Critical wetting

An adhesive should possess a Hquid surface tension that is less than the critical wetting tension of the adherend s surface. 

Fogging is formation of small water droplets (visible condensation) on the surface of a polymer film. Undesirable effects may result from fog formation, such as reduction of clarity and dripping. Incorporation of antifogging agents eliminates the reduction of transparency by migration to the surface and increases the polymer surface critical wetting tension. This results in... 

Lendormi, T., Prevot, C., Doppenberg, F., Foussard, J. N. and Debellefontaine, H. (2001) Sub-critical wet oxidation of municipal sewage sludge comparison of batch and continuous experiments. Water Sci. Technol. 44, 161-169. 

Fig. 18. Log-log plot of the parameter co = l7rwoG j > that controls the behavior of critical wetting [11,220,277,278], vs %/%crit -1, for symmetrical polymer mixtues (NA=NB=N) with chain lengths ranging from N=128 to N=1024, showing the predictions Eqs. (124)-(126). From Werner et al. [266]...

This method has been employed to measure the critical wetting surface tensions of particles of sulfur, silver iodide, methylated glass beads, quartz, paraffin-wax-coated coal, and surfactant-coated pyrite. Generally. Fuerstenau and coworkers [106-115] found that the film flotation technique is sensitive to the surface hydrophobicity and the heterogeneity of the particles. It was found that particle size, particle shape, particle density, film flotation time, and the nature of the wetting liquids have negligible effects on the results of film flotation. But the liquid and the solid particles used in the experiments must not have any chemical interactions. 

Fig. 14.a Composition-depth cf)(z) profiles near the surface (at z=0) of a binary mixture at bulk concentration bi at lower (T—>TW ) and upper (T—>TW+) limit of the first order wetting transition point b,c Cahn constructions with trajectories -2kV< ) plotted for profiles cf)(z) with decreasing (solid lines) and increasing (dashed lines) slopes. Surface boundary condition (Eq. 26) is met at points (marked by ) where surface energy derivative (-dfs/d< ))s (idotted line) intersects trajectories -2kV< > at concentrations reached at the surface. Cahn plot b corresponding to the first order transition depicted in a Cahn construction c typical for a critical wetting trajectories -2kV< > with larger extrema correspond to temperatures T[Pg.37]

In recent work Jerry and Dutta [176] reanalyzed, with the mean field approach, conditions [8] of the second order wetting transition. They have found that critical wetting transition must be accompanied by a prerequisite phenomenon of an enrichment-depletion duality it is expected that the surface is enriched in the given component when bulk composition ( )M is below a certain value Q and is depleted in the same component for >Q. Such an effect, easily predicted by simple lattice theory [177] and observed in Monte Carlo simulations [178, 179], has been very recently determined by us for a real polymer blend [175] (see Sect. 3.1.2.4). 

Henderson, J.R. (1987). Three-dimensional critical wetting and the statistical mechanics of fluids with short-range forces. Mol. Phys., 62, 829—42. 

Fig. 51. Plol of — t)(iiZ)/i)Z z=u versus /j(0) for the cases of a second-order wetting transition (a) and a first-order wetting transition (b). The solution consistent with the boundary condition always is found by intersection of the curve /x2(0) - 1 with the straight line [h + gq.(0)]/y. In case (a) this solution is unique for all choices of k /y (keeping the order parameter g/y fixed). Critical wetting occurs for the case where the solution (denoted by a dot) occurs for y (0) = +1, where then ) i(Z)/dZ z u = I) and hence the interface is an infinite distance away from the surface. For ft] > ftlc the surface is non-wet while for fti > ft C the surface is wet. In case (b) the solution is unique for ft] < ft (only a non-wet state of the surface occurs) and for ft > ft (only a wet state of the surface occurs). For ft > ft] > ft three intersections (denoted by A, B, C in the figure) occur, B being always unstable, while A is stable and C metastable for ft]c > ft > ft and A is metastable and C stable for ft " > ft > ftic. At ftic where the exchange of stability between A and C occurs (i.e., the first-order wetting transition) the shaded areas in fig. 51b are equal. This construction is the surface counterpart of the Maxwell-type construction of the first-order transition in the bulk lsing model (cf. fig. 37). From Schmidt and Binder (1987).

One can show that the second-order wetting transition is characterized by a divergence of the susceptibility xi > in this mean field theory of critical wetting... 

Fig. 32. Phase diagram of the surface plotted in terms of the scaled variables h jy and g/y. For g/y < —2 one observes critical wetting and for g/y > —2 one observes first-order wetting. In the latter regime, mean field theory predicts melaslable wet and non-wet regions limited by the two surface spinodal lines ft, and ft( respectively. Also two quenching experiments arc indicated where starting at a rescaled time r = 0 from a stable state in the non-wet region one suddenly brings the system by a change of fti into the metastahle wet or unstable non-wet region, respectively. From Schmidt and Binder (1987).

Figure 10-5. Effect of concentration on critical wetting temperature for oleophobic films on platinum. (a) n-Octadecyl compounds in cetane. (b) n-Eiscosyl and n-octadecyl acids and alcohols in cetane. (c) n-Octadecyl acid and alcohol in cetane and in dicyclohexyl. Data by Bigelow, Glass and Zisman [181.

Critical Wetting Surface Tension See Critical Surface Tension of Wetting. 

Fig. 17 Swelling induced detachment of carboxylated poly(OEGMA-r-HEMA) brash anchored to a QCM chip via Au-S bond. The swelling behavior depends on the ionic strength and pH of the solution. The polyelectrolyte brash detaches from the substrate beyond a critical wet thickness...

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