Demarcating wetting states in textured microchannels under flow conditions by Poiseuille number

In this work, flow friction in microchannels decorated with micropillars was investigated experimentally, with an interest to understand the wetting transition through two simple means: Poiseuille number and scaling laws. Different wetting states were demarcated by qualitatively assessing the behaviour of Poiseuille number (Po = f.Re, where f is friction factor and Re is Reynolds number), which are further corroborated by confocal microscopy-based measurements and numerical simulations. The wetting transition ensued smoothly with an increase in Re, independent of the gas fraction (a ratio of area covered by the liquid-gas interface to the total projected area), for moderate gas fractions, whereas an early breakdown of the Cassie-Baxter state occurred irrespective of Re at high gas fractions. Additionally, the scaling laws were found to correlate well with the underlying state of the flow. Our observations revealed that the liquid-gas interface exhibits a partial slip, contrary to the common notion that it is shear free. It is inferred that an increase in effective flow area leads to a reduction in flow friction in textured microchannels. The present work underlines three important outcomes. The first is the identification of wetting states in flow conditions shown by tracking the Poiseuille number. The second is that the liquid-gas interface is deduced to behave like a partial slip boundary. The third is that a textured microchannel can be worse than an enlarged dimension microchannel.