UCLA

Superhydrophobic (SHPo) surfaces, which may capture a thin air layer (called plastron) on them underwater, have been studied over the last two decades most notably due to their potential drag-reducing ability for water vessels. Despite many reports of successful drag reduction in laboratory settings, no success has been reported for highly turbulent flows on the open water in natural environment until just two years ago. While reporting around 30% of reduction under a boat, the recent success indicated that the plastron was the culprit for most of the failures and the resulting controversies about SHPo drag reduction. The current study is motivated by our finding that the common practice of confirming the plastron with the silvery sheen appearance is not sufficient and may be seriously misleading for SHPo drag reduction research. Since drag reduction requires a plastron pinned on top of the surface asperities while depinned plastron may still appear bright, we develop a convenient observation strategy that can discern the pinned from the depinned plastron in field studies such as under a boat. By further revamping the 13 foot motorboat retrofitted for drag-reduction experiments, we study the behavior of plastron on micro-trench SHPo surfaces in highly turbulent flows on a natural sea environment. A unified theory is developed and experimentally confirmed to predict the maximum trench length that allows for a full plastron in the typical flow conditions of watercraft. Furthermore, high-performance SHPo surfaces are developed with micro electro mechanical systems (MEMS) technology to maintain a full plastron at typical boat speeds (tested up to 14 knots). In addition to testing the effect of slip length and obtaining about 30% of drag reduction with longitudinal trench SHPo surfaces, transverse trench and aligned post SHPo surfaces are also tested to reveal the effect of transverse slip on drag reduction in the turbulent flows under the boat.