Plastron stability and slip characteristics of finite-length longitudinal micro-trench superhydrophobic surfaces in laminar boundary layer water flow

Abstract

Most theoretical and numerical studies of friction drag reduction on longitudinal micro-trench superhydrophobic (SHPo) surfaces assume the trenches are infinitely long, while they are inevitably of a finite length when implemented on water vehicles. This study employs the volume-of-fluid method to investigate laminar boundary layer flows over SHPo surfaces with finite-length trenches modeled after those tested in previous boat experiments. Each SHPo surface has 90-lm-wide and 10–30-mm-long trenches repeated transversally and separated by 10-lm-wide walls. We quantify how trench length L, depth d, and free-stream velocity U1 influence plastron stability and slip properties. Three distinct plastron states are identified: (i) fully stable, (ii) stable with confined rear wetting, and (iii) unstable due to propagating front wetting. The results show that increasing d promotes confined rear wetting, which leaves the majority of the plastron pinned, whereas increasing L induces front wetting, which propagates downstream and collapses the entire plastron. As long as the plastron remains pinned at the trench top, the SHPo surface provides an essentially full slip effect, even with confined rear wetting. The slip effect enhances with both increasing d and L but remains unaffected by U1, corroborating other studies in the laminar flow regime.