CALCULATION OF A KEY FUNCTION IN THE ASYMPTOTIC DESCRIPTION OF MOVING CONTACT LINES

An important element of the asymptotic description of flows having a moving liquid/gas interface which intersects a solid boundary is a function denoted Q(i) (alpha) by Hocking and Rivers (The spreading of a drop by capillary action, J. Fluid Mech. 121 (1982) 425-442), where 0 < alpha < pi is the contact angle of the interface with the wall. Q(i) (alpha) arises from matching of the inner and intermediate asymptotic regions introduced by those authors and is required in applications of the asymptotic theory. This article describes a new numerical method for the calculation of Q(i) (alpha), which, because it explicitly allows for the logarithmic singularity in the kernel of the governing integral equation and uses quadratic interpolation of the non-singular factor in the integrand, is more accurate than that employed by Hocking and Rivers. Nonetheless, our results show good agreement with theirs, with, however, noticeable departures near alpha = pi. We also discuss the limiting cases alpha -> 0 and alpha -> pi. The leading-order terms of Q(i) (alpha) in both limits are in accord with the analysis of Hocking (A moving fluid interface. Part 2. The removal of the force singularity by a slip flow, J. Fluid Mech. 79 (1977) 209-229). The next-order terms are also considered. Hocking did not go beyond leading order for alpha -> 0, and we believe his results for the next order as a. p to be incorrect. Numerically, we find that the next-order terms are O(alpha(2)) for alpha -> 0 and O(1) as alpha -> pi The latter result agrees with Hocking, but the value of the O(1) constant does not. It is hoped that giving details of the numerical method and more precise information, both numerical and in terms of its limiting behaviour, concerning Q(i) (alpha) will help those wanting to use the asymptotic theory of contact-line dynamics in future theoretical and numerical work.