Drop impact dynamics on hierarchically textured lubricant-infused surfaces

After more than a century since Worthington's 1876 pioneering work, the topic of drop impact and the beautiful postimpact images that emerge continue to inspire scientists and the public alike due to the critical role that impact dynamics plays in cutting-edge engineering applications, with notable examples in spray cooling, spray coating, inkjet printing, pesticide dispersal, and the unlikely field of forensic science. For example, the pitting of turbine blades in steam power plants and the quality of solder bumps on printed circuit boards are directly affected by drop impact dynamics. Driven by recent advances in micro- and nanofabrication, which enabled coating of engineered surfaces with liquid films of controlled thickness, here, we characterize water drop impact dynamics (spreading, retraction, and breakup mechanism) on hierarchically textured silicon micropillars and microholes coated with a few microns thick of lubricant film. We observe that the droplet impact dynamics on overlubricated surfaces is invariant with the surface texture; specifically, micropillars and microholes with the same solid fraction exhibit similar postimpact dynamics. Our experiments show that there is an optimum lubricant layer thickness (≈3-5μm) that maximizes drop breakup and splashing mechanism. Additionally, we observe increased suppression of drop splashing and breakup with lubricant viscosity, which we attribute to viscous dissipation in the wetting ridge. Our experiments also show enhanced drop splashing intensity when the density difference between the drop and lubricant oil increases, suggesting the role of hydrodynamic instability in postimpact dynamics. The new insights gained from this work improve current understanding by informing the relevant material properties and dynamical behaviors of the impacting drop that can be tuned to either suppress or enhance impact-induced drop splashing.