The effects of molecular weight, evaporation rate and polymer concentration on pillar formation in drying poly(ethylene oxide) droplets

Typically, when droplets of dilute suspensions are left to evaporate the final dry deposit is the familiar coffee-ring stain, with nearly all the solute deposited at the initial contact line. Contrastingly, in previous work we have shown that sessile droplets of poly(ethylene oxide) (PEO) solutions form tall central pillars (or monoliths) during a 4-stage drying process. We show that a dimensionless Péclet-type number Pe, a ratio of the competing advective and diffusive motion of the dissolved polymer, which incorporates the effects of evaporation rate, initial concentration c0 and the polymer diffusion coefficient, to determine whether the droplet will form a pillar or a flat deposit. In this work we vary concentration up to c0 = 0.5 and molecular weight Mw between 3.35 kg/mol and 600 kg/mol and find that in ambient conditions with c0 = 0.1 pillars only form for a limited range, 35 ≤ Mw ≤ 200 kg/mol. This observation is in contrast to the the Péclet argument in which high molecular weight polymers with a slow self-diffusion should still form pillars. We present various experimental measurements attempting to resolve this discrepancy: crossover time-scale for viscoelastic behaviour; fast diffusion of an entangled network; and droplet viscosity or contact line friction.