Liquid Patterning Using Droplet Impact on Textured Nonwetting Surfaces.

Controlling the shape and contact area that an impacting droplet makes with a solid substrate has significant implications in numerous industrial processes, including inkjet printing and spray cooling. Here, we report a unique approach that offers an extraordinary ability to precisely control and manipulate the contact shape of a droplet impinging on nonwetting well-structured silicon micropillars. Our experiments show that the wetted Wenzel-type contact area can take on various polygonal shapes, including square, rectangle, hexagon, octagon, and dodecagon, depending on the pillar density (diameter-to-spacing ratio), arrangement (inline versus staggered), and/or the droplet contact angle. Experiments show that inline pillars give rise to a square, rectangle, or octagon shape while staggered pillars give rise to a hexagon, dodecagon, or extended hexagon shape. Rooted in the fundamentals of contact line physics, we develope a closed form unified analytical model that accurately captures the steady-state and transient wetting morphology of the impinging droplet. Furthermore, we show that the model is applicable for analyzing entrapped bubble retraction mechanism during high-velocity droplet impact. Lastly, the outcomes of this study demonstrate the similarity of the shape of the wetted area induced by droplet impact on nonwetting surfaces with that obtained via sessile droplet evaporation on wetting surfaces. The shape selection strategy reported in this study has promising applications in facile microfabrication of lab-on-a-chip devices, polymer-based printed electronics, biomicroarrays, and droplet-based electronics thermal management.