Thin hquid film

J. A.F. Plateau, who first studied their properties. It is the Plateau borders, rather than the thin Hquid films, which are apparent in the polyhedral foam shown toward the top of Figure 1. Lines formed by the Plateau borders of intersecting films themselves intersect at a vertex here mechanical constraints imply that the only stable vertex is the one made from four borders. The angle between intersecting borders is the tetrahedral angle,... 

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian Hquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin Hquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a large number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the lUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

Before determining the degree of stabiUty of an emulsion and the reason for this stabiUty, the mechanisms of its destabilization should be considered. When an emulsion starts to separate, an oil layer appears on top, and an aqueous layer appears on the bottom. This separation is the final state of the destabilization of the emulsion the initial two processes are called flocculation and coalescence (Fig. 5). In flocculation, two droplets become attached to each other but are stiU separated by a thin film of the Hquid. When more droplets are added, an aggregate is formed, ia which the iadividual droplets cluster but retain the thin Hquid films between them, as ia Figure 5a. The emulsifier molecules remain at the surface of the iadividual droplets duiing this process, as iadicated ia Figure 6. 

In the coalescence step, the thin Hquid film between the droplets is destabilized, and a large droplet is formed. Hence, the coalesciag emulsion is characterized by a wide size distribution of the droplets, but no clusters are present (Fig. 5b). FiaaHy, the droplets achieve such a size that they are recognized by the naked eye as a separate phase. A fully separated emulsion consists of an oil layer and an aqueous layer. 

Liquid crystals stabilize in several ways. The lamellar stmcture leads to a strong reduction of the van der Waals forces during the coalescence step. The mathematical treatment of this problem is fairly complex (28). A diagram of the van der Waals potential (Fig. 15) illustrates the phenomenon (29). Without the Hquid crystalline phase, coalescence takes place over a thin Hquid film in a distance range, where the slope of the van der Waals potential is steep, ie, there is a large van der Waals force. With the Hquid crystal present, coalescence takes place over a thick film and the slope of the van der Waals potential is small. In addition, the Hquid crystal is highly viscous, and two droplets separated by a viscous film of Hquid crystal with only a small compressive force exhibit stabiHty against coalescence. Finally, the network of Hquid crystalline leaflets (30) hinders the free mobiHty of the emulsion droplets. 

Thomas and Rice [/. Appl. Mech., 40, 321-325 (1973)] applied the hydrogen-bubble technique for velocity measurements in thin hquid films. DureUi and Norgard [Exp. Mech., 12,169-177 (1972)] compare the flow birefringence and hydrogen-bubble techniques. 

In open tubular colvunns, because the thin hquid film is deposited directly on the wall of the column rather than the sohd supports, the A term is zero, therefore ehminating one of the major contributor to zone broadening. Comparing to packed columns, the resistance to mass transfer is also reduced in both the hquid phase due to the apphcation of very thin film of the stationary phase, and in the mobile phase due to the apphcation of very narrow internal diameter columns. The typical open tubular colmnns have an internal diameter of 0.25 mm and a film thickness of 0.25 pm. A combination of ah these factors makes for the fact that capillary GC columns have much lower plate height value and substantiaUy more theoretical plates. The effect of carrier gas and hnear velocity on capihary column efficiency is illustrated in Figure 4, which shows a family of van Deemter plots for common carrier gases. 

An important advantage of the ATR technique is its applicability to turbid solutions, aqueous solutions included. Suspended particles are surrounded by a thin Hquid film (hydrating shell). This shell forms also the phase boundary to the ATR crystal surface so that the evanescent field will not scattered by the particle. 

Such evaporation singularities can be very easily observed because the thickness profile of a thin hquid film locally reflects the rate of solvent removal. For very... 

Use the technique for obtaining an IR spectrum of a thin-Hquid film as described on page 553 (your instructor will provide you with the experimental... 

Despite these complications, disjoining pressure isotherms such as that shown in Figure 1.12 offer an explanation for much of the behavior of the thin hquid films, formed by aqueous solutions of simple surfactants, subject to an applied capillary pressure. Consider, for example, the regions in those isotherms where < 0. This... 

Kralchevsky, P.A., Diniitrov, K., Ivanov, LB. Thin hquid film physics, in Foams Theory, Measurements and Applications (Pnid homme, R.K., Khan, S.A., eds.), Marcel Dekker, New York, Surfactant Sci. Series, 1996, Vol 57, Chpt 1, p 1. 

A very common column configuration in elution chromatography is simply a tuhular column packed with porous particles, the packings, with or without a bonded liquid phase on the particle surfiices. Other column configurations include capillary columns or open tubular columns, in which a thin hquid film of adsorbents has been applied (or bonded) to the internal surface of the capillaries. A potential variation of this is the microporous hollow fiber membrane based column, wherein the stationary phase is heid in the pores of fiber wall and the eluent is passed through the bore of the fiber (Ding et al., 1989). 

Two processes make it possible to place a liquid in circumstances such as it behaves as if it did not have weight. First process immersion of an olive oil mass in a mixture of water and alcohol of the same density as oil the immersed mass, when it is not adherent with any solid, always takes the sh q)e of a perfect sphere. Boyle and Segner approached this process. - Second process thin hquid films developed in the air the shapes which they affect coincide, without appreciable difference, with those of a full mass without gravity. 3... 

A series of studies of microscopic thin hquid films, both symmetric (foam and 0/Wemulsion films) and asymmetric (wetting films), from aqueous solutions of... 

Ultra-thin Hquid films of PDMS or poly(methyl-hydrodimethyl)siloxane on siHcon substrates were also prepared [50], The silicon wafers were cleaned by immersion in a freshly prepared piranha solution (a mixture of 70% sulfuric acid and 30% hydrogen peroxide), rinsing with deionized water, followed by etching in HF solution. Then the wafers were rinsed with deionized water and blown dry under a stream of nitrogen. The uniform thin films were spread by dipping the clean substrates in dilute solutions of silicone polymers in hexane and withdrawing the wafers at a constant speed. It was shown that solution concentration and withdrawal speed affected the final film thickness. Typical thickness was in the range of 3-8 nm. 

---