One extensively studied approach is to use superhydrophobic coatings ( 3). Thus, finding approaches for making viscous liquids flow faster without the need to increase the driving force is of considerable technological interest. The reduced flow rate due to increased viscosity can be counteracted by increasing the driving pressure gradient, but only to a limited extent, as it will also increase the mechanical stress on the pipe. In a simple model, volumetric flow rate Q inside a pipe, whether a blood vessel or a microfluidic device, scales as Q ∝ Δ P/η in the laminar regime ( 1). Often, it is desirable to have the fluid flow fast and effortlessly. Viscosity plays an important role, as it limits fluid flow and mass transfer at all length scales and in a wide range of systems from industrial processes to biological circulatory networks ( 2). Viscosity η is a measure of resistance of fluid to flow under an external force or pressure gradient Δ P ( 1). Our work demonstrates the unexpected role of the plastron in controlling fluid flow beyond the mere reduction in contact area and friction. The anomalous viscosity-enhanced flow is caused by a viscosity-suppressed deformation of the droplet-air interface and a hydro- and aerodynamic coupling between the droplet and the air trapped within the micro/nanostructures (plastron). This is in stark contrast to the faster moving high-viscosity droplets with nearly vanishing internal flows. Using tracer particles, we show that the low-viscosity droplets are rapidly rotating internally, with flow velocities greatly exceeding the center-of-mass velocity. A gravity-driven, highly viscous glycerol droplet inside a sealed superhydrophobic capillary is moving more than 10 times faster than a water droplet with three-orders-of-magnitude lower viscosity. Here we show that this intuitive rule is not general and can fail for liquids flowing in confined liquid-repellent systems. It is well known that an increased viscosity slows down fluid dynamics.
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