Spreading of a liquid film on a substrate by the evaporation—adsorption process

Abstract

The importance of evaporation followed by multilayer adsorption in comparison to liquid flow at the leading edge of a volatile spreading film is analyzed. Presuming that both flows are functions of the same chemical potential gradient, a dimensionless group ( N) which delineates the relative importance of vapor diffusion flow to viscous flow on the surface is obtained: N = ρ i Dνx (− Aπ) . The relative importance of vapor flow increases with the vapor-pressure dependent partial density, ρ i, and diffusivity, D, of the diffusing vapor, the kinematic viscosity at the liquid, ν, and the distance downstream from the bulk liquid region, x, and decreses with the Hamaker constant, 6π A . Using physical properties the modifiers “volatile” and “nonvolatile” can thereby be put in perspective. Changes in the interfacial force field are a function of A . The spreading velocity due to the vapor diffusion process is obtained and is found to decrease with a decrease in the interfacial force field and the bulk vapor pressure. The infinite stress at the contact line can be easily relieved by evaporation-adsorption in many systems.