Device and method for measuring at least one property of a fluid

Abstract

The invention relates to a device for measuring at least one property of a fluid, comprising: - a cell (1) adapted to receive a volume of fluid, - an optical waveguide (2) comprising an input (20) adapted to be coupled to a light source (3) so as to transmit an optical signal (L) emitted by the light source, - an optomechanical resonator (7) arranged in the cell (1) so as to present at least one principal surface in contact with the fluid, the resonator comprising a suspended element (70) arranged in the vicinity of the optical waveguide (2) so as to allow evanescent coupling between the optical waveguide (2) and the suspended element (70), - a measuring unit (5) arranged at an output (21) of the optical waveguide (2) to receive the output optical signal, and - a processing unit coupled to the measuring unit. Figure for the abstract: Fig. 1

Description

Title of the invention: Device and method for measuring at least one property of a fluid. Technical field

[0001] The invention relates to a device for measuring at least one property of a fluid, as well as a measurement method implemented by means of said device. State of the art

[0002] There are different techniques for characterizing a fluid, based on different properties and different means of measurement.

[0003] Thus, for example, there are different types of viscometers or rheometers in the laboratory for measuring the viscosity of a fluid. However, each device is optimized for a specific type of fluid, so it may be necessary to have different devices to cover a wide range of fluids.

[0004] Moreover, these devices only allow the measurement of viscosity and other devices must therefore be used if it is desired to determine other properties of the fluid, such as its density, its refractive index or its thermal conductivity.

[0005] Finally, laboratory viscometers and rheometers are designed to receive samples of a relatively large volume.

[0006] However, in the field of diagnostics, significant needs are associated with "point-of-care" tests, that is, tests performed near the patient. These tests must be able to be carried out by personnel not necessarily trained in laboratory medicine and therefore require compact and easy-to-use devices.

[0007] Laboratory equipment is not suitable for this type of use.

[0008] There is therefore a need for a measuring device suitable for small fluid samples and capable of determining various properties thereof. Summary of the invention

[0009] The invention aims to remedy the aforementioned problems and to provide a measuring device capable of analyzing samples on the scale of a few microlitres with high precision while being able to determine several different properties of the fluid.

[0010] To this end, a first object of the invention is a device for measuring at least one property of a fluid, comprising:

[0011] - a cell adapted to receive a volume of fluid,

[0012] - an optical waveguide comprising an input adapted to be coupled to a light source so as to transmit an optical signal emitted by the light source,

[0013] - an optomechanical resonator arranged in the cell so as to present at least a main surface in contact with the fluid, the resonator comprising a suspended element arranged in the vicinity of the optical waveguide so as to allow evanescent coupling between the optical waveguide and the suspended element,

[0014] - a measuring unit arranged at an output of the optical waveguide to receive the optical output signal, comprising:

[0015] - a first detection unit configured to measure a low component output signal frequency, and

[0016] - a second detection unit for measuring a radio frequency component of the output signal, and

[0017] - a processing unit coupled to the measuring unit and configured to:

[0018] - from the measurement data of the first detection unit, determine a refractive index and / or thermal conductivity of the fluid, and / or

[0019] - from the measurement data of the second detection unit, determine a viscosity, density and / or compressibility of the fluid.

[0020] According to other advantageous features of the invention, taken independently or in combination where technically feasible:

[0021] - the suspended element is a disk or ring attached to a substrate defining one side of the cell by a central foot;

[0022] - the suspended element has an oblong shape and is attached to a substrate defining one face of the cell by two feet;

[0023] - the suspended element is a nanostructured beam joined at its two ends of a substrate defining two faces of the cell;

[0024] - the suspended element is made of an optomechanical crystal, in particular silicon;

[0025] - the cell has a volume of less than 1 microlitre, preferably less than 1 nanoliter;

[0026] - the cell has at least one dimension less than 1 mm, preferably less than 200 pm;

[0027] - the cell is closed;

[0028] - the cell is a microfluidic channel extending between an inlet orifice and a fluid outlet port;

[0029] - the device further comprises at least one actuation device adapted for make the suspended element vibrate;

[0030] - the device further includes a means for controlling the temperature of the fluid in the cell.

[0031] Another object of the invention is a method for measuring at least one property of a fluid using the device described above. Said method comprises:

[0032] - the introduction of the fluid into the cell of said device,

[0033] - the transmission of an optical signal emitted by the light source through the guide optical wave,

[0034] - the excitation of at least one optical mode of the optomechanical resonator by the evanescent coupling of the suspended element and the optical waveguide,

[0035] - the oscillation of the suspended element according to at least one resonance mode mechanical, said oscillation affecting the signal transmitted by the optical waveguide through evanescent coupling,

[0036] - the measurement, by the first detection unit, of a low-frequency component of the output signal,

[0037] - the measurement, by the second detection unit, of a radio frequency component of the output signal,

[0038] - from the measurement data of the first detection unit, the determination of a refractive index and / or thermal conductivity of the fluid,

[0039] - from the measurement data of the second detection unit, the determination of a viscosity, density and / or compressibility of the fluid.

[0040] According to other advantageous features of the invention, taken alone or in combination where technically feasible:

[0041] - the fluid is static in the cell;

[0042] - the fluid is flowing in the microfluidic channel;

[0043] - the method includes a power scan of the light source;

[0044] - the method includes a wavelength scanning of the light source;

[0045] - the actuation device is activated to set the suspended element into vibration, and the detection unit measures a resonance frequency of the suspended element;

[0046] - the suspended element is activated into vibration by the thermomechanical noise generated by the molecules of the fluid, and the detection device measures a resonance frequency of the suspended element;

[0047] - the method comprises the simultaneous transmission, via the optical waveguide, of two light beams of different wavelengths, each wavelength being associated with the measurement of a low frequency component, respectively radio frequency, of the output signal, and the simultaneous measurement of said components by the first and second detection unit. Brief description of the figures

[0048] Other features and advantages of the invention will become apparent from the detailed description that follows, with reference to the accompanying drawings, in which:

[0049] - the [Fig.1] is a schematic diagram of the measuring device;

[0050] - [Fig.2] is a diagram of the waveguide and optomechanical resonator;

[0051] - [Fig.3] is a partial view of the waveguide and optomechanical resonator allowing evanescent coupling of light circulating in the waveguide;

[0052] - [Fig.4A], [Fig.4B], [Fig.4C], [Fig.4D] and [Fig.4E] illustrate different modes of embodiment of the optomechanical resonator;

[0053] - [Fig. 5] is a scanning electron microscope image of a device electrostatic actuation of the optomechanical resonator;

[0054] - [Fig.6] is a flowchart outlining the measurement protocol;

[0055] - [Fig.7] is an example of the thermo-optical response of a resonator optomechanical according to an embodiment of the invention; the figure gives the optical transmission rate as a function of wavelength for increasing input power.

[0056] For the sake of clarity in the figures, the various elements are not necessarily drawn to scale. Identical elements or elements performing the same function are designated from one figure to another by the same reference symbol and are not fully described each time. Detailed description of implementation methods

[0057] Fig. 1 is a schematic diagram of the measuring device according to the invention.

[0058] The device comprises a cell 1 for receiving a fluid sample. Preferably, the cell is of the microfluidic type, i.e., the cell has at least one dimension less than 1 mm, preferably less than 200 µm. Advantageously, the internal volume of the cell is less than 1 microliter, preferably less than 1 nanoliter. However, for certain applications, the device can be used with a larger fluid volume.

[0059] In some embodiments, the cell is closed, with the sample remaining static within the cell. The sample can be introduced into the cell by means of an external device, for example a syringe.

[0060] In other embodiments, the cell is in the form of a channel through which the sample flows. The channel extends between a sample inlet and a sample outlet. The movement of the sample between the inlet and outlet can be caused, for example, by a pump or any other suitable device.

[0061] The fluid can be a liquid or a gas.

[0062] Among the liquids of interest, we can mention biological fluids, in particular blood plasma, but also polymers, inks or electrolytes used in batteries. The fluid may consist of a material that is in a liquid state at the time of measurement but which subsequently solidifies.

[0063] Among the gases of interest, we can mention hydrocarbon gases, such as methane.

[0064] The cell is advantageously formed in or on a chip. The cell walls can be in particular etched into a silicon substrate or another substrate used in microelectronics or biology, such as a polymer or a ceramic.

[0065] The cell may include means (not shown) for controlling the fluid temperature, to maintain the fluid at a predetermined temperature and / or to vary the fluid temperature in a controlled manner. These means may, for example, take the form of a Peltier element arranged in contact with one face of the cell. Alternatively, these means may take the form of a micro-heater, which is similar to an electrical resistor with dimensions ranging from a few tens to a few hundred micrometers. Such micro-heaters are commonly used in the field of silicon photonics. Finally, as mentioned below, the fluid can be heated locally via the optomechanical resonator arranged in the cell by injecting an optical beam of suitable power into it.

[0066] The device further includes an optical waveguide 2 extending between an input 20 and an output 21. The input of the optical waveguide is coupled to a light source 3, such as a laser, allowing an optical signal to be injected into the optical waveguide.

[0067] The link between the light source and the waveguide can be made by an optical fiber 4 or any other suitable optical linking means.

[0068] The output of the waveguide is coupled to a measuring unit 5, which will be described in detail below. The connection between the waveguide and the measuring unit can be made by means of an optical fiber 60, 61, or any other suitable optical link.

[0069] The device comprises an optomechanical resonator arranged in the cell such that at least one of its principal faces is in contact with the fluid. An optomechanical resonator is understood to have at least one optical mode, i.e., a mode for confining an incident light beam, and one mechanical mode, i.e., a mode of vibration in a resonant state. Confinement of a light beam can typically be achieved at a refractive index gradient within the resonator. Such a difference in refractive index can be obtained by using two different materials within the resonator (for example, alternating regions of silicon and regions of silicon nitride), by imposing a difference in impurity concentration between two regions of the resonator (for example, in the case of silicon oxide), or at the interface between the resonator and another medium, for example, the surrounding fluid.The geometry and material of the optomechanical resonator can thus be chosen to obtain the desired optical and mechanical modes.

[0070] Particularly advantageously, the optomechanical resonator is of micrometric dimensions, i.e., it is suitable for insertion into a microfluidic cell. Thus, the optomechanical resonator has at least one dimension less than 1 mm, preferably less than 100 µm.

[0071] Figure 2 is a perspective diagram of the waveguide and the optomechanical resonator. The cell walls and the light source are not shown in this diagram.

[0072] The waveguide 2 comprises an elongated main body with a rectangular cross-section. The inlet 20 has a flared shape that narrows towards the junction with the main body; conversely, the outlet 21 has a flared shape that widens from the junction with the main body. The inlet and outlet have diffraction gratings (or "grating couplers") whose function is to couple the incident optical beam L, exiting the plane of the waveguide respectively, with the waveguide extending in a plane. In practice, the optical fibers supplying and recovering the optical beam are positioned with a near-normal incidence relative to the optical waveguide.

[0073] The light source (laser) is advantageously tunable within a specific wavelength range. For example, for a GaAs waveguide, the wavelengths are greater than 900 nm. When the waveguide is made of silicon, the wavelengths in which the device operates are between 1100 and 2000 nm and can potentially reach up to 7000 nm. The wide range of possible wavelengths makes it possible to design devices adapted to the fluid being studied. Indeed, one can choose a wavelength that falls within a window of transparency of the fluid; conversely, it may be advantageous to position oneself near an absorption wavelength for the fluid in order to have a means of localized heating or to obtain a spectral signature of the fluid.

[0074] In practice, the device advantageously has a wide operating range (optical resonances can extend over a large wavelength range greater than a few hundred nm), which is mainly limited by diffraction gratings which have a bandwidth of 20-30 nm, for example between 1540 nm and 1570 nm.

[0075] The optical waveguide is arranged at least partially inside the cell in the vicinity of the optomechanical resonator. Preferably, the inlet and outlet of the optical waveguide are arranged outside the cell, with the optical waveguide passing through the cell wall in a fluid-tight manner. Alternatively, the optical waveguide may be arranged entirely within the cell.

[0076] The optomechanical resonator 7 comprises a suspended element 70 (in this illustration, in the form of a disc) and a central foot 71 connecting the suspended element to An unrepresented support. The support is, for example, the cell background, which is parallel to the plane in which the suspended element extends, with the central support extending from the underside of the suspended element. In other embodiments, the central support may extend from the top side of the suspended element, or even from both the underside and the top side of the suspended element.

[0077] In some embodiments, the resonator is immersed in the fluid, such that the two opposite principal faces of the suspended element are in contact with the fluid. In other embodiments, only one principal face of the suspended element is in contact with the fluid.

[0078] The central foot can be solid or hollow. In some embodiments (see [Fig. 4E]), a fluidic channel 72 is arranged in the central foot 71 and passes through the suspended element 70, thus bringing the fluid into contact with the face of the suspended element opposite the central foot. In [Fig. 4E], the face in contact with the fluid is the lower face of the suspended element.

[0079] In the embodiment illustrated in [Fig. 3], the suspended element 70 extends in the same plane as the optical waveguide 2. The suspended element 70 therefore has at least one principal face coplanar with a principal face of the optical waveguide. By principal face, we mean a face of the suspended element having the largest surface area.

[0080] To simplify the fabrication of the suspended element and the optical waveguide, the suspended element and the optical waveguide advantageously have the same thickness, their two main faces being coplanar. However, in other embodiments, the suspended element may be thicker or thinner than the optical waveguide.

[0081] Alternatively, the optical waveguide does not extend in the same plane as the suspended element but can be arranged above or below the suspended element, parallel to it.

[0082] In all cases, the suspended element 70 is arranged at a distance d from the optical waveguide 2 which is chosen to be sufficiently small to allow evanescent coupling of the light between the optical waveguide and the suspended element. The distance d may depend on the wavelength of the optical signal. Generally, the distance d, which is measured between the points closest to the optical waveguide and the suspended element, is on the order of 100 to 200 nm in the infrared range (i.e., around 1500 nm).

[0083] In a manner known per se, evanescent coupling is based on the existence of an evanescent wave, which is a wave whose amplitude decreases exponentially with the distance from the optical waveguide, and which is generated by the passage of the optical signal through the optical waveguide. An optical mode of the suspended element present The evanescent field is therefore excited by the optical signal traveling through the optical waveguide. Conversely, a photon from the optical mode of the suspended element can penetrate the optical waveguide and modify the beam collected at the output of the optical waveguide.

[0084] For this purpose, the suspended element and the optical waveguide are made of a material substantially transparent to an optical signal in the wavelength range emitted by the light source.

[0085] Silicon is a particularly advantageous material due to its optical refractive index in the considered wavelength range and its mechanical properties. Furthermore, it can be etched using well-established techniques in the field of microelectronics. However, other materials can be used as substitutes for silicon, for example gallium arsenide (GaAs) or silicon nitride (SiN).

[0086] The evanescent coupling can be optimized by adjusting, in particular, the width of the optical waveguide and / or the shape of the optical waveguide in the coupling region to maximize the length of the waveguide arranged at a distance d from the suspended element. For example, if the suspended element is rounded, the optical waveguide can have a rounded portion parallel to the suspended element to increase the evanescent coupling effect.

[0087] The suspended element is capable of being set into oscillation by optical, thermal, and / or electrostatic means. The oscillation occurs in the plane of the main surface of the suspended element. Conversely, an oscillation of the suspended element in the plane of its main surface modifies the optical signal transmitted by the optical waveguide and imparts its motion to the amplitude and / or phase of the output optical beam.

[0088] The behavior of the suspended element depends on the fluid with which it is in contact.

[0089] The actuation can be generated deterministically (caused for example by an external actuation) or randomly (caused for example by thermomechanical noise generated by the collision of fluid molecules on the suspended element).

[0090] With regard to optical actuation means, the excitation may result from a intensity modulated optical field present in the suspended element, resulting from evanescent coupling with the optical waveguide.

[0091] With reference to [Fig. 5], the device may include an electrostatic actuation means for the suspended element. This actuation means includes an electrode 8 arranged in the plane of the suspended element. Applying a potential difference between the suspended element and the electrode generates a capacitive force which deforms the suspended element 70 (which has an annular shape in the illustrated embodiment).

[0092] Figures 4A to 4E illustrate, in side view and top view, different embodiments of the optomechanical resonator.

[0093] In some embodiments, the suspended element has a circular shape (disc or ring) and is connected to the support by a central foot.

[0094] Figure 4A illustrates a suspended element 70 having a disc shape and a central foot with a circular cross-section. The diameter of the disc is typically on the order of 2 to 60 pm, preferably on the order of 20 pm. The diameter of the central foot is typically on the order of 500 nm to 10 pm, preferably on the order of 1 pm.

[0095] Fig. 4B illustrates a suspended element 70 having a ring shape and a central foot connected to the ring by radial bridges.

[0096] In other embodiments, as illustrated in [Fig.4C], the suspended element 70 has an oblong shape, consisting of two arcs of a circle connected by straight segments, and is connected to the support by two feet 71 arranged at the center of each respective arc of a circle.

[0097] In other embodiments, the suspended element takes the form of a nanostructured beam embedded at both ends in the support, as illustrated in [Fig. 4D]. The nanostructuring is chosen to enable the generation of localized optical and mechanical vibration modes within the beam. Such a nanostructuring includes, for example, oval patterns connected or not by rectangular patterns. The beam length is typically on the order of 5 to 100 µm, the beam thickness is on the order of 100 to 500 nm, and the beam width is on the order of 100 nm to 1 µm.

[0098] Figure 4E illustrates another embodiment in which the suspended element is in the form of a disc, but the disc and the central foot are traversed from one side to the other by a microfluidic channel. This embodiment is particularly advantageous when only one face of the suspended element needs to be in contact with the fluid. In this case, the microfluidic channel 72 allows the fluid sample to be injected through the disc 70 to bring it into contact with the face opposite the central foot 71.

[0099] Returning to [Fig.1], the unit of measurement 5 comprises two detection units.

[0100] A first detection unit 50 allows a low component to be measured output signal frequency.

[0101] A second detection unit 51 allows a radio frequency component of the output signal to be measured.

[0102] To this end, the output signal of the optical waveguide is separated by a symmetrical or asymmetrical beam splitter 6. A first part of the signal is conducted to the first detection unit 50 by an optical fiber 60 for analysis of the low frequency component of the signal and the other part of the optical signal is conducted to the second detection unit 51 by an optical fiber 61 for analysis of the radio frequency component of the signal.

[0103] The first detection unit 50 comprises a PD photodetector and an analog-to-digital converter ADC arranged at the output of the photodetector.

[0104] The second detection unit 51 comprises successively an optical amplifier EDFA, a photodetector PD, a radio frequency amplifier RFA and a spectral analyzer ESA.

[0105] Fig. 6 is a flowchart showing the operation of the measuring device.

[0106] A first phase I prior to measurement aims to establish and stabilize the operating conditions: ambient temperature (1.1), optical transmission (1.2), fluid level in the microfluidic cell (1.3). In addition, models of the optical, thermal and mechanical response of the optomechanical resonator in contact with different fluids are recorded (1.4).

[0107] A second phase II of data collection is implemented using the unit of measurement.

[0108] A first portion of the optical signal is directed by the beam splitter to the first detection unit, which records (II.1) the low-frequency component of the optical signal. This allows the purely optical response O of the resonator in its linear regime to be measured for a low optical power (LP) of the light source (typically less than 10 pW). This response makes it possible to determine the refractive index of the fluid. When the optical power (HP, typically up to 200 pW or above) is increased, the resonator's response Th is modified by a thermo-optical effect and allows the thermal conductivity of the fluid to be measured by evaluating the heat dissipation within the fluid.

[0109] Figure 7 illustrates this thermo-optical effect on the transmission curve T (in %) as a function of wavelength X (in nm) when the fluid is water. As the optical power Pin is increased between 110 pW and 970 pW, the transmission minimum (and the optical resonance wavelength) shifts towards higher wavelengths. At low power, heating of the structure has a negligible impact on the resonance wavelength, and the cell response resembles a Lorentzian curve. At higher power levels, cell heating alters the resonance wavelength. The amount of light entering the cavity increases with the resonance wavelength, which shifts to the right as the wavelength is swept to the right. The thermo-optical response is also characterized by the existence of a hysteresis loop of the wavelength response when sweeping the wavelength downwards (not shown in this figure).

[0110] In parallel, the second part of the output signal from the optical waveguide is directed by the beam splitter to the second detection unit capable of measuring the RF component (II.2). This allows the mechanical response of the resonator to be measured and the viscosity, compressibility and density of the fluid to be deduced.

[0111] All these measurements are carried out very quickly, typically in a few seconds.

[0112] The measurements are first carried out on a reference fluid (II.3) and then, once the device is calibrated, on the fluid of interest (II.4).

[0113] When using a single light beam, the low-frequency measurement and the RF measurement are performed successively. Since each of these measurements is very rapid, this does not impair the performance of the device.

[0114] Furthermore, it is possible to perform the low-frequency measurement and the radio-frequency measurement simultaneously, for example, by using optical multiplexing to inject two light beams with different wavelengths into the optical waveguide. Each beam implements a different optical mode of the suspended element and is associated with the measurement of the low-frequency component, respectively the radio frequency component. At the output of the optical waveguide, optical demultiplexing can optionally be used to separate the two wavelengths upstream of the photodiodes.

[0115] Another use case for two light beams concerns the injection of optical power into the suspended element so as to heat it and thereby heat the surrounding fluid. This configuration can allow for very localized heating of the fluid, particularly in the case of a microfluidic cell with micrometer dimensions. This optical heating method can be implemented using a second laser, called a pump laser, whose beam is superimposed in the optical waveguide on that of the measurement laser. Filtering (or demultiplexing) means are used to separate the optical signals at the output of the optical waveguide if necessary.

[0116] Finally, a third post-processing phase III is implemented. This post-processing aims to compare the models with measurements previously made with a known fluid (typically water, or a mixture of water and glycerol at different concentrations) (III.1), to update these models (III.2) and to use the models thus calibrated to determine the refractive index, thermal conductivity, density, viscosity and compressibility of the fluid of interest (III.3).

[0117] The frequency response of an oscillating disk can be modeled by the following system of formulas:

[0118]

[0119]

[0120]

[0121]

[0122]

[0123]

[0124]

[0125]

[0126]

[0127]

[0128]

[0129]

[0130]

[0131]

[0132]

[0133]

[0134]

[0135] Equation (1) governs the evolution of the optical field in the cell. Equation (2) governs the propagation of heat as a function of thermal conductivity k. Equation (3) is the modified equation of motion of the optomechanical resonator. a is the optical field in the cell, Pbus is the incident optical power in the optical waveguide, k is the decay of the optical mode, equal to Kabs + Krad + KeX where Kabs, Krad are the loss rates by absorption, scattering and exchange respectively, A = coL - coc is the mismatch between the laser and the cell, x is the position of the suspended element, gom = - dcoc / dx is the optomechanical coupling coefficient, T is the temperature, neff is the effective refractive index of the optical mode, which depends on the refractive index of the disk and the refractive index of its environment. Q(r) = Pabs / V is the heat source, Pabs = Kabslal2 is the optical power absorbed in the suspended disk-shaped element of volume V, is a random Langevin force, cos2 = ks / ms is the angular resonance frequency, ks is the elastic constant of the vibration mode, ms is the associated mass, and the relative mass mr and the depreciation rate are defined as follows:

[0136] where p is the viscosity

[0137] p and ps are the density of the liquid and the solid,

[0138] k is the Fonde propagation vector in the liquid, k being equal to cos / c where c is the speed of sound in the liquid,

[0139] J0(x) and H0(x) are the Bessel and Struve functions, respectively.

[0140] Although only one optomechanical resonator is illustrated in the figures shown, different embodiments of the device may include several optomechanical resonators.

[0141] Thus, the device may comprise at least two optomechanical resonators in the same cell. These resonators may be arranged in the vicinity of the same optical waveguide, thereby providing redundant measurements. Alternatively, the resonators may be arranged in the vicinity of two different waveguides through which different light beams circulate, in order to perform different measurements simultaneously. In other cases, the resonators may be arranged in the vicinity of the same optical waveguide through which, by means of multiplexing, two different light beams circulate, in order to excite different optical modes from one resonator to the other. The resonators are advantageously identical but may optionally be different.

[0142] In other embodiments, the device comprises several cells, each cell comprising at least one optomechanical resonator. Each cell may contain the same fluid, thus enabling redundant measurements or the simultaneous determination of different fluid properties. Alternatively, each cell contains a different fluid, so as to perform simultaneous analyses on several different fluids. Examples

[0143] The device was tested with glycerol (CsHsOs) as the reference fluid.

[0144] Tests were then carried out with decan-l-ol (CH3(CH2)sCH2OH), which exhibits non-Newtonian behavior with a frequency-dependent viscosity in the ultra-high frequency (UHF) range.

Claims

Demands

1. A device for measuring at least one property of a fluid, comprising: - a cell (1) adapted to receive a volume of fluid, - an optical waveguide (2) comprising an input (20) adapted to be coupled to a light source (3) so as to transmit an optical signal (L) emitted by the light source, - an optomechanical resonator (7) arranged in the cell (1) so as to present at least one principal surface in contact with the fluid, the resonator comprising a suspended element (70) arranged in the vicinity of the optical waveguide (2) so as to allow evanescent coupling between the optical waveguide (2) and the suspended element (70), - a measuring unit (5) arranged at an output (21) of the optical waveguide (2) to receive the output optical signal, comprising: - a first detection unit (50) configured to measure a low-frequency component of the output signal,and - a second detection unit (51) for measuring a radio frequency component of the output signal, and - a processing unit coupled to the measurement unit and configured to: - from the measurement data of the first detection unit, determine a refractive index and / or a thermal conductivity of the fluid, and / or - from the measurement data of the second detection unit, determine a viscosity, a density and / or a compressibility of the fluid.

2. Device according to claim 1, wherein the suspended element (70) is a disk or a ring attached to a substrate defining a face of the cell by a central foot (71).

3. Device according to claim 1, wherein the suspended element (70) has an oblong shape and is attached to a substrate defining a face of the cell by two feet (71).

4. Device according to claim 1, wherein the suspended element (70) is a nanostructured beam attached at its two ends to a substrate defining two faces of the cell.

5. Device according to any one of claims 1 to 4, wherein the suspended element (70) is made of an optomechanical crystal, in particular silicon.

6. Device according to any one of claims 1 to 5, wherein the cell (1) has a volume of less than 1 microlitre, preferably less than 1 nanolitre.

7. Device according to any one of claims 1 to 6, wherein the cell (1) has at least one dimension less than 1 mm, preferably less than 200 pm.

8. Device according to any one of claims 1 to 7, wherein the cell (1) is closed.

9. Device according to any one of claims 1 to 7, wherein the cell (1) is a microfluidic channel extending between a fluid inlet orifice and a fluid outlet orifice.

10. Device according to any one of claims 1 to 9, further comprising at least one actuation device (8) adapted to vibrate the suspended element (70).

11. Device according to any one of claims 1 to 10, further comprising a means for controlling the temperature of the fluid in the cell (1).

12. A method for measuring at least one property of a fluid, comprising: - placing the fluid in the cell (1) of a device according to any one of claims 1 to 11, - transmitting an optical signal (L) emitted by the light source (3) through the optical waveguide (2), - exciting at least one optical mode of the optomechanical resonator (7) by the evanescent coupling of the suspended element (70) and the optical waveguide (2), - setting the suspended element (70) into oscillation according to at least one mechanical resonance mode, said oscillation affecting the signal transmitted by the optical waveguide (2) through the evanescent coupling, - measuring, by the first detection unit (50), a low-frequency component of the output signal, - measuring, by the second detection unit (51), a radio-frequency component of the output signal, - from the measurement data of the first detection unit,the determination of a refractive index and / or thermal conductivity of the fluid, - from the measurement data of the second detection unit, the determination of a viscosity, density and / or compressibility of the fluid.

13. A method according to claim 12 in its dependent relationship with claim 8, wherein the fluid is static in the cell (1).

14. A method according to claim 12 in its dependent relationship with claim 9, wherein the fluid is flowing in the microfluidic channel.

15. A method according to any one of claims 12 to 14, comprising a power scanning of the light source.

16. A method according to any one of claims 12 to 14, comprising a wavelength scanning of the light source.

17. A method according to any one of claims 12 to 16 in its dependent relationship with claim 10, wherein the actuation device is activated to vibrate the suspended element (70), and wherein the sensing unit measures a resonance frequency of the suspended element.

18. A method according to any one of claims 12 to 16, wherein the suspended element is activated into vibration by the thermomechanical noise generated by the fluid molecules, and wherein the sensing device measures a resonance frequency of the suspended element.

19. A method according to any one of claims 12 to 18, comprising the simultaneous transmission, by the optical waveguide, of two light beams of different wavelengths, each wavelength being associated with the measurement of a low-frequency component, respectively radio frequency, of the output signal, and the simultaneous measurement of said components by the first and second detection unit.

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