Wetting and Design Guidelines for Bio-Inspired Liquid Diodes

Spontaneous unidirectional transport of liquid has a wide range of potential applications, from microfluidics, to medical devices in which fluid collection is needed, to printing, condensing, filtering and lubrication apparatuses, water-harvesting and drug delivery (Li et al., 2019, 2021; Stone et al., 2004). Currently, available technologies feature an external energy source and/or moving parts to drive the flow and fluid rectifiers such as Tesla valves require high Reynold’s numbers or non-Newtonian fluids.<br/>In nature, several systems achieve passive unidirectional transport of liquids with remarkable efficiency and precision, exploiting capillary spontaneous flow. For example, some fleas feature a unique organ called Spermatheca, which stores the male’s sperm until the right conditions for egg laying are found. It is hypothesized that liquid can only flow in one direction inside the organ, without the ability to flow back. This system inspired the fabrication of biomimetic liquid diodes (Buchberger et al., 2018).<br/>In this study, we present 3D-printed passive liquid diodes, inspired by the rat flea. We modulate a structural feature, called pitch, which widens the working window of the device by introducing more degrees of freedom in the system.<br/>Detailed flow analysis provided by Lattice Boltzmann Method (LBM) simulations led us into correctly adjusting the design for the best performances in terms of diodicity and fluid velocity. By varying the pitch height and the contact angle of the liquid, we obtained a comprehensive flow behaviour phase diagram, which outlines a designated region for diodicity.<br/>Furthermore, in addition to the dependence on the contact angle, we found an interesting relation between the flow velocity and the pitch height, which also results in different pinning behaviors.<br/>With this, we provide specific guidelines for the design of liquid diodes with improved performance.