Thin liquid film

Chapter 5 (Section 6) described the conditions when a thin liquid film could become unstable and rupture and thereby cause coalescence of bubbles or drops. Instability was possible when the film became thin enough (less than 100 nm) for the disjoining pressure effects to be significant. However, considraable time may be required for the film to drain to this thickness, so that the rate of drainage has an important influence on the coalescence rate. The literatnre on expmmental and theoretical aspects of thin film drainage is extensive (Exerowa and Kruglykov, 1998 Ivanov and Dmitrov, 1988). 

More than a century ago Reynolds (1886) solved for the rate of decrease in the thickness h of the liquid layCT separating two solid, parallel disks bring pushed toward each other. His analysis is readily adapted to the case of a small circular film of radius R with sufficient surfactant present to eliminate lateral flow at the film surfaces  

Experiments on drainage of individual films ean also be used to estimate A (Sheludko, 1967). In aqueous films with high eleetrolyte contents and in films of organic liquids, H, is neghgible and H f/t) can be obtained from Equation 7.80 and data on the time dependence of h. The results confirm that H j varies inversely with (cf. Equation 5.111) and provide values of A.  

If the adsorbed surfactant is unable to completely prevent lateral motion at the film surfaces, it is still possible to obtain an analytical solution for the rate of drainage of a film of uniform thickness. The most recent of these analyses (Singh et al., 1996), which employs an improved boundary condition on the surface velocity at the film perimeter based on results of numerical simulations of drainage of nonuniform films, yields the following expression for the ratio V  

If the surfactant film is coherent and has a high surface shear viscosity e, the film will continue to drain while maintaining its axisymmetric shape, although 

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Thus it is necessary to find alternative approach to describe the physical mechanism of two-side filling of conical capillaries with hquids. Theoretical model of film flow in conical dead-end capillary is based on the concept of disjoining pressure II in thin liquid film [13]... 

In the context of the structural perturbations at fluid-solid interfaces, it is interesting to investigate the viscosity of thin liquid films. Eaily work on thin-film viscosity by Deijaguin and co-workers used a blow off technique to cause a liquid film to thin. This work showed elevated viscosities for some materials [98] and thin film viscosities lower than the bulk for others [99, 100]. Some controversial issues were raised particularly regarding surface roughness and contact angles in the experiments [101-103]. Entirely different types of data on clays caused Low [104] to conclude that the viscosity of interlayer water in clays is greater than that of bulk water. 

Demirel A L and Graniok S 1996 Relaxations in moleoularly thin liquid films J. Phys. Condens. Matters 9537-9... 

Figure 6.15 The infrared vibrational spectrum of crotonaldehyde. The parts marked (a), (b) and (c) refer to a 10 per cent (by volume) solution in CCI4, a 1 per cent solution in CCI4, and a thin liquid film, respectively. [Reproduced, with permission, from Bowles, A. J., George, W. O. and Maddams, W. F J. Chem. Soc. (B), 810, 1969]...

J. N. Israelachvili, P. M. McGuiggan, A. M. Homola. Dynamic properties of molecularly thin liquid films. Science 240 189-191, 1988. 

M. O. Robbins, D. Andelman, J.-F. Joanny. Thin liquid films on rough or heterogeneous solids. Phys Rev A 45 4344—4354, 1991. 

The theory of seaweed formation does not only apply to solidification processes but in fact to the completely different phenomenon of a wettingdewetting transition. To be precise, this applies to the so-called partial wetting scenario, where a thin liquid film may coexist with a dry surface on the same substrate. These equations are equivalent to the one-sided model of diffusional growth with an effective diffusion coefficient which depends on the viscosity and on the thermodynamical properties of the thin film. 

Foam formation in a boiler is primarily a surface active phenomena, whereby a discontinuous gaseous phase of steam, carbon dioxide, and other gas bubbles is dispersed in a continuous liquid phase of BW. Because the largest component of the foam is usually gas, the bubbles generally are separated only by a thin, liquid film composed of several layers of molecules that can slide over each other to provide considerable elasticity. Foaming occurs when these bubbles arrive at a steam-water interface at a rate faster than that at which they can collapse or decay into steam vapor. 

In order to develop the above burn-out mechanism further, it will be necessary to know more about the entrainment and deposition processes occurring. Experimentally, it is likely that these processes will be very difficult to measure separately and under conditions comparable to those prevailing in a boiling channel. From analysis of their film flow-rate data, Staniforth et al. (S8) have deduced that under burn-out conditions, the deposition of liquid droplets from the vapor core plays an important part in reinforcing the liquid film, particularly at high mass velocities. At low mass velocities, they conclude that deposition and entrainment rates must be nearly equal, and, therefore, since a thin liquid film can be expected to be tenacious and give rise to very little entrainment, they argue that both deposition and entrainment tend to zero near the burn-out location with low mass velocities. 

The effect of surface contamination and the wettability between the tube wall and the fluids were also studied experimentally. It has been shown that a stable annular flow and gas slug formation with a stable thin liquid film formed between the tube wall and gas slugs, which appeared at high velocities under carefully treated, clean... 

At a given flow condition, different flow patterns were observed which can be classified into five distinct patterns depending on the interfacial configuration liquid alone (or liquid slug), gas core with a smooth thin liquid film, gas core with a smooth thick liquid film, gas core with a ring-shaped liquid film, and gas core with a deformed interface. 

With increasing superficial gas velocity the gas core with a thin liquid film was observed. The flow pattern, displayed in Fig. 5.14c (the second, third and fourth channels from the top), indicates that as the air velocity increased, the liquid droplets entrained in the gas core disappeared such that the flow became annular. 

Surface force apparatus has been applied successfully over the past years for measuring normal surface forces as a function of surface gap or film thickness. The results reveal, for example, that the normal forces acting on confined liquid composed of linear-chain molecules exhibit a periodic oscillation between the attractive and repulsive interactions as one surface continuously approaches to another, which is schematically shown in Fig. 19. The period of the oscillation corresponds precisely to the thickness of a molecular chain, and the oscillation amplitude increases exponentially as the film thickness decreases. This oscillatory solvation force originates from the formation of the layering structure in thin liquid films and the change of the ordered structure with the film thickness. The result provides a convincing example that the SFA can be an effective experimental tool to detect fundamental interactions between the surfaces when the gap decreases to nanometre scale. 

Berman, A., and Israelachvili, J. N., "Surface Forces and Microrheology of Molecularly Thin Liquid Films, Handbook of Micro/Nanotribology, 2nd ed., B. Bhushan, Ed., CRC Press, Boca Raton, FL, 1999. 

An interesting phenomenon that some gas microbubbles emerged in the thin liquid film in a nanogap under an EEF was observed by Luo s group [78-80]. They have investigated the influence factors on and the mechanism of the emergence of these micro-bubbles. 

Matsuoka, H., andKato, T., "An Ultral-thin Liquid Film Lubrication Theory—Calculation Method of Solvation Pressure and Its Application to the EHL Problem, Trans. ASME, J. Tribol, Vol. 119,1997, pp. 217-226. 

Pressure between solid walls separated by ultra-thin liquid film is then expressed as follows ... 

Thin liquid films on a fluid surface were also employed for the construction of protein arrays [40]. The construction of a tightly chemically bound protein monolayer onto a solid support required detailed systematic study involving careful optimization of reaction conditions and comparison of the efficacy of several alternatives [46]. 

P 12] A falling film micro reactor was applied for generating thin liquid films [6]. A reaction plate with 32 micro channels of channel width, depth and length of 600 pm, 300 pm and 66 mm, respectively, was used. Reaction plates made of pure nickel and iron were employed. The micro device was equipped with a quartz window transparent for the wavelength desired. A 1000 W xenon lamp was located in front of the window. The spectrum provided ranges from 190 to 2500 nm the maximum intensity of the lamp is given at about 800 nm. 

When a thin liquid film with a thickness of approximately 2 pm prepared by spin coating of a 15% benzene solution of polymer 1 was irradiated with a 500-W Xe-Hg lamp for 300 s in air, a transparent solid film was obtained. The UV spectrum of this solid film shows that an absorption at 235 nm due to phenyldisilanyl units vanishes after UV-irradiation (Figure 1). This clearly indicates that photolytic cleavage of silicon-silicon bonds leading to the cross-linking occurred. Similar photolysis of the thin liquid films under a nitrogen atmosphere again afforded transparent solid films whose UV spectra show no absorption at 235 nm due to phenyldisilanyl units. 

We have investigated the photochemical behavior of polymer 1 under various conditions in air and found that UV irradiation of the thin liquid films with a thickness of less than 10 pm, indeed produced transparent solid films. However, when the films with a thickness of 100 pm were irradiated with a mercury lamp, cross-linking leading to the solid films occurred only on the surface of the films, but inside remained as liquid after prolonged irradiation. In thses cases, tha surface of the films was found to be slightly opaque. Therefore, most of the light would not be transmitted to the inside of the films. 

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