Drops, Bubbles and Interfacial Fluid Mechanics

When a drop impacts a micropost array at a sufficiently high velocity that is often quantified using the dimensionless Weber number, it penetrates the structure and wets the surface by displacing the air, rather than forming a completely bouncing Fakir drop. Understanding the drop-surface contact dynamics is key to designing surfaces with tailored wetting characteristics. For example, a small solid-liquid contact area is desirable for reducing drag and ice adhesion whereas a large solid-liquid contact area is beneficial for spray cooling. In this work, we demonstrate the ability to precisely control the shape and size of the contact area that a droplet makes when impacting a well-designed micropost array. Our preliminary results reveal that the contact area can take on various shapes, including square, rectangle, hexagon, octagon, and dodecagon, depending on the micropost diameter, height, and arrangement. Furthermore, our experiments show that the density of the microposts plays a pivotal role in influencing the resulting post-impact wetting pattern. Importantly, we used the fundamentals of wetting science and local contact line dynamics to develop an analytical model that accurately captures the solid-liquid contact shape, area, and 2D pattern. These findings provide valuable insights into controlling liquid deposition and patterning at the microscale, with potential applications in microfluidics, lab-on-a-chip, inkjet printing, and thermal management.