Superhydrophobic Surfaces & Applications
MIT SEMINAR SERIES IN MANUFACTURING AND PRODUCTIVITY
Place: Room 33-116 Time: 12:00 P.M. Thursday, March 18th, 2008
Dr. Kripa Varanasi
General Electric Global Research
The main theme of my research is in the development of nanotechnology-enabled systems that can significantly enhance the performance of energy, aviation, and desalination systems. My talk will focus on the design of lotus-leaf inspired superhydrophobic surfaces for a variety of applications. We study the wetting energetics and wetting hysteresis of water and oil droplets as a function of surface texture and surface energy and find three wetting regimes on these surfaces: equilibrium Cassie at small feature spacing, equilibrium Wenzel at large feature spacing, and an intermediate state at medium feature spacing. Surprisingly, we observe minimum wetting hysteresis not on surfaces that exhibit Cassie wetting but rather on surfaces in the intermediate regime. We argue that the droplets on these surfaces are metastable Cassie droplets whose internal Laplace pressure is insufficient to overcome the energy barrier required to homogeneously wet the surface. These metastable Cassie droplets show superior roll-off properties because the effective length of the contact line that is pinned to the surface is reduced. We then develop a model that can predict the transition between the metastable Cassie and Wenzel regimes by comparing the Laplace pressure of the drop (or the water-hammer pressure in the case of a impacting drop) to the capillary pressure associated with the wetting energy barrier of the textured surface. The resistance to droplet roll-off on textured surfaces can be obtained as a product of the length of contact line in contact with the surface with a “pinning parameter” that can be obtained from roll-off measurements on a smooth surface. Together these models can be used to optimize texture design for droplet-shedding and droplet-impact resistant surfaces.
Next, we study the behavior of superhydrophobic surfaces under phase change in an environmental SEM. We find that these surfaces promote dropwise condensation but result in a mixture of Cassie and Wenzel drops. Heat transfer measurements indicate significant enhancement in the heat transfer coefficient on these surfaces when compared to baseline film condensation surfaces. Further optimization of the surface leads to a hybrid hydrophobic-hydrophilic architecture similar to the one found on a Namib beetle. Finally, I will discuss the flip side of superhydrophobic surfaces viz., superhydrophilic surfaces and characterize their wetting properties.
About the Speaker
Dr. Varanasi is a lead research scientist in the Nanotechnology & Energy & Propulsion technology programs at GE Research, NY. He is the PI for GE’s DARPA Thermal Ground Plane program. He received his B.T. from IIT Madras & his MS (ME & EECS) & Ph.D from MIT. The focus of his research is in the development of nano-engineered surfaces that enhance performance in energy, aviation, & desalination systems. He has research programs on superhydrophobic, superhydrophilic, & hybrid surfaces. He has obtained new results in wetting interactions, phase-change heat transfer, icing, nucleation & materials design. Dr. Varanasi has filed 15 patents in this area. He has won NIST ATP & DARPA TGP awards to apply this technology to steam turbines, condensers, & advanced electronic cooling applications. Other research interests include quantum dots & bandgap engineering, low-wave-speed media, & nano-scale motion systems. Rewards he has receive at GE include Technology Project of the Year (‘05) for metallic superhydrophobic surfaces & Best Patent Award for heat transfer enhancement via superhydrophobic/philic surfaces.
