Biomimetic Engineered Surfaces for Directional Liquid Transport

仿生工程表面上單向液體傳輸的研究

Student thesis: Doctoral Thesis

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Award date11 Jan 2019

Abstract

Directional and passive liquid transport on a solid surface is of scientific significance and technical importance in a wide range of applications, including microfluidics, printing, oil-water separation, water harvesting and heat transfer management. Although this field of research has blossomed over the past three decades, the developed strategies are far from controlling a desired fluidic transport with a high degree of fidelity and efficiency. The principal detriment to the generation of such a preferential fluidic motion generally arises from the unwanted contact line pinning that thwarts its motion. Fortunately, nature provides human a high diversity of directional surfaces for liquid transport by taking advantage of surface topography or chemistry. For instance, many living organisms, such as spider, lizard, shorebird, cactus, pitcher plant, desert beetle and so on, develop robust and passive engineered systems to transport liquid for survive under diverse environments. Thus, the aim of my thesis is to advance our fundamental understanding of liquid transport in nature, imitate the exquisite morphology and composition to attain superior functions of liquid transport, even go beyond nature in some special cases.

First, we revealed the more complex picture of droplet transport on the peristome surface of pitcher plant. In addition to the presence of the asymmetric arch-shaped microcavity with gradient wedge-corners and sharp edges, the structural gradient in the first-tier microgroove of the pitcher’s rim also plays an important role in the regulation of the directional droplet transport. Moreover, the directional liquid transport only occurs in a limited condition. Without the intricate control of the interplay between its multiscale structures and multiscale sources of water, as well as the dynamic conditions of water, the preferential directional droplet transport will collapse. The new transport phenomenon and the mechanisms we revealed will provide important insights for the design of asymmetric morphologies for droplet manipulation.

Second, inspired by hierarchical structures on the pitcher’s peristome, we proposed a new method of liquid transportation based on a unique topological structure that breaks the contact line pinning through efficient conversion of excess surface energy to kinetic energy at the advancing edge of the droplet whilst simultaneously arresting the reverse motion of the droplet via strong pinning. What results is a novel topological liquid diode that allows for a rapid, directional, and long-distance transport of virtually any kind of liquid without the input of external energy. The design of this novel liquid diode shifts away from the conventional paradigm in which a continuous gradient of wettability is invariably used to generate droplet motion.

Third, we further demonstrated that our topological liquid diode also permits a spontaneous, long-distance and unidirectional liquid transport under a wider range of isothermal and non-isothermal situations. In particular, when a thermal gradient is applied against the spreading direction, the liquid diode also displays a stabilized unidirectional liquid transport, although with a slight decrease in the spreading distance and velocity. We also elucidate that the spreading behavior (average velocity and rectification coefficient) vary almost linearly with the gradient of temperature without the presence of liquid-gas phase change on the liquid diode.

In summary, we systemically investigated the fluidic system on the peristome of pitcher plant. Getting inspiration from pitcher plant, a novel topological liquid diode was firstly proposed to allow for the long-distance, unidirectional and passive liquid transport. Furthermore, this peculiar liquid diode can function in the wide range of temperatures, and even overcome the undesired Marangoni flow from thermal gradient. We believe that our work on the directional liquid transport will yield important insights to the fundamental understanding of fluid dynamics and therefore spur a wide range of applications.