Effect of the Initial Solute Concentration on the Flow Pattern Inside an Evaporating Polymer Solution Droplet on a Substrate

Abstract

The internal flow of an evaporating polymer solution droplet on a substrate is experimentally studied. The flow visualization is carried out. The effect of the initial polymer concentration is further investigated. A polystyrene-acetophenone (PS-Ap) and a polystyrene-anisole (PS-Ani) solution are employed as the droplet. A nylon powder is mixed with the droplet for the visualization by a YAG-laser sheet light. The droplet evaporates after the settlement on the substrate. Without the polymer dissolved in the solvent, complicated flow is observed in both droplets. For the cases with dissolved polymer, the flow pattern is rectified. In the PS-Ap droplet, the source flow is observed for the initial solute mass fraction c0 = 0.005 – 0.20. This convection becomes strong as c0 increases. The mechanism of the flow inside the PS-Ap droplet can be understood by the combination of the natural convection and Marangoni convection due to the differences of the temperature and the solute concentration. As for the PS-Ani droplet, the evaporation process and the flow pattern are affected by c0. For the dilute solution (0 < c0 < 0.03), the contact angle decreases during the contact line receding. The observed flow pattern becomes similar to that in the PS-Ap droplet. At c0 = 0.08 – 0.2, the decline of the contact angle is remarkable and the direction of the internal flow becomes inverse. This flow mechanism cannot be clarified, but it may have the relations with the decreasing contact angle.

Multiscale assembly of solution-processed organic electronics: the critical roles of confinement, fluid flow, and interfaces

“…Surface tension gradients arise naturally due to solute concentration gradients and/or temperature gradients. The strengths of these forces relative to viscous forces are characterized by the Marangoni numbers given in equations (3.5) and (3.6), respectively [122].…”

“…where γ represents surface tension, d 0 represents the initial droplet diameter, T is temperature, C is concentration, μ is viscosity, α is thermal diffusivity, and D AB is the mass diffusivity of the solute. As the Marangoni number increases, forces induced by the surface tension gradient outweigh viscous resistance and circulatory flow will occur [122]. For most liquids, surface tension decreases with temperature linearly for small perturbations [107].…”