THE INTERPRETATION OF DYNAMIC CONTACT ANGLES MEASURED BY THE WILHELMYPLATE METHOD

We present an analysis for properly interpreting apparent dynamic cont act angles measured using the Wilhelmy plate method at low capillary n umbers, Ca. This analysis removes the ambiguity in current dynamic mea surements which interpret data with the same formula as static measure ments. We properly account for all forces, including viscous forces, a cting on the plate as it moves into or out of a liquid bath. Our main result, valid at O(1) as Ca --> 0, relates the apparent dynamic contac t angle to material-dependent, geometry-independent parameters necessa ry for describing dynamic wetting of a system. The special case of the apparent contact angle = pi/2 was solved to O(Ca). This O(Ca) solutio n can guide numerical work necessary for higher Ca's and arbitrary val ues of the apparent contact angle. These results make the Wilhelmy pla te a viable method for determining material parameters for dynamic spr eading. (C) 1997 Academic Press