Method for evaluating droplet removal capability

Abstract

Droplets 200 are dropped on surfaces of samples 100A, 100B, 100C, the samples 100A, 100B, 100C are inclined after the droplets 200 are dropped, a hysteresis and the size of a contact portion when each of the samples 100A, 100B, 100C is inclined are measured, and the droplet removal capability of each of the samples 100A, 100B, 100C is evaluated by using the measured values of the hysteresis and the size of the contact portion.

Description

Droplet removal performance evaluation method

[0001] The present disclosure relates to a method for evaluating droplet removal performance.

[0002] The static contact angle has been used as an index to evaluate the water and oil repellency of water and oil repellent materials, but the relative merits of static contact angles do not necessarily correspond to the relative merits of droplet removal (Non-Patent Document 1). Non-Patent Document 2 shows an example where a water droplet rolls off even when tilted by just a few degrees, even when the static contact angle is approximately 160°, and an example where the water droplet does not slide off even when tilted by 90°.

[0003] Therefore, it is effective to evaluate the droplet removability by hysteresis, which is defined as the difference between the advancing contact angle and the receding contact angle (Non-Patent Document 3). The smaller the hysteresis, the higher the droplet removability can be evaluated.

[0004] "What is sliding angle?", [online], Kyowa Interface Science Co., Ltd., [Retrieved April 25, 2023], Internet <URL: https: / / www.face-kyowa.co.jp / science / theory / what_sliding_angle.html> Atsushi Hozumi and four others, "Development trends of liquid-repellent treatments focusing on liquid sliding properties," Surface Technology, Surface Finishing Association of Japan, September 2016, Vol. 67, No. 9, pp. 452-460. "Liquid removal evaluation," [online], Kyowa Interface Science Co., Ltd., [Retrieved April 25, 2023], Internet <URL: https: / / www.face-kyowa.co.jp / science / lite rature / contactangle07.html〉 "Research introduction: Micro-liquid handling device," [online], Kyushu Institute of Technology, [Retrieved April 25, 2023], Internet〈URL: https: / / www.life.kyutech.ac.jp / ~yasuda / jp / research / liquidhandling / index.html〉 "What is contact angle (wettability)?", [online], Kyowa Interface Science Co., Ltd., [Retrieved April 25, 2023], Internet〈URL: https: / / www.face-kyowa.co.jp / science / theory / what_contact_angle / 〉

[0005] If the contact area of ​​the droplet increases with increasing tilt angle, it may be impossible to accurately determine droplet removability based on hysteresis alone. The contact area may expand downward due to gravity, but if the force of the sample attracting the droplet is greater than the force of repelling it, or if the sample has a special pattern (Non-Patent Document 4), the contact area may also expand upward.

[0006] The present disclosure has been made in view of the above, and aims to more appropriately evaluate droplet removability.

[0007] A method for evaluating droplet removal performance according to one aspect of the present disclosure involves dropping a droplet onto the surface of a material to be evaluated, tilting the material to be evaluated after dropping the droplet, measuring the hysteresis of the droplet and the size of the contact area between the droplet and the material to be evaluated when the material to be evaluated is tilted, and evaluating the droplet removal performance of the material to be evaluated using the measured values ​​of the hysteresis and the size of the contact area.

[0008] According to the present disclosure, the removability of droplets can be evaluated more appropriately.

[0009] FIG. 1 is a diagram showing an example of an advancing contact angle and a receding contact angle. FIG. 2 is a diagram showing an example of an advancing contact angle and a receding contact angle. FIG. 3 is a diagram showing an example of an advancing contact angle and a receding contact angle. FIG. 4 is a flowchart showing an example of a process flow of a method for evaluating droplet removal performance. FIG. 5 is a diagram showing an example of hysteresis and a normalized value of the hysteresis. FIG. 6 is a diagram showing an example of a contact area and a normalized value of the contact area. FIG. 7 is a diagram showing an example of a droplet removal index. FIG. 8 is a diagram showing an example of the configuration of an evaluation apparatus. FIG. 9 is a diagram showing an example of measurement results of hysteresis and a contact area. FIG. 10 is a diagram showing an example of hysteresis and a normalized value of the hysteresis. FIG. 11 is a diagram showing an example of a contact area and a normalized value of the contact area. FIG. 12 is a diagram showing an example of a droplet removal index. FIG. 13 is a diagram showing an example of the hardware configuration of an evaluation apparatus.

[0010] An example in which droplet removability cannot be accurately determined by hysteresis alone will be described with reference to FIGS. 1 to 3. FIG.

[0011] A droplet 200 is dropped onto the surfaces of Sample 100A, Sample 100B, and Sample 100C, and the tilt angle α is increased. The advancing contact angle θa and the receding contact angle θr are measured at the tilt angle α just before the droplet 200 slides off. The hysteresis is calculated as θa - θr. For Sample 100A in Figure 1, θa = 100°, θr = 100°, and the hysteresis θa - θr = 0°. For Sample 100B in Figure 2, θa = 120°, θr = 70°, and the hysteresis θa - θr = 50°. Since the hysteresis of Sample 100A is smaller than that of Sample 100B, Sample 100A can be evaluated as having better droplet removal performance.

[0012] 3, θa = 100°, θr = 100°, and the hysteresis θa - θr = 0°. However, the contact area of ​​the droplet 200 is larger for sample 100C than for sample 100A (Lc > La). Comparing only the hysteresis, the droplet removal performance of sample 100A and sample 100C is evaluated to be the same.

[0013] However, in Sample 100C, if it is considered that the force (gravity) acting on the droplet 200 is used to increase the contact area of ​​the droplet 200, it is considered that the force required to remove the droplet 200 is smaller in Sample 100C than in Sample 100A. In other words, it is considered that Sample 100C has lower droplet removal performance than Sample 100A.

[0014] Hysteresis alone does not adequately evaluate droplet removal performance because it does not monitor the anisotropic increase in contact area, which occurs when the force attracting the droplet on the sample is greater than the force repelling it, causing the contact area to expand upward.

[0015] Therefore, in the method for evaluating droplet removal performance of this embodiment, the droplet removal performance of the water- and oil-repellent material was evaluated using the hysteresis and the size of the contact area. Specifically, the droplet removal performance was evaluated using the following formula as a new droplet removal index.

[0016]

[0017] Here, the normalized hysteresis value is a value obtained by normalizing the hysteresis measured for each sample to a range from 0 to 1. The normalized size of the contact area is a value obtained by normalizing the size of the contact area between the droplet and the sample to a range from 0 to 1. The size of the contact area is the contact area between the droplet and the sample, or the contact distance when the contact area between the droplet and the sample is viewed from the side. The normalized size of the contact area may be a value obtained by normalizing the amount of change in the size of the contact area when the sample is tilted.

[0018] [Method for Evaluating Droplet Removal Performance] The process flow of the method for evaluating droplet removal performance of this embodiment will be described with reference to the flowchart in Fig. 4. In the following, each step will be described as being performed by an apparatus, but some or all of the steps may also be performed by a person.

[0019] In step S11, the device drops a droplet onto the sample to be evaluated, and while photographing the droplet, changes the tilt angle of the sample from 0° to 90° in 1° increments. The droplet is photographed so that the contact angle of the droplet and the size of the contact area between the droplet and the sample can be determined. The tilt angle is not limited to 0° to 90°, and may be varied between any tilt angles.

[0020] In step S12, the device measures the advancing contact angle, receding contact angle, and contact area at the tilt angle before the droplet slides off. If the droplet does not slide off, the device measures the advancing contact angle, receding contact angle, and contact area when the tilt angle is 90°.

[0021] The advancing contact angle, receding contact angle, and contact area can be measured using the image captured in step S11. The area of ​​a circle whose diameter is the length of the contact area on the image may be used as the contact area. The length of the contact area between the droplet and the sample may be used instead of the contact area. The amount of change in the contact area or the amount of change in the length of the contact area may also be used. When the amount of change is used, the size of the contact area is measured without tilting the sample. In the present disclosure, the contact area may be interpreted as the length of the contact area.

[0022] The processes of steps S11 and S12 are performed for all samples. The processes of steps S11 and S12 may be performed multiple times for each sample to calculate an average value of the measured values, and the average value may be used as the measurement result for that sample. Alternatively, droplets may be dropped at multiple locations on each sample, measurements may be taken at each location, and the average value of the measured values ​​may be calculated.

[0023] In step S13, the device normalizes the hysteresis and contact area. For example, the hysteresis of each sample is normalized by setting 0° as 0 and the maximum hysteresis among the samples as 1. The contact area of ​​each sample is normalized by setting the minimum as 0 and the maximum contact area among the samples as 1. During normalization, the operator may determine the maximum value of the hysteresis and the maximum value of the contact area.

[0024] Figure 5 shows an example of the hysteresis and normalized hysteresis values ​​for Sample A, Sample B, and Sample C. Hysteresis is calculated by subtracting the receding contact angle from the advancing contact angle. In the example shown in Figure 5, the hysteresis of Sample B is maximum at 50°, so for Samples A, B, and C, the hysteresis from 0° to 50° is converted to a value between 0 and 1, and the hysteresis is normalized.

[0025] Figure 6 shows an example of the contact area and normalized value of the contact area for sample A, sample B, and sample C. In the example of Figure 6, the contact area of ​​sample C is the largest at 20, so for samples A, B, and C, the contact area values ​​from 0 to 20 are converted to values ​​from 0 to 1 to normalize the contact area.

[0026] In step S14, the apparatus substitutes the normalized hysteresis value and the normalized contact area value of each sample into the above-mentioned formula to derive the droplet removal index for each sample. Figure 7 shows examples of the droplet removal index for samples A, B, and C.

[0027] In step S15, the device compares the droplet removal indexes of each sample and evaluates the droplet removal performance of each sample. The higher the droplet removal index calculated in step S14, the higher the droplet removal performance of the sample can be evaluated. In the example of FIG. 7, sample A can be evaluated as having the highest droplet removal performance, sample C as having the next highest droplet removal performance after sample A, and sample B as having the lowest droplet removal performance. The operator may determine a reference value for the droplet removal index and evaluate the droplet removal performance of each sample by comparing the droplet removal index of each sample with the reference value. If evaluation were performed using only hysteresis, the droplet removal performance of sample A and sample C would be evaluated as being similar, but by using the evaluation method of this embodiment, the difference in droplet removal performance between sample A and sample C can be more accurately evaluated.

[0028] [Evaluation Apparatus] An example of the configuration of an evaluation apparatus for droplet removal performance will be described with reference to Fig. 8. The evaluation apparatus 10 shown in the figure includes an input unit 11, an analysis unit 12, a calculation unit 13, and an evaluation unit 14.

[0029] The input unit 11 inputs a still image or a video of the droplet. When a video is input, the input unit 11 identifies the frame in the video at which the droplet begins to slide down, and transmits the previous frame to the analysis unit 12. The input unit 11 may be equipped with a camera to capture the droplet.

[0030] The analysis unit 12 analyzes the captured image of the droplet to determine the advancing contact angle, the receding contact angle, and the contact area. The analysis unit 12 may display the image and accept input from the operator of the advancing contact angle, the receding contact angle, and the line segment indicating the contact area.

[0031] The calculation unit 13 obtains the normalized values ​​of the hysteresis and the contact area, and calculates the droplet removal index for each sample.

[0032] The evaluation unit 14 evaluates the droplet removal performance of each sample based on the droplet removal index of each sample.

[0033] [Example] With reference to Figs. 9 to 12, an example in which the droplet removal performance of Samples 1, 2, and 3 was evaluated will be described.

[0034] A 6 μL droplet of water was used. Droplets were placed at three locations on each sample. The tilt angle was changed in 1° increments from 0° to 90°. The advancing and receding contact angles were measured 1000 ms after the tilt angle was changed. Since the droplet did not slide off between 0° and 90° for any sample, the advancing and receding contact angles, as well as the length of the contact area, were measured at tilt angles of 0° and 90°. Images of the droplets were taken and the tangent method described in Non-Patent Document 5 was applied to measure the advancing and receding contact angles. The change in the length of the contact area was used as the contact area. The image at a tilt angle of 0° and the image at a tilt angle of 90° were superimposed to determine the change in the length of the contact area.

[0035] As shown in Figure 9, the hysteresis and contact area size were determined for droplets at three locations for each sample, and the average value was calculated. The average value for each sample was used to calculate the droplet removal index.

[0036] 10 and 11 show the normalized values ​​of the hysteresis and contact area size for each sample. Fig. 12 shows the droplet removal index for each sample. Since Sample 3 had the largest droplet removal index, Sample 3 was evaluated as having high droplet removal performance, while Samples 1 and 2 were evaluated as having low droplet removal performance.

[0037] In the above, the advancing contact angle and the receding contact angle are measured by the sliding method, but the advancing contact angle and the receding contact angle may also be measured by the expansion / contraction method.

[0038] As described above, the droplet removal performance evaluation method of this embodiment involves dropping droplet 200 onto the surfaces of samples 100A, 100B, and 100C, tilting samples 100A, 100B, and 100C after dropping droplet 200, measuring the hysteresis and the size of the contact area when samples 100A, 100B, and 100C are tilted, and evaluating the droplet removal performance of samples 100A, 100B, and 100C using the measured values ​​of hysteresis and the size of the contact area. This allows for more appropriate evaluation of droplet removal performance.

[0039] The evaluation device 10 described above can be, for example, a general-purpose computer system including a central processing unit (CPU) 901, a memory 902, a storage 903, a communication device 904, an input device 905, and an output device 906, as shown in Fig. 13. In this computer system, the evaluation device 10 is realized by the CPU 901 executing a predetermined program loaded onto the memory 902. This program can be recorded on a computer-readable non-transitory recording medium such as a magnetic disk, an optical disk, or a semiconductor memory, or can be distributed via a network.

[0040] 10 Evaluation device 11 Input unit 12 Analysis unit 13 Calculation unit 14 Evaluation unit 100A, 100B, 100C Sample 200 Droplet

Claims

1. A method for evaluating droplet removal performance, comprising: dropping a droplet on a surface of a material to be evaluated; tilting the material to be evaluated after dropping the droplet; measuring the hysteresis of the droplet and the size of a contact area between the droplet and the material to be evaluated when the material to be evaluated is tilted; and evaluating the droplet removal performance of the material to be evaluated using the measured values ​​of the hysteresis and the size of the contact area.

2. A method for evaluating droplet removal performance according to claim 1, comprising: The droplet removal performance of the evaluation target material is evaluated using the droplet removal performance evaluation method.

3. A method for evaluating droplet removal performance according to claim 2, wherein the normalized value of the size of the contact portion is the amount of change in size of the contact portion when the material to be evaluated is tilted.

4. A method for evaluating droplet removal performance as described in claim 1, wherein the size of the contact portion is the contact area between the droplet and the material to be evaluated or the contact distance between the contact portion between the droplet and the material to be evaluated when viewed from the side.