Wenzel Cassie-Baxter transition

Antifogging Self-cleaning Superhydrophobicity Wenzel Cassie-Baxter transition Superoleophobicity Re-entrant angle Directional wetting Unidirectional wetting... 

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]. 

Before proceeding further we note that the problem of drops on top of rough surfaces with pillars or protrusions is different from the one in which rough surfaces have cavities. In case of rough substrates (made of hydrophilic materials) with pillars, a Cassie-Baxter state will typically transition to a Wenzel state by simply displacing the air which is part of the ambient, thus trapping of air is not possible. 

Note that is the change in energy corresponding to the transition from the Cassie-Baxter state to the Wenzel state. Since all air in both the Cassie-Baxter and Wenzel states is at the ambient conditions, there is no contribution to from the pressure terms in equation (14). The only contribution comes from the surface energy change. 

The Cassie-Baxter-Wenzel theory [44, 47,48] defines the critical contact angle value on the smooth surface above this value, the Cassie-Baxter model is more stable wetting state and below this value the Wenzel model is the most stable wetting state. If the measured contact angle on a smooth surface is lower than this critical value and if the superhydrophobic behaviour is observed, the transition between the two models should be possible like, for example, with the LDPE surface treated in both plasmas. In this case, the roughness factor is 1.043, the contact angle on the dried surface reaches a value of 171° and the contact angle on the same surface partially wetted with water vapour or dipped in water is only 140°. 

Bormashenko E, Bormashenko Y, Stein T, Whyman G, Bormashenko E. (2007) Why do pigeon feathers repel water Hydrophobicity of Pennae, Cassie—Baxter wetting hypothesis and Cassie-Wenzel capillarity-induced wetting transition. / Colloid Interf Sci 311 212-216. 

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