What is contact angle

The contact angle refers to the angle θ between the tangent of the gas-liquid interface at the intersection of gas, liquid, and solid phases and the solid-liquid boundary line, and is a measure of the degree of wetting.
If θ<90 °, the solid is hydrophilic, meaning that the liquid can wet the solid. The smaller the angle, the better the wettability; If θ>90 °, the solid is hydrophobic, meaning that the liquid does not wet the solid and can easily move on the surface without entering the capillary pores.
The wetting process is related to the interfacial tension of the system. When a droplet of liquid falls on a horizontal solid surface and reaches equilibrium, the contact angle formed between it and the interfacial tension follows the Young's equation below:
γs,g = γs,l + γg,l×cosθ
It can predict the following wetting conditions:
1) When θ=0, * wetting;
2) When θ<90 °, partially wet or wet;
3) When θ=90 °, it is the boundary between wetting and non wetting;
4) When θ>90 °, it does not wet;
5) When θ=180 °, * does not wet.
The height h of liquid rise and fall in capillary phenomenon. The positive or negative sign of h represents rise or fall.
The wetting liquid rises and the contact angle is acute; The liquid does not infiltrate and the contact angle is obtuse.
Rising height h=2 * surface tension coefficient/(liquid density * gravitational acceleration g * liquid surface radius R).
Rising height h=2 * surface tension coefficient * cos contact angle/(liquid density * gravitational acceleration g * capillary radius r).
Wettability issues are closely related to mining flotation, petroleum extraction, textile printing and dyeing, pesticide processing, photosensitive film production, paint formulation, waterproofing, washing, and other related fields.
The contact angle is the angle at which a liquid/vapor interface meets the solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Most often the concept is illustrated with a small liquid droplet resting on a flat horizontal solid surface. The shape of the droplet is determined by the Young Relation. The contact angle plays the role of a boundary condition. Contact angle is measured using a contact angle goniometer. The contact angle is not limited to a liquid/vapour interface; it is equally applicable to the interface of two liquids or two vapours.
There are usually two existing testing methods for contact angle: one is the shape image analysis method; The second method is the weighing method The latter is commonly referred to as a wetting balance or a permeation contact angle meter But currently, the most widely used and accurate measurement method is still the shape image analysis method
The principle of the shape image analysis method is to drop liquid droplets onto the surface of a solid sample, obtain the shape image of the droplets through a microscope lens and camera, and then use digital image processing and some algorithms to calculate the contact angle of the droplets in the image
The method of calculating contact angle is usually based on a specific mathematical model, such as a droplet being considered as a part of a sphere or cone, and then the contact angle value is calculated by measuring specific parameters such as width/height or by direct fitting. The Young Laplace equation describes the relationship between the internal and external pressure difference of a closed interface and the curvature and tension of the interface, which can be used to accurately describe the contour of an axisymmetric droplet and calculate its contact angle.