CN115436228A - System and method for measuring viscosity of micro-upgrading liquid sample - Google Patents
System and method for measuring viscosity of micro-upgrading liquid sample Download PDFInfo
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Abstract
The invention discloses a system and a method for measuring viscosity of a micro-upgrading liquid sample, which comprises the following steps: the device comprises a pumping unit, a micro-fluidic chip, an imaging camera and a controller; the micro-fluidic chip is provided with a micro-channel for accommodating a liquid sample; the micro-fluidic chip is arranged in a manifold connector, the pumping unit is connected with the manifold connector, the surface of the manifold connector is provided with a visualization port and a port for pressure measurement, and the port is connected with a pressure sensor through a capillary tube; the imaging camera is used for imaging the flowing process of the liquid sample in the micro-fluidic chip micro-channel through the visualization port; and the pressure sensor and the imaging camera are respectively connected with the controller. The invention can realize effective and accurate measurement of the viscosity of the micro-upgraded liquid sample, and the pressure sensor is arranged outside the micro-fluidic chip, thereby having no influence on the flow of the liquid sample in the micro-channel and having accurate measurement result.
Description
Technical Field
The invention relates to the technical field of viscosity measurement of micro-upgrading liquid samples, in particular to a system and a method for measuring viscosity of a micro-upgrading liquid sample.
Background
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Viscosity is a key physical parameter of a fluid that quantitatively measures the resistance characteristics of the fluid as it flows. In general, viscosity is defined as the ratio of the shear stress to the shear rate of a fluid. On a microscopic scale, viscosity is related to the internal friction of the constituent molecules of a fluid against each other as the fluid flows. Viscosity not only can help predict and understand the behavior of various fluids commonly found in daily life, it can also be of great benefit to the proper design of the relevant industrial process. In fact, viscosity is an important parameter in a wide range of production and living areas (e.g., petroleum and lubricants, cosmetics, food and beverages, hemp oil, batteries, and numerous biological fluids).
To achieve measurement of the fluid viscosity, it is necessary to select an appropriate measurement method according to the test conditions (e.g., temperature of the measurement environment, volume of sample available for measurement, etc.) and the related knowledge of the approximate viscosity of the fluid. Three common commercial viscometers currently include: capillary, coaxial cylinder, and ball drop. Capillary viscometers are generally in the shape of a U-tube that forces a fluid under test to flow based on a pressure differential, the dynamic viscosity measured being proportional to the time the fluid flows between two specified points on the tubing; in a coaxial cylinder viscometer, sample fluid flows between two concentric cylinders in a laminar flow and generates a shearing process, and a sample can be calculated based on the torque on the inner cylinder under the action of viscous force; falling ball viscometers are commonly used for transparent newtonian fluids and operate on the principle that the time required for a sphere of known composition to fall in a tilted tube filled with a sample fluid is directly related to the viscosity of the fluid.
Each of the three viscometers described above has advantages and disadvantages. For example, capillary viscometers occupy less space and are less expensive than coaxial drum viscometers. However, when the sample fluid has a high viscosity or contains solid particles having a diameter comparable to that of the capillary, the capillary viscometer becomes difficult to clean and even unusable. The coaxial cylinder viscometer is easy to use, the required measurement time is short, the shear rate in the measurement process can be accurately controlled, but the maintenance cost of the coaxial cylinder viscometer is generally higher, and the coaxial cylinder viscometer needs to be thoroughly disassembled and cleaned before operation. The falling ball viscometer requires a shorter measurement time, lower cost and more convenient operation than the capillary viscometer, however, this type of viscometer has a limited number of liquids to be tested and the sample to be tested must be a transparent newtonian fluid.
It is worth noting that in many application scenarios, sample volume (sometimes referred to as "minimum sample volume" or "sample working volume") is one of the important usage requirements that the aforementioned viscometers cannot ignore. The required sample volume for commercial viscometers is typically between 0.5mL and 500mL, and when the sample volume to be measured is insufficient or multiple measurements are required, particularly for liquid samples that are difficult to obtain or have high economic costs, the various viscometer types currently used are difficult to use. Therefore, there is a great need for an apparatus and method that can measure the viscosity of a sample in minute volumes (on the order of microliters). In addition, the novel viscometer is expected to have the advantages of short measurement time, good economy, small occupied space and the like.
Microfluidics is a science and technology oriented to integrated microchannel systems (with critical channels having characteristic lengths between hundreds of nanometers and hundreds of micrometers), involving very small volumes (typically 10 a) -6 -10 -18 L) precise transport and handling of fluids. The initial goal of microfluidic technology was to miniaturize biological or chemical analysis equipment; to date, this technique and related platforms have been widely used in capillary electrophoresis, liquid chromatography, chemical reactions and synthesis, biochemical analysis, and the like. The microfluidic technology has great application potential in solving the contradiction that the sample to be measured is small in volume but the sample required by the viscometer is large in volume.
Although the prior art discloses a technique for viscosity measurement using microfluidic channels, in which a monolithic pressure sensor array is embedded in a channel wall, a measurable pressure drop is caused by a geometric change in a flow channel (or a constricted region), so that the viscosity of a microfluidic liquid can be measured; however, the incorporation of a pressure sensor array within the microchannel walls may introduce surface roughness, leading to an overestimation of the shear stress of the sample, which degrades the measurement accuracy of the viscometer.
Disclosure of Invention
In order to solve the problems, the invention provides a system and a method for measuring the viscosity of a micro-upgrading liquid sample, and the effective and accurate measurement of the viscosity of the micro-upgrading liquid sample is realized.
In some embodiments, the following technical scheme is adopted:
a system for measuring viscosity of a micro-scale liquid sample, comprising: the device comprises a pumping unit, a microfluidic chip, an imaging camera and a controller; the micro-fluidic chip is provided with a micro-channel for accommodating a liquid sample; the micro-fluidic chip is arranged in a manifold connector, the pumping unit is connected with the manifold connector, the surface of the manifold connector is provided with a visualization port and a port for pressure measurement, and the port is connected with a pressure sensor through a capillary tube; the imaging camera is used for imaging the flowing process of the liquid sample in the micro-fluidic chip micro-channel through the visual port; the pressure sensor and the imaging camera are respectively connected with the controller.
As a further scheme, the depth of the micro-channel is in micron order, and the width of the micro-channel is at least n times of the height of the micro-channel, so as to generate the slit laminar flow; n is more than or equal to 10.
As a further scheme, the microfluidic chip is provided with a packaging layer, the packaging layer is provided with at least two openings, the openings are communicated with the micro-channels, and the aperture of each opening is not larger than the width of the micro-channel.
As a further scheme, the pumping unit is used for providing pushing pressure for the liquid sample loaded in the micro-channel; the outlet of the air cavity of the pumping unit, an interface of the manifold connector and a measuring port of the pressure sensor are communicated; the pressure sensor is used for measuring the pressure difference between the front and the back of the liquid sample.
In a further aspect, the capillary tube is rigid and is made of one or a combination of two or more of PEEK, stainless steel, quartz glass, perfluoroalkoxy alkane polymer, fluorinated ethylene propylene copolymer, and ethylene-tetrafluoroethylene copolymer.
As a further scheme, the liquid sample in the microchannel flows forwards under the action of the pumping unit, an image of the liquid sample in a flow balance state is obtained, a characteristic receding contact angle is obtained by measuring a receding contact angle in each time step and performing arithmetic averaging, further, the capillary pressure difference at the liquid-gas interface at the rear part of the microchannel is calculated, and the capillary pressure difference is subtracted from the total pressure difference measured by the pressure sensor, so that the effective pressure difference for pushing the liquid sample to flow is obtained;
calculating a shear force based on the effective pressure differential.
As a further scheme, the controller analyzes an image acquired by the imaging camera to obtain the average flow speed of the liquid sample under the equilibrium state flow; based on the average flow velocity and the microchannel dimensions, a volumetric flow rate of the liquid sample within the microchannel at equilibrium is derived, and a shear rate is calculated based on the volumetric flow rate.
In other embodiments, the following technical solutions are adopted:
a method for measuring viscosity of a micro-upgraded liquid sample, comprising:
loading a liquid sample into a microchannel of the microfluidic chip;
starting a pumping unit, enabling a liquid sample in the micro-channel to flow under the pressure of the pumping unit, measuring the pressure difference between the front and the back of the liquid sample in real time through a pressure sensor, and calculating the shearing force;
when a liquid sample flows in the microchannel, the liquid sample is in a section of liquid plug, and the flow process of the liquid plug type liquid sample is recorded in real time by an imaging camera;
analyzing an image acquired by an imaging camera, and calculating a shearing rate;
and calculating the dynamic viscosity of the liquid sample based on the ratio of the shearing force to the shearing rate.
As a further scheme, the liquid sample in the microchannel flows forwards under the action of the pumping unit, an image of the liquid sample in a flow balance state is obtained, a characteristic receding contact angle is obtained by measuring a receding contact angle in each time step and performing arithmetic averaging, further, the capillary pressure difference at the liquid-gas interface at the rear part of the microchannel is calculated, and the capillary pressure difference is subtracted from the total pressure difference measured by the pressure sensor, so that the effective pressure difference for pushing the liquid sample to flow is obtained;
calculating a shear force based on the effective pressure differential.
As a further scheme, analyzing an image acquired by an imaging camera to obtain the average flow speed of the liquid sample under the equilibrium state flow; based on the average flow velocity and the microchannel dimensions, a volumetric flow rate of the liquid sample within the microchannel at equilibrium is derived, and a shear rate is calculated based on the volumetric flow rate.
Compared with the prior art, the invention has the beneficial effects that:
(1) The invention can realize effective and accurate measurement of the viscosity of the micro-upgraded liquid sample, and the pressure sensor is arranged outside the micro-fluidic chip, thereby having no influence on the flow of the liquid sample in the micro-channel and having accurate measurement result.
(2) The micro-channel on the micro-fluidic chip has a small depth-to-width ratio, can realize slit laminar flow, and simultaneously greatly reduces the effect of capillary pressure difference in the depth direction and the influence on the accuracy of final viscosity measurement.
Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
Drawings
FIG. 1 is a schematic diagram of a system for measuring viscosity of a micro-upgraded liquid sample according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a microfluidic chip according to an embodiment of the present invention;
FIGS. 3 (a) - (c) are schematic structural views of a manifold coupler according to an embodiment of the present invention;
FIG. 4 is a schematic view of the assembly of the manifold connector and the microfluidic chip according to the embodiment of the present invention;
FIG. 5 is a schematic diagram of a method for quantitatively determining an interfacial capillary pressure difference between a liquid sample and air when the liquid sample flows through a microchannel according to an embodiment of the present invention;
FIG. 6 is a graph showing the change of total pressure difference before and after a liquid sample with time according to an embodiment of the present invention;
FIG. 7 is a schematic diagram of the measurement results of the apparatus of the present invention and a commercial viscosity measuring apparatus;
wherein, 1, a system for measuring the viscosity of a micro-upgraded liquid sample, 2, a pumping unit, 3, a direct current power supply, 4, a pressure sensor, 5, an imaging camera, 6, a controller, 7, a manifold connector, 8, a capillary tube, 9, a data connecting line, 10, a micro-fluidic chip, 11, a first opening, 12, a second opening, 13, a third opening, 14, a fourth opening, 15, a micro-channel, 16, a liquid sample, 17 visualization window, 18 manifold connector upper port, 19 small hole, 20 annular groove, 21 rectangular groove, 22 boss, 23 rectangular groove, 24 shallow groove, 25 microfluidic chip and manifold connector assembly, 26 first interface, 27 second interface, 28 third interface, 29 solid wall, 30 receding dynamic contact angle, 31 initial baseline pressure.
Detailed Description
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
Example one
In one or more embodiments, a system 1 for measuring viscosity of a micro-upgraded liquid sample is disclosed, which, in conjunction with fig. 1, specifically comprises: a pumping unit 2, a microfluidic chip 10, an imaging camera 5 and a controller 6. Wherein, the microfluidic chip 10 is rectangular; a microchannel 15 having a small aspect ratio for accommodating a liquid sample 16 to be measured; the width of the microchannel 15 should be at least n times its height to produce a slot-laminar flow; n is more than or equal to 10. The microfluidic chip 10 has an encapsulation layer with 2-4 openings, each opening having a pore size no greater than the microchannel width, and the openings of the encapsulation layer are aligned with the microchannels 15 on the microfluidic chip.
Specifically, referring to fig. 2, the microfluidic chip has a straight microfluidic channel (i.e., microchannel) 15 for micro-scale liquid sample loading, flow and viscosity measurement thereof, the microchannel having a width of not more than 1.5mm and a depth in the order of micrometers, and an aspect ratio of about 0.1. The microfluidic chip has an encapsulation layer on its upper portion, and after the microfluidic chip is encapsulated by the encapsulation layer, two small holes (or "openings") are formed at the end of the microfluidic channel, which are respectively a first opening 11 and a fourth opening 14, and the aperture of the two openings is smaller than the width of the microfluidic channel. Furthermore, the microfluidic channel has no more than two openings on its encapsulation layer between the first opening 11 and the fourth opening 14, wherein the second opening 12 near the first opening 11 is used to help quantify the liquid sample 16 loaded to the microfluidic chip, and the other third opening 13 (if any) near the fourth opening 14 is used to connect to the measurement port of the pressure sensor 4 and ultimately serve for pressure measurement. In this embodiment, the microfluidic chip 10 has the four openings in the packaging layer.
During viscosity measurement, the microfluidic chip 10 is placed inside the manifold connector 7; the manifold connector 7 is composed of an upper part and a lower part, and is rectangular as a whole; when measuring the viscosity, the upper part and the lower part of the manifold connector 7 are tightly closed, and air sealing is formed at the opening of the packaging layer of the microfluidic chip; the air seal may be achieved by using an O-ring rubber.
The upper part of the manifold connector 7 is provided with not less than 2 interfaces for connecting with a pressure sensor measuring port; during viscosity measurement, a connecting nut for connecting the capillary tube 8 is arranged at each interface; the inner wall of the bore at each interface has threads that conform to the coupling nut. The capillary 8 is rigid and has no obvious elasticity, and the material of the capillary can be any one or the combination of two or more of PEEK, stainless steel, quartz glass, perfluoroalkoxy alkane Polymer (PFA), fluorinated ethylene propylene copolymer (FEP) and ethylene-tetrafluoroethylene copolymer (ETFE); the inner diameter of the capillary tube is 1-5000 μm, and the outer diameter is 0.1-10mm.
The manifold coupler 7 is also provided with a visualization window 17 in the upper part for image capture of the liquid sample 16 by the imaging device during viscosity measurement; the visualization window 17 is in the shape of one of a rectangle and a circle or a combination of the two; the visualization window 17 is rectangular in shape and when the microfluidic chip is placed in the manifold connector 7, the visualization window 17 is parallel to the microchannel of the microfluidic chip 10.
Specifically, referring to fig. 3 (a) - (c), the manifold connector 7 is composed of two parts (upper and lower parts), and the microfluidic core is placed in the manifold connector at the time of viscosity measurement, and the upper and lower parts are tightly closed to form an assembly, as shown in fig. 4. The first opening 11, the third opening 13 and the fourth opening 14 of the microfluidic chip are respectively aligned with the three ports of the upper part of the manifold connector in the center, and an air seal is formed between the two through a special seal (in the embodiment, a rubber O-ring is used as a seal). In this embodiment, a flat bottom flange-free nut (with a capillary sealing collar) having an internal bore and adapted to fit the capillary tube is mounted in the manifold coupler upper port 18 to allow connection of the capillary tube to the manifold coupler.
It should be noted that the design depth of the manifold coupler upper portion port 18 is similar to the nut thread depth, and the design bore diameter is suitable for threading to produce a thread matching this type of nut. The bottom of the port 18 of the upper part of the manifold connector is provided with a small hole 19, the design depth of which is about 5mm, and the manifold connector can ensure enough structural safety when the connecting nut and the capillary tube are screwed into the port 18. The pore diameter of the small pore 19 corresponds to the first opening 11, the second opening 12, the third opening 13, and the fourth opening 14 of the microfluidic chip. The manifold coupler is provided in its upper portion with a visualization window 17 (as shown in fig. 3 (a)) for image capture of the liquid sample 16 as it flows through the microchannel 15. Alternatively, the manifold coupler 7 is 3D printed. Fig. 3 (b) is a schematic bottom view of the upper part of the manifold coupler 7, which has an annular groove 20 for the placement of the aforementioned O-ring. When the microfluidic chip is loaded to the manifold connector 7 and the upper and lower portions of the manifold connector are closed, an air seal is formed between the first opening 11, the third opening 13, and the fourth opening 14 of the microfluidic chip and the corresponding interface of the manifold connector. Figure 3 (c) is a top view of the lower portion of the manifold coupler, which is provided with a boss 22 and fits into a rectangular recess 21 in the upper portion, to facilitate the two-part closure of the manifold coupler. In addition, the manifold connector is provided with a rectangular groove 23 in the lower part for facilitating loading of the microfluidic chip, and a plurality of shallow grooves 24 in the inner side for facilitating disassembly of the manifold connector after the measurement is finished.
Fig. 4 is a top view of an assembly 25 of a microfluidic chip and manifold connector. It should be noted that the second opening 12 of the microfluidic chip is open when the liquid sample 16 is loaded, and is sealed during the loading of the microfluidic chip to the manifold connector and the viscosity measurement. Furthermore, the first port 26 (positive pressure end of the pressure measurement) and the second port 27 (low pressure end of the pressure measurement) of the manifold connector are connected to the measurement ports of the pressure sensor respectively through capillary tubes, which in this embodiment avoids the use of elastic capillary tubes, which may cause certain pressure measurement errors. The communication between the first port 26, the pumping unit 2 and the measurement port of the pressure sensor 4 is achieved by using a three-way joint. In this embodiment, the third port 28 (the end open to the atmosphere) is open to the atmosphere by a length of polymer capillary tubing (same outside diameter as the capillary tubing 8) fitted through one of the aforementioned coupling nuts.
The measuring port of the pressure sensor is connected with the interface of the upper part of the manifold connector; the pressure sensor is driven by a direct current power supply; the measurement accuracy of the pressure sensor is not more than +/-1% of the measurement range, and the direct-current power supply is one of an external battery or a USB port of a computer.
A measuring port of the pressure sensor, an outlet of the air chamber of the pumping unit, and an interface of the manifold connector are communicated with each other, and the communication can be realized by using a three-way joint or a three-way valve in cooperation with a capillary tube.
The pumping unit is provided with an air cavity capable of outputting pressure outwards, and is in a fixed flow mode during viscosity measurement, and the volume flow rate of output air can be adjusted within the range of 1-1000 microlitres/minute.
An imaging camera is mounted over the visualization window of the manifold coupler; the imaging camera may be one of a commercial video camera, an OEM type optical sensor; the image capturing speed is not lower than 30 sheets/second.
The controller is used for receiving the data measured by the pressure sensor and the imaging camera, and is used for storing, analyzing and calculating the viscosity of the liquid sample 16; the controller can be one of a notebook computer, a desktop computer, a single board computer, a tablet computer and a smart phone.
In the process of viscosity measurement, the pressure difference between the upstream and the downstream of the liquid sample 16 in the microchannel 15 of the microfluidic chip 10 is measured by the pressure sensor 4, and the measurement data is transmitted to the controller 6 provided with a data acquisition card. The imaging camera 5 is positioned directly above the manifold coupler 7 and when the liquid sample 16 flows within the microchannel 15 and appears in the visualization window 17 of the manifold coupler 7, its flow image is captured and the relevant image is transmitted and saved to the controller 6.
Due to the relative positional relationship of the imaging camera 5 and the manifold coupler 7, there may be visual deviations in the camera 5 in the dimensional or geometric length measurements, so it is necessary to quantitatively determine the measurement deviations of the camera 5 prior to viscosity measurements. In this embodiment, a straight line segment parallel to the microchannel 15 and having a known length is drawn at the top of the microfluidic chip 10, and then a photograph is taken by the camera 5, and according to the known length of the line segment and the length of the line segment imaged by the camera 5, a length correction coefficient C is obtained as follows:
wherein L is known Is the known length of the straight line segment, L captured Is the length of the line measured by the camera 5. When determining the length of the liquid sample 16 within the micro channel 15, the sample length measured by the camera 5 must be multiplied by the correction factor to obtain the true length of the liquid sample 16. If the phase isWhen the machine 5 is moved relative to the manifold coupler 7 and the visual difference and the length correction coefficient C are changed, the above operation must be performed again, and a new correction coefficient is determined by equation 1-1.
The specific method for measuring viscosity by using the device of the embodiment is as follows:
a micro-scale liquid sample 16 is first loaded into the microchannel 15 of the microfluidic chip 10, in this example using a pipette. After the loading of the liquid sample 16 is completed, the second opening 12 of the microfluidic chip 10 is sealed by the sealing tape, the microfluidic chip is held horizontal, and loaded into the manifold connector 7, and then the upper and lower portions of the manifold connector 7 are closed.
The pumping unit 2, the pressure sensor 4 and the imaging camera 5 are simultaneously activated, wherein the pumping unit 2 is set to a fixed flow (about several tens to several hundreds of microliters/minute) mode. Since the air pressure of the upstream side of the liquid sample 16 gradually rises, when the air pressure reaches a certain critical value, the liquid sample 16 loaded in the micro flow channel 15 starts to flow, and the sample liquid plug 16 flows from left to right in the micro flow channel 15 with time, and the flow process is captured by the camera 5. Fig. 5 is a schematic view of the liquid sample 16 (hatched area) flowing through the microchannel 15 (area between two parallel horizontal lines) from left to right. The liquid sample 16 has two interfaces: a front gas-liquid interface, and a rear gas-liquid interface. In this embodiment, the microchannel is hydrophobic, and the gas-liquid interface at the front has an advancing dynamic contact angle (measured at the three-phase contact point of air, liquid and solid channel wall 29) close to zero. On the other hand, in a condition where the sample is in flow (quasi) equilibrium, the rear liquid-gas interface is subjected to air pressure, and the interface exhibits an anomalous morphology (i.e., exhibits hydrophilic behavior), so the receding dynamic contact angle 30 should be measured by analyzing flow images and/or video of the liquid sample 16.
In the actual analysis process, a video part of the liquid sample 16 in a flow equilibrium state is taken, a characteristic receding contact angle θ is obtained by measuring a receding contact angle in each time step and performing arithmetic averaging, and then the capillary pressure difference at the rear liquid-gas interface can be calculated according to a famous Young-Laplace equation, that is:
where σ is the surface tension of the liquid sample,
is the hydraulic radius of the channel and can be calculated from the following equation:
where H and W are the height and width of the microchannel, respectively.
Theoretically, as a propulsive force for the liquid sample 16 to flow in the microchannel 15, the total pressure difference generated by the pumping unit 2 needs to overcome three resistances: flow viscosity forces (closely related to viscosity), frictional resistance (mainly caused by channel roughness and physical adhesion), and interfacial capillary pressure differences. To minimize measurement errors due to frictional resistance, in this embodiment, the surface of the microchannel 15 is coated to be smooth and hydrophobic. The interfacial capillary pressure difference is calculated by the formulas (1-2) and (1-3), and is further subtracted from the total pressure difference measured by the pressure sensor to obtain an effective pressure difference for pushing the liquid sample to flow, namely, a real pressure difference for balancing the viscous force applied to the sample.
The kinematic viscosity (. Mu.) of the fluid sample 16 is defined as the shear force (. Tau.) w ) And shear rate
In a ratio of (i) to (ii)
Wherein the apparent shear τ of the channel solid wall 29 w And shear rate
Calculated by the following formula, respectively:
wherein Δ P is the total pressure difference measured for the liquid sample 16, Δ P capillary For the calculated interfacial capillary pressure difference, L is the actual length of the sample 16 in the microchannel 15 (corrected for the visual deviation), Q is the volume flow rate of the liquid sample 16 in the microchannel 15, and can be calculated from the following formula
Wherein, the first and the second end of the pipe are connected with each other,
is the average velocity of the liquid sample 16 as it flows in the microchannel 15 in a (quasi-) equilibrium state.
The viscosity mu is calculated by post-processing the acquired image, video and pressure measurement data. First, by analyzing the relevant video, the time period during which the liquid sample 16 is moving at a constant or near constant speed is determined. For the (quasi-) equilibrium time period when the liquid sample 16 flows through the microchannel 15, based on image software, markers are placed in frames at equal intervals of video to track the leading edge of the liquid sample, and then the speed of the liquid sample is calculated; at each time step, the dynamic contact angle and sample length were also calculated using the imaging software. The arithmetic mean of these three parameters is substituted into the above equations to calculate the shear force, shear rate, volume flow rate and capillary pressure difference, respectively. Figure 6 shows a typical plot of measured total pressure differential over time. It is worth noting that the viscosity should be calculated using the average of the pressure data when the sample is flowing at a constant speed. Furthermore, to apply the shear stress equations (1-5) to calculate viscosity, the aforementioned average pressure needs to be subtracted by the initial baseline pressure 31.
To verify the effectiveness of the viscosity measuring device of this example, the viscosity of four aqueous solutions of glycerol (glycerol) with different mass concentrations (10 wt% -70 wt%) was measured by using 12 microliter of liquid sample, and compared with a cone-plate rheometer (500 microliter of sample is required for one measurement) on the market, as shown in fig. 7. The result shows that the viscometer and the viscosity measurement method provided by the invention have relatively consistent measurement results with commercial rheometers and have very good measurement accuracy. Aiming at the viscosity measurement of the micro-upgrading liquid sample, the invention realizes effective and accurate measurement. It should be noted that aqueous glycerol solutions are newtonian fluids whose viscosity is independent of the applied shear rate, and therefore the viscosity measurements can be compared to commercial rheometers without taking into account deviations in shear rate.
Example two
In one or more embodiments, a method for measuring viscosity of a micro-scale liquid sample is disclosed, comprising the processes of:
(1) Loading a liquid sample into a microchannel of the microfluidic chip;
(2) Starting a pumping unit, enabling a liquid sample in the micro-channel to flow under the pressure of the pumping unit, measuring the pressure difference between the front and the back of the liquid sample in real time through a pressure sensor, and calculating the shearing force;
(3) When a liquid sample flows in the microchannel, the liquid sample is in a section of liquid plug, and the flow process of the liquid plug type liquid sample is recorded in real time by an imaging camera;
(4) Analyzing an image acquired by an imaging camera, and calculating a shearing rate;
(5) And calculating the dynamic viscosity of the liquid sample based on the ratio of the shearing force to the shearing rate.
Specifically, a liquid sample in the micro-channel flows forwards under the action of a pumping unit, an image of the liquid sample in a flow balance state is obtained, a characteristic receding contact angle is obtained by measuring a receding contact angle in each time step and performing arithmetic averaging, the capillary pressure difference at a liquid-air interface at the rear part of the micro-channel is further calculated, and the capillary pressure difference is subtracted from the total pressure difference measured by a pressure sensor, so that the effective pressure difference for pushing the liquid sample to flow is obtained;
calculating a shear force based on the effective pressure differential.
Analyzing an image acquired by an imaging camera to obtain the average flow speed of the liquid sample under the equilibrium state flow; based on the average flow velocity and the microchannel dimensions, a volumetric flow rate of the liquid sample within the microchannel at equilibrium is derived, and a shear rate is calculated based on the volumetric flow rate.
The viscosity of a liquid sample is determined by the ratio of the shear force and the shear rate.
Although the embodiments of the present invention have been described with reference to the accompanying drawings, it is not intended to limit the scope of the invention, and it should be understood by those skilled in the art that various modifications and variations can be made without inventive changes in the technical solutions of the present invention.
Claims (10)
1. A system for measuring viscosity of a micro-upgraded liquid sample, comprising: the device comprises a pumping unit, a microfluidic chip, an imaging camera and a controller; the micro-fluidic chip is provided with a micro-channel for accommodating a liquid sample; the micro-fluidic chip is arranged in a manifold connector, the pumping unit is connected with the manifold connector, the surface of the manifold connector is provided with a visualization port and a port for pressure measurement, and the port is connected with a pressure sensor through a capillary tube; the imaging camera is used for imaging the flowing process of the liquid sample in the micro-fluidic chip micro-channel through the visual port; and the pressure sensor and the imaging camera are respectively connected with the controller.
2. The system for measuring viscosity of a micro-scale liquid sample of claim 1, wherein the depth of the micro-channel is in the order of micrometers, and the width of the micro-channel is at least n times its height to produce a slot laminar flow; n is more than or equal to 10.
3. The system for measuring the viscosity of a micro-scale liquid sample according to claim 1, wherein the microfluidic chip is provided with an encapsulation layer, the encapsulation layer is provided with at least two openings, the openings are communicated with the micro-channels, and the aperture of each opening is not larger than the width of the micro-channel.
4. The system for measuring the viscosity of a micro-upgraded liquid sample as claimed in claim 1, wherein the pumping unit is used for providing a pushing pressure for the liquid sample loaded in the micro-channel; the outlet of the air cavity of the pumping unit, an interface of the manifold connector and a measuring port of the pressure sensor are communicated; the pressure sensor is used for measuring the pressure difference between the front and the back of the liquid sample.
5. The system for measuring viscosity of a micro-scaled liquid sample of claim 1, wherein the capillary tube is rigid and is made of one or a combination of two or more of PEEK, stainless steel, silica glass, perfluoroalkoxy alkane polymer, fluorinated ethylene propylene copolymer, and ethylene tetrafluoroethylene copolymer.
6. The system for measuring the viscosity of a micro-scale liquid sample according to claim 1, wherein the liquid sample in the micro-channel flows forward under the action of the pumping unit, an image of the liquid sample in a flow equilibrium state is obtained, a characteristic receding contact angle is obtained by measuring a receding contact angle in each time step and performing arithmetic averaging, further, a capillary pressure difference at a liquid-gas interface at the rear part of the micro-channel is calculated, and the capillary pressure difference is subtracted from a total pressure difference measured by the pressure sensor, so that an effective pressure difference for pushing the liquid sample to flow is obtained;
calculating a shear force based on the effective pressure differential.
7. The system for measuring the viscosity of a micro-scale liquid sample according to claim 1, wherein the controller analyzes the image obtained by the imaging camera to obtain the average flow velocity of the liquid sample under the equilibrium flow; based on the average flow velocity and the microchannel dimensions, a volumetric flow rate of the liquid sample within the microchannel at equilibrium is derived, and a shear rate is calculated based on the volumetric flow rate.
8. A method for measuring viscosity of a micro-scaled liquid sample, comprising:
loading a liquid sample into a microchannel of the microfluidic chip;
starting a pumping unit, enabling a liquid sample in the micro-channel to flow under the pressure of the pumping unit, measuring the pressure difference between the front and the back of the liquid sample in real time through a pressure sensor, and calculating the shearing force;
when a liquid sample flows in the microchannel, the liquid sample is in a section of liquid plug, and the flow process of the liquid plug type liquid sample is recorded in real time by an imaging camera;
analyzing an image acquired by an imaging camera, and calculating a shearing rate;
and calculating the dynamic viscosity of the liquid sample based on the ratio of the shearing force to the shearing rate.
9. The method according to claim 8, wherein the liquid sample in the micro channel flows forward under the action of the pumping unit, an image of the liquid sample in a flow equilibrium state is obtained, a characteristic receding contact angle is obtained by measuring a receding contact angle in each time step and performing arithmetic averaging, further, a capillary pressure difference at a liquid-gas interface at the rear of the micro channel is calculated, and the capillary pressure difference is subtracted from a total pressure difference measured by the pressure sensor, so that an effective pressure difference for pushing the liquid sample to flow is obtained;
calculating a shear force based on the effective pressure differential.
10. The method for measuring the viscosity of a micro-scale liquid sample according to claim 8, wherein the image obtained by the imaging camera is analyzed to obtain the average flow velocity of the liquid sample under the equilibrium state flow; based on the average flow velocity and the microchannel dimensions, a volumetric flow rate of the liquid sample within the microchannel at equilibrium is derived, and a shear rate is calculated based on the volumetric flow rate.
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