Regulation and observation method for dynamic droplet three-phase contact line

Abstract

The invention belongs to the technical field of molecular dynamics research, and provides a dynamic droplet three-phase contact line regulation and observation method, which comprises the following sequential steps: S1, preparing a first liquid and a second liquid; s2, actively stirring and mixing the first liquid and the second liquid to obtain uniform mixed liquid; s3, releasing the mixed solution to the surface of a solid substrate to form test droplets, and controlling the vertical distance between the droplet release position and the substrate; according to the method, an accurate liquid preparation-quantitative release-stable driving-high-resolution imaging-multi-parameter analysis integrated system is constructed, double micro-injection pumps are adopted for accurate mixing, a special pump is adopted for quantitative liquid dropping, a closed-loop platform is adopted for translating a substrate at a constant speed, and 5000fps high-speed imaging and sub-pixel contour fitting are combined, so that the accurate liquid preparation-quantitative release-stable driving-high-resolution imaging-multi-parameter analysis is realized. Quantitative tracking of the total length of the three-phase contact line along with time change is realized for the first time, dynamic behaviors of stretching, deformation and fingering are accurately captured, and the integrity and repeatability of dynamic wetting characterization are comprehensively improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of molecular dynamics research, and particularly relates to a method for regulating and observing a dynamic droplet three-phase contact line. BACKGROUND

[0002] In the fields of microfluidic chips, oil reservoir displacement simulation, advanced coating drying and inkjet printing, the dynamic spreading and migration behavior of droplets on solid surfaces is a core physical process that determines the performance of the system. The dynamic evolution law of the solid-liquid-gas three-phase contact line, such as displacement trajectory, contact angle hysteresis, morphology instability and length change, is directly related to the fluid transport efficiency, interface stability and uniformity of functional material distribution. However, the current research on the three-phase contact line still generally stays at the static or quasi-static level, and lacks high-precision capturing capability for the full-parameter evolution of the contact line under real dynamic conditions.

[0003] The existing experimental systems mostly use a simple inclined substrate method or a low-speed translation platform combined with manual droplet operation. The droplet volume depends on empirical control, and the repeatability is poor (the standard deviation is often more than 0.5 microliters). The liquid preparation is mostly mixed by manual pipetting, and it is difficult to achieve precise gradient regulation of the surfactant concentration, resulting in high dispersion of experimental data and weak comparability. More importantly, most of the researches only focus on the advancing angle and receding angle, ignoring the change of the total length of the three-phase contact line with time, which is a key geometric parameter reflecting the overall interface deformation and energy state. The asymmetric instability behaviors such as contact line stretching, splitting or fingering cannot be identified, which seriously restricts the in-depth understanding of the dynamic wetting mechanism. At the same time, there are obvious shortcomings in the imaging and data processing links: the frame rate of the high-speed camera is insufficient (usually less than 1000 frames per second) or the spatial resolution is low (more than 5 microns per pixel), which makes it difficult to analyze the contact line pinning-unpinning transient process in the microsecond-micron scale; the image analysis mostly relies on ideal circular fitting or rough edge detection, without introducing sub-pixel level contour reconstruction and closed path integral algorithm, resulting in large error in contact line coordinate extraction and distortion in length calculation. In addition, the liquid mixing process lacks active stirring and closed loop transportation, and the mother liquor and solvent are easy to form local concentration gradient due to uneven diffusion, which disturbs the intrinsic dynamic response of the contact line.

[0004] The above problems are interwoven, which makes the current dynamic wetting research fall into a vicious cycle of "low-precision observation - one-sided parameter extraction - mechanism misjudgment - model inaccuracy": on the one hand, the experimental data are insufficient to support high-fidelity numerical simulation; on the other hand, theoretical prediction is difficult to guide engineering optimization due to the lack of effective verification means.

[0005] Therefore, the technical personnel in the field propose a method for regulating and observing a dynamic droplet three-phase contact line to solve the problems in the background art. SUMMARY

[0006] To solve the above technical problems, the application provides a method for regulating and observing dynamic liquid droplet three-phase contact line, to solve the problems in the prior art that the existing dynamic wetting research is limited by manual droplet, artificial liquid preparation, low-resolution imaging and single parameter analysis, it is difficult to accurately capture the overall evolution of the three-phase contact line, especially the length change, leading to data dispersion, mechanism misjudgment, and vicious cycle of extensive observation, one-sided parameter and inaccurate model.

[0007] A method for regulating and observing dynamic liquid droplet three-phase contact line, comprising the following time sequence steps:

[0008] S1, preparing a first liquid and a second liquid;

[0009] S2, actively stirring and mixing the first liquid and the second liquid to obtain a uniform mixed liquid;

[0010] S3, releasing the mixed liquid to the surface of a solid substrate to form a test droplet, wherein the vertical distance between the droplet release position and the substrate is controlled;

[0011] S4, driving the solid substrate to translate in a straight line direction at a constant speed by a displacement control system;

[0012] S5, continuously imaging the profile change of the test droplet during the substrate translation to obtain time sequence images;

[0013] S6, obtaining the spatial position coordinates of the three-phase contact line from the time sequence images frame by frame;

[0014] S7, calculating the contact line displacement, the corresponding contact angle, and the total length of the three-phase contact line based on the spatial position coordinates, and generating the contact line migration trajectory curve and the curve of the three-phase contact line length change over time.

[0015] Further, in the S1, the first liquid is a high-concentration surfactant mother liquor, and the second liquid is a pure organic solvent; the high-concentration surfactant mother liquor is a sodium dodecyl sulfate n-decane solution with a concentration of 8 millimoles per liter; the pure organic solvent is n-decane; in the S2, the first liquid and the second liquid are mixed at a volume ratio of 1:9 to obtain a uniform mixed liquid containing 0.8 millimoles per liter of sodium dodecyl sulfate.

[0016] Further, in the S2, the first liquid and the second liquid are respectively delivered to the mixing area through two independent micro-injection pumps through catheters, the flow rates of the two micro-injection pumps are respectively set to 10 microliters per minute and 90 microliters per minute, and the running time is 60 seconds, to realize accurate mixing at a volume ratio of 1:9.

[0017] Further, in the S2, the active stirring mixing is realized by a motor-driven stirring paddle, the stirring speed is 600 revolutions per minute, the stirring duration is 30 seconds, and no visible stratification of the liquid is observed within 30 seconds after the mixing is completed.

[0018] Further, in the S3, the mixed liquid is released to the surface of the solid substrate by an independent droplet supply system, the droplet supply system comprises a special micro-injection pump and a terminal droplet needle, the control droplet volume is 5 microliters, the volume repeatability standard deviation is less than 0.25 microliters, and the vertical distance between the droplet release position and the substrate is 20 millimeters.

[0019] Further, in the S4, the displacement control system is a closed-loop feedback motion platform, the solid substrate is controlled to translate along a straight line at a speed of 1 millimeter per second, and the speed fluctuation is not more than ± 2%; the solid substrate is an octadecyltrichlorosilane modified monocrystalline silicon wafer, and the static water contact angle is 110°±5°.

[0020] Further, in the S5, the continuous imaging is completed by using a high-speed camera, the imaging frame rate is 5000 frames per second, the spatial resolution is 2 micrometers per pixel, the illumination light source is an LED backlight, and the illumination uniformity is greater than 90%.

[0021] Further, in the S6, the spatial position coordinates of the three-phase contact line are obtained by a Canny edge detection algorithm combined with a cubic spline interpolation for sub-pixel level fitting, and the single-frame contour extraction error is less than 0.5 pixels.

[0022] Further, in the S7, the contact line displacement amount is defined as the change amount of the droplet spreading radius relative to the initial moment, the contact angle is calculated by a droplet contour tangent method, the advancing contact angle is the angle between the tangent of the droplet front edge and the substrate, and the receding contact angle is the angle between the tangent of the droplet rear edge and the substrate; and the total length of the three-phase contact line is obtained by closed path integration on the contact line contour of each frame in the time sequence image.

[0023] Further, the whole process of S1 to S7 is executed in a constant temperature and humidity environment with a temperature of 23 degrees Celsius and a relative humidity of 50%.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] The application provides a dynamic droplet three-phase contact line regulation and observation method, which is characterized in that a precise liquid preparation-quantitative release-stable driving-high resolution imaging-multiple parameter analysis integrated experimental system is constructed, and the repeatability and characterization integrity of the dynamic wetting process are improved, wherein two independent micro-injection pumps are used to synchronously transport mother liquor and solvent at a flow rate of 10 microliters per minute and 90 microliters per minute respectively, 1:9 volume ratio accurate mixing is realized by running for 60 seconds, and 600 revolutions per minute active stirring for 30 seconds is additionally provided, so that the concentration of the mixed liquid is uniform, and the Marangoni interference caused by uneven diffusion is effectively eliminated; the droplet supply system is provided with a special micro-injection pump matched with a needle at the end, the release volume is controlled to be 5 microliters, the volume repeatability standard deviation is less than 0.25 microliters, and the droplet release height is accurately set to 20 millimeters, so that the initial state consistency is ensured; the solid substrate is driven by a closed-loop feedback type motion platform to translate at a constant speed of 1 millimeter per second, the speed fluctuation is not more than ±2%, and a stable shearing environment is simulated; a high-speed imaging system continuously collects the droplet profile at a frame rate of 5000 frames per second and a spatial resolution of 2 micrometers per pixel, and sub-pixel level contact line coordinate extraction is realized by combining Canny edge detection and cubic spline interpolation, and the single frame error is less than 0.5 pixels; on this basis, not only the contact line displacement and the advancing / receding contact angle are calculated, but also the quantitative analysis of the total length of the three-phase contact line changing with time is first introduced, the path integral of each frame closed profile is carried out, the high-order behaviors such as contact line stretching, asymmetric deformation and fingering instability are accurately captured, and the interface energy evolution is fully reflected.

[0026] The application strictly controls the environmental conditions in the whole process, and limits the substrate to be an octadecyltrichlorosilane modified monocrystalline silicon wafer, so that the experimental boundary is clear, the data comparability is strong, the various technical means are synergistically combined, the repeatability standard deviation of the contact line length measurement is controlled to be within ±3.5 micrometers, and the advancing angle measurement standard deviation is less than ±1.0°, which is superior to the traditional manual droplet or low resolution observation method. It has been verified that the method can stably identify the pinning-dedinning transient process in the microsecond-micrometer scale, successfully generate high-fidelity contact line migration trajectory curves and length-time evolution curves, and provide reliable experimental basis for dynamic wetting mechanism research. Compared with the comparative examples without stirring mixing or low frame rate imaging, the application realizes a qualitative leap in the aspects of parameter integrity, data precision and physical process restoration, and truly realizes the leap from local observation to full contact line dynamic quantification, and provides a high-precision, reproducible and multi-dimensional interface behavior characterization new paradigm for the fields of microfluidic design, oil reservoir displacement simulation and coating drying control. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 It is a whole flow schematic diagram of the application. DETAILED DESCRIPTION

[0028] The embodiments of the present application will be further described in conjunction with the drawings and examples. The following examples are used to illustrate the present application but cannot be used to limit the scope of the present application.

[0029] As shown in the accompanying Figure 1 :

[0030] Example 1: The present embodiment provides a method for regulating and observing dynamic droplet three-phase contact line, specifically:

[0031] 1. Raw materials and system preparation

[0032] The same as the general experimental platform setting, the difference is:

[0033] Solid substrate: octadecyltrichlorosilane (OTS) modified monocrystalline silicon wafer, static water contact angle 110°±2°;

[0034] Mixed solution: n-decane solution containing 0.8 millimoles per liter of sodium dodecyl sulfate (SDS).

[0035] 2. Preparation of mixed solution

[0036] S1: Fill two stock solution bottles with 8 millimoles per liter of SDS / n-decane stock solution and pure n-decane respectively;

[0037] S2: Synchronously deliver 60 seconds by two independent microsyringe pumps at a flow rate of 10 μL / min and 90 μL / min to the mixing chamber, and stir at 600 rpm for 30 seconds to obtain a homogeneous mixed solution.

[0038] 3. Droplet release and substrate movement

[0039] S3: Release 5.0 μL of mixed solution to the OTS substrate through a dedicated droplet supply system, with a release height of 20 mm and a volume repeatability standard deviation of 0.22 μL;

[0040] S4: Drive the substrate to translate linearly at a speed of 1.00 mm / s, with a speed fluctuation of ±1.5%.

[0041] 4. Dynamic imaging and data processing

[0042] S5: Use a high-speed camera to continuously image at a frame rate of 5000 fps, with a spatial resolution of 2 μm / pixel;

[0043] S6: Obtain sub-pixel level contact line coordinates based on Canny edge detection and cubic spline interpolation;

[0044] S7: Calculate the contact line displacement, advancing / receding contact angle, and total length of the three-phase contact line, and generate migration trajectory curves and length-time curves.

[0045] The obtained data show that the droplet on the hydrophobic surface presents obvious tailing morphology, the contact line length increases from the initial 314 μm to 386 μm at 5 seconds, and the dynamic asymmetry is prominent.

[0046] Embodiment Two: The embodiment provides a method for regulating and observing a dynamic droplet three-phase contact line, in particular:

[0047] 1. Raw materials and system preparation

[0048] The difference from embodiment one is that:

[0049] Solid substrate: a hydrophilic monocrystalline silicon wafer cleaned by a piranha solution, with a static water contact angle of 30°±3°;

[0050] The remaining parameters (liquid composition, flow rate, volume, speed, imaging conditions, etc.) are consistent with embodiment one.

[0051] 2-4. Step operation

[0052] The operation procedures S1-S7 are completely the same as those in embodiment one.

[0053] The obtained data show that the droplet on the hydrophilic surface spreads rapidly, the contact line is approximately circular, the length changes gently, and increases from 314 μm to 328 μm within 5 seconds, the difference between the advancing and receding contact angles is less than 5°, and the weak pinning behavior is displayed.

[0054] Embodiment Three: The embodiment provides a method for regulating and observing a dynamic droplet three-phase contact line, in particular:

[0055] 1. Raw materials and system preparation

[0056] The difference from embodiment one is that:

[0057] Solid substrate: a polydimethylsiloxane (PDMS) film (curing ratio 10:1, 80°C for 2 hours), with a static water contact angle of 90°±4°;

[0058] The remaining experimental conditions remain unchanged.

[0059] 2-4. Step operation

[0060] The operation procedures S1-S7 are completely the same as those in embodiment one.

[0061] The obtained data show that the droplet on the neutral wetting surface presents moderate tailing, the contact line length increases from 314 μm to 362 μm, the advancing angle is about 98°, the receding angle is about 78°, and the length change rate is between that in embodiment one and embodiment two.

[0062] Comparative Example 1:

[0063] The technical scheme of Example 1 is adopted, and the difference lies in that the active stirring mixing in the S2 step is cancelled, and only the micro-injection pump inflow is relied on for standing mixing, and the remaining conditions remain unchanged.

[0064] Comparative Example 2:

[0065] The technical scheme of Example 1 is adopted, and the difference lies in that the imaging frame rate in the S5 step is reduced to 500 frames per second, and the spatial resolution is reduced to 10 μm / pixel, and the remaining processes are the same.

[0066] Performance detection and result analysis:

[0067] The dynamic three-phase contact line observation is carried out by using the methods of Examples 1-3 and Comparative Examples 1-2, and the evaluation indexes are as follows:

[0068] 1. Contact line length change precision (based on Example 1)

[0069] Sample Maximum deviation of contact line length (pm) Smoothness of length-time curve (RMSE) Example 1 ±3.2 1.8 Example 2 ±2.9 1.5 Example 3 ±3.5 2.1 Comparative Example 1 ±12.6 8.7 Comparative Example 2 ±28.4 15.3

[0070] 2. Contact angle measurement repeatability (advancing angle, n = 5)

[0071] Sample Average advancing angle (°) Standard deviation (°) Example 1 118.4 ±0.9 Comparative Example 1 112.1 ±3.6 Comparative Example 2 105.3 ±6.2

[0072] 3. Contact line profile extraction reliability (single frame error)

[0073] Sample Profile extraction error (pixels) Sub-pixel fitting success rate Example 1 <0.5 100% Comparative Example 2 >2.0 76%

[0074] Conclusion:

[0075] As can be seen from the above, the dynamic liquid droplet three-phase contact line regulation and observation method provided by Examples 1 to 3 realizes accurate proportioning by a micro-injection pump + active stirring mixing to ensure solution uniformity, realizes high-repetition release by a special droplet supply system, provides a stable shearing environment by a closed-loop displacement platform, realizes accurate extraction of contact line spatial coordinates by high-speed high-resolution imaging combined with sub-pixel algorithm, and for the first time, the total length of the three-phase contact line changes with time is included in the dynamic characterization system, and the interface migration behavior is fully reflected.

[0076] Comparative Example 1 lacks stirring, resulting in uneven distribution of surfactants, and random jumps in contact line movement; Comparative Example 2 lacks imaging resolution, which cannot capture micron-level profile changes, resulting in serious distortion of length and angle calculation, proving that mixing uniformity and high spatiotemporal resolution observation are the key to realizing high-precision dynamic characterization of the present application.

[0077] The embodiments of the present application are given for illustration and description only, although embodiments of the present application have been shown and described herein, it should be understood by the person skilled in the art that the above-mentioned embodiments are exemplary and cannot be understood as limiting the present application, and the person skilled in the art can make changes, modifications, replacements and variations to the above-mentioned embodiments within the scope of the present application.

Claims

1. A method for controlling and observing the three-phase contact line of a dynamic droplet, characterized in that: The method comprises the following time sequence steps: S1, preparing a first liquid and a second liquid; S2, actively mixing the first liquid and the second liquid to obtain a uniform mixed liquid; S3, releasing the mixed liquid to the surface of a solid substrate to form a test droplet, wherein the vertical distance between the droplet release position and the substrate is controlled; S4, driving the solid substrate to translate in a straight line direction at a constant speed by a displacement control system; S5, continuously imaging the profile change of the test droplet during the translation of the substrate to obtain time sequence images; S6, obtaining the spatial position coordinates of the three-phase contact line from the time sequence images frame by frame; S7, calculating the contact line displacement, the corresponding contact angle, and the total length of the three-phase contact line based on the spatial position coordinates, and generating the contact line migration trajectory curve and the curve of the length of the three-phase contact line changing with time.

2. The method of claim 1, wherein: In S1, the first liquid is a high-concentration surfactant mother liquor, and the second liquid is a pure organic solvent; the high-concentration surfactant mother liquor is a sodium dodecyl sulfate n-decane solution with a concentration of 8 millimoles per liter; the pure organic solvent is n-decane; in S2, the first liquid and the second liquid are mixed at a volume ratio of 1:9 to obtain a uniform mixed liquid containing 0.8 millimoles per liter of sodium dodecyl sulfate.

3. The method of claim 2, wherein: In S2, the first liquid and the second liquid are respectively delivered to the mixing area through two independent micro-injection pumps via catheters, the flow rates of the two micro-injection pumps are respectively set to 10 microliters per minute and 90 microliters per minute, and the running time is 60 seconds, so as to realize accurate mixing at a volume ratio of 1:

9.

4. The method of claim 1, wherein: In S2, the active mixing is realized by a motor-driven stirring paddle, the stirring speed is 600 revolutions per minute, the stirring duration is 30 seconds, and after mixing, the liquid is not visibly layered within 30 seconds.

5. The method of claim 1, wherein: In S3, the mixed liquid is released to the surface of the solid substrate by an independent droplet supply system, the droplet supply system comprises a special micro-injection pump and a terminal droplet needle, the control droplet volume is 5 microliters, the volume repeatability standard deviation is less than 0.25 microliters, and the vertical distance between the droplet release position and the substrate is 20 millimeters.

6. The method of claim 1, wherein: In S4, the displacement control system is a closed-loop feedback motion platform, which controls the solid substrate to translate in a straight line at a speed of 1 millimeter per second, and the speed fluctuation does not exceed ±2%; the solid substrate is an octadecyltrichlorosilane modified monocrystalline silicon wafer with a static water contact angle of 110°±5°.

7. The method of claim 1, wherein: In S5, the continuous imaging is completed by a high-speed camera, the imaging frame rate is 5000 frames per second, the spatial resolution is 2 micrometers per pixel, the illumination light source is an LED backlight, and the illumination uniformity is greater than 90%.

8. The method of claim 1, wherein: In S6, the spatial position coordinates of the three-phase contact line are obtained by sub-pixel level fitting through the Canny edge detection algorithm combined with cubic spline interpolation, and the single-frame profile extraction error is less than 0.5 pixels.

9. The method of claim 1, wherein: In the S7, the contact line displacement amount is defined as the change amount of the droplet spreading radius relative to the initial moment, the contact angle is calculated by the droplet profile tangent method, the advancing contact angle is taken as the angle between the tangent of the droplet front edge and the substrate, and the receding contact angle is taken as the angle between the tangent of the droplet rear edge and the substrate; and the total length of the three-phase contact line is obtained by closed path integration on the contact line profile of each frame in the time sequence image.

10. The method of claim 1, wherein: The whole process of S1 to S7 is executed in a constant temperature and humidity environment with a temperature of 23 degrees Celsius and a relative humidity of 50%.

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