Liquid Infused Surfaces (LIS) have impressive capabilities for repelling liquid droplets and therefore have potential applications in numerous sectors due to their anti-biofouling and self-cleaning properties. LIS are produced by imbibing a textured solid substrate with a lubricant. The lubricant layer creates a smooth surface that prevents the droplet from being pinned on surface inhomogeneities.  Droplets on LIS are surrounded by a lubricant wetting ridge (Fig. 1a), which was hypothesised to govern the friction force experienced by the droplet. This hypothesis was applied to explain experimental results, yet it had not been verified directly. Using computational fluid dynamics simulations to compute the viscous dissipation, we directly showed that the energy lost due to the friction force is primarily dissipated within the wetting ridge. Most of the viscous dissipation is localized close to the solid texture at the bottom of the wetting ridge (Fig. 1b). We found that this is the case regardless of the geometry of the solid texture. Our results confirmed that the wetting ridge is the main source of friction when the lubricant viscosity is greater than the drop viscosity. 

Directional and self-propelled flow in open channels has a variety of applications, including microfluidic and medical devices, industrial filtration processes, fog-harvesting, and condensing apparatuses. Here, we present versatile three-dimensional-printed liquid diodes that enable spontaneous unidirectional flow over long distances for a wide range of liquid contact angles (CAs). Typically, we can achieve average flow velocities of several millimeters per second over a distance of tens to hundreds millimeters. The diodes have two key design principles. First, a sudden widening in the channels' width, in combination with a small bump, the pitch, ensure pinning of the liquid in the backward direction. Second, an adjustable reservoir with differing expansion angles, the bulga, is introduced to manipulate the liquid velocity. Using a combination of experiments and lattice Boltzmann simulations, we provide a comprehensive analysis of the flow behavior and speed within the channels depending on CAs, pitch heights, and bulga angles. This provides guidelines for the fabrication of bespoke liquid diodes with optimal design for their potential applications. As a feasibility investigation, we test our design for condensation of water from fog and subsequent transport uphill.