Poisson’s ratio is found at the point where the one or more rheological properties determined by the method according to the invention agree with the rheometric 5 data obtained from the second rheometric technique. When the one or more rheological properties agrees with the rheometric data, the value of the Poisson’s ratio may be stored (e.g. in a memory) and / or provided as an output (e.g. displayed to a user). As explained above, the second rheometric technique may be capillary rheometry. io The method described above may be implemented by the processing system described herein. Features of any aspect or embodiment described herein may, wherever 15 appropriate, be applied to any other aspect or embodiment described herein. Where reference is made to different embodiments or sets of embodiments, it should be understood that these are not necessarily distinct but may overlap. Brief description of the drawings Embodiments of the invention will now be described, by way of example, with reference to the drawings, in which: Fig. 1 shows a system in accordance with an embodiment of the invention; Fig. 2 is a flowchart of part of a method embodying the invention, wherein the force data and indentation depth data are measured; Fig. 3 is a flowchart showing method steps associated with determining the one or more rheological properties of the viscoelastic sample, according to an embodiment of the invention; Fig. 4 is a graph showing data from the force sensor and the position sensor during indentation of the viscoelastic sample; Fig. 5A and Fig. 5B are schematic side and front views of an example measurement apparatus; Fig. 6A shows an exploded view of a temperature-controlled sample holder for holding the viscoelastic sample; Figs. 6B to 6D show different views of various parts of the sample holder with example dimensions; Fig. 7 A is a schematic drawing of the indenter tip and viscoelastic sample before indentation; Fig. 7B is a schematic drawing of the indenter tip and viscoelastic sample during indentation; Fig. 8 is a schematic representation of the indenter deforming the free surface of a viscoelastic sample labelled with parameters; Fig. 9A and Fig. 9B show the indenter tip indenting the viscoelastic sample; Fig. 10 shows an example indenter tip; Fig. 11 shows an example visual output of the method according to embodiments of the invention displaying the rheological properties of the viscoelastic sample; Fig. 12 is a graph showing the agreement between rheological properties measured using the invention and measured using a capillary rheometer; and Fig. 13 is a flowchart of a variant of the method described herein for use in determining an unknown Poisson’s ratio of a material. Detailed Description One or more embodiments of the invention will now be described with reference to the drawings. Fig. 1 shows a system 100 in accordance with an embodiment of the invention. The system 100 has an indenter 101 and a processing system 102 for rheological measurement of a viscoelastic sample 103. The processing system 102 may be provided separate to or as part of the indenter 101. However, for the purposes of example only, the system 100 of Fig. 1 shows a separate processing system 102. In one example embodiment, the processing system 102 is a computer running software comprising instructions which cause the processing system 108 to carry out certain steps of the method described herein. The processing system 102 has a processor 108, an input / output interface 107, and storage 109 (e.g. memory). The system 100 also has a sample holder 130 for holding the viscoelastic sample 103 (e.g. Syndopal 300 - a syndet base product). An indenter tip 111 having a truncated cone geometry forms part of the indenter 101. The system has an indentation mechanism 105 arranged to urge the indenter tip 111 toward the sample holder 130 so that the indenter tip 111 indents the viscoelastic sample 103. The indenter 101 has a force sensor 104, connected to the indenter tip 111, to acquire force data, i.e. the normal force at the indenter tip 111. The indenter 101 also has a position sensor 106 (i.e. an encoder sensor) to acquire indentation depth data of the indenter tip 111 during indentation of the viscoelastic sample 103. The processing system 102 is configured to determine, from the force data and the indentation depth data, one or more rheological properties of the viscoelastic sample 103. The processing system 102 may optionally comprise a connection for controlling the indenter 101, e.g. the processing system may control the indentation mechanism and thus, the movement of the indenter tip 111. Figs. 2 and 3 together depict the overall method according to an embodiment of the invention. Fig. 2 is a flowchart 200 of an indentation method which results in force data and indentation depth data being obtained and Fig. 3 is a flowchart 300 showing steps associated with determining the one or more rheological properties of the viscoelastic sample 103 from the force data and indentation depth data. The operation of the system will now be described with reference to the system 100 and flowcharts 200, 300 of Figs. 1, 2 and 3. In step 201, the viscoelastic sample 103, e.g. solid soap or solid syndet, is placed in the sample holder 130. The indentation mechanism 105, in step 202, urges the indenter tip 111 toward the sample holder 130 to indent the viscoelastic sample 103. During indentation of the viscoelastic sample 103, the force sensor 104 acquires force data and the position sensor 106 acquires indentation depth data as shown in step 203. During the test, the temperature of the viscoelastic sample 103 is controlled by means of a temperature-controlled sample holder 130. The temperature-controlled sample holder is described in more detail with reference to Figs. 6A-6D and Figs. 9A-9B below. In the embodiment described herein, the indentation process is achieved by means of an indenter tip 111 having the shape of a truncated cone (as shown in Figs. 5A, 7A-B, 8, 9A-B and 10 and described in the accompanying description below). The indenter 101 is controlled, e.g. via a control panel on the indenter or via software running on the processing system 102, to urge the indenter tip 111 toward a free upper surface of the viscoelastic sample 103 and to stop when the force sensor 104 reads a low threshold value (e.g. 0.1N). The position sensor 106 resets the position value to a zero value and then the indenter tip 111 performs a finite indentation (e.g., d = 500 mm) at a fixed speed (e.g., 1000 mm / s). Once the position sensor 106 reads that the indenter tip 111 has reached the desired indentation value, the indenter tip 111 is held static for a predetermined duration (e.g., 100 seconds). After this duration, the indenter tip 111 is retracted away from the viscoelastic sample 103. During the indentation process described above, the normal force F(t) measured by the force sensor 104 (i.e. the normal force at the indenter tip 111) is recorded along with the indentation depth d(t). An example of a typical indentation measurement acquired by the force sensor 104, 512 and position sensor 106, is shown in Fig. 4. The diagram is a graph showing force 402 and indentation depth 401 against time, the time axis being logarithmic. The diagram 400 thus shows the temporal behaviour of the indentation 401 and the force response 402 of the viscoelastic sample 103. The force data and indentation depth data are sent to the processing system 102 (e.g. continuously during the measurement) so that the one or more rheological properties of the viscoelastic sample 103 can be determined. An algorithm, e.g. performed by software running on the processing system 102, for determining one or more rheological properties of the viscoelastic sample will now be described. Turning to the flowchart 300 of Fig. 3, in step 301, the processing system 102 receives, as input, the force data F(t) and indentation depth data d(t) measured by the indenter 101. In step 302, the force data and indentation depth data are interpolated using a (e.g. non-overshooting cubic) spline interpolation method 308. In step 303, a deformation function is calculated using the interpolated indentation depth data (d(t)) and other inputs including the geometry 310 of the indenter tip 111 (b and 0) and the Poisson’s ratio (v) of the viscoelastic sample 103 under test. For example, for viscoelastic materials such as solid soap, solid syndet and rubber the Poisson’s ratio can be approximated as 0.5. As shown in Fig. 8, b represents the radius of the truncated end (i.e. indentation base) of the indenter tip 111 and 6 represents the inclination angle of a side surface (or wall) bounding the truncated end of the truncated conical indenter tip 111. To calculate the deformation function, the following expression 309 of instantaneous deformation is used A(d, b, ey. tan(0) _ A(d, b, 0) — ——— *(d* tan(0) + b)2 b (d * tan(0) + b) (A) ( b A] ( b \(d * tan(0) + bp. \(d * tan(0) + by This equation 309 (Equation A) has been derived for the first time by the Inventor, the derivation of which is provided in detail below. Virtual oversampling is performed on the force and deformation functions F(t) and A(t), in step 304, using an oversampling factor 311 of 500. However, it should be appreciated that oversampling may be performed prior to the calculation of the deformation function. Oversampling is performed by sampling not only at the exact data points but also at a plurality of equally spaced points (i.e. equally spaced in logarithmic space) therebetween. The Applicant has found an oversampling factor of 500 to be most suitable for viscoelastic samples made of solid soap or syndet. However, a range of between 10 to 1000 is preferred for viscoelastic samples as these oversampling factors provide an acceptable trade-off between obtaining valuable information from the data and giving weight to noise. The Fourier Transforms of each of the oversampled force function F(t) and deformation function A(t) are taken in step 305 to give F(a>) and A(a)). At step 306, the complex shear modulus G*(w) is calculated using the following expression 312 (equation B): G*(w) A(o)) (B) When G*(o>) is determined, the following equation (Equation C) can be used to obtain the frequency dependent elastic modulus G’(w) and the frequency dependent loss modulus G”(w) of the viscoelastic sample. G*(w) = G'(aA) + (C) The real part of Equation C gives the frequency dependent elastic modulus G’(w) and the imaginary part of Equation C gives the frequency dependent viscous modulus G”(w), which are output as the rheological properties of the viscoelastic sample, in step 306. This data may be output via a graph 1103 on a computer display 1100 as shown in Fig. 11. Furthermore, the frequency dependent complex viscosity of the viscoelastic sample may be calculated using the below equation (Equation D), which is also output via a graph 1104 on a computer display 1100 as shown in Fig. 11. G*(w) G'(o)) + iG" (o)} G"^ G'^} = —--=-----:------=--i---- io) io) o) 0) = rf O) - iv” (") (D) Fig. 11 shows a LabVIEW executable which implements the algorithm described above and displays rheological properties to a user via graphical user interface 1100. Four log-log graphs 1101, 1102, 1103, 1104 are shown in Fig. 11. The measurements taken from the measurement apparatus, i.e. the force F(t) and indentation depth d(t), are shown in the top-left graph 1101. The bottom left graph 1102 shows the oversampled force function F(t) and the deformation function A(t). The interface 1100 allows the user to input information relating to the geometry of the indenter tip 111 which is used for determining the rheological properties of the viscoelastic sample 103. The interface has an input field 1105 for the diameter (2b) of the truncated circular surface, and an input filed 1107 for the inclination angle 9 of the side surface bounding the truncated circular surface. An input field 1106 for Poisson’s ratio is also provided, here it is taken to be 0.5 (e.g. the Poisson’s ratio for rubber is 0.5) but can be changed depending on the viscoelastic material being tested. The outputs provided in the example shown in Fig. 11 are the frequency dependent elastic modulus G’(co) and the frequency dependent loss modulus G”(w) 1103 (i.e. the top-right graph) of the viscoelastic sample, from which the real and imaginary parts of the complex viscosity 1104 can be derived (bottom-right graph) by means of Equation D. Fig. 12 shows overlapping log-log plots of Shear Viscosity (Pas) against shear rate (s-1), and Dynamic Viscosity (Pas), i.e. the real part of Equation D (if (w)), against angular frequency (rad / s). The graph shows dynamic viscosities 1201, 1202, obtained in accordance with the invention and shear viscosities 1203 obtained using a capillary rheometer. In this example, the viscoelastic sample under test is Syndopal 300 - a syndet base product. The graph shows an agreement over the overlapping range of 10° to 102 rad / s between the rheological measurements 1201, 1202 performed in accordance with the invention and the rheological measurements 1203 obtained by the capillary rheometer. This shows that the method described herein obtains the same results significantly faster using less costly components and provides access to information at lower frequencies than the capillary method. The measurement time for obtaining the rheological measurements 1203 using the capillary rheometer was found to be several hours with the addition of a few hours for the data analysis. The measurement time for obtaining the rheological measurements 1201, 1202 according to the embodiment of the present invention was significantly faster in comparison - i.e. 60 seconds with a few seconds for data analysis. Furthermore, it is apparent from the graph that the technique embodying the invention achieves rheological measurement over a broader range of frequencies in significantly less time than the capillary rheometer. Fig. 13 is a flowchart 1300 of a variant of the method described herein. The method of Fig. 13 can be used to determine an unknown Poisson’s ratio of a material. It should be appreciated that the method may include any of the steps from the flowchart of Fig. 3. As shown in Fig. 13, in step 1301 force data and indentation depth data is input to the processing system. An initial test value of Poisson’s ratio, v, is also provided. This is initial test value may be input by an operator of the system or may be preprogrammed. In step 1302, a deformation function is generated from the test value of Poisson’s ratio and geometric data 1307 such as radius of the indentation base, b, and the inclination angle, 0 using equation A. In step 1303, one or more rheological properties are determined - e.g. a frequency dependent viscosity of the viscoelastic sample. Then in step 1304, rheometric data - e.g. a shear rate dependent viscosity of a similar viscoelastic sample - from a second technique (e.g. capillary rheometry) is compared with the one or more rheological properties determined by steps 1301 to 1303 to determine if there is an agreement. If there is not an agreement, the method proceeds to step 1306 and the test value of Poisson’s ratio, v, is changed and a deformation function, A(t), is generated (i.e. calculated) using the new test value. The method then proceeds to step 1303 to determine the one or more rheological properties and then to step 1304 where again, the results are compared. If the results agree at step 1304, the processing system provides the new test value of Poisson’s ratio, v, as an output. The results that may be compared could be the viscosities shown in Fig. 12. Various features of the measurement apparatus and method for determining rheological properties of the viscoelastic sample will now be described in more detail with reference to Figs. 5 to 10. Figs. 5A and 5B show an example apparatus 500 which can be adapted with the addition of a sample holder (not shown) to the base 522, an indentation mechanism (not shown) and a position sensor (not shown) to perform the measurement of force and indentation depth for determining the rheological properties of the viscoelastic sample 103. The apparatus 500 has an indenter tip 511 and a force sensor 512 which can be moved vertically using the indentation mechanism 514. The apparatus 500 may be controlled via a control panel 513. The indentation mechanism 514, 105 enables vertical motion of the indenter tip. In the example described herein, the indentation mechanism (not shown) comprises a motorized micrometric screw coupled to the position sensor (e.g. an encoder sensor) 106 that enables vertical motion of the indenter tip 511. An exploded view of the temperature-controlled sample holder 600 is shown in Fig. 6A. The temperature control is achieved using an electrical heating plate 603. The plate 603 may comprise electrically conducting strips or wires arranged within the plate 603 which heat up when a current is passed through (as shown in the plate 903 of Figs. 9A-B). At the bottom there is a platform 601, which allows the sample holder 600 to be secured to the indenter apparatus and elevates the testing area and fixes the viscoelastic sample 103 in the correct spatial position. As shown in Fig. 6D, the platform 621 has a flared base defining four holes 622a-d which allow the sample holder 600 to be bolted to existing holes in the indenter base 522, fixing the position of the sample holder 500 and elevating the remaining components, providing both the necessary height for the samples to be easily reachable by the indenter, as well as thermal insulation of the heating plate 603 from the indenter base. The platform has a bolt 602 protruding from the top of the platform 601 for mounting the remaining components thereto. Mounted to the platform 601 are the electrically heated plate 603, a rotatable sample tray 604, a sample cover 621 which each have a respective circular (central) hole shaped and sized for receiving the bolt 602 therethrough. These components 603, 604, 605 are held in place using a fastener 606 with an internal screw thread corresponding to the external screw thread of the bolt 602. The fastener 606 prevents the sample tray 604 from spinning during indentation and ensures that all components are tightly pressed together, eliminating any unwanted movement which can interfere with the measurement. The rotatable sample tray 604 has ten recesses (e.g. 620) and so can hold ten viscoelastic samples at once. Each recess 620 has a shape of a short cylinder. The viscoelastic samples are preferably moulded to shape of the recesses before indentation testing is done. For example, the viscoelastic samples may be heated to become more fluid and are moulded to the shape of the recess. Alternatively or additionally, the viscoelastic sample can be flattened by a hydraulic press. This preparation may allow the samples to have consistent dimensions and a flat exposed sample surface. Thus, the samples take the shape of small cylinders having a diameter of approximately 10 mm and a height of approximately 2 mm. As shown in Figs. 6A and 6B, the recesses 620 are spaced uniformly around the central aperture of the sample tray 604. Between each recess is an opening 640 for inserting a temperature probe. As shown in Figs. 6A and 6C these openings allow the temperature probe to be inserted through the side of the sample tray 604. Above the sample tray is a sample cover 605 having a cutout (i.e. gap) 621 sized and shaped to expose a single recess at a time. A different recess can be exposed by rotation of the sample holder 640. The sample cover 605 provides insulation to the viscoelastic samples which are not undergoing testing, helping to control the temperature and other conditions such as humidity. Fig. 7A and Fig. 7B show a closer view of the indenter tip 711 indenting the viscoelastic sample 717 in the temperature-controlled sample holder 704. Both show the indenter tip 711 having a truncated cone geometry, with a circular indentation base 716 and a side surface 715, bounding the indentation base 716, oriented at an inclination angle 0 relative to the indentation base 716 and the viscoelastic sample 717 in an exposed recess of the sample holder 704. The recess of the sample holder has a circular recess base 719 and a recess wall 718 bounding the recess base 719. Fig. 7A shows the viscoelastic sample having a flat top surface with the indenter tip 711 approaching the recess of the sample holder 704. For some viscoelastic materials, a flat top surface may be obtained by warming the sample before allowing it to settle in the recess of the sample holder 704. Fig. 7B shows the indenter tip 711 once it has reached the predetermined indentation depth and deformed the viscoelastic sample 717. Fig. 8 is a schematic representation of the indenter 811 deforming the free surface of a viscoelastic sample 817 labelled with parameters a, b, d, 9 and Fn. The indenter tip 811 has a truncated cone geometry with a circular indentation base 816 and a side surface 815, bounding the indentation base 816, oriented at an inclination angle 9 relative to the indentation base 816. As can be seen in Fig. 8, a is the effective contact radius between the indenter tip 811 and the sample 817, b is the radius of the blunt end (i.e. the indentation base) 816 of the indenter tip 811. Fig. 9A and Fig. 9B show photographs of the indenter tip indenting the viscoelastic sample 917. Both Figs. 9A and 9B show the indenter tip 911 and the force sensor 912, the platform 601, electrical heater plate 603, the rotatable sample tray 604 with viscoelastic samples (e.g. Syndopal 300 917) contained in the recesses (e.g. the recess 620 contains the sample under test 917). Fig. 9B shows the same features shown in Fig. 9A, with the addition of the sample cover 905 and the fastener 906 securing the components of the temperature-controlled sample holder together. The cutout 921 of the sample holder 905 exposes a single recess 620, thus exposing the viscoelastic sample 917 under test. Furthermore, Fig. 9B shows the measurement apparatus mid-indentation - i.e. the indenter tip 911 has been urged downwards by the indentation mechanism to indent the viscoelastic sample 917. Fig. 10 shows a close-up view of the indenter tip of Figs. 9A-B. The indenter tip 911 is again shown with a truncated cone geometry having the circular indentation base 916 with a diameter of approximately 1mm and the sloped side surface 915 bounding the indentation base 916. The indenter tip 911 may be connected to the force sensor 912 via a connecting end 950 distal to the indentation base 916. For example, the indenter tip 911 may be screwed to the force sensor 912. As shown in Fig. 10 the total length of the indenter tip 911 is between 1 and 2 cm. Therefore, the Applicant has found that by applying a finite indentation to a viscoelastic material and analysing the temporal behaviour of the force sensed at an indenter tip and the indentation depth of the indenter tip it is possible to unveil the frequency-dependent rheological properties of the materials across an extensive range of frequencies. A more detailed derivation of the mathematics used in the embodiment described herein is presented below. The analytical method is based on the following constitutive equation: t F(t) = J (1(1 - T)A(T)dT (1) — 00 Where F(t) is the load force, G(t) is the shear relaxation modulus of the viscoelastic sample and A(t) is the time derivative of A(d(t)). The latter is the material’s ‘instantaneous’ deformation introduced by the Hertz’s model and it is a function of the indentation depth (d(t)), which for a rigid indenter having a shape of a truncated cone on an incompressible linearly elastic half-space has the following expression: _ fb\ d(a, 0) = a * tan(0) * cos 1 — = < / >0 * a * tan(0) (2) \a) Where a, b and 0 are the effective contact radius, the radius of the blunt end of the truncated conical indenter tip and the inclination angle, respectively (see Fig. 8); whereas, <p0 = cos-1(b / a). At each instant, it is then possible to write: £ F(d) = 7!----77 * tanW «2(0o + cos 0O sin 0O) ¢1 — v^) 2G = —---- * tan(0) a (0O + cos0o sin0o) (1 - v) (3) where E is the Young’s modulus of the material and v is the Poisson’s ratio (commonly assumed to be 0.5). The left side of the above equation has been obtained by assuming E - 26(1 + v). From Equation 3 it is possible to infer the expression of the material’s ‘instantaneous’ deformation: A(d, b, 0) (1-v) * tan(0) a2 (0O + cos 0O sin 0O) This would require the knowledge / measurement of the parameter a to be evaluated. However, this latter parameter is not easy to be measured in real experiments; but to a first approximation it can be expressed as function of the indentation depth, d(t). This is possible by assuming that the material wets the truncated cone up to a height equal to d(t) (see Fig. 8). This assumption leads to the following expression of A(d, b, 0): tan(0) _ A(d, b, 0) = ------r * (d * tan(0) + b)2 (1-v) (d * tan(0) + b) (5) ■ r -1 z b a + sin cos I — -----———7—) \(d * tan(0) + b)J b (d * tan(0) + b") Which is only function of d(t) and parametric in (b, 0). By performing the Fourier transform of the Equation 1, one obtains: F(a)) = G (te)ia)A (^) = 6*(r0M(ro) (6) Where F(to), G(w) and Z(<») are the Fourier transforms of F(t), G(t) and A(f), respectively. From the above equation the frequency dependent complex shear modulus + the viscoelastic sample can be calculated where the real part G'O) is the elastic modulus and the imaginary part 5 G”(6o) is the viscous modulus. These are related to the complex viscosity: G*(oj) G'(m) + iG"(a>) G"(o>) G^oj) n*(a>) =-----=---------------=--i----- io) io) 0)0) = This method is based on the interpolation of the finite data set by means of a piecewise-linear function. In particular, the general validity of the proposed 10 procedure makes it equally applicable to find the Fourier transform of any time-dependent function ^(t) that vanishes for negative t, sampled at a finite set of data points 0k>9k\ where k = 1 — N, which extend over a finite range, and need not be equally spaced: -^2^(60) = i^(0) + (1- N _ 1 (8) L\tk-tk J 1 } ’ k=2 15 where is the gradient of ^(t) extrapolated to infinite time and g(0) is the value of g(t) extrapolated to t = 0 from above.

Claims

1. A system for rheological measurement of a viscoelastic sample, the system comprising:a sample holder;an indenter tip;a force sensor configured to measure a force at the indenter tip;an indentation mechanism arranged to urge the indenter tip toward the sample holder;a position sensor for measuring an indentation depth of the indenter tip; anda processing system;wherein the system is configured to:receive a viscoelastic sample into the sample holder;use the indentation mechanism to urge the indenter tip toward the sample holder to indent the viscoelastic sample;acquire force data using the force sensor and acquire indentation depth data using the position sensor during indentation of the viscoelastic sample;send the force data and indentation depth data to the processing system;wherein the processing system is configured to determine, from the force data and the indentation depth data, one or more rheological properties of the viscoelastic sample.

2. The system of claim 1, wherein the one or more rheological properties of the viscoelastic sample comprises a frequency dependent complex shear modulus, G*(w), of the viscoelastic sample.

3. The system of claim 2, wherein the one or more rheological properties comprises a frequency dependent complex viscosity, rfO). of the viscoelastic sample.

4. The system of claim 2 or 3, wherein the processing system is configured to output one or more of the list comprising:(i) a frequency dependent elastic modulus, G’(w), of the viscoelastic sample;(ii) a frequency dependent viscous modulus, G”(w), of the viscoelastic sample;(iii) a frequency dependent complex shear modulus, G*(co), of the viscoelastic sample;(iv) a frequency dependent complex viscosity, q* (oo), of the viscoelastic sample;(v) a real part of the frequency dependent complex viscosity, and (vi) an imaginary part of the frequency dependent complex viscosity,5. The system of any preceding claim, wherein each of the force data and the indentation depth data are measured at a plurality of time points and the processing system is configured to generate a force function, and / or an indentation depth function, from the force data and / or the indentation depth data.

6. The system of any preceding claim, wherein the processing system is configured to generate a deformation function of the viscoelastic sample using the indentation depth data and information relating to a shape of the indenter tip.

7. The system of claim 6, wherein the processing system is configured to calculate a Fourier Transform of the force function and a Fourier Transform of the deformation function for use in determining the one or more rheological properties of the viscoelastic sample.

8. The system of claim 7, wherein the processing system is configured to calculate a frequency dependent complex shear modulus of the viscoelastic sample by dividing the Fourier Transform of the force function by the Fourier Transform of the deformation function.

9. The system of any preceding claim, wherein the indenter tip comprises a truncated cone shape comprising an indentation base and a side surface bounding the indentation base oriented at an inclination angle relative to the indentation base.

10. The system of any of claims 6 to 9, wherein the indenter tip comprises a truncated cone shape comprising a circular indentation base and a side surface bounding the indentation base oriented at an inclination angle relative to the circular indentation base and the processing system is configured to calculate the deformation function of the viscoelastic sample using the following equation: / l(d. = • tan(0) + bV . {cos"1 +Sin [cOS-1 (-----y- -)1 + (-----y- -)ljL \(d*tan(0)+b) / J \(d*tan(fl)+b) / Jwherein A(d, b, ()) represents instantaneous deformation of the viscoelastic sample as a function of d, b and 0, and wherein d is an indentation depth of the indenter tip, b is a radius of the circular indentation base, 6 is the inclination angle and v is a Poisson’s ratio of the viscoelastic sample.

11. The system of any preceding claim, wherein the processing system is configured to:perform interpolation on the force data and / or indentation depth data; and perform virtual oversampling on interpolated functions derived from the force data and / or indentation depth data.

12. The system of any preceding claim, wherein the viscoelastic sample has a power-law index from 0.1 to 0.4 in the power-law region of a graph of log(viscosity) against log(shear rate).

13. The system of any preceding claim, wherein the sample holder is a temperature-controlled sample holder.

14. The system of claim 13, wherein the temperature-controlled sample holder comprises:a platform;a heating plate;a rotatable sample tray comprising a plurality of recesses, wherein each recess is for holding the viscoelastic sample; anda sample cover arranged to expose a single recess of the plurality of recesses at a time.

15. The system of claim 14, wherein the platform, the heating plate, the rotating sample tray and the sample cover each comprise a respective aperture for receiving a common elongate member therethrough.

16. The system of claim 14 or 15, wherein the platform, the heating plate, the rotatable sample tray and the sample cover are secured together via the common elongate member and a fastener.

17. The system of any preceding claim, wherein the sample holder for receiving the viscoelastic sample comprises at least one cylindrical recess having a diameter greater than its height; wherein a maximum indentation depth is equal to or less than half of the height of the recess.

18. A method for rheological measurement of a viscoelastic sample, wherein the method comprises using a measurement apparatus, the measurement apparatus comprising:a sample holder;an indenter tip;a force sensor configured to measure a force at the indenter tip;an indentation mechanism arranged to urge the indenter tip towardthe sample holder; anda position sensor for measuring an indentation depth of the indenter tip, the method comprising:receiving a viscoelastic sample into the sample holder;using the indentation mechanism to urge the indenter tip toward the sample holder to indent the viscoelastic sample;acquiring force data using the force sensor and acquiring indentation depth data using the position sensor during indentation of the viscoelastic sample;sending the force data and indentation depth data to a processing system; andusing the processing system to determine, from the force data and the indentation depth data, one or more rheological properties of the viscoelastic sample.

19. The method of claim 18, comprising moving the indenter tip into the viscoelastic sample by a predetermined distance at a constant speed and, when the predetermined distance is reached, holding the indenter tip static for at least 100 seconds.

20. A measurement apparatus for rheological measurement of a viscoelastic sample, the measurement apparatus comprising:an indenter tip;a force sensor configured to measure a force at the indenter tip;an indentation mechanism arranged to urge the indentation tip toward the sample holder;a position sensor for measuring an indentation depth of the indenter tip; anda temperature-controlled sample holder comprising:a platform;a heating plate for controlling the temperature of the viscoelastic sample;a rotatable sample tray comprising a plurality of recesses, wherein each recess is for holding the viscoelastic sample; anda sample cover arranged to expose a single recess of the plurality of recesses at a time.

21. A temperature-controlled sample holder for an apparatus for rheological measurement of a viscoelastic sample, comprising:a platform;a heating plate for controlling the temperature of the viscoelastic sample;a rotatable sample tray comprising a plurality of recesses, wherein each recess is for holding the viscoelastic sample; anda sample cover arranged to expose a single recess of the plurality of recesses at a time.

22. A computer implemented method for rheological measurement of a viscoelastic sample comprising:receiving force data from a measurement apparatus, wherein the force data relates to a force measured at an indenter tip by a force sensor of the measurement apparatus during indentation of a viscoelastic sample;receiving indentation depth data from the measurement apparatus, wherein the indentation depth data relates to an indentation depth of the indenter tip determined by a position sensor of the measurement apparatus during indentation of the viscoelastic sample; anddetermining from the force data and the indentation depth data, one or more rheological properties of the viscoelastic sample.

23. A processing system for rheological measurement of a viscoelastic sample, the processing system comprising a memory and one or more processors, wherein the memory stores software which, when executed by the one or more processors causes the processing system to:receive force data from a measurement apparatus, wherein the force data relates to a force measured at an indenter tip by a force sensor of the measurement apparatus during indentation of a viscoelastic sample;receive indentation depth data from the measurement apparatus, wherein the indentation depth data relates to an indentation depth of the indenter tip determined by a position sensor of the measurement apparatus during indentation of the viscoelastic sample; anddetermine from the force data and the indentation depth data, one or more rheological properties of the viscoelastic sample.

24. A software tool for rheological measurement of a viscoelastic sample, the software tool comprising instructions for:receiving force data from a measurement apparatus, wherein the force data relates to a force measured at an indenter tip by a force sensor of the measurement apparatus during indentation of a viscoelastic sample;receiving indentation depth data from the measurement apparatus, wherein the indentation depth data relates to an indentation depth of the indenter tip determined by a position sensor of the measurement apparatus during indentation of the viscoelastic sample; anddetermining from the force data and the indentation depth data, one or more rheological properties of the viscoelastic sample.

25. A computer-implemented method of determining a Poisson’s ratio of a viscoelastic sample, the method comprising:receiving force data from a measurement apparatus, wherein the force data relates to a force measured by a force sensor of the measurement apparatus during indentation of a viscoelastic sample;receiving indentation depth data from the measurement apparatus, wherein the indentation depth data relates to an indentation depth of an indenter tip determined by a position sensor of the measurement apparatus during indentation of the viscoelastic sample; anddetermining from the force data and the indentation depth data, using an initial test value of a Poisson’s ratio of the viscoelastic sample, one or more rheological properties of the viscoelastic sample;receiving rheometric data representative of one or more rheological properties of a similar viscoelastic sample, the data being obtained using a second rheometric technique;changing the value of the Poisson’s ratio until the one or more rheological properties agrees with the rheometric data.

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