The contact angle is a widely used method when studying the liquid-solid interface.
For example, when a liquid is placed on a solid surface, the following two situations may occur: 1. The liquid spreads on the solid surface ( that is, so-called wetting occurs), 2. The liquid withdraws (shrinks), trying to limit or It reduces its contact with the solid surface ( ie no wetting occurs). Wetting of liquid on solid surface can be judged by measuring the contact angle formed by the liquid-solid interface, see Figure 21-5. The figure shows the situation when a drop of liquid is placed on a horizontal solid surface, and the angle θ is generally called the contact angle of the liquid. This angle is generally measured through a liquid and can range from 0° to 180°.
The size of the contact angle can be determined by surface tension: the size of the liquid contact angle formed by a liquid placed on a flat solid surface can be determined by the three surface tensions acting on the liquid-solid interface. The first force is the surface tension σ1 of the liquid, and its function is to pull the liquid away from the interface end in the direction tangent to the liquid surface (the angle formed by the surface tension of the liquid and the solid surface can be defined as the contact angle of the liquid). The second force is the interfacial tension σa1 , which exists in the contact between the solid surface and the liquid. The effect of this force is also to pull the liquid away from the interface, but its direction only refers to the solid surface. The third force is the surface tension σ A of the solid surface, which pulls the liquid-solid interface end, and the direction is opposite to the interfacial tension.
In Figure 21-5, σ A is pulled to the left, σa1 is pulled to the right, and σ1 is pulled to the upper right, and its component force to the right is equal to σ1 cos θ , and the following formula expresses their relative forces:
Generally speaking, when the contact angle is 0, the solid is effectively wetted by the liquid (such as mineral oil on a metal surface). When the contact angle is greater than 90°, it means that the liquid cannot spread on the solid surface (for example, mercury is placed on a glass plate - about 140°, water is placed on paraffin - about 100~115°).
