Black film holes

In a foam with thin (black) films the rate of diffusion transfer is influenced to a great extent by the adsorption layers that delay the dissolution and diffusion of the surfactant, especially in the presence of additives such as dodecanol. At low surfactant concentrations the permeability of the adsorption layers depends also on the presence of fluctuation holes in the NBFs (see Section 3.5). 

In order to apply the hole-nucleation theory of bilayer stability of Kashchiev-Exerowa [27] involving quantitative interpretation of the W(C) dependence (probability for observation of black films vs. surfactant concentration), the black films from amniotic fluid should be bilayer films. This is proved experimentally by two dependences Y hw) (Fig. 11.1) and hw(Cei) (Fig. 11.2). As it can be seen in Fig. 11.1, the equivalent film thickness is 8 nm and does not change with the increase in IT (which is the difference between the pressures in the... 

Let us summarise the conditions of formation of a microscopic foam film in order to serve the in vivo situation. These are film radius r from 100 to 400 pm capillary pressure pa = 0.3 - 2.5-102 Pa electrolyte (NaCl) concentration Ce 0.1 mol dm 3, ensuring formation of black films (see Section 3.4) and close to the physiological electrolyte concentration sufficient time for surfactant adsorption at both film surfaces. Under such conditions it is possible also to study the suitable dependences for foam films and to use parameters related to formation and stability of black foam films, including bilayer films (see Section 3.4.4). For example, the threshold concentration C, is a very important parameter to characterise stability and is based on the hole-nucleation theory of bilayer stability of Kashchiev-Exerowa. As discussed in Section 3.4.4, the main reason for the stability of amphiphile bilayers are the short-range interactions between the first neighbour molecules in lateral and normal direction with respect to the film plane. The binding energy Q of a lipid molecule in the foam bilayer has been estimated in Section 11.2. 

The stability of foams and emulsions depends critically on whether formation of a stable Newton black film or a hole leading to coalescence is favored. Kabalnov and Wen-nerstrom (4) addressed this question by developing a temperature-induced hole nuclea-tion model applicable to emulsions. They point on that the coalescene energy barrier is strongly affected by flic spontaneous monolayer curvature. The aufliors consider a flat emulsion film, covered by a saturated surfactant monolayer, in thermodynamic equi-... 

In the mechanism we have just described, the films thin down in their upper portion—the region of the so-called black film. They eventually burst, typically by way of a hole nucleating around a speck of dust. 

If the short-range repulsive disjoining pressure is large enough, the black foam films are stable. There are two types of black foam films common and Newtonian. While the common black films are the thicker type of black films (from about 5 to 20 nm in thickness), the Newtonian black (NB) films are bimolecular thin films (less than 5 mn in thickness). A mechanism of rupture of NB films is considered as a process of new phase nucleation in a two-dimensional system [105 108]. There exist in the film elementary vacancies (unoccupied positions of surfactant molecules) moving randomly, which associate to form clusters of vacancies called holes. A hole can grow up by fluctuations to a critical size and become a nucleus of a hypothetical two-dimensional phase of vacancies. Further spontaneous growth of the nucleus leads irreversibly to the rupture of the film. When the rupture of NB film is due to formation of holes in it by a nucleation mechanism, it has been shown that the mean film lifetime r depends on the monomer surfactant concentration C as ... 

The foam film c is formed in the middle of a biconcave drop b, situated in a glass tube of radius R, by withdrawing liquid from it (A and B) and in the hole of a porous plate g (C) (Figure 8.2). A suitable tube diameter in A and B is 0.2-0.6 mm and the film radius ranges from 100 to 500 nm. In C, the hole radius can considerably smaller, in the range of 120 pm and the film radius is 10 pm. When the film thins to form the so-called black film, black spots can be observed under the microscope. 

If now the capillary pressure increases to p so that it exceeds the maximum at C in the isotherm then the film will drain to another unstable region and will jump from C to form another stable film at D. Equilibrium will again be established when the film thins to thickness where pf = II la ( 2) Such films are extremely thin, being little more than bilayer leaflets of thickness in the region of 5 nm. They are usually termed Newton black films. Further increases in capillary pressure to pf > n j (/ 3) should simply lead to film rupture. However, thickness fluctuations cannot exist in such thin films. It has therefore been argued by Kashchiev and Exerowa [56] that rupture occurs by nucleation of holes caused by thermal fluctuations in these films. 

Hooke, R. (1672) On Holes (Black Film) in Soap Bubbles, Communications to the Royal Society, March 28. Also see History of the Royal Society, Birch, T. (A. Millard, London), Vol. Ill, p. 29 (1757). 

Mueller et al. [6] discovered in 1962 that when a small quantity of a phospholipid (2% wt/vol alkane solution) was carefully placed over a small hole (0.5 mm) in a thin sheet of Teflon or polyethylene (10-25 pm thick), a thin film gradually forms at the center of the hole, with excess lipid flowing towards the perimeter (forming a Plateau-Gibbs border ). Eventually, the central film turns optically black as a single (5 nm-thick) bilayer lipid membrane (BLM) forms over the hole. Suitable lipids for the formation of a BLM are mostly isolated from natural sources, e.g.,... 

The film is in contact with the cylindrical brass frame of the camera and is held in position by springs 8. Sharp edges E terminate the exposed part of the film abruptly. Light is excluded by a brass cover which fits over the whole camera the X-rav beam is admitted through a hole covered with black paper. Further details of the construction and use of Debye- Scherrer powder cameras can be found in a paper by... 

A solution of brain lipids was brushed across a small hole in a 5-ml. polyethylene pH cup immersed in an electrolyte solution. As observed under low power magnification, the thick lipid film initially formed exhibited intense interference colors. Finally, after thinning, black spots of poor reflectivity suddenly appeared in the film. The black spots grew rapidly and evenutally extended to the limit of the opening (5, 10). The black membranes have a thickness ranging from 60-90 A. under the electron microscope. Optical and electrical capacitance measurements have also demonstrated that the membrane, when in the final black state, corresponds closely to a bimolecular leaflet structure. Hence, these membranous structures are known as bimolecular, black, or bilayer lipid membranes (abbreviated as BLM). The transverse electrical and transport properties of BLM have been studied usually by forming such a structure interposed between two aqueous phases (10, 17). 

The black flap keeps light from entering the camera. Once the flap is lifted, light reflected from an object enters the camera and strikes the film. After optimal light has entered the camera, the flap is lowered to close the hole and block out excess light. 

Another approach to explain foam film rupture has been developed by de Vries [101] who proposed to consider film rupture as a result of fluctuational formation of holes (black spots) in it - nuclei of critical size (see Section 3.4). This idea was used in the analysis of the... 

Microscopic foam films are most successfully employed in the study of surface forces. Since such films are small it is possible to follow their formation at very low concentrations of the amphiphile molecules in the bulk solution. On the other hand, the small size permits studying the fluctuation phenomena in thin liquid films which play an important role in the binding energy of amphiphile molecules in the bilayer. In a bilayer film connected with the bulk phase, there appear fluctuation holes formed from vacancies (missing molecules) which depend on the difference in the chemical potential of the molecules in the film and the bulk phase. The bilayer black foam film subjected to different temperatures can be either in liquid-crystalline or gel state, each one being characterised by a respective binding energy. 

Theoretical analysis of sheeting in the drainage of thin liquid films has been conducted in [359]. Sheeting dynamics and hole formation (i.e. black spot formation) was described by non-linear hydrodynamic stability analysis based on the equilibrium oscillatory structural component of disjoining pressure. The effect of stepwise thinning, accompanied by formation of holes , was described qualitatively. It is rather arguable whether the term holes for a black spot is appropriate since in 1980 holes in NBF were described as lack of molecules. The use the same term for two different formations is at least confusing. Besides, to have a hole in a CBF is almost as to have a hole in the sea water . 

Black foam films appear in thermodynamically non-equilibrium films in the form of black spots (see Section 3.2.2.2). These clearly expressed thin regions (but not holes as named in [e.g. 35,381]) expand, fuse and occupy the whole film area. Thus, CBF and NBF reach an equilibrium state. This process can be most distinctly observed in microscopic foam films (see Fig. 3.14). 

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