Contact angle metastable

FIG. 6.11 Schematic energy diagram for metastable states corresponding to different contact angles. 

As a general rule, metastable states are predicted for patched surfaces. For such surfaces, Horsthemke and Schroder (1981) proposed the following expressions for advancing and receding contact angles ... 

In the metastable configuration, the reaction product layer does not extend beyond the edge of the drop (Figure 2.24.b). As a consequence, the final contact angle 0F, calculated by considering the effect of a small displacement of the triple line around the equilibrium position, is ... 

Whatever the composition of the Ni-Si alloy, the adsorption of Ni, r, at the alloy/SiC interface is positive, corresponding to YNi/XNi values of 1.5 to 2. Enrichment of the interface in Ni indicates that interactions between Ni and SiC at the interface are stronger than those between Si and SiC. The work of adhesion of pure Ni on SiC in metastable equilibrium (i.e., for a supposed non-reactive Ni/SiC system), evaluated in Appendix I, is W 1 = 3.17 J/m2. This value is reported in Figure 7.6 along with the corresponding values of work of immersion and contact angle. 

In this equation, Qm is the molar surface area, m i is a structural parameter defined in Section 1.1 (see Figure 1.3) and A is the regular solution parameter of Ni-Si alloy defined by equation (4.3). From the slope of the osL(XNi) curve for XNi— 0, the adsorption energy is found to be E i,(f ) = —8.2 kJ/mole. Thus, in equations (1.2), all the quantities are known (or can be easily estimated), except W and Wf 1 which represent respectively the work of adhesion and the work of immersion of pure liquid Ni in metastable equilibrium with SiC (i.e., for a supposed non-reactive pure Ni/SiC system). The values deduced from equation (1.2) are Wj4 = 3.17 J/m2 and W = —1.35 J/m2 for pure Ni. They are reported in Figure 7.6 along with the corresponding value of contact angle. 

Hence, regardless of the accuracy of the technique employed, a range of NaCl concentration (0.2 to 0.334 mol dm"3) is distinguished in which the 9 Cei) curves follow a different course. This is due to the metastable state of CBF. In this concentration range the microscopic CBF transform into NBF after certain time. Fig. 3.63 illustrates the contact angles of NBF in the metastable area, limited by the dashed lines. The values of contact angles obtained with microscopic NBF [323] are close to those obtained with macroscopic NBF [252], The slight difference is most probably due to the different surfactant concentration in the initial solution. 

The transition from one black film type to another for films from non-ionic surfactants does not involve a sharp change in contact angles and in this case no region of metastability is... 

Fig. 2. A simplified drawing of the Gibbs energy curve for a real wetting system. Each minimum defines a metastable state. The lowest minimum defines the most stable state and the most stable, apparent contact angle (MSAPCA). The lowest and highest APCAs that are associated with a metastable state are the receding contact angle (RCA) and the advancing contact angle (ADCA), respectively.

The phenomenon of contact angle hysteresis can be caused by surface roughness or chemical surface heterogeneity. The free energies involved are greater than kT (otherwise no metastable states could exist) or in case of vibrations greater than the vibration energy [67]. 

Most liquid films which are formed during film-coating are metastable or instable. This especially involves coating with aqueous suspensions. With ceramic membrane support coatings with a wet coating as thin as 1000 nm, instability can already occur at relatively low contact angles. 

Wetting hysteresis (or contact angle hysteresis) is defined as the ability of a liquid to form on a solid surface several stable (or metastable) contact... 

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