Contact angle measurement is usually accomplished through the use of optics or force tensiometers. Water droplet contact angle measuring instruments can be classified into static, dynamic, and roughness corrected contact angles.

What is the principle of the water droplet contact angle measuring instrument? Today, let's analyze together and delve into the classification and operation principles of droplet angles!

The contact angle is a quantitative measure of the wetting of a solid by a liquid. Geometrically, the contact angle is defined as the angle formed by the liquid at the three-phase boundary where the liquid, gas, and solid intersect. There are three different forces acting on the three-phase contact point between the solid, liquid, and gas, or in some cases between the solid, liquid, and liquid.

1. Static contact angle: Measure the static (i.e. fixed droplet) contact angle when the droplet is on the surface and the three-phase boundary is not moving. The static contact angle is the most commonly measured wettability value to date. It is most suitable for relatively smooth and uniform surfaces. The static contact angle is also used to define the surface free energy of the substrate (i.e. the surface tension of the solid), providing a fast, simple, and quantitative method for measuring wettability.

2. Dynamic contact angle: When the three-phase boundary moves, the dynamic contact angle can be measured, which is called the forward angle and backward angle. As these terms imply, the forward contact angle is measured when the leading edge of the droplet advances, while the backward contact angle refers to the measurement when the leading edge of the droplet retreats. On an ideal surface, these two values are close to each other. However, in most cases, the measured contact angle depends on the direction in which the contact wire moves. Contact angle hysteresis is the difference between the forward and backward contact angles. Contact angle hysteresis is mainly caused by surface chemical and morphological inhomogeneity, absorption of solution impurities on the surface, or solvent swelling, rearrangement, or alteration of the surface. Roll angle refers to the tilt angle at which a droplet begins to move. Low roll angle is related to low contact angle hysteresis.  

3. Roughness correction contact angle: The contact angle is affected by roughness. Surfaces with the same chemical composition but different roughness will have different contact angles. The roughness correction contact angle is the contact angle value obtained by removing the influence of roughness. When determining the surface free energy, roughness corrected contact angles should be specifically used.

Optical tensiometer for contact angle measurement: Optical tensiometers are used to measure static, dynamic, and roughness corrected contact angles. The main components of an optical tensiometer are a camera, a distributor for dispensing droplets, a sample stage, and a light source for illuminating droplets on the sample stage. The range of optical tensiometers ranges from fully manual systems to fully automatic instruments.   

Optical tensiometers are sometimes referred to as optical or contact angle goniometers, contact angle meters, or droplet shape analyzers. Basically, all of these mean similar types of instruments, which we call optical tensiometers.

Static contact angle of optical tensiometer

Static droplet measurement is the most commonly used method for measuring contact angle. In practice, place the droplet on a solid surface and record the image of the droplet. Then, the static contact angle is defined by fitting the Yang Laplace equation around the droplet, but other fitting methods such as circles and polynomials can also be used. It can also measure inverted fixed droplets called captured bubbles. Capture bubble measurement is particularly used to study the wettability of contact lenses. It is also applicable to other highly hydrophilic materials, in which the standard fixed droplet method is challenging. In addition, it is also very commonly used in measuring the contact angle of submerged objects.

The following are commonly used application areas for droplet angles:

1. Material research and development: The hydrophobic properties of nano coatings will be further optimized, making the surface more hydrophilic and hydrophobic. For example, some lens glass surfaces are less prone to fogging.

2. Electronic manufacturing: PCB surface cleanliness testing (contact angle<60 ° indicates no oil residue), as well as evaluating the performance of semiconductor wafer surfaces.

3. Biomedical industry: The hydrophobicity of medical catheters is tested through droplet angle testing to assess the compatibility of blood and the adhesion of biomaterials to cells.

4. Coatings and Printing: The droplet angle measuring instrument can evaluate the spreading effect of ink on paper and plastic surfaces, as well as the effect of droplet contact angle on printing clarity and coating adhesion.

5. The new energy industry, such as lithium batteries and solar panel surfaces, can conduct anti fouling performance tests on product surfaces.