In situ monitor of superhydrophobic surface degradation to predict its drag reduction in turbulent flow

Abstract

In situ monitoring is the most insightful technique to examine superhydrophobic surface degradation as it provides real-time information on the liquid–solid interface in a continuous, noninvasive manner. Using reflecting-pixel intensity, we introduced a simple method to characterize in situ the air-plastron over a superhydrophobic surface in a turbulent channel flow. Prior to the turbulent experiments, a no-flow hydrostatic test was carried out to determine a critical absolute pressure under which the surfaces are able to maintain the air layer for a prolonged period of time. Pressure-drop and velocity measurements were conducted in a series of turbulent flow tests. Resulting from the coupling effects of normal and shear stresses over the plastron, the air layer was progressively lost with flow time which caused the drag ratio (i.e., the friction factor ratio between superhydrophobic and smooth surfaces) to increase. Meanwhile, the average pixel intensity also increased with time and exhibited a consistent trend with the drag ratio evolution. At a fixed near-wall y/h location (within the viscous sublayer), the velocity increased with time since the shear stress increased. However, a velocity measurement at the center of the channel exhibited a decrease, consummate with an overall downward shift of the velocity profile. Both pressure-drop and velocity results were observed to be correlated with the average pixel intensities of the images captured over the surfaces, and therefore, this is a suitable proxy measure of the plastron. This technique is confirmed to be valid for monitoring the air layer and, hence, predicting the consequent loss of drag reduction.