Evaporation of active drops: Puncturing drops and particle deposits of ring galaxy patterns

By virtue of self-propulsion, active particles impart intricate stresses to the background fluids. We propose that this active stress can be utilized to greatly control evaporation dynamics of active drops. We discover a new phenomenon of puncturing of the active drops, where the air-liquid interface of the drop undergoes spontaneous tearing and there occurs a formation of a new three-phase contact line due to the liquid-air interface hitting the liquid-solid interface through evaporation-driven mass loss. Post puncturing, we see an inside-out evaporation of the drop, where the new contact line sweeps towards the pinned outer contact line of the drops, contrasting regular drops that straightaway shrink to zero volume with self-similar shape. Furthermore, we describe how the activity inside the drops can manipulate the three-phase contact-line dynamics, which for contractile drops can result in an up to 50% enhanced lifetime of the drop and 33% quicker evaporation for extensile drops. By analyzing the flux distribution inside the drop, we gain insights on nonintuitive deposition patterns (e.g., ring galaxy type deposits that demonstrate controllable spatial gradients in the concentrations of the deposited particles) of active particles, which are oftentimes biological substances or bimetallic nanoparticles of interest. Finally, we argue that such unique evaporation and particle deposition dynamics can be leveraged for altering the lifetime of drops for bioapplications and for creating customized thin-film deposits with potential three-dimensional printing applications.