Wenzel regime

Patterned SiCOH surfaces exhibited liquid penetration (Wenzel regime), resulting in the expected increase of contact angle with pattern depth. 

Single surfactant solutions have been studied at SH surfaces as a function of concentration and the presence of salts, with a resulting switch from the Cassie-Baxter to the Wenzel regime which is a required feature for many technological processes [4]. 

FIGURE 7.2 Wetting of a liquid drop on a textured surface, (a) Wenzel regime, (b) Cassie-Baxter regime. 

The homogeneous wetting regime is accurately described by the classical theory of Wenzel, which relates the measured angle, 0r = 0 with the Young angle, 0 ... 

SLA, the LLR functionalized with a pure CHj-terminated SAM shows extremely high dynamic and static contact angles, an indication of Cassie-type wetting. The papillae are able to support the drop, even in the absence of the tubular wax of the original leaf. As soon as there is a hydrophilic contribution present in the SAM, Wenzel-type wetting occurs. Note the high standard deviations, which arise from the fact that in some measurements, the Cassie state still persisted, whereas in others, the drop penetrated into the structures. Both types of behavior were sometimes even observed on the same sample, which is an indication of the presence of metastable states. An explanation for this metastable Cassie regime... 

It becomes clear that, contrary to the predictions of Wenzel s law, we cannot induce total wetting 6 = 0) simply by means of a surface texture. Since we are in a partial wetting regime Oe 0), there will always remain islands that emerge above the absorbed film. 

Stability of the Cassie wetting regime and the Cassie-Wenzel transition have been extensively studied in recent years. 

Application of various external stimuli, such as pressure or vibration, could convert Cassie wetting state into the Wenzel one. " It has been accepted for a long time that the Cassie wetting regime corresponds to a higher energetic state, and the Cassie-Wenzel transition is irreversible. Acmally the situation is more complicated... 

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).

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