Researchers in the Department of Mechanical Engineering at the University of Hong Kong (HKU) have made key breakthroughs in the manipulation of droplets. Droplet manipulation is critical in many fields and often encompasses both in-plane and out-of-plane droplet control. For the in-plane control, directed, long-range self-propulsion of fluid on solid surfaces is fundamental to thermal management, desalination, materials self-delivery, and numerous other applications. For the out-of-plane control, the instantaneous, reliable fluid deposition on solid surfaces, especially those with liquid repellency, is essential for agricultural sprays, insecticides, spray cooling, and cosmetics.
Conventionally, asymmetries such as chemical inhomogeneity and topological anisotropy are pre-patterned on the target surface to enable precise droplet transport. Without this, droplets tend to propel isotropically without well-defined direction. RGC postdoctoral fellow Dr. Xin Tang, Ph.D. student Mr. Wei Li, and Chair Professor Liqiu Wang, all from HKU Mechanical Engineering, have found that when a cold/hot or volatile droplet is liberated on a lubricated piezoelectric crystal (lithium niobate) at ambient temperature, the droplet instantaneously propels for a long distance (which can be ~50 times the droplet radius). Unlike random motion on homogeneous surfaces, droplets on the crystal self-propel in furcated routes. Depending on the crystal plane that interfaces with the droplet, the self-propulsion can be unidirectional, bifurcated, and even trifurcated. In the absence of any macro-/micro-asymmetry, the intrinsically orientated liquid motion is fueled by cross-scale thermo-piezoelectric coupling, a phenomenon originating in the anisotropy of crystal structure.
On non-wetting surfaces, an impacting droplet resembles a mass-spring system that instantaneously rebounds in ~10 ms, a consequence of the air pockets or vapor cushions in promoting liquid levitations. To break the repellency and enhance deposition, special polymers are conventionally dissolved in the liquid, but these unwanted additives tend to toxify and contaminate the solid. Dr. Xin Han, a Postdoctoral fellow, Dr. Xin Tang, and other team members led by Professor Wang have presented a conceptually different strategy by capping the droplet with a preferentially wetting liquid overlayer. The overlayer resembles a viscous damper with a roller support attached to the mass-spring system. Upon impact, the overlayer suppresses the out-of-plane rebounding through viscous dissipation. Moreover, the droplet’s post-deposition state can be switched between being immobilized and sliding by tuning the overlayer volume, enabling rich fluid controls on the repellent surfaces, including superhydrophobic, superomniphobic, and superheated types.
The surprising furcated self-propulsion entitled “Furcated Droplet Motility on Crystalline Surfaces” has been published in Nature Nanotechnology. Link: https://www.nature.com/articles/s41565-021-00945-w.
The exceptionally enhanced droplet deposition entitled “Slippery Damper of an Overlay for Arresting and Manipulating Droplets on Nonwetting Surfaces” has been published in Nature Communications. Link: https://www.nature.com/articles/s41467-021-23511-3.
Artistic representation of trifurcated droplet motility atop a piezoelectric crystal surface.
Schematics and images showing the enhanced droplet deposition using the lubricant overlayer.