A Convention of UFOs

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Sushant Anand
Adam Paxson
Kripa Kiran Varanasi
Massachusetts Institute of Technology
Cambridge, MA, USA

A Convention of UFOs
Image by Sushant Anand/MIT

On superhydrophobic surfaces, droplets rest atop surface textures in a Cassie state and can be shed easily due to reduced pinning. However during condensation, condensing drops grow in Wenzel pinned state. By impregnating the solid surface with a lubricant this limitation can be overcome. Condensing surfaces were fabricated using standard photolithography with well-defined cubical microposts of silicon(a= 10μm,b= 10μm, and h= 10μm) and these were etched further to produce nanograss features. Subsequently they were silanized using a low-energy silane to render them hydrophobic. The textured surface was impregnated with an ionic liquid ([BMIm+][Tf2N-]) by using a controlled dip coating procedure. Experiments for observing microscale condensation growth were performed in an Environmental SEM. The surfaces were titled by 15º to the vertical and the ESEM experiments were conducted under identical conditions (pressure 1000 Pa, substrate temperature 4.5 ºC, beam voltage 25 kV, and beam current 1.7 nA). Drops condense on impregnated surfaces in Cassie state and microscale drops even with low contact angle zip across the surface giving an appearance of hovering UFOs (such as shown in image). A wetting ridge is formed around a water droplet due to interfacial force balance. The remarkable shedding behavior seen on these surfaces make them promising for enhanced condensation heat transfer.

Usage Information

This image may be freely reproduced with the accompanying credit: S. Anand/MIT

References

Anand, S.; Paxson A. T.; Dhiman R.; Smith J. D.; Varanasi K. K. Enhanced Condensation on Lubricant-Impregnated Nanotextured Surfaces. ACS Nano 2012 6 11 10122-10129. URL: (http://pubs.acs.org/doi/abs/10.1021/nn303867y)

Smith, J. D.; Dhiman, R.; Anand, S.; Reza-Garduno, E.; Cohen, R. E.; McKinley, G. H.; Varanasi, K. K., Droplet mobility on lubricant-impregnated surfaces. Soft Matter 2013, 9, (6), 1772-1780. URL: (http://pubs.rsc.org/en/content/articlehtml/2013/sm/c2sm27032c)