Counterintuitive Ballistic and Directional Liquid Transport on a Flexible Droplet Rectifier

Abstract

Achieving the directional and long-range droplet transport on solid surfaces is widely preferred for many practical applications but has proven to be challenging. Particularly, directionality and transport distance of droplets on hydrophobic surfaces are mutually exclusive. Here, we report that drain fly, a ubiquitous insect maintaining nonwetting property even in very high humidity, develops a unique ballistic droplet transport mechanism to meet these demanding challenges. The drain fly serves as a flexible rectifier to allow for a directional and long-range propagation as well as self-removal of a droplet, thus suppressing unwanted liquid flooding. Further investigation reveals that this phenomenon is owing to the synergistic conjunction of multiscale roughness, structural periodicity, and flexibility, which rectifies the random and localized droplet nucleation (nanoscale and microscale) into a directed and global migration (millimeter-scale). The mechanism we have identified opens up a new approach toward the design of artificial rectifiers for broad applications.

Controllable preparation of an ice cream-shaped hollow sphere array

“…In the hydrothermal environment, the liquid metal placed in the designed substrate would move upwards due to the capillary effect. 13,14 The formation of a dense oxide film on its surface prevents further oxidation since gallium is easily oxidized in the presence of oxygen. When the liquid droplet reaches the equilibrium position, the ZnO layer, encasing EGaIn liquid metal, would react with the aqueous solution to start constructing petal-like nanostructures on the surface.…”

Recent Growth of Wettability Gradient Surfaces: A Review

“…The evolution of WGSs stems from the living surfaces, for example, the desert beetle [4], drain fly [5], cactus spines (consisting of both wettability gradient and physical shape gradient) [6], spider silk [7] (consisting of both wettability gradient and physical shape gradient), Salvinia molesta [8], and bamboo leaf [9]. Upon looking into the surface morphology of desert beetle, different topographical patterns are observed where the condensed water adopts the dropwise form in a primary zone (low wetting waxy zone), which then moves in the direction of a secondary zone (high wetting non-waxy zone) until it reaches the mouth [10].…”

Nanoarray-Embedded Hierarchical Surfaces for Highly Durable Dropwise Condensation

“…Therefore, the film-like droplets that pin on the surface will undergo the irreversible Cassie-Wenzel wetting transition, leading to the phenomenon of flooding that declines the heat transfer efficiency of the surface [19,20]. In efforts to maintain the Cassie state of the droplet, previous studies have focused on decreasing the solid-liquid contact area [21,22] or the number of droplet nucleation sites [23,24] by designing increasingly smaller arrays or approximate arrays, from micro to nanoscale size, such as pine needle shapes [6], microconical architectures [25], nanoscaffolds [26], microratchet arrays [27], and vertical arrays of carbon nanotube (CNTs) [28,29]. These surfaces indeed exhibit an enhancement in dropwise condensation by collecting a volume of water three times higher than that in the case of plate surfaces [30], achieving a 100% higher heat flux than that on the plane hydrophobic surface [31], and reaching a directional transport efficiency of approximately 80% for tiny droplets [27].…”