Controlling the morphology and slippage of the air–water interface on superhydrophobic surfaces

In this work, experiments on controlling the air–water interface morphology by regulating air film internal pressure to achieve adjustable slip length on superhydrophobic surfaces (SHS) were carried out. Fluorinated silica particles with the diameter ranging from 0.5 to 40 microns were sprayed on perforated substrates to obtain different microstructures on SHS. The air film internal pressure was regulated through micro-holes on substrate which helps the plastron sustain under 50 kPa hydrostatic pressure. In the experiment, the variation of the air–water interface morphology was obtained by a total light refection method. Results showed that the reflection intensity on the air–water interface was positively correlated with the air film internal pressure, which reflected that the shape of meniscus changed from concave to flat. The effective slip length on SHS was determined through particle image velocimetry measurements in a fully developed laminar channel flow with a Reynolds number of 500. It was found that with the same Laplace pressure ( P L ) of the air–water interface, the effective slip length increases as the microstructure average spacing ( R sm ) increases and decreases as the root mean square roughness ( R rms ) increases. Moreover, a positive relationship between the effective slip length and an empirical parameter including air–water interface morphology and microstructure roughness was obtained. Results showed that a large microstructure average spacing, a low roughness height and a flat air–water interface morphology are pivotal for SHS with random microstructure to obtain maximum slip length in laminar flow. Our research demonstrates that the method of adjusting the internal pressure of the air film cannot only improve the pressure resistance of the air–water interface but also help to improve the drag reduction effect of SHS. Graphic abstract