| The contact line region is studied at the molecular level. Several problems are addressed. (i) An equation relating the dynamic contact angle to the contact line velocity is derived. The force leading to the motion is found to depend on contact line velocity, surface tension, the width of the fluid/fluid interface, the viscosity in the fluid/solid interface, and the slip length. The dynamic contact angle develops a stress in the contact line region which is modeled as an interfacial pressure. This pressure leads to contact line motion on the solid through a Maxwell-type slip condition. (ii) The vicinal fluid in the fluid/solid interface behaves as a non-Newtonian fluid and leads to the contact angle depending non-linearly on the contact line velocity. The theory agrees well with measurement by the Wilhelmy plate method. It also predicts a discrepancy observed in the theory by Dussan, Rame, & Garoff. It is suggested that in contrast to some current theories, the actual contact angle changes from its static value. (iii) The density distribution in the contact line region is studied for both the static and the dynamic cases. For the static case, an approximate analytic distribution is presented. Also, a methodology for predicting the contact angle is presented. (iv) It is also shown that microscopic roughness reduces the slip motion of the Maxwell model. Exact solutions are given for the case of sinusoidal roughness. |
