Cassie-Baxter

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The other type of superhydrophobicity is the Cassie-Baxter form, wherein the liquid does not fully penetrate the surface topography, but instead will sit atop surface asperities and air pockets. For nonwetting surfaces, this is a more likely scenario, since it may be more energetically favorable for the hquid to be in contact with air than a repelhng surface. This type of wetting can be described by... 

When air pockets are present on a rough surface, the Cassie-Baxter equation can also be applied to the contact angle estimate of a water drop on such a surface. Since the contact angle of water in air is equal to 180°, by taking 02 = 180°, which corresponds to (cos ft = -1), Equation (676) then becomes... 

The modified Cassie-Baxter equation was successfully used to interpret some of the contact angle data reported for heterogeneous surfaces. 

The situation where the hquid does not penetrate into the spaces created by the surface roughness is termed the Cassie-Baxter regime. In this case the hquid is suspended at the tops of the surface pillars, the air-water interface is supported... 

Polymer honeycomb films can be also used as templates to produce micropattemed inorganic/organic structures by external deposition of NPs or chemical vacuum deposition CVD. The selective assembly of the Si02 NPs into the pores is strongly impacted by the wettability and thus the Cassie-Baxter/ Wenzel transition was demonstrated as the key factor of such an approach [182]. Cadmium tellurium quantum dots were deposited onto polystyrene honeycomb film leading to red emitting films [183]. Recently, surface-enhanced Raman Scattering SERS substrates were also prepared by Ag NPs adsorption onto poly (AW-dimethylaminoethyl methacrylate) PDMAEMA surface [184]. 

A treatment similar to Wenzel s has been given by Cassie and Baxter [69] for chemically heterogeneous surfaces. For the case of a surface consisting of two types of patches, for which the intrinsic contact angles are 0] and 02, respectively, minimization of the system free energy leads to the Cassie-Baxter equation ... 

Figure 2. AmpUflcation of hydrophobicity due to surface roughness. In the Wenzel model, the liquid confonnaUy follows the surface topography (a), while in the Cassie-Baxter model, the air remains in the lower regions of the topographic features (b). In (a), r represents the ratio of the actual surface area of the rough substrate to the nominal surface area. In (b), fg is the sohd fraction that is in contact with the liquid under the droplet.

Wetting, Lotus effect, super-hydrophobicity, water repellence, contact angle, Wenzel equation, Cassie-Baxter equation, roU-off angle... 

Cassie-Baxter equations for the surface topographies shown in Fig. 1 are developed and listed. 

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