Thin films rupture

It is noteworthy that low concentrations of an ionic surfactant can increase emulsion stability as a result of the simultaneous manifestation of three mechanisms. First, the depth of the secondary potential minimum decreases owing to the electrostatic repulsion that is accompanied by a xj decrease. Second, the transition from the secondary minimum through an electrostatic barrier and into the primary minimum extends the coalescence time. Third, the time of true coalescence, i.e., the time necessary for thin-film rupture increases because of electrostatic repulsion as well (27, 63). 

Nonlinear dynamics and breakup of free-surface flows is rewieved in the recent paper by Eggers (1997). The thin film rupture is considered also by Ida and Miksis (1996) and these authors [Ida and Miksis (1995)] are considered also the dynamics of a lamella in a capillary tube. A wavy free-surface flow of a viscous film down a cylinder is considered by A.L. Frenkel (1993). 

R. Blossey, Thin film rupture and polymer flow, Phys. Chem. Chem. Phys., 10, 5177-5183 (2008). 

The speed of wetting has been measured by running a tape of material that is wetted either downward through the liquid-air interface, or upward through the interface. For a polyester tape and a glycerol-water mixture, a wetting speed of about 20 cm/sec and a dewetting speed of about 0.6 cm/sec has been reported [37]. Conversely, the time of rupture of thin films can be important (see Ref. 38). 

The rupture process of a soap film is of some interest. In the case of a film spanning a frame, as in Fig. XIV-15, it is known that rupture tends to originate at the margin, as shown in the classic studies of Mysels [207, 211]. Rupture away from a border may occur spontaneously but is usually studied by using a spark [212] as a trigger (a-radia-tion will also initiate rupture [213]). An aureole or ridge of accumulated material may be seen on the rim of the growing hole [212, 214] (see also Refs. 215, 216). Theoretical analysis has been in the form of nucleation [217, 218] or thin-film instability [219]. 

G. Reiter. Unstable thin polymer films rupture and dewetting process. Langmuir 9 1344-1351, 1993. 

Reiter, G. (1993) Unstable thin polymer films Rupture and dewetting processes. Langmuir, 9, 1344-1351. 

If a critical film thickness is not reached during film drainage, the drops separate from each other. Conversely, if the critical film thickness is reached, the film ruptures—as a result of van der Waals forces—and the drops coalesce. This generally occurs at thin spots, because van der Waals forces are inversely proportional to h (Verwey and Overbeek, 1948). The value of bent can be determined by setting the van der Waals forces equal to the driving force for film drainage, giving (Verwey and Overbeek, 1948)... 

The basic mechanism of dryout almost invariably involves the rupture of a residual thin liquid film, either as a microlayer underneath the bubbles or as a thin annular layer in a high-quality burnout scenario. Bankoff (1994), in his brief review of significant progress in understanding the behavior of such thin films, discussed some significant questions that still remain to be answered. 

As drops of this dispersed phase collect near the separation interface, they will flocculate into a closely packed mass which can best be described by the term liquid-liquid foam. Each drop is surrounded by a thin film of the continuous phase. The film between two adjacent drops can rupture and the two combine by coalescence in the foam layer. Only those drops near the general phase boundary can coalesce into the general drop phase layer. The residence time in the flocculation zone can be many minutes, and considerable mass transfer may occur there. 

In addition to electric charge between particles other factors are in some cases operative in preventing actual contact, thus the medium may be strongly adsorbed by the surface, and the thin film may not readily be displaced on collision of the two neutral particles, in other cases a tough elastic film may be formed, possessing definite mechanical strength and necessitating a violent impact to ensure rupture. 

In order to understand the basis for the prevention of bubble coalescence and hence the formation of foams, let us examine the mechanical process involved in the initial stage of bubble coalescence. The relatively low Laplace pressure inside bubbles of reasonable size, say over 1 mm for air bubbles in water, means that the force required to drain the water between the approaching bubbles is sufficient to deform the bubbles as illustrated in Figure 8.2. The process which now occurs in the thin draining film is interesting and has been carefully studied. In water, it appears that the film ruptures, joining the two bubbles, when the film is still relatively thick, at about lOOnm thickness. However, van der Waals forces, which are attractive in this system (i.e. of air/water/air), are effectively insignificant at these film thicknesses. 

It was found that increasing Ca caused the yield stress and yield strain to increase, along with cell deformation at the yield point. At sufficiently high values of Ca, cell distortion is so severe that film thinning and rupture can occur, resulting in mechanical failure of the foam (Fig. 6). This implies the presence of a shear strength for foams and HIPEs. The initial orientation of the cells was also found to affect the stress/strain behaviour of the system in the presence of viscous forces [63]. For some particular orientations, periodic flow was not observed for any value of Ca. 

Liquid films which form between approaching drops or bubbles are important structural elements of dispersed systems. The stability of these films controls the dispersion stability because the drops or bubbles cannot coalesce until the intervening film ruptures. The drainage and stability of thin liquid films attracted the attention of scientists already centuries ago [5,6]. 

When two emulsion drops or foam bubbles approach each other, they hydrodynamically interact which generally results in the formation of a dimple [10,11]. After the dimple moves out, a thick lamella with parallel interfaces forms. If the continuous phase (i.e., the film phase) contains only surface active components at relatively low concentrations (not more than a few times their critical micellar concentration), the thick lamella thins on continually (see Fig. 6, left side). During continuous thinning, the film generally reaches a critical thickness where it either ruptures or black spots appear in it and then, by the expansion of these black spots, it transforms into a very thin film, which is either a common black (10-30 nm) or a Newton black film (5-10 nm). The thickness of the common black film depends on the capillary pressure and salt concentration [8]. This film drainage mechanism has been studied by several researchers [8,10-12] and it has been found that the classical DLVO theory of dispersion stability [13,14] can be qualitatively applied to it by taking into account the electrostatic, van der Waals and steric interactions between the film interfaces [8]. 

Figure 2. A Schematic representation of the stages whereby a spreading particle causes local film thinning leading to film rupture [1].

A positive Hamaker constant corresponds to an attractive force between the silicon oxide-polystyrene and the polystyrene-air interfaces. This implies that the film is not stable. If it is thin enough and has a chance, for instance when annealing, the film ruptures and holes are formed. 

Fig. 30. Different types of the momentary morphologies which are typically observed during wetting (a-c) and dewetting (d-f) events on a solid flat substrate, a droplet, b spherical cap with a precursor film.c thin film (eventually with a multilayer structure), d thin liquid film, e ruptured film with rims at the dewetting front,f droplets...

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