A liquid viscosity measuring device and method based on free vibration of liquid drops

Abstract

The present application relates to a kind of liquid viscosity measuring device and method based on liquid drop free vibration, belong to the technical field of fluid mechanics.The device includes hydrophilic circular platform and piezoresistive cantilever beam array, piezoresistive cantilever beam array includes multiple piezoresistive cantilever beams, any piezoresistive cantilever beam includes two cantilever beams and two piezoresistive sensors, bearing microcolumn is located above the free end of cantilever beam, bearing microcolumn is located in the periphery of hydrophilic circular platform, and bearing microcolumn is not connected with hydrophilic circular platform, and bearing microcolumn is used to carry the edge of liquid drop;Piezoresistive sensor is located at the root of cantilever beam fixed end, and piezoresistive sensor is used to detect the force acting on bearing microcolumn, and cantilever beam fixed end is rigidly connected with device silicon layer.The present application uses piezoresistive cantilever beam to detect the liquid drop free vibration of target liquid caused by buzzer, and calculates the viscosity according to the free vibration attenuation characteristic of target liquid.The present application device is small in size, and the volume requirement of target liquid is small, and accuracy and sensitivity are high.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of fluid mechanics, more particularly, to a liquid viscosity measuring device and method based on free vibration of liquid droplet, in particular, to a measuring device and method for measuring liquid viscosity by measuring the attenuation rate of free vibration of liquid droplet. BACKGROUND

[0002] Liquid viscosity is an important physical parameter for evaluating its flow properties, and the detection of liquid viscosity is widely used in the fields of chemistry, medicine, biotechnology and material science. The measurement of liquid viscosity has guiding significance for product quality control, research and development of new materials, and clinical diagnosis of related human pathological states. For example, plasma viscosity is very important in blood clotting tests, and viscosity is also an important indicator for characterizing antibody solutions and protein drugs and DNA analysis.

[0003] Traditionally, the measurement method of liquid viscosity is mostly based on large volume samples, including rotor viscometer, capillary viscometer, falling ball viscometer, etc. These methods usually require milliliter level sample amount for effective measurement. However, when the sample is scarce or the cost is high, such as biological samples or newly developed drugs, the practical application of these methods is subject to certain constraints.

[0004] In the latest development of micro-rheology, a viscosity meter based on microfluidic technology has been developed, which is an instrument that uses microfluidic channels to measure liquid. These viscosity meters can be generally divided into two categories: one uses a syringe pump to control the flow rate, which requires a relatively large amount of target liquid (20 microliters) to accurately measure the viscosity of the liquid; the other method uses capillary pressure to drive the target liquid, and detects the speed of the target liquid filling the microchannel to measure the viscosity. This method only requires nanoliters of liquid for measurement, but the change in channel wettability will affect the flow, and the change in channel wettability after measurement will limit the reuse of the viscosity meter, which is relatively high in cost. SUMMARY

[0005] The present application provides a device and method for detecting liquid viscosity, which uses a piezoresistive cantilever beam to detect the free vibration of liquid droplets generated by the target liquid caused by a buzzer, and calculates the viscosity of the target liquid according to the free vibration attenuation characteristics of the target liquid. The device of the present application has small volume, small volume requirement of target liquid, high accuracy and sensitivity, and solves the problems of high volume requirement, bulky and inconvenient measurement instrument, and difficulty in repeated use in the measurement of target liquid viscosity.

[0006] According to a first aspect of the present application, there is provided a liquid viscosity measuring device based on free vibration of liquid droplet, comprising a hydrophilic circular platform for attaching a fixed liquid droplet and a piezoresistive cantilever beam array comprising a plurality of piezoresistive cantilever beams, any of which comprises two cantilever beams and two piezoresistive sensors, a bearing micro-column is located above the free end of the cantilever beam and at the periphery of the hydrophilic circular platform, and the bearing micro-column is not connected to the hydrophilic circular platform.

[0007] The free end of the cantilever beam is located on both sides of the bottom surface of the bearing micro-column, and the piezoresistive sensor is located at the root of the fixed end of the cantilever beam, the piezoresistive sensor is used to detect the force of the liquid droplet acting on the bearing micro-column, and the fixed end is rigidly connected to the device silicon layer.

[0008] A buzzer is arranged below the hydrophilic circular platform, and the buzzer is used to output sound waves to excite the liquid droplet to vibrate.

[0009] Preferably, the hydrophilic circular platform is obtained by depositing a hydrophilic material on the top device silicon layer.

[0010] Preferably, the top device silicon layer further has an intermediate insulating layer and a bottom substrate silicon layer in sequence.

[0011] Preferably, the vertical distance between the buzzer and the hydrophilic circular platform is 3-5 mm.

[0012] According to another aspect of the present application, there is provided a method for measuring the viscosity of a liquid using any of the devices, comprising the following steps:

[0013] S1: Dropping the target liquid on the hydrophilic circular platform to form a target liquid droplet;

[0014] S2: When the target liquid droplet is balanced on the bearing micro-column, the piezoresistive sensor receives the pressure of the bearing micro-column and converts it into an analog electrical signal, which is then converted into a digital signal, which is processed by a digital signal processor and then converted into an analog signal;

[0015] S3: After the buzzer receives the analog signal in step S2, the buzzer outputs sound waves to excite the liquid droplet to vibrate;

[0016] S4: After the buzzer stops outputting sound waves, the target liquid droplet vibrates freely, and the pressure generated by the free vibration of the target liquid droplet is transmitted to the piezoresistive sensor through the bearing micro-column and the cantilever beam;

[0017] S5: The piezoresistive sensor converts the pressure into an analog electrical signal, which is then converted into a digital signal, and the digital signal processor performs exponential fitting on the maximum peak data points in a single period of the decay vibration electrical signal of the digital signal to obtain the decay rate a; according to the formula The target liquid kinematic viscosity υ is calculated, wherein a is the attenuation rate, k is the proportional relationship coefficient, and V is the target liquid drop volume; the target liquid dynamic viscosity μ is calculated according to the formula μ = υρ, wherein ρ is the target liquid density and υ is the target liquid kinematic viscosity.

[0018] Overall, compared with the prior art, the above technical solutions conceived by the present application mainly have the following technical advantages:

[0019] (1) The entire device of the present application is based on a micro-electromechanical system, and the entire device is only centimeter-sized, portable and not heavy, and is convenient to carry and use.

[0020] (2) The present application measures the viscosity of the target liquid by measuring the free vibration of the liquid drop, thereby realizing the small volume requirement (about 2-3 microliters) of the target liquid, which is about eight times smaller than the liquid volume required by the currently commercially available portable viscometer.

[0021] (3) The buzzer of the present application is used to output a certain time (about 1s) and a specific frequency (close to the first-order natural frequency of the target liquid drop, about 100Hz) sound wave to excite the liquid drop to generate a first-order modal vibration (forced), and after the buzzer stops outputting the sound wave, the liquid drop only performs a first-order modal free vibration, avoiding the interference of other order modal vibrations, thereby improving the reliability of the entire device. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 It is a schematic view of a liquid drop vibration attenuation rate excitation detection device of the present application.

[0023] Figure 2 It is a schematic view of a liquid drop vibration attenuation rate excitation detection device of the present application.

[0024] Figure 3 It is a three-dimensional schematic view of a piezoresistive cantilever beam and a bearing micro column of the present application.

[0025] Figure 4 It is a schematic view of an electrical signal for detecting forced vibration and free decay vibration and a fitting curve.

[0026] Figure 5 It is a work flow chart of the entire system of the present application.

[0027] In all the drawings, the same reference signs are used to represent the same elements or structures, wherein: 1 is a hydrophilic circular truncated cone, 2 is a piezoresistive cantilever beam array, 3 is a piezoresistive cantilever beam, 4 is a bearing micro column, 5 is a cantilever beam, 6 is a piezoresistive sensor, 7 is a buzzer, 8 is a top device silicon layer, 9 is an intermediate insulating layer, and 10 is a bottom substrate silicon layer. DETAILED DESCRIPTION

[0028] In order to make the objects, technical solutions and advantages of the present application clearer, further detailed description will be made to the present application in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and not used to limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0029] The liquid viscosity measuring device based on free vibration of liquid droplet includes a disc substrate (top device silicon layer 8, middle insulating layer 9, bottom substrate silicon layer 10) for placing a liquid droplet with a specific volume, the top device silicon layer 8 of the disc substrate has a hydrophilic circular platform 1 (the upper surface is coated with a hydrophilic material by atomic layer deposition, the diameter is 2 mm), a piezoresistive cantilever beam array 2 and a bearing micro column 4 (50 μm x 50 μm x 50 μm, a total of 24, of which 8 are located above the free end of the piezoresistive cantilever beam), the hydrophobic side of the bearing micro column 4 (the upper surface is coated with a hydrophilic material by atomic layer deposition, and the four side surfaces are coated with a hydrophobic material by atomic layer deposition) can ensure that the liquid droplet attached to the surface thereof has a large contact angle (greater than 120 degrees); the hydrophilic upper surface of the bearing micro column 4 and the hydrophilic circular platform 1 can fix the target liquid droplet and prevent it from moving horizontally. The force change under the large contact angle is more obvious, which can ensure the accurate determination of the decay rate of the free vibration of the liquid droplet. The center bottom of the hydrophilic circular platform 1 is provided with a buzzer 7 for outputting an acoustic wave with a specific frequency (close to the first-order natural frequency of the target liquid droplet, about 100 Hz) for a certain time (about 1 s) to make the target liquid droplet vibrate in the first-order mode (forced). The continuous vibration of the liquid droplet can make the target liquid droplet completely wet the upper surface of the bearing micro column 4, so that the liquid droplet is pinned at the edge of the bearing micro column 4, ensuring that the liquid droplet does not slip, and after the buzzer 7 stops outputting the acoustic wave, the target liquid droplet performs free vibration in the first-order mode.

[0030] In some embodiments, the piezoresistive cantilever beam array 2 is in the form of a'm' son distribution, which is composed of 8 piezoresistive cantilever beams 3, each of which is composed of two cantilever beams 5 and two piezoresistive sensors 6. The bearing micro column 4 is located at the periphery of the hydrophilic circular platform 1, and the bearing micro column 4 is not connected with the hydrophilic circular platform 1, and the bearing micro column 4 is used to support the edge of the liquid droplet. The free ends of the two cantilever beams 5 are respectively located on both sides of the bottom surface of the bearing micro column 4, and the piezoresistive sensors 6 are located at the roots of the fixed ends of the cantilever beams 5. The piezoresistive sensors 6 are used to detect the force acting on the bearing micro column 4, and the plurality of piezoresistive sensors 6 can more accurately detect the change of the force on the upper surface of the bearing micro column 4, and the fixed end of the cantilever beam 5 is rigidly connected with the top device silicon layer 8.

[0031] The entire device of the present application is based on micro-electro-mechanical system. The buzzer 7 outputs an acoustic signal with a time (about 1s) close to the inherent frequency (about 100Hz) of the target liquid droplet to stimulate the target liquid droplet to generate a first-order modal vibration (forced). After the buzzer 7 stops outputting the acoustic signal, the target liquid droplet performs a first-order modal free vibration. The free vibration of the target liquid droplet on the surface of the bearing micro-column 4 generates a changing force, which causes the resistance value of the piezoresistive sensor 6 to change, thereby forming an analog electrical signal change of vibration and attenuated vibration. The analog electrical signal change is converted into a digital signal by an application-specific integrated circuit (ASIC) through an analog / digital converter. The change of the digital signal with time can describe the attenuated vibration of the free vibration of the target liquid droplet. The digital signal processor obtains the attenuation rate of the free vibration of the target liquid droplet by exponentially fitting the maximum amplitude data points in each cycle of the digital signal through an exponential attenuation model. The kinematic viscosity of the target liquid is estimated by the corresponding relationship between the obtained attenuation rate of the target liquid droplet and the kinematic viscosity, and finally displayed on the display.

[0032] Figure 1 and Figure 2 The present application is a schematic diagram of a liquid droplet vibration attenuation rate excitation detection device. The volume of the target liquid droplet (usually 2-3 microliters) needs to be fixed (such as controlled by a pipette); the hydrophilic circular platform 1 is used to carry and fix the target liquid droplet, so that the bottom surface of the target liquid droplet and the contact surface of the bearing micro-column 4 are on the same horizontal plane and prevent the target liquid droplet from moving randomly; the bearing micro-column 4 has a total of 24, arranged in a ring shape, which can effectively promote the target liquid droplet to form a large contact angle on its surface, facilitating the accurate capture of the free vibration of the target liquid droplet by the piezoresistive cantilever beam 3; the piezoresistive sensor 6 is made of micro-electro-mechanical technology and is used to detect the size and change of pressure, which will cause the piezoresistive sensor to deform, and the deformation will cause the piezoresistive sensor to change in resistance, eventually producing a change in analog electrical signal. Since the piezoresistive sensor 6 under this device is formed by micro-machining technology, it needs to be calibrated by an experimental device before formal use; the top device silicon layer 8 provides the necessary physical support for the micro-electro-mechanical structure, and the above-mentioned piezoresistive cantilever beam 3, bearing micro-column 4 and hydrophilic circular platform 1 are formed by micro-machining technology on the top device silicon layer 8; the middle insulating layer 9 can electrically isolate the top device silicon layer 8 and the bottom substrate silicon layer 10 to reduce parasitic capacitance, improve the speed and performance of the device, and prevent carriers from flowing from the top device silicon layer 8 into the bottom substrate silicon layer 10, thereby reducing power consumption. The bottom substrate silicon layer 10 mainly provides mechanical support to ensure the structural integrity of the entire wafer. It can ensure that the wafer has sufficient strength and stability, and can also effectively conduct and dissipate the heat of the device to prevent overheating of the device. The buzzer 7 can produce a certain time (about 1s) of acoustic waves with a specific frequency (close to the first-order inherent frequency of the liquid droplet, about 100Hz) to make the target liquid droplet produce a first-order inherent frequency (about 100Hz) of free vibration.

[0033] Figure 3 A schematic diagram of a three-dimensional structure of a piezoresistive cantilever beam and a load-bearing micro-pillar, the piezoresistive cantilever beam 3 is composed of two cantilever beams 5 and two piezoresistive sensors 6, and the load-bearing micro-pillar 4 is above the free end of the piezoresistive cantilever beam 3, and the fixed end is connected to the top device silicon layer 8. The piezoresistive cantilever beam 3 is the key component of the device. The piezoresistive cantilever beam is made as follows:

[0034] (1) Select a suitable substrate material (silicon, SI), and perform oxidation treatment on the surface to form an insulating layer of silicon dioxide (SIO2), and attach a layer of silicon wafer (device silicon layer) on the insulating layer. The above structure is called silicon on insulating substrate (SOI structure). This structure insulates the silicon wafer layer from the substrate silicon layer, has low substrate leakage current, reduces parasitic capacitance, and has the advantages of noise isolation.

[0035] (2) Dope the top device silicon layer on the insulating layer (silicon dioxide) by ion implantation to change the electrical properties of the silicon material.

[0036] (3) Deposit chromium (Cr) and gold (Au) layers on the top device silicon layer. These metal layers provide a medium for forming a cantilever pattern later.

[0037] (4) Form a cantilever pattern through photolithography and wet etching of the metal layer. This step defines the shape and size of the cantilever.

[0038] (5) Use plasma reactive ion etching technology (ICP-RIE technology) to etch the device silicon layer with Au / Cr layer as a mask to form a cantilever structure.

[0039] (6) Pattern and wet etch the chromium (Cr) and gold (Au) layers to expose the piezoresistive sensors at the root of the cantilever.

[0040] (7) Use plasma reactive ion etching technology (ICP-RIE technology) to pattern and etch the processing layer silicon to create a through hole below the cantilever.

[0041] (8) Finally, use hydrogen fluoride acid vapor to etch the silicon dioxide (SIO2) layer to release the cantilever. Through the above technology, the final Figure 3 structure is formed, which can transmit the capillary force generated by the free vibration of the target liquid droplet on the load-bearing micro-pillar surface, causing the piezoresistive sensor to deform and form a changing electrical signal.

[0042] Figure 4is a schematic diagram of the forced vibration and free decay vibration of the target droplet to be processed by the digital signal processor. The piezoresistive sensor converts the free vibration of the target droplet into an electrical signal (analog signal), and through analog / digital conversion, a digital signal is formed which can be processed by the digital signal processor. The digital signal processor obtains the decay rate a by exponentially fitting (y = ce -at ) the maximum amplitude data points in each cycle of the electrical signal curve of the decay vibration.

[0043] Figure 5 is a workflow diagram of the entire system. The workflow of the entire system is specifically introduced (the pre-processing process such as signal amplification and filtering is omitted).

[0044] According to the formula When the volume of the target droplet is fixed, the decay rate a is approximately proportional to the kinematic viscosity . In order to obtain the calibration coefficient, glycerol / water mixtures with different mixing ratios can be set. The viscosity range of the glycerol / water mixtures with different mixing ratios is 1-30 mPa·s (10 groups), and the decay rate of the free vibration of each group of droplets is read out by using the experimental equipment. Through the corresponding relationship between the kinematic viscosity υ and the decay rate a of the 10 groups of glycerol / water mixtures with different mixing ratios, linear fitting calibration is carried out by using a computer, and the corresponding proportional relationship k can be obtained, which provides a complete formula for obtaining the kinematic viscosity according to the free vibration decay rate. Note: The volume of the droplet in the calibration process needs to be consistent with the volume of the target droplet to be measured.

[0045] The detection personnel first take a small amount of target liquid through the collection device, and then quantitatively drop the target liquid taken by the pipette (control the target liquid drop volume to be consistent with the fixed volume in calibration) to the upper surface of the hydrophilic cylinder to form a target liquid drop, and ensure that the target liquid drop edge covers the surface of the bearing micro-column. When the piezoresistive sensor senses the pressure of the target liquid drop, an analog electrical signal converted by the pressure of the target liquid drop is output to the application specific integrated circuit (ASIC), and the application specific integrated circuit (ASIC) forms a digital signal that can be processed by the digital signal processor through analog / digital signal conversion. After being processed by the digital signal processor, a certain frequency electrical signal is formed, and the electrical signal is converted into an analog signal that can work through analog / digital signal conversion. The buzzer emits a certain frequency (close to the first-order natural frequency of the liquid drop, about 100 Hz) sound wave for a certain time (about 1 s) to excite the first-order modal vibration (forced) of the target liquid drop (the first-order natural frequency is near 100 Hz). After the buzzer stops outputting the sound wave, the target liquid drop performs a first-order modal free vibration. The bearing micro-column and cantilever beam transmit the pressure change generated by the vibration to the piezoresistive sensor, and the piezoresistive sensor converts the change of the deformation into a change analog electrical signal. The application specific integrated circuit (ASIC) converts the changing analog electrical signal into a digital signal that can be processed by the digital signal processor through analog / digital signal conversion. The digital signal processor obtains the decay rate a by exponentially fitting (y=ce -at ) the maximum amplitude data points in each period of the electrical signal curve of the attenuated vibration, and calculates the target liquid drop kinematic viscosity The target liquid drop kinematic viscosity digital signal is converted into an analog signal that can be executed by the actuator (display) through analog / digital signal conversion, and finally the target liquid kinematic viscosity is displayed through the display. Through the density of the target liquid, the dynamic viscosity μ of the target liquid can be obtained.

[0046] The above is a detection device based on a micro-electro-mechanical system, which measures the viscosity of the target liquid by detecting the free vibration of the target liquid drop. This device only needs about 2-3 microliters (about 8 times smaller than the current commercial portable viscometer) of liquid drop to accurately measure the viscosity of the target liquid. In addition, the device can be used multiple times. The entire device is based on a micro-electro-mechanical system, small in size and capable of meeting the requirements of portability, and can be mass-produced through micro-electro-mechanical system manufacturing technology using existing integrated circuit production lines, thereby reducing costs.

[0047] Those skilled in the art will readily understand that the above description is only a preferred embodiment of the present application, and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method of measuring the viscosity of a liquid based on the free oscillation of a droplet of the liquid, the method comprising: The liquid viscosity measuring device comprises a hydrophilic circular platform (1) for fixing a liquid droplet and a piezoresistive cantilever beam array (2) comprising a plurality of piezoresistive cantilever beams (3), each of which comprises two cantilever beams (5) and two piezoresistive sensors (6), a bearing micro column (4) being located above the free end of the cantilever beam (5) and at the periphery of the hydrophilic circular platform (1), and the bearing micro column (4) is not connected to the hydrophilic circular platform (1); the upper surface of the bearing micro column (4) is coated with a hydrophilic material by atomic layer deposition, and the four side surfaces are coated with a hydrophobic material by atomic layer deposition, the hydrophobic side surface of the bearing micro column (4) ensures that the liquid droplet attached to its surface has a contact angle greater than 120 degrees. The free end of the cantilever beam (5) is located on both sides of the bottom surface of the bearing micro column (4), and the piezoresistive sensor (6) is located at the root of the fixed end of the cantilever beam (5), which is used to detect the force of the liquid droplet acting on the bearing micro column (4), and the fixed end is rigidly connected to the device silicon layer (8). A buzzer (7) is arranged below the hydrophilic circular platform (1), which is used to output sound waves to excite the liquid droplet to vibrate. The liquid viscosity measurement comprises the following steps: S1: 2-3 microliters of target liquid is added on the hydrophilic circular platform (1) to form a target liquid droplet; S2: When the target liquid droplet is balanced on the bearing micro column (4), the piezoresistive sensor (6) receives the pressure of the bearing micro column (4) and converts it into an analog electrical signal, which is then converted into a digital signal, which is processed by a digital signal processor and then converted into an analog signal; S3: After the buzzer (7) receives the analog signal in step S2, the buzzer (7) outputs sound waves to excite the liquid droplet to vibrate in the first order mode; S4: After the buzzer (7) stops outputting sound waves, the target liquid droplet vibrates freely in the first order mode, thereby avoiding the interference of other order mode vibrations, and the pressure generated by the target liquid droplet vibrating freely in the first order mode is transmitted to the piezoresistive sensor (6) through the bearing micro column (4) and the cantilever beam (5); S5: The piezoresistive sensor (6) converts pressure into an analog electrical signal. After the analog electrical signal is converted into a digital signal, the digital signal processor performs exponential fitting on the maximum peak data point of the decay vibration electrical signal within a single cycle of the digital signal to obtain the decay rate. According to the formula The kinematic viscosity of the target liquid was calculated. ,in: The attenuation rate, This is the proportionality coefficient. The target droplet volume; according to the formula The dynamic viscosity of the target liquid was calculated. ,in For the target liquid density, The target liquid kinematic viscosity.

2. The method of measuring the viscosity of a liquid based on the free vibration of a liquid droplet according to claim 1, wherein, The hydrophilic circular platform (1) is obtained by depositing a hydrophilic material on the top device silicon layer (8).

3. The method of measuring the viscosity of a liquid using the liquid droplet free vibration based liquid viscosity measuring apparatus as claimed in claim 2, wherein, The bottom of the top device silicon layer (8) further has an intermediate insulating layer (9) and a bottom substrate silicon layer (10) in sequence.

4. The method of measuring the viscosity of a liquid based on the free vibration of a liquid droplet as claimed in claim 1, wherein, The vertical distance between the buzzer (7) and the hydrophilic circular platform (1) is 3-5 mm.

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