Capillary-wave-driven droplet spreading on particles: Curvature-induced pseudo-hydrophobicity at low Weber numbers

We investigate droplet–particle interactions in the low Weber number regime, which play dominant roles in many engineering applications, such as bioprinting and cell spray processes, but have been less explored compared to impacts in the moderate-to-high Weber number regimes. We demonstrate curvature-induced pseudo-hydrophobicity, which can be understood as a hydrophobic-like dynamic response that emerges on intrinsically hydrophilic curved targets without any change in the intrinsic equilibrium contact angle. More specifically, on a highly curved surface, the difference between the apparent and equilibrium contact angles during the initial spreading stage decreases, resulting in a reduced capillary force. This effect is analogous to that induced by increased hydrophobicity. Three major parameters that typically characterize spreading dynamics in the low Weber number regime –namely, (i) the spreading extent, (ii) the propagation of capillary waves, and (iii) the film thickness– are quantitatively examined in detail. These characteristic features on a highly curved hydrophilic surface mimic those typically observed on a hydrophobic substrate, including reduced spreading extents, weakened capillary waves, recovery of the film thickness, thicker coatings, and reduced energy dissipation. This previously overlooked curvature-induced effect provides new insights into the low-Weber-number droplet spreading dynamics and suggests a geometric route for manipulating droplet–particle interactions.