Films fluctuations

Surfactants also reduce the coalescence of emulsion droplets. The latter process occurs as a result of thinning and disruption of the liquid film between the droplets on their close approach. The latter causes surface fluctuations, which may increase in amplitude and the film may collapse at the thinnest part. This process is prevented by the presence of surfactants at the O/W interface, which reduce the fluctuations as a result of the Gibbs elasticity and/or interfacial viscosity. In addition, the strong repulsion between the surfactant layers (which could be electrostatic and/or steric) prevents close approach of the droplets, and this reduces any film fluctuations. In addition, surfactants may form multilayers at the O/W interface (lamellar liquid crystalline structures), and this prevents coalescence of the droplets. 

This refers to the process of thinning and disruption of the liquid film between the droplets, with the result that two or more droplets fuse into a larger droplet. The limiting case for coalescence is the complete separation of the emulsion into two distinct liquid phases. The driving force for coalescence is the surface or film fluctuations this results in a close approach of the droplets whereby the van der Waals forces are strong and prevent their separation. 

Interest in the dynamics of film fluctuations was prompted by the search for a theory of film stability. At that time (1966) no experiments on thermal excited surfaces waves had been performed. 

The first theory for film fluctuations resembles Cahn s theory of spinodal decomposition of unstable bulk systems. A very simple mechanism was adopted for the liquid flow. It was assumed that because of the presence of the soap monolayers, the film surfaces were stagnant (see Section 11) and that the film liquid was pumped back and forth through a slab with thickness h according to Reynolds s law ... 

One of the earliest methods for reducing coalescence is to use mixed surfactant films. These will increase the Gibbs elasticity and/or interfacial viscosity. Both effects reduce film fluctuations and, hence, reduce coalescence. In addition, mixed surfactant films are usually more condensed and hence diffusion of the surfactant molecules from the interface is greatly hindered. An alternative explanation for enhanced stability using surfactant mixture was introduced by Friberg and coworkers [67] who considered the formation of a three-dimensional association structure (liquid crystals) at the oil/water interface. These liquid crystalline structures prevent coalescence since one has to remove several surfactant layers before droplet-droplet contact may occur. 

The investigations of Stage and Bose [65] carried out in laboratoiy rectification columns show that the thickness of the wall film fluctuates and does not reach equilibrium state. The difference between the results of Weisman [64] and of Stage tmd Bose [65] is explained [66] with condensation of the vapour on the column wall. 

If rupture and coalescence take place (Figure 9.7c), they occur in an area of the adsorbed monolayer thinned out by the mechanical action of the approaching drops in stretching the monolayer as a result of deformation and increases in the interfacial area and/or of depletion of emulsifier due to desorption. Understanding the behavior of such lamellar films and the role of surfactants in their action requires an understanding of the forces involved in interactions across the film and the kinetic aspects of film fluctuations. 

A general prerequisite for the existence of a stable interface between two phases is that the free energy of formation of the interface be positive were it negative or zero, fluctuations would lead to complete dispersion of one phase in another. As implied, thermodynamics constitutes an important discipline within the general subject. It is one in which surface area joins the usual extensive quantities of mass and volume and in which surface tension and surface composition join the usual intensive quantities of pressure, temperature, and bulk composition. The thermodynamic functions of free energy, enthalpy and entropy can be defined for an interface as well as for a bulk portion of matter. Chapters II and ni are based on a rich history of thermodynamic studies of the liquid interface. The phase behavior of liquid films enters in Chapter IV, and the electrical potential and charge are added as thermodynamic variables in Chapter V. 

On compression, a gaseous phase may condense to a liquid-expanded, L phase via a first-order transition. This transition is difficult to study experimentally because of the small film pressures involved and the need to avoid any impurities [76,193]. There is ample evidence that the transition is clearly first-order there are discontinuities in v-a plots, a latent heat of vaporization associated with the transition and two coexisting phases can be seen. Also, fluctuations in the surface potential [194] in the two phase region indicate two-phase coexistence. The general situation is reminiscent of three-dimensional vapor-liquid condensation and can be treated by the two-dimensional van der Waals equation (Eq. Ill-104) [195] or statistical mechanical models [191]. 

Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

Nonintrusive Instrumentation. Essential to quantitatively enlarging fundamental descriptions of flow patterns and flow regimes are localized nonintmsive measurements. Early investigators used time-averaged pressure traverses for holdups, and pilot tubes for velocity measurements. In the 1990s investigators use laser-Doppler and hot film anemometers, conductivity probes, and optical fibers to capture time-averaged turbulent fluctuations (39). 

J. L. Harden, D. Andelman. Thermal fluctuations of thin wetting films on disordered solids. Langmuir 5 2547-2551, 1992. 

For diffuse and delocahzed interfaces one can still define a mathematical surface which in some way describes the film, for example by 0(r) = 0. A problem arises if one wants to compare the structure of microemulsion and of ordered phases within one formalism. The problem is caused by the topological fluctuations. As was shown, the Euler characteristic averaged over the surfaces, (x(0(r) = 0)), is different from the Euler characteristics of the average surface, x((0(r)) = 0), in the ordered phases. This difference is large in the lamellar phase, especially close to the transition to the microemulsion. x((0(r)) =0) is a natural quantity for the description of the structure of the ordered phases. For microemulsion, however, (0(r)) = 0 everywhere, and the only meaningful quantity is (x(0(r) = 0))-... 

In general, the requirements of heat resistance limit film thickness and therefore corrosion resistance. This is a particular problem when surfaces fluctuate between hot and cold. Coatings should be selected carefully, depending on the exact maximum temperature that will be experienced. Wherever possible, conventional materials should be used. The majority of air-oxidation coatings will be satisfactory up to 95°C and epoxies up to 175°C continuous dry heat. 

The irons are most useful in environments containing a plentiful supply of oxygen or oxidising agents anaerobic or reducing conditions may lead to rapid corrosion. Physical effects such as abrasion or sudden dimensional changes induced by temperature fluctuations may rupture the film and allow corrosion to take place. The iron will also be subject to corrosion by solutions containing anions, such as those of the halides, which can penetrate surface films relatively readily. 

Figure 6-14. Average domain size vs. inverse deposition temperature Tor different film thicknesses. Error bars represent the mean absolute error and straight lines the best lit for each film thickness. Doited line is the locus of the transition from grains to lamellae. Data for 50-nm films are estimated from the correlation length of the topography fluctuations. Adapted from Ref. [501.

Process contamination. Sticky films (LP boilers) Varnish (HP boilers) Steam discoloration FW pH fluctuations Acid corrosion Stable foams leading to carryover... 

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