Liquid Zener diodes†

Abstract

The Zener diode (Z-diode) is often used as a voltage regulator, and is designed to operate in reverse breakdown. Namely, it can conduct current in the reverse direction when a certain voltage, known as the Zener voltage, is applied. Analogous to electric diodes, liquid diodes are microscale surface structures that promote spontaneous unidirectional liquid flow. In nature, they are used to increase water collection and uptake, for reproduction, and feeding. While such structures are usually deterministic, and used to guide liquids in specific directions while preventing backflow, the ability to render them responsive to external stimuli holds great potential for actuating and manipulating pump-free liquid flow in capillary networks. Inspired by electric Z-diodes, we demonstrate a local break in the diodic nature of flexible liquid diodes, in response to global compression or bending. Under specific conditions, flexural deformations trigger the formation of a capillary bridge in the reverse flow direction. This localized actuated flow is confined to where the liquid front is halted, preserving the integrity of liquid diodes elsewhere. This innovative concept showcases sequence-dependent liquid propagation within capillary networks, opening avenues for implementing network memory, reaction-dependent actuation, and the design of dynamic capillary networks.