Cassie-Baxter wetting model

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°. 

Superhydrophobicity, wetting of metals, hierarchical patterns, Cassie-Baxter model... 

Several frequentiy used models for wetting have been tested with the acquired data. The models investigated were the Cassie— Baxter approximation, the Furmidge equation for roll-off angles, and a proposition from Patankar for receding contact angles. 

Wenzel s equation applies to what is called homogeneous wetting, and it can only be applied to homogeneous, rough surfaces. Surfaces having heterogeneous character can be modeled with the Cassie-Baxter model [43] ... 

Surface Tension, Capillarity and Contact Angle, Rgute 8 Effect of surface stnictures on wetting behavior, (a) Wenzel s model where the liquid penetrates between the structures, (b) Cassie-Baxter modet where water does not wet the surface between the structures... 

The Young equation cannot be used directly to explain the effect of surface roughness on the wettability of a material because it is valid only for ideal smooth solid surfaces. There are two wetting models that are proposed when a water droplet sits on rough surfaces, these are the Wenzel model and the Cassie-Baxter model. 

Wettability of a rough surface can be described by the Wenzel [7] model if the surfaces are completely wetted by the liquid into the protrusions on the surface. The Cassie-Baxter [8,9] model gives an idea about wettability of rough hydrophobic surfaces. In the case of such surfaces, the air trapped into hierarchical roughness prevents water penetration into the surface protrusions. 

Figure 4.8 Schematics of a motion of the contact hne of a hquid droplet sitting on a solid surface, leading to a corresponding free energy variation for a smooth surface (Young model, a), complete penetration of the liquid into the recessed features of a pattern (Wenzel model, b), and for heterogeneous wetting regimes with absence of penetration (Cassie-Baxter model, c) and for partial penetration (d). In the Wenzel model, rfp = AB + BC + CD + DE)I AB + CD) according to the letters in (b). In the Cassie-Baxter model, (j) = (AB)I(AB + BE) according to the letters in (c).

In the heterogeneous wetting regime, air can instead be trapped inside the features underneath the liquid drop, which ideally stands on the top of the nanostructures. This situation (schematized in Figure 4.8c) is described by the Cassie-Baxter model. The free energy variation following the contact line displacement is ... 

In the context of the fimdamental relation between surface topography and wetting behavior as modeled by Wenzel or Cassie and Baxter one has to consider the relevance of geometrical parameters for the complex porous and textured textile substrate. It is of interest in this context that a paper by Hsieh et al. [26], who studied the wetting of water and ethylene... 

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