CN108982299A - A kind of micro-structure surface wetting state judgment method based on total reflection principle - Google Patents

Abstract

The micro-structure surface wetting state judgment method based on total reflection principle that the invention discloses a kind of, when drop is in Cassie state, when with light from liquid incident air, if incidence angle is greater than critical angle of incidence, light will be totally reflected in gas-liquid surface, if reflection light can be observed, then drop is in Cassie state, conversely, drop is in Wenzel state, therefore this method passes through the drop wetting state for judging the surface of solids, can reflect the hydrophobic characteristic of the surface of solids.Compared to AFM equipment, this method only needs laser and corresponding observation device, and price is lower, and is able to satisfy precision needs.

Description

A method for judging the wetting state of microstructured surfaces based on the principle of total reflection

technical field

The invention belongs to the technical field of surface wetting property measurement, and in particular relates to a method for judging the wetting state of a microstructure surface based on the principle of total reflection.

Background technique

According to the degree of wetting of the droplet on the rough surface with micron-scale microstructure, the wetting state of the droplet can be divided into two types: Cassie and Wenzel states, as shown in Figure 1(a) and Figure 1(b), in general , the liquid in the Cassie state flows away from the solid surface more easily. Therefore, the wetting state of liquid droplets on the solid surface directly reflects the ease of liquid flow on the solid surface. This is because when the fluid is in the Cassie state, due to the reduction of the interaction area between the fluid and the solid, the force of the solid on the fluid is also reduced, thereby resulting in reduced resistance to fluid flow. Since different surface structures have different effects on reducing the resistance of the flow process, the water repellency of a solid surface to a liquid can be determined by judging the wetting state of a droplet on a solid surface. Therefore, it is of great significance to propose an experimental method for judging the wetting state of liquid on different solid surfaces.

At present, the conversion of Cassie and Wenzel mainly adopts the method of obtaining local surface characteristics by AFM. Although this method can obtain the distribution of liquid on the surface very accurately, it requires the support of the AFM system, so it is very expensive.

Contents of the invention

In view of this, the purpose of the present invention is to provide a method for judging the wetting state of the microstructure surface based on the principle of total reflection, which can judge the wetting state of the droplet on the surface of the structure, and the measurement result of the method is accurate, and the operation method , low price, easy to popularize and apply.

A method for judging the wetting state of a microstructure surface based on the principle of total reflection of the present invention comprises the following steps:

Step 1. Put the droplet on the surface of the microstructure;

Step 2, using light to irradiate the liquid droplets at an angle greater than the critical incident angle; wherein, the critical incident angle is the critical incident angle at which total reflection occurs when the light enters the air from the liquid droplet;

Step 3. Observe the reflected light. If total reflection occurs, the surface of the microstructure is in a Cassie wet state.

Preferably, the droplet adopts Cu(OH) ₂ suspension.

Preferably, the light is laser light.

Preferably, in the step 2, the light enters the droplet at an angle greater than the critical incident angle of 5°-10°.

Preferably, the incident point of the light on the droplet is constantly changed, and when the number of total reflections of the light emission reaches the set number, it is determined that the total reflection phenomenon has occurred, that is, it is in the Cassie wetting state at this time.

Preferably, a microscope is used to observe the propagation of the light.

Preferably, a CCD is used to record the propagation of the light.

The present invention has following beneficial effects:

The present invention proposes a method for detecting the wetting state of the microstructure surface (Cassie and Wenzel wetting state) based on the principle of total reflection. At the critical incident angle, the light will be totally reflected on the gas-liquid surface. If the reflected light can be observed, the droplet is in the Cassie state. Otherwise, the droplet is in the Wenzel state. Therefore, this method judges the wetting state of the droplet on the solid surface, It can reflect the water-repellent properties of the solid surface. Compared with AFM equipment, this method only needs laser and corresponding observation equipment, the price is lower, and it can meet the accuracy requirements.

Description of drawings

Figure 1(a) is a schematic diagram of the Cassie state, and Figure 1(b) is a schematic diagram of the Wenzel state;

Fig. 2 is a schematic diagram of surface parameters of a rectangular array structure;

Figure 3 is a schematic diagram of the principle of total reflection;

Figure 4 is a schematic diagram of the experimental device for judging the wetting state based on the principle of total reflection;

Figure 5. Schematic diagram of droplet measurement locations in the experiment.

Among them, 1-stage, 2-laser, 3-holding device, 4-structured surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the accompanying drawings and examples.

As shown in Figure 1(b), the droplet is in the Cassie wetting state in the solid with surface microstructure. When the droplet is in the Cassie state, there is a gas layer between the droplet and the solid surface, and the gas layer is located in the gap of the surface structure. In between, that is, the liquid in the droplet does not enter the voids of the microstructure; on the contrary, as shown in Figure 1 (a), in the Wenzel wetting state, the liquid in the droplet enters the voids of the microstructure. In order to facilitate the explanation of the specific steps of the method, the specific steps of the judging method of the present invention will be described in detail below by taking a surface with a regular rectangular array structure as an example. Figure 2 is a schematic diagram of the surface of a rectangular array structure, p is the distance between the centers of two adjacent structures, h is the height of the microstructure, and b is the length of the structure. When a liquid is dropped on the surface, if the droplet is in the Cassie state, the liquid-air interface can be specifically enlarged as shown in Figure 3.

As shown in Figure 3, when light is incident at a certain incident angle θ ₁ on the plane x, θ ₂ is the refraction angle. Since the refractive index of light in water is n ₁ , its refractive index in air is n ₂ , given by The law of refraction shows that:

n ₁ sinθ ₁ = n ₂ sinθ ₂

Since n ₁ >n ₂ , so θ ₂ >θ ₁ , considering that θ ₂ must be less than 90°, therefore, the maximum value of θ ₁ can only be:

θ ^* ₁ = asin(n ₂ /n ₁ )

θ ^* ₁ is the critical incident angle, when the incident angle is greater than θ ^* ₁ , the light will be totally reflected at the liquid-gas interface. Since the droplet is in the Cassie state, when the light is incident on the droplet at an angle greater than the critical incident angle, the light will undergo total reflection due to the presence of an air layer in the microstructure gap; therefore, by observing whether the incident light is totally reflected The reflection phenomenon can judge the wetting state of the droplet. However, pure water cannot see the light path, so the present invention considers adding PS fluorescent particles into the water to form a suspension. When light passes through the suspension, it will be scattered due to the fluorescent particles, so that the propagation of the light can be seen . In this embodiment, Cu(OH) ₂ suspension is used, and the concentration is as light as possible, using 10 ⁻⁸ nmolar).

Figure 4 is a schematic diagram of the experimental setup. The stage 1 is used to place the transparent structure surface 4 carrying liquid droplets. The transparent structure surface 4 is suspended on the stage 1 to allow light to pass through. It can move along the z direction and the x direction, and the movement accuracy in the z direction is 1mm , the motion accuracy in the x direction is 5 μm; wherein, the z direction is along the vertical direction, and the x-y plane is a horizontal plane; the laser 2 is fixed by a clamping device 3, and the angle between it and the optical axis of the laser 2 is 90°, and the clamping device 3 can be moved along The x-z plane rotates and moves along the y direction, the rotation angle accuracy is 1°, the y direction movement accuracy is 5μm, and the incident angle of the laser light is φ, which changes according to the rotation angle ψ. The CCD and microscope are placed on the side of the droplet to observe and record the light passing through the droplet.

The specific measurement steps are as follows:

1. Drop 1ml-2ml PS solution on the transparent structure surface 4;

2. The transparent structural plane 4 carrying water droplets is placed on the stage 1, and the z-axis position of the stage 1 is adjusted so that it is at the same height as the microscope and within the observation range of the microscope. At the same time, adjust the x-direction position of the stage 1 so that it is located at the center of the observation field of view;

3. Adjust the rotation angle ψ so that 90-ψ is about 5-10° larger than the critical incident angle, and turn on the laser 2. At this time, move the clamping device 3 along the y direction so that the incident laser is incident on the right side of the droplet , and adjust the x-direction position of the stage 1 at this time, so that the laser is located at the front of the droplet, and the final laser incident position is shown in Figure 5 (the points are randomly selected in the figure, but the selection is based on the uniform distribution on the plane);

4. At this time, the path of the laser in the droplet can be recorded by the CCD, and numbered in Figure 1; since the microstructure is a periodically distributed void structure, the incident laser light may be irradiated on the protrusions of the microstructure, and the light will not occur at this time. Total reflection phenomenon, so it is necessary to move the incident point of the laser multiple times so that it can be incident into the groove of the microstructure, that is, in the air layer, so that the total reflection phenomenon can be observed. Therefore, move the laser position to position b in Figure 5, and record the laser path again, numbered in Figure 2; record the laser paths of the 16 positions in Figure 5 in sequence, Figures 1, 2, 3...15, 16.

5. When more than a certain number of pictures show total reflection, the droplet is in the Cassie state on the surface.

To sum up, the above are only preferred embodiments of the present invention, and are not intended to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (7)

1. A method for judging the microstructure surface wetting state of a total reflection principle, is characterized in that, comprises the steps: Step 1. Put the droplet on the surface of the microstructure; Step 2, using light to irradiate the liquid droplets at an angle greater than the critical incident angle; wherein, the critical incident angle is the critical incident angle at which total reflection occurs when the light enters the air from the liquid droplet; Step 3. Observe the reflected light. If total reflection occurs, the surface of the microstructure is in a Cassie wet state. 2. the method for judging the wet state of a structure surface of a kind of total reflection principle as claimed in claim 1, is characterized in that, described droplet adopts Cu (OH) _Suspension liquid. 3. The method for judging the wetting state of a structural surface based on the principle of total reflection as claimed in claim 2, wherein the light is laser light. 4. The method for judging the wetting state of a structural surface based on the principle of total reflection as claimed in claim 1, wherein in said step 2, the light enters the droplet at an angle greater than the critical incident angle of 5°-10° . 5. The method for judging the wetting state of a structural surface based on the principle of total reflection as claimed in claim 1, wherein the incident point of the light on the droplet is constantly changed, and when the number of times of total reflection of the light emission reaches the set value times, it is determined that the total reflection phenomenon occurs, that is, it is in the Cassie wet state at this time. 6. The method for judging the wetting state of a structural surface based on the principle of total reflection as claimed in claim 3, wherein a microscope is used to observe the propagation of light. 7. The method for judging the wetting state of a structural surface based on the principle of total reflection as claimed in claim 6, wherein a CCD is used to record the propagation of light.