Design of hydrophobic surfaces for liquid droplet control

Abstract

During rainfall, small water droplets can be observed to roll off hydrophobic umbrella surfaces at diff erent speeds. A droplet on a round hydrophobic surface does not always slide down on the surface easily at a small tilt angle, but even if it does, large moving acceleration is not always observed. What determines the motion of droplets on solid surfaces?Diff erences in droplet shape on solid surfaces are often explained by variations in wettability. Th e wetting of a solid surface has persisted through the ages as a research subject at the border between physics and chemistry. Wettability control of a solid against a liquid has been widely explored in industry for application in daily life because it can engender various physical and chemical phenomena related to solids and liquids. Wettability is an important property of solid surfaces from both fundamental and practical aspects. Th e degree of adhesion of liquid droplets onto a solid surface is commonly described in terms of the contact angle (θ), which is given by Young's equation aswhere γ sv , γ sl , and γ lv represent the interfacial free energies per unit area of the solid-gas, solid-liquid and liquid-gas interfaces. Although the defi nition and measurement of the contact angle are simple and easy, the nature of wettability is complex because it is related to three phases: solid, liquid and gas. Moreover, various factors aff ect the surface wettability of a solid. When one factor of a solid surface such as roughness is altered, other factors such as the surface topography or its chemical composition often change simultaneously. Th erefore, it is usually diffi cult to discuss the results of surface wettability obtained from experiments while retaining academic rigor. Th e main research interest for industrial materials is the behavior of millimeter-size droplets on a solid surface of more than a few centimeters in area. Th erefore, the discussion of wettability among samples with great diff erences in surface character is sometimes conducted with ignorance of the infl uence of other characteristics.Hydrophobic coatings are extremely important for wettability control. Th e coatings are anticipated for various industrial uses such as anti-wetting, anti-snow (or ice), anti-rust and reduced friction resistance by reducing solid-liquid interaction. Generally, a hydrophobic surface is one on which water forms a round droplet that is easily removed. Recent advances in probe microscopy and surface spectroscopy and the wider availability of such techniques have revealed that macroscopic surface hydrophobicity, especially dynamic hydrophobicity, is aff ected by nanolevel characteristics such as structure, chemical composition and their alignment and homogeneity. Th e expected properties for a hydrophobic surface cannot always be obtained unless precise design and control of the solid surface are applied. Th is report presents the results of recent studies on the design of hydrophobic surfaces for droplet removal or control from the viewpoint of materials science....

Combining the lotus leaf effect with artificial photosynthesis: regeneration of underwater superhydrophobicity of hierarchical ZnO/Si surfaces by solar water splitting

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] In recent years, the non-wetting property of superhydrophobic surfaces submerged in water has attracted much attention because it has potential applications in drag reduction, anti-fouling, anti-corrosion, waterproof devices, microchannels, anti-icing and other non-wetting related applications. [16][17][18][19][20][21][22][23][24][25] Practical applications of non-wetting surfaces, however, have been impeded by the limited stability of their underwater superhydrophobicity.…”

A bio-inspired flexible fiber array with an open radial geometry for highly efficient liquid transfer

“…20 Moreover, open fibrous arrays enjoy the advantages of easy preparation, low cost and low hydrodynamic resistance. 14,19 However, manipulating liquid in open fibrous systems is rarely reported and remains a significant challenge, 21 because capillary coalescence is frequently encountered when fiber array interacted with a liquid. 22 Here, we describe the special dynamic wetting behavior of dandelion pappi.…”

Superwetting of TiO 2 by light-induced water-layer growth via delocalized surface electrons

“…The phenomenon has been attributed to production of surface radical groups (1,(3)(4)(5) or photo-oxidation of hydrophobic surface contaminants (6)(7)(8), but its origin remains controversial (9,10) in part because conventional contact-angle analysis alone cannot fully account for the complex surface wettability (11). To address the fundamental mechanism responsible for superwetting, one has to go beyond the contact-angle measurement that reflects the specific surface properties and to probe the temporal growth dynamics of the water layer itself adsorbed on TiO 2 under wide-spectrum illumination of visible and near-infrared (NIR) in addition to UV light.…”