Droplet-surface interactions are common to a plethora of natural and industrial processes due to their ability to rapidly exchange energy, mass, and momentum. Droplets are particularly of interest due to their large surface-to-volume ratios and hence enhanced transport properties. For example, coalescence-induced droplet jumping on superhydrophobic surfaces has recently received much attention for its potential to enhance heat transfer, anti-icing, and self-cleaning performance by passively shedding microscale water droplets. To study droplet jumping, researchers typically use a two-camera setup to observe the out-of-plane droplet motion, with limited success due to the inability to resolve the depth dimension using two orthogonal cameras. Here we develop a single-camera technique capable of providing three-dimensional (3D) information through the use of focal plane manipulation, termed "focal plane shift imaging" (FPSI). We used FPSI to study the jumping process on superhydrophobic surfaces having a wide range of structure length scales (10 nm < l < 1 μm) and droplet radii (3 μm < R < 160 μm). We benchmarked the FPSI technique and studied the effects of droplet mismatch, multidroplet coalescence, and multihop coalescence on droplet jumping speed. Furthermore, we were able to resolve the full 3D trajectory of multiple jumping events, to show that, unlike previously theorized, the departure angle during droplet jumping is not a function of droplet mismatch or number of droplets coalescing prior to jumping. Rather, angular deviation arises due to in-plane motion postcoalescence governed by droplet pinning. The outcomes of this work both elucidate key fundamental aspects governing droplet jumping and provide a powerful imaging platform for the study of dynamic droplet processes that result in out-of-plane motion such as sliding, coalescence, or impact.
Electrifying both stationary and mobile systems requires ultra-compact, lightweight power electronics and electric machines. Increasing the volumetric and gravimetric density of these systems is constrained, however, by the capacity to remove heat from these assemblies. A promising method for extracting heat is jumping droplet condensation, which can address both spatially and temporally changing hotspots. Yet, disagreement exists in the literature about the maximum attainable heat flux for water-based, droplet jumping devices such as vapor chambers, with values ranging from 5 to 500 W/cm2. Here, using thermal measurements and optical imaging in pure vapor conditions, we directly observe the hydrodynamics occurring inside of a jumping droplet vapor chamber. Our experiments show that flooding is the key obstacle limiting jumping droplet mass flux to hot spots, limiting heat transfer to less than 15 W/cm2. These results indicate that past works reporting high heat fluxes benefited from other hot spot cooling pathways such as previously observed liquid bridges formed due to flooding. To test our hypothesis, we characterize progressive flooding on a variety of structured surfaces ranging in length-scale from 100 nm to 10 μm. Progressive flooding was delayed by decreasing the length-scale of the surface structures, which supports recent observations in the literature. Our work not only helps to understand the wide variability of past results quantifying droplet jumping heat transfer, but also provides design guidelines for the development of surfaces that are capable of maintaining enhanced jumping droplet condensation.
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