Transition wetting

Figure B3.6.3. Sketch of the coarse-grained description of a binary blend in contact with a wall, (a) Composition profile at the wall, (b) Effective interaction g(l) between the interface and the wall. The different potentials correspond to complete wettmg, a first-order wetting transition and the non-wet state (from above to below). In case of a second-order transition there is no double-well structure close to the transition, but g(l) exhibits a single minimum which moves to larger distances as the wetting transition temperature is approached from below, (c) Temperature dependence of the thickness / of the enriclnnent layer at the wall. The jump of the layer thickness indicates a first-order wetting transition. In the case of a conthuious transition the layer thickness would diverge continuously upon approaching from below.

The mean field treatment of such a model has been presented by Forgacs et al. [172]. They have considered the particular problem of the effects of surface heterogeneity on the order of wetting transition. Using the replica trick and assuming a Gaussian distribution of 8 Vq with the variance A (A/kT < 1), they found that the prewetting transition critical point is a function of A and... 

For the system studied in [174], it turns out that the oil/water interface is not wetted by the microemulsion, even though the latter is weakly structured. Hence fluctuations do shift the wetting transition beyond the disorder... 

D. E. Sulhvan, M. M. Telo da Gama. Wetting transition and multilayer adsorption at fluid interfaces. In C. A. Croxton, ed. Fluid Interfaeial Phenomena. New York Wiley, 1986. 

This approximation amounts to truncating the functional expansion of the excess free energy at second order in the density profile. This approach is accurate for Lennard-Jones fluids under some conditions, but has fallen out of favor because it is not capable of describing wetting transitions and coexisting liquid-vapor phases [105-107]. Incidentally, this approximation is identical to the hypemetted chain closure to the wall-OZ equation [103]. 

So far we only have considered adsorption phenomena in the submonolayer range. It is well known, however, that for certain substrate-adsorbate partners it is possible to observe multilayer adsorption phenomena or adsorption of fluid films which may undergo wetting transitions from a microscopic to a macroscopic thickness. 

Fig. 28. Typical Monte Carlo equilibrium arrangements of atoms adsorb on the center rectangular lattice illustrating the wetting transition A C->A B C. Systems of sizes L X Af = 20 X 60 are used at (a) ktTI J = A, /f/ Jj = 2.8 (b) k,r/ J, = 1.4, fl/l JjI = 2.3 and (c) k,TI J = 2.8. The solid lines show the borders of the A and C domains. Case (c) is rather close to the transition to the disordered phase.

George T, Ladha IK, Garrity DP, Buresh RJ. 1994. Legumes as nitrate catch crops during the dry-to-wet transition in lowland rice cropping systems. Agronomy Journal 86 267-273. 

The energy of adhesion between hexadecane droplets stabilized in water by SDS in the presence of NaCl is shown in Fig. 2.35. It is observed that the adhesion depends strongly on the temperature and on the salt concentration. For a given salt concentration, there is a well defined temperature, T, above which there is no adhesion. As the behavior of the surface energy changes at T, this temperature can be referred to as a wetting transition temperature [109]. The dependence of T versns the salt concentration is plotted on Fig. 2.36. 

Rgure 2.39. Evolution of the wetting transition temperature T (c) for macroscopic interfaces and gelation temperature Tg(c) of emulsions of various droplet sizes. (Adapted from [113].)... 

D. Chatain and P. Wynblatt, Experimental Evidence for a Wetting Transition in Liquid Ga-Pb Alloys,... 

Grunwaldt et al. (2000) XAS Cu/ZnO Wetting transition during chemical potential change, structural dynamics + + + Methanol synthesis... 

In this section we return to the situation sketched in Fig. Id, namely a polymer mixture (A,B) confined between two different planar surfaces, such that the lower surface prefers species A, and the upper surface prefers species B. We are interested in the temperature region in between the critical temperature of phase separation in the bulk and the wetting transition temperatures of the two surfaces (i.e., yw < % < in Fig 6 in principle, one must for film thicknesses D—... 

Attempts to improve wetting by non-reactive pure metals such as Cu by the addition of ferrous metals such as Ni or Fe have been unsuccessful (Naidich 1981). In contrast, improvements in wetting have been achieved by adding carbide-forming elements such as Cr or Ti. Additions of Cr to Cu above a critical value of the molar fraction of Cr, XCr, produce a sharp wetting transition (Figure 8.10) owing to the formation of a continuous layer of wettable Cr carbides in accord with the reaction ... 

Ti additions to Cu and also to Sn, Ag (Naidich and Kolesnichenko 1968), Ga (Naidich and Chuvashov 1983) or Ni-Pd (Kritsalis et al. 1991) also produce a nonwetting/wetting transition due to the formation of a wettable reaction product (TiC). Note that although there is some uncertainty about the wettability of TiC by pure non-reactive metals, it is clearly established that TiC is wetted by these metals when they contain even low concentrations of Ti leading to the formation of hypostoichiometric TiC at the interface (see Section 7.2, Figure 7.32). An example... 

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