Mechanism of gas depletion on super-hydrophobic surfaces in turbulent flows

A textured super-hydrophobic (water-repelling) surface provides an exciting opportunity to potentially reduce the friction drag in turbulent flows, which could lead to substantial energy-savings in the marine industry. This drag reducing property is related to the presence of a gas layer trapped between the liquid flow and the textured surface. Unfortunately, the trapped gas could be depleted and carried away by turbulent flows, causing a failure of drag reduction.

The aim of this project is to better understand the mechanism of gas depletion when the super-hydrophobic surface is subject to turbulent flows. Ultimately, the results of this research will guide the development of passive and active approaches to sustain the drag reduction by super-hydrophobic surfaces in highly turbulent flows. In addition to training graduate and undergraduate students, the education plan will provide engineering experiences for local high-school students through University of Massachusetts Dartmouth’s Upward Bound program and Spotlight program.

Outreach activities through social media and local museums will raise public awareness of the development of novel drag reduction technologies in addressing energy and environmental challenges.

The mechanism of gas depletion on a super-hydrophobic surface caused by turbulent flows is poorly understood due to a lack of experimental data. This project aims to fill this knowledge gap through start-of-the-art experimental measurements. The first project goal is to characterize the spatial and temporal variations of the shape of gas-liquid interface in turbulent flows using Reflection Interference Contrast Microscopy and Digital Holographic Microscopy.

The results will illustrate how the interface deforms, vibrates, and finally detaches from the surface textures. The second goal is to develop predictive models of the wall pressure fluctuation and the critical condition for gas depletion. The wall pressure fluctuation, which is the main force causing interface deformation, will be estimated based on the resolved interface shape and the Young-Laplace equation.

The velocity field in the inner part of the turbulent boundary layer will also be measured by Holographic Particle Tracking Velocimetry. The predictive models will consider various factors including texture size, texture geometry, Reynolds number, Weber number, and initial interface shape. The third goal is to evaluate the effectiveness of hierarchical structure and gas injection for sustaining the drag reduction in high-Reynolds number turbulent flows.

The experimental results will validate the key assumptions used in past computational simulations. The research outcomes will advance the knowledge of the complex interplay between turbulent flows and super-hydrophobic surface.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.