Dr. Patricia Weisensee, Assistant Professor in the Mechanical Engineering & Materials Science department at Washington University in St. Louis.

We experience and rely on droplets almost every single day of our lives: some more familiar (the shower in the morning, the rain on our way to work, printing a documents once at work, etc.), some less obvious (atmospheric water harvesting, thermal management & power generation, materials manufacturing and processing). Yet we hardly ever think about them – we take them for granted. In this talk I will show that droplets aren’t only ubiquitous in both nature and industrial processes, but also a complex and fascinating research subject that still holds many mysteries to be solved. In my research group, we are especially interested in the coupling of fluid dynamics, heat transfer, and phase change (condensation or evaporation) during the interaction of droplets with solid or other liquid surfaces.

In this presentation, I will introduce two examples of such interactions: 1) droplet impact and evaporation/boiling dynamics on heated surfaces, and 2) dropwise condensation on so-called lubricant-infused surfaces. Using a combination of advanced imaging techniques, ranging from high-speed optical and infrared (IR) imaging to high-speed interferometry and scanning confocal fluorescence microscopy, we study the interplay of droplet dynamics and heat transfer in these systems. For example, I will show that water condensation on lubricant-infused surfaces can lead to significantly increased water collection rates due to an extremely high droplet mobility compared to bare metal surfaces. Lubricant wetting ridges surrounding droplets introduce an attractive capillary force, leading to self-propelled and gravity-independent droplet motion, which efficiently clears the surface for frequent re-nucleation. On the other hand, restricting the mobility of droplets can be advantageous during droplet evaporation. Interestingly, when droplets impact a heated surface, the creation of additional contact lines, either through wettability-patterning the surface or the formation of an entrapped air bubble, does not significantly alter the heat transfer performance. Instead, convection (at early times) and conduction (at later times) dominate heat transfer, meaning that the addition of – for example – metallic posts with high thermal conductivity on the surface can effectively increase heat transfer rates in these scenarios.