Superhydrophobic surfaces, liquid-solid

The lotus effect has inspired scientists to design superhydrophobic surfaces for applications such as self-cleaning windows and water-repellent clothing. To understand the lotus effect and other phenomena involving liquids and solids, we must understand intermoiecuiar forces, the forces that exist between molecules. Only by understanding the nature and strength of these forces can we understand how the composition and structure of a substance are related to its physical properties in the liquid or solid state. 

The wettability of solid surfaces is a veiy important properly of surface chemistiy, which is controlled by both the chemical composition and the geometrical microsttuc-ture of surface [21-23], When a liquid droplet contacts a solid surface, it will sptead or remain as droplet with the formation of angle between the liquid and solid phases. Contact angle (CA) measurements are widely used to characterize the wettability of solid surface. Surface with a water CA greater than 150° is usually called superhydrophobic surface. On the other hand, when the CA is lower than 5°, it is called superhy-drophilic surface. Fabrication of these surfaces has attracted considerable interest for both fundamental research and practical studies [23-25]. 

The fact that tuning the chemical nature alone of the soUd is unable to provide friction reduction beyond the submicrometer scale has led to the suggestion that one should try to get rid of the actual solid-liquid boundary by coating the surface with a bubble (a gas layer). Such a situation, where gas is trapped at the solid interface and partially replaces the solid-liquid contact, can be achieved in specific conditions (see Section 2.1) with the use of the so-called superhydrophobic surfaces. Such surfaces, which combine surface roughness and nonwettability to achieve unique static properties with water contact angles close to 180°, were indeed recently predicted [17] to exhibit also super-lubricating characteristics. 

Figure 1. Sketch of the hydrodynamic flow close to a superhydrophobic surface in the Fakir state in the limit of vanishing solid fraction. V is the liquid velocity far away from the solid-gas interface, L the lateral period of the roughness pattern and a the width of a single solid post. The local slip length b, which is 0 at the liquid-solid interface because of the viscous dissipation, tends to infinity at the liquid-gas interface.

Furthermore, the measurements of sliding angles show that although the modified samples are superhydrophobic, a small water droplet (5 pi) on their surface cannot roll off when the samples are tilted to any angle, even upside down, i.e., it is firmly pinned on such surface. Generally, the three-phase (air-liquid-solid) contact line plays an important role in sliding behavior of water droplets [37-39], and a... 

An important conclusion from the above analysis is that the surface textures that optimize transverse flow can significantly differ from those optimizing the effective (forward) slip. It is well known, and we additionally demonstrated before, that the effective slip of a superhydrophobic surface is maximized by reducing the solid-liquid area fraction transverse flow in superhydrophobic channels is maximized by stripes with a rather large solid fraction, tpi = 0.5, where the effective slip is relatively small. 

In this way, a low-surface-energy, chemically inert lubricant forms a physically smooth and chemically homogeneous lubricating film on the structured surface, which leads to low contact angle hysteresis and a strongly reduced adhesion of the test liquids (e.g., water) to be repelled. The schematic diagram presented in Figure 11 illustrates the physical action principle of slippery liquid infused structured surfaces in comparison to superhydrophobic surfaces based on composite solid-air interfaces. 

Equation (645) shows that contact angle is a thermodynamic quantity, which can be related to the work of adhesion and interfacial free energy terms. When 6 values are small, the work of adhesion is high and considerable energy must be spent to separate the solid from the liquid. If 0 = 0°, then W L = 2yv if 0 = 90°, then W L = yLV, and if 0 = 180°, then W1L = 0, which means that no work needs to be done to separate a completely spherical mercury drop from a solid surface (or a water drop from a superhydrophobic polymer surface), and indeed these drops roll down very easily even with a 1° inclination angle of the flat substrate. 

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