Prewetting


The thermodynamics of Ae wetting transition as the critical point is approached has been treated for simple fluids [10-12] and polymer solutions [13], and the influence of vapor adsorption on wetting has been addressed [14]. Widom has modeled the prewetting transition with a van der Waals-like theory and studied the boundary tension between two coexisting surface phases [15]. The line tension at the three phase contact line may diverge as the wetting transition is approached [16]. The importance of the precursor film (see Section X-7A) on wetting behavior was first realized by Marmur and Lelah in 1980 in their discovery of the dependence of spreading rates on the size of the solid surface [17]. The existence of this film has since received considerable attention [1, 18, 19].  [c.466]

Screening is a process whereby particles are presented in an appropriate manner to a series of apertures of fixed dimensions that allow finer particles to pass through into the undersize, and coarser particles to be retained in the oversize. In addition to sizing, screens are used for desliming, ie, the removal of very small particles from much larger ones by draining or rinsing dewatering (qv), ie, the removal of water from particles by draining and prewetting, ie, the addition of water to the particles by spraying.  [c.435]

A variation is appHed where warps for denims are first dyed with sulfur black or blue and then with indigo. The sulfur dye in its reduced form is added to the prewetting or scouring bath. The yam is passed through and rinsed before passing to the part of the machine where the indigo is appHed by the traditional method of successive stages of impregnation and air oxidation. The sulfur dye improves the appearance and reduces the quantity of indigo required.  [c.171]

In all the above mentioned processes of coffee decaffeination, changes occur that affect the roast flavor development. These changes are caused by the prewetting step, the effects of extended (four hours plus) exposure at elevated temperature as required to economically extract the caffeine from whole green beans, and the post-decaffeination drying step.  [c.389]

Carbon should be prewetted prior to being placed in the test columns. Backwashing the carbon at low rates (2.5 m/hr) does not remove the air. Rates that would expand the bed 50 percent or 15-30 m/hr, are required. The liquid used for prewetting can either be water, if it is compatible with the liquid to be treated, or a batch of the liquid to be treated which has been purified previously. There are three types of carbon systems (1) fixed beds, (2) pulse beds, and (3) fluidized beds, and these can be used singly, in parallel, or in combination. The majority of systems are either fixed or pulse beds. The two basic types of adsorbers which can be designed to operate under pressure or at atmospheric pressure are the moving or pulse bed and the fixed bed. Either can be operated as packed or expanded beds.  [c.308]

In the case of bulk systems it has been established that the increase of the association energy results in a gradual increase of the critical temperature and slight widening of the coexistence region. Plots of such phase diagrams for several systems can be found in Refs. 13-17,114. On the other hand, the effects of association on adsorption have received less attention. In particular, to our best knowledge, the prewetting transition [49] in such systems has been studied only in Refs. 40,41. Namely, in Ref. 41, results from the density functional theory concerning the influence of association on wettability of solid surfaces have been reported. We discuss here some of the results obtained in Ref. 40.  [c.219]

Since the prewetting transition may occur only for weakly attractive surfaces [146], we must choose an appropriate value for the parameter This value has been set as follows e = 3 /3/2 j = 6. This corresponds to a relative strength of the fluid-fluid and fluid-surface potential minima close to that for Ar in contact with sohd carbon dioxide [147]. The second parameter in Eq. (144), zq, was set to 0.8[c.219]

We observe that association leads to pronounced differences in the behavior of the system. In particular, we find that for the thick film the increase of the association energy leads to the increase of the height of the first local density peak. In contrast, the height of the local density maximum decreases at the region of small adsorption (below the prewetting transition). One can say that for increasing association energy the film structure becomes more ordered and its layered structure is more visible.  [c.220]

There are two characteristic temperatures the wetting temperature, T% and the surface critical temperature 7%. Below the wetting temperature, the adsorption remains finite up to a density equal to the bulk coexistence density. In the case of the existence of the first-order prewetting, there is a jump discontinuity in the adsorption isotherms at temperatures between T% and T c- The first-order prewetting transition terminates at the surface critical temperature above this temperature the thick film development is continuous. The surface phase diagrams have been evaluated for different values of the association energy [40]. These calculations indicate that the main effect of association is the shift of the wetting temperature towards higher values.  [c.220]

In the case of simple fluids, the wetting behavior of fluid-solid interfaces has been discussed in detail [49,158]. The most interesting issue is the determination of state conditions at which the adsorbed film undergoes a surface wetting transition. In the case of the first-order wetting transition the prewetting line lies very close to the bulk gas-liquid coexistence fine, which makes simulational studies difficult. The most convenient simulational technique applied to study the wetting behavior is the method of isobaric-isothermal ensemble, introduced by Finn and Monson [159,160]. The wetting of nonassociating fluids has been also studied using molecular dynamics simulations [161].  [c.229]

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  [c.279]

Thus, an increase in the distribution width (A) leads to a decrease in the critical temperature of the prewetting point. Besides, under the simplifying assumption that the heterogeneity effects are directly felt only by the particles from the first layer, they found that the wetting transition, which is first order in a pure system, becomes continuous in the presence of randomly distributed heterogeneities. This, of course, occurs only when the heterogeneity becomes strong enough. Infinitesimal heterogeneity does not change the character of the wetting transition, according to the mean-field predictions at least.  [c.279]

Nicolaides D and Evans R 1989 Nature of the prewetting critical-point Phys. Rev.L 63 778-81  [c.2286]

Thermosetting Plastics. Thermosets ate processed only once, using heat and pressure to form semifinished or finished articles. The coloration of these plastics is generally accompHshed using paste color dispersions. The colorants are worked into Hquid resins before curing. Ball mills are commonly used to color prewetted mol ding powders prior to hardening. Dispersions for coloring epoxies are requited to be free of water because the presence of water affects the curing role and hardeners of the resin. The dark color of phenoHc resins restricts the coloration of these resins. Pigment dispersions used for thermosetting plastics are typically inorganic browns, reds, and greens. Coloration of unsaturated polyester and multiacrylic resin is performed with pigment—plasticizer (DOP) pastes. Occasionally, the pigments are directly dispersed into a small amount of monomer.  [c.515]

Fig. 17 shows the adsorption isotherms of all (undimerized and dimerized) particles. Except for a very fast increase of adsorption connected with filling of the first adlayer, the adsorption isotherm for the system A3 is quite smooth. The step at p/k T 0.28 corresponds to building up of the multilayer structure. The most significant change in the shape of the adsorption isotherm for the system 10, in comparison with the system A3, is the presence of a jump discontinuity at p/k T = 0.0099. Inspection of the density profiles attributes this jump to the prewetting transition in the  [c.230]

The results provide new insight into the behavior of assoeiating fluids near a solid surfaee. The most interesting finding is the observation of the prewetting transition due to assoeiative forees. The prewetting transition does not lead to signifieant ehanges of the preferable orientations of adsorbed dimers. Of eourse, the moleeules loeated in higher layers are eon-siderably less ordered, but they remain preferentially oriented in the same manner in the first, in the seeond, as well as in the third layer.  [c.233]


See pages that mention the term Prewetting : [c.2267]    [c.404]    [c.233]    [c.278]   
Computational methods in surface and colloid science (2000) -- [ c.219 , c.220 , c.221 , c.222 , c.223 , c.224 , c.225 , c.226 , c.227 , c.230 , c.233 , c.278 ]