ABSTRACT
The dynamics of liquid spreading on a solid surface can be described by the change in the contact angle 9 (Fig. 1). Full spreading occurs when 0 0. The variation in the contact angle of a liquid laid on a solid surface is conditioned by the following forces [3]: 1) cohesive forces on the liquid; 2) interaction forces between molecules of solid and liquid (adhesive forces); 3) attractive (molecule) forces of solid at the perimeter of the drop; and 4) gravity forces. The first two kinds of forces resist the liquid spreading and the last two encourage it. The equilibrium (without taking gravity forces into consideration) is described by Young's equation 1Cos = s – sl where 0 is the contact angle, 1 is the surface tension of liquid, ys is the surface free energy of solid, and s1 is the interfacial tension. The spreading of oil drop is therefore the result of the acting force related to the perimeter unit of the liquid-solid interphase circle = s — sl — 1 cos 0m, where 0m is the momentary value of the contact angle. Because the value of ys1 s relatively small, the spreading dynamics depends on the difference between s and 1.
