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R&D Profile: Hierarchical Superhydrophobic Surfaces Resist Water Droplet Impact K. Varanasi, MIT, US and T. Deng, GE Global Research Center, US

Droplet impingement on surfaces is a ubiquitous phenomenon in many industrial applications. In applications such as ice formation in aircraft and wind turbine external surfaces, raindrops on windshields, droplet-induced erosion, moisture-induced efficiency losses in power turbines, etc, there is significant advantage to engineer surfaces with anti-wetting properties.

Overview courtesy of Kripa K. Varanasi, MIT, US and Tao Deng, Ming Hsu, Nitin Bhate, GE Global Research Center, US

Recent interest in the lotus-inspired superhydrophobic surfaces has been intense. Most of these studies, however, characterized the wetting behavior of these surfaces using sessile droplet techniques and therefore neglected the dynamic interactions that occur during the impact process. At NSTI 09, we report on the dynamic interactions of impinging droplets on a variety of superhydrophobic surfaces captured using a high-speed camera. We find that effective contact angle, though a useful measure of hydrophobicity cannot be used to fully predict the droplet-impact resistance of superhydrophobic surfaces.

To predict droplet impact resistance, we propose a simple pressure balance model in which dynamic wetting pressures related to droplet impact such as the effective water hammer pressure and Bernoulli pressure (where C is the speed of sound and V is the velocity of the droplet) be compared to the antiwetting capillary pressure PC associated with the superhydrophobic surface. The relative strength of these wetting and anti-wetting pressures dictates the extent of surface wetting. To verify this pressure balance model, we present snapshots of droplet impact on various superhydrophobic surfaces to explicitly show the effect of surface texture on droplet impact resistance and recoil efficacy (Figure 1). We perform droplet impact experiments on various surfaces: (1) sparse microtextured hydrophobic surface with PC < PB < PWH that leads to extensive texture wetting (2) dense microtextured hydrophobic surface with PB < PC < PWH that leads to partial texture wetting (3) hierarchical surface with nanodendrites on microposts (mimics the lotus leaf structure) and a nanoporous surface that exhibit complete droplet recoil as the capillary pressure exceeds both the water hammer and Bernoulli pressures (PB < PWH < PC). This fundamental understanding can aid surface design for droplet impact resistance in a variety of applications.

References
K.K. Varanasi, T. Deng, M. Hsu, N. Bhate, “Design of Superhydrophobic Surfaces for Optimum Roll-off and Droplet Impact Resistance,” ASME IMECE, 2008

T. Deng, K.K. Varanasi, M. Hsu, N. Bhate, C. Keimel, J. Stein, M. Blohm, “Non-wetting of Impinging Droplets on Textured Surface,” Applied Physics Letters, 94, 133109, 2009

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Figure 1. Dynamic interactions of 1mm diameter droplets with a variety of surfaces captured using a high-speed camera (a) Microtextured surface consisting of 15um posts spaced apart by 150um – droplet does not recoil (b) Partial drop recoil on microtextured surface consisting of 15um posts spaced apart by 5um (c) Complete drop recoil on hierarchical texture comprising of 3um posts with 100nm dendritic structures (d) Complete drop recoil on metal-oxide nanoporous surface with ~38nm pores.

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Kripa K. Varanasi, Massachusetts Institute of Technology, Cambridge, MA

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Tao Deng, GE Global Research Center, Niskayuna, NY

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