Abstract
Condensation on lubricant-infused micro- or nanotextured superhydrophobic surfaces exhibits remarkable heat transfer performance owing to the high condensation nucleation density and efficient condensate droplet removal. When a low surface tension lubricant is used, it can spread on the condensed droplet and “cloak” it. Here, we describe a previously unobserved condensation phenomenon of satellite droplet formation on lubricant-cloaked water droplets using environmental scanning electron microscopy. The presence of satellite droplets confirms the cloaking behavior of common lubricants on water such as Krytox oils. More interestingly, we have observed satellite droplets on BMIm ionic liquid-infused surfaces, which is unexpected because BMIm was used in previous reports as a lubricant to eliminate cloaking during water condensation. Our studies reveal that the cloaking of BMIm on water droplets is theoretically favorable due to the fast timescale spreading during initial condensation when compared to the long timescale required for dissolution of the lubricant in water. We utilize a novel characterization approach based on Raman spectroscopy to confirm the existence of cloaking lubricant films on water droplets residing on lubricant-infused surfaces. The selected lubricants include Krytox oil, ionic liquid, and dodecane, which have drastically different surface tensions and polarities. In addition, spreading dynamics of cloaking and noncloaking lubricants on water droplets show that ionic liquid has the capability to mobilize water droplets spontaneously owing to cloaking and its relatively high surface tension. Our studies not only elucidate the physics governing cloaking and satellite droplet condensation phenomena at micro- and macroscales but also reveal a subset of previously unobserved lubricant–water interfacial interactions for a large variety of applications.
Highlights
Condensation is a phenomenon commonly found in many natural and industrial processes, including atmospheric dew formation,[1] power generation,[2,3] natural gas processing,[4,5] water harvesting,[6,7] and refrigeration.[8]
As satellite droplets condensed on the cloaking lubricant layer, the presence of satellite droplets was observed to be independent of the structure on the substrate as long as the lubricant could be stabilized on the surface
Figure 3a−c show environmental scanning electron microscopy (ESEM) images of micro/nanostructures, the samples were coated with trichloro(1H,1H,2H,2H-perfluorooctyl)silane (TFTS) by chemical vapor deposition (CVD) in a vacuum desiccator to turn from superhydrophilic to superhydrophobic
Summary
Condensation is a phenomenon commonly found in many natural and industrial processes, including atmospheric dew formation,[1] power generation,[2,3] natural gas processing,[4,5] water harvesting,[6,7] and refrigeration.[8] Depending on surface tensions of the condensate and surface energy of the surface, two distinct condensation mechanisms can occur, either dropwise or filmwise condensation. Dropwise condensation is preferable for most applications owing to its higher heat transfer performance.[9−11] extensive studies have been conducted to encourage or “promote” dropwise condensation by surface modification, including hydrophobic coating[12−14] and micro- or nanotexturing.[14−22] For liquids with a low surface tension The low surface tension of the lubricant helps to achieve dropwise condensation, it causes problems of cloaking at the condensate−vapor interface.[26,27] The lubricant can spread on the condensate droplet thereby encapsulating it and forming a thin cloaking layer, which adds heat and mass transfer resistance and hinders growth of the droplet
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