Dissipation in the dynamics of a moving contact line: effect of the substrate disorder

We have studied the dynamics of the contact line of a viscous liquid on a solid substrate with macroscopic random defects. We have first characterized the friction force f <Subscript>0</Subscript> at microscopic scale for a substrate without defects; f <Subscript>0</Subscript> is found to be a strongly nonlinear function of the velocity U of the contact line. In presence of macroscopic defects, we find that the applied force F(U) is simply shifted with respect to f <Subscript>0</Subscript>(U) by a constant: we do not observe any critical behavior at the depinning transition. The only observable effect of the substrate disorder is to increase the hysteresis. We have also performed realistic numerical simulation of the motion of the contact line. Using the same values of the parameters as in the experiment, we find that the experimental data is qualitatively well reproduced. In light of experimental and numerical results, we discuss the possibility of measuring a true critical behavior. Copyright EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2004