Date of Award

6-25-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Applied Science

First Advisor

Alexandru Biris

Abstract

In this work, anti-icing superhydrophobic (SHP) tungsten nanorod (WNR) surfaces were fabricated. The fabrication of WNR surfaces as rough surfaces was carried out using the Glancing Angle Deposition (GLAD) technique. Using the magnetron- sputtering deposition method, the deposition Ar pressure, Ar flow rate, and substrate tilting angle were varied to fabricate WNRs with different surface morphologies and porosities. The surface energy of the WNR films was lowered by coating them with a nano-layer (10-15nm) of Teflon AF2400 using an effusion cell. Static contact angle (SCA) measurements of the sessile water droplet gently dispensed on surfaces were used to characterize the static wetting properties of those surfaces. After surface treatment of the fabricated WNRs with different spacing, heights, and nanorod natural pyramidal tips, surfaces with tunable hydrophobic properties with SCAs ranging from 120° to 160° were obtained. The well-known classical Wenzel and Cassie models were used to predict the observed CAs. The proposed geometrically modified Cassie model showed consistent agreement with the observed high SCAs. The dynamic study of the wetting properties of the fabricated surfaces was carried out using water droplet evaporation on the surfaces. The kinetics of water droplet evaporation showed a significant influence of surface wetting properties, morphology, and porosity on the three modes of evaporation. Moreover, the contact angle hysteresis (CAH) of the surfaces was measured by the dynamic approach of adding/withdrawing water to/from the surfaces, respectively. The SHP-WNR films maintained a relatively low CAH of 30 degrees. The CAHs obtained by both kinetics of evaporation and the dynamic approach were consistent. Finally, the ability of the supercooled SHP-WNRs, kept at a temperature of -10 °C, to repel supercooled water droplets with a subzero temperature as low as -10 °C was tested. The SHP-WNRs' surfaces were able to repel supercooled water droplets released from a height of 15 mm with an impact velocity of 0.54 m/s. Calculations of energy dissipation in the course of bouncing due to the water droplets' physical properties and the surface wetting properties were performed. In addition to surface CAH and water droplet vibration, a significant energy loss due to an increase in supercooled water droplet viscosity as a function of its temperature was observed.

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