Resuspension Dynamics of Micro-droplets: Deformation, Sliding, and Escape on Inclined Surfaces

Quantifying droplet resuspension from inclined surfaces is essential for predicting fomite-mediated pathogen spread, optimizing spray-coating, and enhancing environmental decontamination. We examine the escape dynamics of 1 µL water droplets on nine discrete tilt angles (0°–90°) using a custom 3D-printed stage that imparts controlled pre-impact velocities via free fall. High-speed imaging (4000 fps) reveals two regimes: at low angles, droplets deform asymmetrically before detachment; at high angles, they slide downslope with motion governed by the approach velocity relative to the critical escape threshold. Our analysis shows that the critical escape velocity decreases gently with increasing tilt up to a boundary angle of ~39°, beyond which it diverges—signaling a limit where resuspension becomes prohibitively difficult. Sliding measurements demonstrate that the maximum travel distance occurs when the substrate velocity closely matches the escape threshold, and collapses sharply for velocities above this point. Despite these kinematic differences, the volume of fluid resuspended remains nearly constant across all angles, with only a slight reduction at higher velocities—attributed to suppressed ligament formation and increased residue retention. To interpret these observations, we introduce an energy model enforcing conservation of total energy: initial kinetic energy partitions into altered surface energy from droplet deformation (angle-dependent) and viscous dissipation during sliding, culminating in conditions for spherical detachment. This framework captures the non-monotonic dependence of escape velocity on tilt angle and offers predictive capability for resuspension under varied substrate inclinations. Our findings extend droplet dynamics understanding to inclined geometries, with direct implications for controlling resuspension in practical settings.