Nature-inspired Surfaces for Directional Transport of Droplets at Extreme Conditions

基於仿生材料的液滴極端環境下定向輸運的研究

Student thesis: Doctoral Thesis

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Award date6 Sep 2018

Abstract

The directional and spontaneous transport of liquid droplets on surfaces is ubiquitous in nature and essential to numerous practical applications. While extensive advances in scientific and technical technologies make it possible to mimic intriguing functionalities inherent in natural directional surfaces, our ability to rationally tailor the droplet motions, especially at various harsh environments remains challenging. This challenge becomes further server owing to the complexity imposed by phase change process, which dedicates the distinct droplet dynamics including droplet generation format, the droplet length scale, the triple-phase interfaces, and the driving force. This thesis is to develop various bio-inspired structures with tailored interfacial properties and explore how the structural topography promotes the triple-phase interactions involving different time and length scales, which may have potential applications in the fields of heat transfer, anti-fogging, ice formation/retardation, microfluidics and so on.

When a droplet is subjected to a hot surface with temperature higher than a critical value, the so-called Leidenfrost point, a continuous vapor layer form underneath the droplet, which highly compromises the heat transfer rate. Thus, in the first part of my research, I created a gradient surface that directed the droplet to a specific location and eliminated the large heat transfer resistance associated with the undesirable vapor layer. Through the elegant control of structural topography and the imposed temperature range, two concurrent thermal states, i.e. Leidenfrost and contact boiling can be manifested in a single droplet. Such a Janus thermal state finally generates a preferential motion of droplet toward the region with high heat transfer, which has promising applications especially those related to thermal management.

In another aspect, the insulating vapor layer beneath the droplet eliminates the undesired interfacial hydrodynamic resistance, which provides potential applications in drag reduction, fluidic device, energy conversion and so on. Despite extensive progresses, the-state-of-art structures as well as our fundamental understanding of Leidenfrost self-propulsion remain quite limited. In this regard, we designed a novel surface with two-tier asymmetry to allow the tunable self-propulsion of Leidenfrost droplet. Dictated by the cooperation and competition between the asymmetries of individual unit (first-order) and collective lattice (second-order), the evaporating vapor under the Leidenfrost droplet can be rectified into different velocities and even distinct directions, giving rise to a tunable driving force to drive the droplet accordingly. The rich physics underlying such a tunable and spontaneous transport will not only advance our fundamentally understanding, but also find plenty applications in the field of drag reduction, fluidic devices and so on.

Meanwhile, more challenges emerge at another extreme condition, namely the temperature lower than dew point. Under such condition, the occurrence of continuous condensation results in the formation of small droplets, which are associated with large contact line pinning even on refined superhydrophobic surfaces. Without timely departure, these droplets tend to accumulate on these surfaces, which dramatically compromise their original water repellency. Thus, in the final part of my research, we explore the unique ballistic droplet transport mechanism developed by the natural drain fly, which functions well even under high humidity. Further studies show that this remarkable feature is owing to the elegant conjunction of structural topography, hydrophobicity, geometric regularity and flexibility, which allows the rectification of random and localized vapor condensation to a directional and global transport of droplets. The new transport mechanism we have identified opens up a new approach toward the design of artificial rectifiers for broad applications in harsh environments.