Large Drag Reductions with Superhydrophobic Surfaces Sustainable in Turbulent Boundary Layer Flows

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Kim, Chang-Jin 1336966 Utilizing recently developed superhydrophobic (SHPo) surfaces that can sustain the nonwetting state even in high-speed flows, the UCLA team proposes to elucidate how they experience such a reduced drag (by > 50%) in turbulent flows. The drag reduction will be characterized both experimentally and numerically over a wide range of Reynolds numbers in turbulent boundary layer flows, which represent water vehicles in motion. The ultimate utility of the developed SHPo surfaces will be confirmed by the field tests in marine environment. Intellectual Merit : Despite numerous studies on SHPo surfaces over the past decade, challenges remain in obtaining SHPo surfaces deployable for real applications. Although drag reduction is the most attractive feature anticipated from the SHPo surfaces, no surface has ever demonstrated a drag reduction under a field condition, such as in outdoor water. While the drag reduction requires the SHPo surface to be in a dewetted state as a necessary condition,the dewetted state is very fragile underwater. No matter what one does, the trapped gas would eventually be diffused out to the surrounding water, eventually making the surface wetted. Recently the PI's lab has dramatically increased the robustness of underwater SHPo surfaces by developing a semi-active SHPo surface that sustained the dewetted state for month,) even in tens of meters of deth. In addition, they obtained over 50% drag reduction on some passive SHPo surfaces in turbulent boundary layer flows. These surfaces were also found nonwetting even in high-speed flows. Empowered by these recent breakthroughs, the UCLA team proposes to find the underlying physics of the large drag reduction on SHPo surfaces in turbulent flows, and establish the drag reduction over a wide range of Reynolds numbers (5x10E5 - 2x10E7). Ultimately, they would demonstrate a drag reduction under field conditions, using a small boat. Broader Impacts : Reduction of friction drag by flowing liquids has long been an elusive goal in fluid engineering. If the drag reduction is sustainable practical conditions, the energy savings alone will bring about significant benefits to the economy and the environment. Rhe proposed research aims to explain how drag is reduced on certain SHPo surfaces in flows that represent the general traveling conditions of most water vehicles. The results will be widely disseminated through lab websites, magazines, blogs, and regular media. The research results will be integrated in curricula and programs at UCLA, especially the Ph.D. Major Field of MEMS and nanotechnology. Furthermore, by providing attractive tasks for non-specialists, the project will promote education for undergraduate and high school students. For example, undergraduate students will help develop the field-testing apparatus through many programs at the UCLA engineering school. High school teachers and students can help field-test the SHPo surfaces in their local coastal water, strengthening the outreach programs of UCLA California NanoSystems Institute (CNSI).