Research On The Wettability Of Biomimetic Superhydrophobic Surfaces And Their Applications

During the long process of evolution, biological populations have developed many unique and excellent surfaces through natural selection. Micro-and nanostructured surface is one of the typical representatives. Because of the unique arrangement and compositions of micro-and nanostructures, such surfaces tend to exhibit some unexpected functions. For example, the superhydrophobic characteristic exhibited by lotus leaf makes water droplets easily roll down the leaf and take away contaminants; the superior floating capability provided by the multilevel micro-nano structures on mosquito legs allows mosquitos to stand on and take off from water surface freely; multi-level structured gecko toe surface has a strong adsorption capability which enables the gecko to crawl on smooth vertical wall; the tiny scals, alined in the direction of fluid flow, covering the skin of fast-swimming sharks were found to be very effective in drag reduction. Inspired by these extraordinary biological structures from nature, a series of studies on the characteristics of micro-and nanostructured surfaces have been launched and applied to many designs and manufactures of functional materials. In the last decade, based on the concept of bionics, explorations and researches on micro-and nanostructured surface characteristics and their multi-functional applications have drawn extensive attention and great interests.Superhydrophobic surface is one typical kind of micro-and nanostructured surfaces. The trapped air layer in its microstructure can effectively reduce the contact area between liquid and solid surface, and this advantage of the superhydrophobic surface can achieve many disarable functions, including self-cleaning, superbuoyancy, drag reduction, anti-icing and so on. This paper here, inspired by biological micro-and nanostructured surfaces, studies the wettability of superhydrophobic surfaces and their applications from a mechanical point of view. The main content of this paper is composed of the following aspects:In Chapter2, the structural surface wettability is studied. The surface with micropillar structure is taken as an example to discuss the influence of geometric morphology parameters on the surface wettability. The principles of size selection of hydrophobic surface microstructure are analyzed from three aspects, including minimum energy principle, force balance and geometric conditon. When defining the height of the microstructure, we found that a hydrophobic surface cannot be necessarily obtained even though the height of posts is greater than the maximum sagging depth of the meniscus (the first height criteria). The surface can be held steady in a state of Cassie model only if the height condition (the second height criteria) derived from the minimum energy principle is satisfied. Meanwhile, a novel method is proposed to manufacture superhydrophobic surface:Assembling microneedles, blades or other components with small curvature top to form a rough surface and modifying the surface with low energy coating.This method has obtained national patent.In Chapter3, the dynamic response of the droplet impacting on structured rough surface is analyzed by using commercial software Fluent. The numerical method of VOF (volume of fluid) model for the two phase flow is used in the three-dimensional simulation. Considering the cycle arranged micropillar surface, we discuss the influence of contact angle, height of microstructure and initial velocity on the spreading of droplet. The simulation results are basically consistent with the theoretical analysis in the second chapter. We also analyzed the pressure effect, and found that an effective method to prevent the liquid entering during droplet impacting is to reduce the size of microstructure.In Chapter4, based on the digital microfluidics system for single droplet, a series of experiments are studied. According to the situation of needle electrode, the control technology is defined as contact mode (the needle electrode is inserted into the droplet) and non-contact mode (the needle electrode is suspended over the top of the droplet), respectively. In the contact mode, the electrowetting and electric field induced oscillation are observed due to the good insulating property of Cu(CH3(CH2)12COO)2superhydrophobic surface. However, the inserting of needle electrode is not good for the droplet oscillation. In the non-contact mode, we found that the AC voltage can also induce droplet oscillation after the needle electrode is suspended over the top of the droplet. And the influences of droplet size, voltage amplitude and frequency on the oscillation shape of droplet are discussed experimentally. A new method to achieve rapid transfer and mixing of droplets with the control of electric field is proposed. This method has obtained national patent.In Chapter5, inspired by water striders and other small insects, a superhydrophobic grille structure for drag reduction is designed and fabricated. A self build open-water cycle measurement system is used to measure the resistance value of the flat miniature boat and the one with superhydrophobic grille structure. The experimental results show a considerable drag reduction due to the use of superhydrophobic grille structure. Because the buoyancy is applied by surface tension, the form drag and friction drag of superhydrophobic grille structure can be eliminated drastically. Meanwhile, the periodic superhydrophobic grilles also display a considerable loading capacity, which can be used in the design of micro water surface robots.