Air bubble entrapment mechanism during droplet impact on well-defined silicon micropillar arrays

A droplet approaching a rigid surface deforms at its base due to the pressure buildup in the surrounding air beneath it, which leads to the entrapment of an air film that eventually contracts to form an air bubble at the center of the impinging droplet. Even though this is a classical fluid mechanics problem that has been studied for decades, the evolution of the air bubble, particularly on textured surfaces, is not fully understood to date. Experimentally characterizing and studying the dynamics of the entrapped air bubble on such micro/nanotextured surfaces is challenging since the surface texture introduces additional complexity. Here, we show that the micropillar density and arrangement have a significant impact on the nature and evolution of the entrapped air bubble. High-speed images at 20,000 frames-per-second show that a single central air bubble forms on sparse silicon micropillars, whereas fragments of microbubbles form on dense micropillars. Interestingly, high-resolution images captured at 50× magnification show that the fragmented air bubbles are located atop silicon micropillars, an observation that has been overlooked in past studies. Furthermore, the high-resolution images captured during post-impact droplet spreading and retraction show that the microbubbles are radially distributed in distinct zones depending on the pillar array geometry, arrangement, and density. The insights gained from this work inform the relevant surface design parameters that can be tuned to manipulate the mechanics and nature of the entrapped air disk beneath an impacting droplet.