Self-cleaning surfaces are a class of materials with the inherent ability to remove any debris or bacteria from their surfaces in a variety of ways. The self-cleaning functionality of these surfaces are commonly inspired by natural phenomena observed in lotus leaves, gecko feet, and water striders to name a few. The majority of self-cleaning surfaces can be placed into three categories:
superhydrophobic superhydrophilic photocatalytic.
History The first instance of a self-cleaning surface was created in 1995. Paz et al. created a transparent titanium dioxide (TiO2) film that was used to coat glass and provide the ability for the glass to self-clean. The first commercial application of this self-cleaning surface, Pilkington Activ, was developed by Pilkington glass in 2001. This product implements a two-stage cleaning process. The first stage consists of photocatalysis of any fouling matter on the glass. This stage is followed by the glass becoming superhydrophilic and allowing water to wash away the catalyzed debris on the surface of the glass. Since the creation of self-cleaning glass, titanium dioxide has also been used to create self-cleaning nanoparticles that can be incorporated into other material surfaces to allow them to self-clean.
Surface characteristics The ability of a surface to self-clean commonly depends on the hydrophobicity or hydrophilicity of the surface. Whether cleaning aqueous or organic matter from a surface, water plays an important role in the self-cleaning process. Specifically, the contact angle of water on the surface is an important characteristic that helps determine the ability of a surface to self-clean. This angle is affected by the roughness of the surface and the following models have been developed to describe the "stickiness" or wettability of a self-cleaning surface.
Young's model
Young and colleagues proposed Young's model of wetting that relates the contact angle of a water droplet on a flat surface to the surface energies of the water, the surface, and the surrounding air. This model is typically an oversimplification of a water droplet on an ideally flat surface. This model has been expanded upon to consider surface roughness as a factor in predicting water contact angle on a surface. Young's model is described by the following equation:
cos ( θ 0 ) = ( γ SA − γ SL γ LA ) {\displaystyle \cos(\theta _{0})=\left({\frac {\gamma _{\text{SA}}-\gamma _{\text{SL}}}{\gamma _{\text{LA}}}}\right)}
where
θ 0 {\displaystyle \theta _{0}} is the contact angle of water on the surface
γ SA {\displaystyle \gamma _{\text{SA}}} is the surface energy of the surface-air interface
γ SL {\displaystyle \gamma _{\text{SL}}} is the surface energy of surface–liquid interface
γ LA {\displaystyle \gamma _{\text{LA}}} is the surface energy of liquid–air interface
Wenzel's model When a water droplet is on a surface that is not flat and the surface topographical features lead to a surface area that is larger than that of a perfectly flat version of the same surface, the Wenzel model is a more accurate predictor of the wettability of this surface. Wenzel's model is described by the following equation:
cos ( θ ) = R f c o s ( θ 0 ) {\displaystyle \cos(\theta )=R_{\text{f}}cos(\theta _{0})}
where
θ {\displaystyle \theta } is the contact angle of water predicted by Wenzel's model
R f {\displaystyle R_{\text{f}}} is the ratio of surface area of rough surface to the surface area of a flat projection of the same surface
Cassie–Baxter model For more complex systems that are representative of water-surface interactions in nature, the Cassie–Baxter model is used. This model takes into consideration the fact that a water droplet may trap air between itself and the surface that it is on. The Cassie–Baxter model is described by the following equation:
cos ( θ CB ) = R f cos ( θ 0 ) − f LA ( R f cos ( θ 0 ) + 1 ) {\displaystyle \cos(\theta _{\text{CB}})=R_{\text{f}}\cos(\theta _{0})-f_{\text{LA}}(R_{\text{f}}\cos(\theta _{0})+1)}
where
θ CB {\displaystyle \theta _{\text{CB}}} is the contact angle of water predicted by the Cassie–Baxter model
f LA {\displaystyle f_{\text{LA}}} is the liquid–air fraction, the fraction of the liquid droplet that is in contact with air
Mechanisms
Use of water
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