Nucleation, growth and surface movement of a condensing sessile droplet

The near-equilibrium microscale transport processes in a system composed of a small condensing sessile drop subsequently removed by capillarity into a corner meniscus were studied. Experimental results on the following three stages for ethanol with a small contact angle on heat-treated fused quartz were analyzed: droplet nucleation, droplet growth and the transfer of the liquid from the drop to the corner meniscus. The dynamic character of the experiment allowed the heterogeneous condensation/removal cycle to be related to the chemical potential gradient, which is a function of the excess free energy (due to the liquid shape) and an excess temperature. The liquid thickness profile and contact angle were obtained using image analyzing interferometry in which the interference fringes were recorded using a video camera attached to a microscope. The capillary pressure, the polar and apolar components of the spreading coefficient, and the excess free energy were obtained from the equilibrium thickness profile data and the extended Young–Laplace equation. Using the derivative of the excess free energy per unit volume, flat film thicknesses in the film thickness range 0.6<δ<6.2 nm were found to be unstable. Transient profile data also gave the nucleation rate, the condensation heat flux and the liquid flow field on the substrate. The temperature field was related to the condensation flux and the pressure field using a Kelvin–Clapeyron (chemical potential) model for intefacial mass flux. Non-measurable interfacial temperature differences of the order of 10−4 K can generate easily measurable changes in the excess free energy. Extremely small interfacial temperature differences have a large effect on transport processes. We find that there are sufficient interfacial models and experimental measurement techniques available to describe the nucleation, growth, and surface removal of a condensing partially wetting sessile droplet.