Abstract
Super-hydrophobic textured surfaces reduce hydrodynamic drag in pressure-driven laminar flows in micro-channels. However, despite the wide usage of non-Newtonian liquids in microfluidic devices, the flow behaviour of such liquids was rarely examined so far in the context of friction reduction in textured super-hydrophobic micro-channels. Thus, we have investigated the influence of topologically different rough surfaces on friction reduction of shear-thinning liquids in micro-channels. First, the friction factor ratio (a ratio of friction factor on a textured surface to a plain surface) on generic surface textures, such as posts, holes, longitudinal and transverse ribs, was estimated numerically over a range of Carreau number as a function of microchannel constriction ratio, gas fraction and power-law exponent. Resembling the flow behaviour of Newtonian liquids, the longitudinal ribs and posts have exhibited significantly less flow friction than the transverse ribs and holes while the friction factor ratios of all textures has exhibited non-monotonic variation with the Carreau number. While the minima of the friction factor ratio were noticed at a constant Carreau number irrespective of the microchannel constriction ratio, the minima have shifted to a higher Carreau number with an increase in the power-law index and gas fraction. Experiments were also conducted with aqueous Xanthan Gum liquids in micro-channels. The flow enhancement (the flow rate with super-hydrophobic textures with respect to a smooth surface) exhibited a non-monotonic behaviour and attenuated with an increase in power-law index tantamount to simulations. The results will serve as a guide to design frictionless micro-channels when employing non-Newtonian liquids.
Highlights
Super-hydrophobic (SH) micro- and nano-textured surfaces have been a subject of significant interest in a plethora of applications (Geyer et al 2020; Sharma et al 2018; Hwang et al 2018; Jiang et al 2020; Gaddam et al 2021; Chen et al 2019), where the existence of a heterogeneous wetting state namely the Cassie-Baxter state helps in achieving the desired functionality
We focus our efforts on understanding the flow behaviour and estimating the friction reduction of shear-thinning liquids on SH textures
Once the Newtonian regime is established at the high shear rates (γ > 103 s−1), the friction factor ratio (FFR) is observed to plateau towards constant FFR curves
Summary
Super-hydrophobic (SH) micro- and nano-textured surfaces have been a subject of significant interest in a plethora of applications (Geyer et al 2020; Sharma et al 2018; Hwang et al 2018; Jiang et al 2020; Gaddam et al 2021; Chen et al 2019), where the existence of a heterogeneous wetting state namely the Cassie-Baxter state helps in achieving the desired functionality. In flow settings where SH-textured surfaces are involved, in microscale laminar flows, the Cassie–Baxter state helps in hydrodynamic drag reduction (Li et al 2017; Ko et al 2020) and enhanced thermo-hydraulic performance (Dilip et al 2018; Sharma et al 2020). Since the velocity slip induced by the entrapped gas pockets on an SH-textured surface reduces the
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