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

Abstract: Spilling tea or coffee leads to a tell-tale circular stain after the droplet dries, known as the "coffee ring effect". The evaporation of suspension droplets is a complex physical process, and predicting and controlling the particle deposit patterns from sessile droplet evaporation are essential for many industrial processes, such as ink-jet printing and crop-care applications. In this article, we systematically investigate the effect of surface wettability on the evaporation dynamics of a particle-laden droplet, focussing on the contact line stick-slip behaviour, the hydrodynamic flow of the suspended particles, and the resulting particle deposit after evaporation. We use substrates with different wettabilities, ranging from hydrophilic to hydrophobic, 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. Correspondingly, the particle deposit transitions from the typical coffee-ring pattern to a central stain as the substrates's hydrophobicity increases. Finally, 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 significantly alter hydrodynamic flow within drying droplets, thereby affecting the resulting particle deposit.