Evaporatively-driven Marangoni instabilities of volatile liquid films spreading on thermally conductive substrates

Laser confocal microscopy is used to non-invasively investigate the steady and unsteady evolution of viscous microdroplets on solid substrates. Three characteristic dynamical regimes of spreading drops (viscous-capillary, viscous-inertia-capillary, and inertia-capillary) are studied using this non-invasive optical technique. It is shown that the dynamics of each regime depend on the Ohnesorge number, Oh = mu/(rhoRsigma)(1/2), and on the relative magnitudes of the droplet height, radius, compared with the capillary length, l(cap) = rootsigma/rhog. The power-law relationships between the extent of spreading and elapsed time that are extracted from the experiments are in excellent agreement with available analytical results. We also study the onset and evolution of surface instabilities of the slightly volatile liquid films as they spread across the thermally-conductive surfaces. When the fluid droplet is a volatile silicone oil and the surface is a smooth silicon wafer, an evaporatively-driven thermocapillary instability leads to onset of a time-dependent free surface motion. Below a certain critical thickness (similar to20 mum), waves can be observed on the free surface of the film, and the confocal technique is used to measure the amplitude, the frequency, and non-linear evolution of these waves. We interpret these waves in terms of evaporatively-driven Marangoni instabilities induced by surface tension gradients close to the moving contact line. Experiments show that the amplitude and the critical onset thickness of the disturbances vary with the viscosity and the volatility of the liquid, and also with the surface roughness and thermal diffusivity of the substrate. The critical onset conditions for this evaporatively driven instability can be characterized by a dimensionless interfacial thermal resistance, 93, which has to be larger than a critical value at the onset of instability. We also demonstrate that this evaporatively-driven Marangoni instability can be eliminated by reducing the volatility of the liquid or the thermal diffusivity of the substrate. (C) 2002 Elsevier Science B.V. All rights reserved.