Contact angles thin-liquid films

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. 

In this work, microscale evaporation heat transfer and capillary phenomena for ultra thin liquid film area are presented. The interface shapes of curved liquid film in rectangular minichannel and in vicinity of liquid-vapor-solid contact line are determined by a numerical solution of simplified models as derived from Navier-Stokes equations. The local heat transfer is analyzed in term of conduction through liquid layer. The data of numerical calculation of local heat transfer in rectangular channel and for rivulet evaporation are presented. The experimental techniques are described which were used to measure the local heat transfer coefficients in rectangular minichannel and thermal contact angle for rivulet evaporation. A satisfactory agreement between the theory and experiments is obtained. 

An interesting approach for obviating the contact angle problem has been proposed by Padday and Pitt I The method ctm be applied for drops separated from the surface by a thin liquid film, as sketched in fig. 1.14. Depending on the densities of the two fluids, it can be used for surfaces emd interfaces, in the sessile or captive mode. The authors showed that for this situation... 

In conclusion, it should be noted that the width of the transition region between a thin liquid film and Plateau border is usually very small — below 1 pm. That is why the optical measurements of the meniscus profile give information about the thickness of the Plateau border in the region r > (Figure 5.16). Then, if the data are processed by means of the Laplace equation (Equation 5.101), one determines the contact angle, a, as discussed above. Despite that it is a purely macroscopic quantity, a characterizes the magnitude of the surface forces inside the thin liquid film, as implied by Equation 5.148. This has been pointed out by Derjaguin and Princen and Mason. ... 

Fig. 2D.5. Contact angle between the flat part and the Plateau border of a thin liquid film, a) defmition of jj, b) definition of 0,...

Beside direct disjoining pressure measurements an interferometric determination of this contact angle provides another way of examining thin liquid films. The Young equation applied here reads... 

The concept for the rupture of a free liquid film or a thin liquid film on a solid substrate is used in some applications, for example in the flotation process. Scheludko et al. (1968) have published first contact angle measurements for liquid films. 

Fig. 1 Rim formation in the course of de wetting of a thin liquid film. At the front position F of the rim, the contact angle assumes Its dynamic value and at the rear position R it takes the value .

The emulsion drops in floes and creams are separated with thin liquid films, whose rupture leads to eoalescence and phase separation. At equilibrium the area of the films and their contact angle are determined by the surface forces (disjoining pressure) acting across the films (Sec. lll.A.l). Several ways of breakage of these emulsion films have been established capillary-wave meehanism, pore-nucle-ation mechanism, solute-transport meehanism, barrier mechanism, etc. (Sec. 111.A.2). 

The design of an optimum interface is strongly dependent on the pore structure of the active layer of the gas diffusion electrode. According to Fig. 6, the liquid electrolyte has to penetrate into the pores and to wet the pores, so that a thin layer of electrolyte covers the pore wall (low contact angle). This electrolyte film should be as thin as possible to allow a short diffusion path for the reactant gases to exist. A high solubility of the reactant gases in the electrolyte film is also favourable. 

Huisman, H.F. and Mysels, K.J., Contact angle and the depth of the free-energy minimum in thin liquid films. Their measurement and interpretation, J. Phys. Chem., 73, 489, 1969. 

A thin liquid film lies on the solid surface which forms the floor of a narrow horizontal slit. Through the slit, air is blown at a steady rate. The air is seen to exert a constant shear stress on the liquid surface, thus the film thickness varies linearly with the distance from the leading edge, which is also the contact line (Derjaguin et al., 1944 Levich, 1962). Very close to the contact line the profile changes to retain the equilibrium contact angle at the contact line. The equations of motion and continuity under the lubrication theory approximation reduce to (Neogi, 1982) (see Problem 7.13)... 

Figure 6. Zoomed-in chronophotographs of the impact region, when a hydrophobic sphere (static contact angle 6>q 115°) is falling on an air-water interface at different impact velocities compared with the air entrainment threshold f/ (a) U = 2.4 m/s < f/ and (b) U = 5.0 m/s > f/. The thin liquid film that develops and rises along the sphere in both cases either gathers at the pole to encapsulate the sphere (low velocity), or is ejected from the sphere thus creating an air cavity behind it (high velocity).

The application area of surface and colloid science has increased dramatically during the past decades. For example, the major industrial areas have been soaps and detergents, emulsion technology, colloidal dispersions (suspensions, nanoparticles), wetting and contact angle, paper, cement, oil recovery (enhanced oil recovery [FOR] and shale oil/gas reservoir technology), pollution control, fogs, foams (thin liquid films), food industry, biomembranes, membranes, and pharmaceutical industry. 

The thin liquid films formed in foams or emulsions exist in permanent contact with the bulk liquid in the Plateau border, encircling the film. From a macroscopic viewpoint, the boundary between the film and Plateau border is treated as a three-phase contact line the line, at which the two surfaces of the Plateau border (the two concave menisci sketched in Fig. 16), intersect at the plane of the film—see the right-hand side of Fig. 16. The angle, 0o, subtended between the two meniscus surfaces represents the thin-film contact angle. 

In the case of secondary thin liquid films, stabilized by an ionic surfactant (h = I12 in Fig. 17a), the measured contact angles are considerably larger than the theoretical value predicted if only van der Waals attraction is taken into account [262]. The experimentally detected additional attraction in these very thin films (A <= 5 nm) can be attributed to short-range ionic correlation effects [263] as well as to the discreteness of the surface charge [2,261,341]. 

The modification of the fluid interfaces due to surfactant adsorption strongly influences the interactions between fluid particles (droplets, bubbles) in dispersions. Frequently a thin liquid film is formed in the zone of contact of two fluid particles. The contact angle at the periphery of such a film is a measure for the interaction of the two opposite surfactant adsorption monolayers. When the latter adhere to each other, a hysteresis of the contact angle is observed, irrespective of the fact that the fluid interfaces are molecularly smooth. The properties of the thin liquid films are important for the flocculation in dispersions and the deposition (attachment-detachment) of particles at surfaces see Sec. V. 

Lateral capillary forces also occur when the particles are partially immersed in a thin liquid film on a solid support (Figure 5.18) [590,595]. In this case, they are called immersion forces. Immersion forces on solid surfaces occur always when suspensions of solid particles are dried. In the last state of evaporation, the particles are only partially immersed in the thinning liquid film and attractive immersion forces lead to an aggregation of particles (Figure 5.19). Immersion forces can be used to self-assemble particles in two-dimensional arrays [595, 596]. The deformation of the liquid surface is related to the wetting properties of the particles, that is, to the position of the contact line and the contact angle rather than the weight. For this reason, also very small particles such as proteins are affected. 

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