Changing the flow profile and resulting drying pattern of dispersion droplets via contact angle modification

The evaporation of suspension droplets is a complex physical process, and prediction and control over particle deposit patterns obtained from sessile droplet evaporation are essential for many industrial processes such as ink-jet printing or crop-care applications. We present a systematic investigation of the effect of surface wettability on the evaporation dynamics of a particle-laden droplet, including the contact line stick-slip, hydrodynamic flow of the particles and deposit after evaporation. We tuned the wettability of glass slides using silanisation; and measured the flow inside the drying droplets using fluorescent tracer particles. We found that the internal flows shift from predominantly outward flow for low contact angles to predominantly inward flow for large contact angles. Additionally, upon increasing the substrates hydrophobicity, the dried deposit gradually changes from the typical coffee-ring to a central stain. Last, we corroborate these experimental findings with dynamic density functional theory, modelling the droplet evaporation process and stick-slip behaviour of the contact line. Our investigation suggests that the wettability of the substrate can substantially alter hydrodynamic flow within drying droplets and therefore the resulting particle deposit.