Buckling instability induced by polymer solution drying

Abstract

The large shape distortions that occur during the drying of sessile drops of polymer solution are shown to be related to a buckling instability. As solvent evaporates, polymers accumulate near the vapor/drop interface and, depending on the experimental conditions, can form a glassy skin which bends as the volume it encloses decreases. A comparison of the times that characterize drying kinetics and glassy skin formation enables us to predict instability occurrence. Good agreement is found with measurements performed at different polymer concentrations, drop volumes and drying rates.

Growth of solid conical structures during multistage drying of sessile poly(ethylene oxide) droplets

“…During this initial period when the volume loss is constant and A decreasing, the average evaporative flux across the interface must be increasing, which is predicted to occur as θ decreases 7,10 .…”

“…on 8% curve close to t/ t 0 = 0.5) due to bright reflections from the top of the droplet leading to a cusped profile and a peak position which jumps around. For comparison, the upper curve shows data for the skin buckling model with dextran 10 in which the surface area remains constant during the growth phase.…”

“…Alain and Pauchard 10 used the model system of the branched aqueous polymer dextran to investigate the additional complexities that arise as polymer solutions evaporate. In this case, the increase in polymer concentration at the droplet's edge, due again to the outward flux of water, resulted in a phase change: on the surface of the liquid droplet a glassy skin with spherical cap geometry formed which was flexible and permeable, but also incompressible.…”

A minimal model for solvent evaporation and absorption in thin films

“…Therefore, a value of D ∞ can be straightforwardly estimated using (22) after the rate on the left-hand side has been calculated from the experimental data and λ 0 is approximated by π/2.…”

“…Remarkably, Eckersley & Rudin's model is able to capture these two stages without explicitly considering the spatial variation of the film composition. Extended models that are based on the diffusion equation [17,18,19,20] have been used to study concentration gradients throughout the film and, in particular, the onset of viscous or brittle solute-rich skins below the free surface [21,22]. The formation of a skin is expected to occur when the depleted solvent cannot be replenished sufficiently quickly due to local decreases in the rate of mass diffusion.…”