Manipulating droplet jumping on hot substrates with surface topography: from vibration to explosion

Liquid droplet rapid and facile detachment from a substrate surface has received increasing attention due to its broad applications in a variety of fields such as anti-icing, surface self-cleaning, and thermal management. During the past several decades, many research efforts have been made for rapid droplet detachment by designing various functional substrates with complex macro/micro/nanostructures or resorting to costly external stimuli such as electrical, photothermal or magnetic fields. But our understating of the inherent and intricate droplet-substrate interactions still remains elusive, impeding simple designs of engineered surfaces for agile droplet manipulations. Here we introduce a simple but effective method to manipulate droplet jumping behaviors on micro-pillared superhydrophobic substrates at moderate superheat of about 30 °C by controlling the vapor bubble growth thereon. We find that the vapor bubble growth at the droplet base can be transferred from the heat-transfer-controlled slow growth mode to the inertia-controlled rapid growth mode by simply increasing the substrate micropillar height from 20 um to 80 um. As opposed to the relatively slow vibration jumping in seconds, the change of vapor bubble growth to the inertia-controlled mode at the droplet base leads to the prompt droplet out-of-plane explosion jumping in milliseconds. The rapid sessile droplet detachment stems from the vapor bubble explosion at the droplet base on the hot substrate, during which the vapor bubble expanding velocity can reach as high as ~4 m/s. Our observations in this study unveil the mechanism of droplet rapid detachment from a hot micro-structured surface and shed lights on engineered surface design avoiding the damage of vapor explosion.