The contact angle of a droplet on a solid surface mainly depends on the surface energies of the solid and liquid and the interfacial energy of the liquid and solid.
The liquid droplet is acted upon by the following equilibrium forces on the solid surface, and the resultant force at the three-phase junction is zero.
When a drop of liquid falls on a solid surface, the following will happen:
(1)The liquid may completely spread on the solid surface to form a water film. In this case, the liquid completely wets the solid. As shown in (a) in Figure 1
(2)The liquid may form into droplets. In this case, the tangent between the solid surface and the liquid edge forms an angle θ, which is called the contact angle.
When 0°<θ<90°, as shown in (b) in Figure 1, the liquid partially wets the solid;
When 90°<θ<180°, as shown in (c) in Figure 1, the liquid does not wet the solid.
When droplets fall on the surface of the fabric, only two situations will occur:
(1)When the contact angle is less than 90°, the droplets begin to take the shape of Figure 1(b), but in extremely short After time, the droplets will spread around and penetrate into the fabric.
(2)When the contact angle is greater than 90°, the droplet takes the shape of Figure 1(c). The larger the contact angle, the longer the retention time. When the fabric is tilted, the droplets will roll off.
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