The fluid diode: tunable unidirectional flow through porous substrates.

Many important applications in fluid management could benefit from unidirectional transport through porous media via a simple, large-area, low-cost coating treatment; in essence, a fluid diode demonstrated herein for water using common cellulosic paper substrates. In electronics, the diode is an electrical component with asymmetric current transfer characteristics. A light (<2 g/m(2)) superhydrophobic conformal coating applied onto one side of a porous substrate is shown to create a liquid transport function analogous to the electronic diode, facilitating fluid movement in one direction under negligible penetration pressures, but opposing it in reverse up to greater pressures. The phenomenon is driven by capillary action and can be observed using any similarly-thin fluid barrier applied on only one side (i.e., wettability contrast) of an absorbent porous matrix. Diodic action and liquid transport rates are shown to be highly tunable, determined by fiber diameter and spacing, in combination with coating deposition amount. As an example, the device is used to separate an oil/water mixture, relying upon the surface tension differences of the mixture constituents, and may be implemented in multicomponent fluid filtration/separation technologies.