RELAXATION MODES OF THE CONTACT LINE OF A LIQUID SPREADING ON A SURFACE

THE dynamics of spreading of liquids on solid surfaces are important in several practical and industrial processes, such as painting, lubrication and oil recovery in a porous medium. In all these situations, the motion of the 'contact line' at the edge of the advancing fluid is perturbed by the roughness and chemical contamination of the solid, which distort the line and give rise to poorly understood hysteresis effects. To elucidate these effects, one must begin by studying simplified processes, for example the time response to a perturbation of the line on an ideal surface. Here we describe a study of the dynamics of a contact line under controlled conditions of partial wetting, in which we determine the relationship between the characteristic decay time and the wavelength of a perturbation imposed on the line. When relaxation is controlled by the fluid viscosity, there is good agreement with theoretical predictions, but we observe deviations at long wavelengths which we ascribe to gravitational effects that weaken the contact line's elastic response.