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

Spilling tea or coffee leads to a tell-tale circular stain after the drying of the droplet. This phenomenon was termed after the latter example as the "coffee ring effect". 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. In this article, we present a systematic investigation of the effect of surface wettability on the evaporation dynamics of a particle-laden droplet, including the effect on the contact line stick-slip, the hydrodynamic flow of the suspended particles and the resulting particle deposit after evaporation. We tune the wettability of glass slides using silanisation and quantify the internal flow during the evaporation by tracking fluorescent tracer particles. We find that the internal flow shifts from a predominantly outward flow towards the contact line for low contact angles to an inward flow for large contact angles. Additionally, the corresponding deposit gradually changes from the typical coffee-ring to a central stain upon increasing the hydrophobicity of the substrate. 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.