Investigation on the convection flow effect on the surface and depth of the evaporative polymeric solutions via PIV method

Abstract

Convection is the main reason for surface roughness in both polymeric and non-polymeric films obtained by the drying process (solvent evaporation). It is important to find the origin to control this phenomenon which leads to the creation of a variety of surface patterns that are formed on the polystyrene/methyl ethyl ketone solution in various thicknesses and concentrations. In this research, the convection flow was observed and analyzed at the surface and depth of the polymeric fluids using Particle Image Velocimetry method. Counter-rotational flow was observed in the cross section of each cell and the flow direction was found to be ascending in the cell center and descending around it. The non-dimensional Rayleigh number (based on buoyancy) and Marangoni number (based on surface tension) were calculated to determine the convection dominant mechanism. The critical value of each number is the threshold for starting instabilities. According to the calculations based on initial assumptions and solving the one-dimensional mass transfer equation in a layer of the solution as well as controlling the effective parameters such as concentration, thickness and viscosity. Rayleigh convection was observed in a certain thickness and concentration of the samples, while concentration Marangoni convection was only observed in some of them. However, both concentration and thermal Marangoni were found in most samples.