Optical investigation of flow rate induced drag reduction in partially substrateless microchannels with opposing shear-free meniscus surfaces

Conventional drag-reducing microchannels with superhydrophobic or liquid-infused surfaces are often limited by complex fabrication processes, mechanical fragility, and chemical hazards. This study introduces a partially substrateless microchannel (PSM) featuring a single slit or a set of parallel slits spanned by shear-free air/water meniscus interfaces on both the top and bottom walls. We investigate the impact of opposing slits and the effect of increasing slit number on flow behavior and drag reduction. White light interferometry was exploited to capture the meniscus topography, which enabled a detailed analysis of the spatial pressure distribution. Our results demonstrate a nonlinear increase in drag reduction with growing flow rate Q. Intriguingly, for PSM with a high shear-free meniscus surface ratio, the pressure drop Δp saturates at high Q, resembling the behavior of an inviscid flow. To understand this unique behavior, simulations were conducted taking the spatial variation of the radius of curvature of the meniscus into account. They revealed the formation of virtual flow tubes in the region between opposing slits with plateau-shaped velocity distributions exploiting the complete height of the microchannel for low loss flow. In contrast to the assumptions of most theoretical approaches on microchannel flow, we observed a notable influence of the finite width of the PSM on the velocity distribution. With the intention to maximize the shear-free meniscus surface, a PSM with 59.5% shear-free and only 40.5% no-slip surface led to an excellent 71% drag reduction at high Q.