Ideal Solid Surfaces
An ideal solid surface is one that is flat, rigid, perfectly smooth, chemically homogeneous, and has zero contact angle hysteresis. Zero hysteresis implies that the advancing and receding contact angles are equal. In other words, there is only one thermodynamically stable contact angle. When a drop of liquid is placed on such a surface, the characteristic contact angle is formed as depicted in Fig. 1. Furthermore, on an ideal surface the drop will return to its original shape if it is disturbed. The following derivations apply only to ideal solid surfaces. In other words, they are only valid for the state in which the interfaces are not moving and the phase boundary line exists in equilibrium.
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