Contact angle Cassie-Baxter states

Table 1 lists the equilibrium values of 0h at different values of Co (i e. at different values of 9q) for = 0.1739 and 0e = 70°. Once % is known, equation (16) or (17) can be used to obtain the equilibrium value of. The first two columns of Table 1 specify the size of the spherical cavity. An interface will remain pinned at the top edge of the spherical cavity if 180° 0e 0o (see [31] for details). Thus, whenever this condition is satisfied the Cassie-Baxter state is possible in the cavities because the local contact angle condition is satisfied at the top edge of the spherical cavities. The corresponding value of Ou is equal to Oq. These values for 0 = 0o are listed in Table 1. For 0q > (= 70°), the local contact angle condition cannot...

To explore the entire energy landscape, Ecav could be plotted as a function of and f. The extrema on this landscape will be the equilibrium solutions, discussed above, of which some will be the stable solutions. In this work, we will select a probable path in this landscape. The values of Ecav will be plotted for this path. To this end, assume that the liquid-air interface is first in the Cassie-Baxter state. As the liquid-air interface moves toward the other equilibrium states, the value of 9u will increase until it reaches the next available equilibrium state given in Table 1. At each intermediate state, between the equilibrium states, it will be assumed that the interface is spherical, the gas pressure is such that it is in accordance with the ideal gas law (equation (16)) and that the interface is in mechanical equilibrium (equation (17)). Thus, equations (16) and (17) are satisfied, however, equation (18) is not. This implies that in the intermediate states the local contact angle condition... 

In this work we have identified the various possible states within the cavities and compared their energies. In case of the experiments of Abdelsalam et al. [15], the Cassie-Baxter state in the cavities (see Fig. 1) is possible only for (fo > 0.67. However, Abdelsalam et al. [15] report an apparent contact angle corresponding to the... 

Cassie-Baxter formula for all cases. The following conclusion is apparent from the comparison between theory and experiments in Fig. 6 even if the experimentally observed contact angle matches the Cassie-Baxter formula, it does not necessarily imply that the cavities below the drop are also in the Cassie-Baxter state (see Fig. 1). A possible scenario is discussed below. 

In light of the above discussion, the experimental data of Abdelsalam et al [15] at smaller values of i iay be considered. Initially, the deposited drop may trap air in the Cassie-Baxter state in the cavities. The advancing front of the drop attains the apparent contact angle corresponding to the Cassie-Baxter state. Since the... 

In addition to its influence on surface reactivity, surface structure is also seen to affect wettability on the micrometer scale, as is best illustrated by the lotus effect (see Chapter 3b). The lotus leaf is superhydrophobic, i.e. has a water contact angle of about 160°, thanks to the combination of the waxes on the surface with a characteristic dual micrometer- and nanometer-scale surface topography. Without the structure, the wax chemistry would only impart mild hydrophobicity to the surface. Superhydrophobicity comes about only when a water droplet is in contact with a rough surface with a substantial enclosure of air beneath the drop (Figure 9). This is the so-called Cassie-Baxter state, named after the authors of the work that described the contact angle of water droplets in this state by means of the equation ... 

If the liquid, on the other hand, sits on top of the surface features without penetrating the valleys , air will be enclosed between the droplet and the substrate and the liquid/air interface increases (cf [4,5]). In this Cassie-Baxter state , the solid/liquid interface approaches a minimum. The further increase of surface area (i.e. spreading of the droplet) is hindered for energetic reasons [5]. The apparent contact angle observed xmder these conditions is usually described by the simplified equation... 

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