Water droplet attraction and coalescence on liquid-crystal-infused textured and porous surfaces

Abstract

Coalescence of droplets on liquid-infused surfaces has been extensively investigated for isotropic lubricants, where interfacial and hydrodynamic responses are well described by geometry-based and mass-spring models. However, the corresponding dynamics on anisotropic lubricating films, such as liquid crystals (LCs), remain largely unexplored. In this work, we report the use of high-speed imaging to study the attraction and coalescence of millimetre-sized water droplets on two classes of substrates, covered with a thin LC overlayer: LC-infused textured surfaces (LCITS) and LC-infused porous surfaces (LCIPS). On both substrates, the droplets coalesce through three stages over approximately one minute: long-range capillary-mediated attraction, drainage of the lubricant within the wetting ridge, and final merging accompanied by in-plane oscillations of the formed droplet. On LCITS, the initial approach velocities and post-merging dynamics are broadly consistent with the geometry-based mass–spring model developed for oil-impregnated surfaces of a similar type. However, on LCIPS, where a thicker lubricating film produces a larger wetting ridge, we observe substantially reduced attraction and merging velocities, no oscillations were resolved within our temporal resolution at the first velocity peak, and drainage times strongly influenced by evaporation. In the final stage, the peak velocity mainly depends on the LC mesophase and is nearly independent of droplet size, while the oscillation period scales approximately with the square root of the droplet radius. These results clarify how the porous LC scaffold and enlarged wetting ridge alter droplet-droplet interactions and coalescence dynamics relative to textured silicone substrates.