Wetting in the Cassie-Baxter State

Wetting in the Cassie-Baxter State 4.3.1 The Lotus Effect... 

Figure 6. This graph shows the results of the uncoated and coated tapered SU8 pillars. The figure is separated in two different sections. The first block on the left side shows the CA values on the uncoated SU8 pillars (probe). The second block shows the SU8 pillars coated with Au and ODT Additionally, because of the different wetting states achieved here, the uncoated SU8 structures are labeled W for the Wenzel-state and CB for the Cassie-Baxter-state.

Figure 12.6 shows a water droplet in a heterogeneous wetting state. The liquid in this state only contacts the top of the protrusions, leaving air trapped into the grooves [3]. This wetting state is called the Cassie-Baxter state. It is described by the following equation ... 

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°. 

For the nanoparticle surfaces, a similar mixed wetting state as described above is assumed [11, 17]. Only on silica sphere arrays decorated by gold nanoparticles we observe CA > 150°, reduced hysteresis and SA < 5°, which are characteristic for superhydrophobic substrates. Although a detailed characterization of the wetting mechanism on these hierarchical surfaces lies outside the scope of this work, we assume that the droplets on the substrates with hierarchical roughness are neither in the Wenzel nor Cassie-Baxter state. Most likely they reside in a mixed state as presented by... 

A schematic of a drop partially wetting a solid surface in the case of a solid in the Young, Wenzel, and Cassie-Baxter states is shown in Figs. 10.1a, 10.1b, and 10.1c, respectively. 

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