Thin films from nuclei

Various thin films of formvar (about 50 — 100 A thick) were prepared by standard procedures. An ECS spectrum (which is a function of energy transfer AE from the electron to the impinging nucleus) is shown in Fig. 11. As is done in NCS investigations [Sears 1984 Watson 1996], the electron spectra were fitted with Gaussians. The "background" of the peaks is mainly due to interactions of the incident electrons with electrons in the sample. 

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 ... 

Sphemlites in thin films are visible when viewed between crossed polars under an optical microscope. They appear as Maltese crosses due to the alignment of chains within lamellae that radiate from the nucleus. An optical micrograph of sphemlites grown in a thin film of high density polyethylene is shown in Figure 41. 

The recollless fraction, that Is, the relative number of events In which no exchange of momentum occurs between the nucleus and Its environment. Is determined primarily by the quantum mechanical and physical structure of the surrounding media. It Is thus not possible to observe a Mossbauer effect of an active nucleus In a liquid, such as an Ion or a molecule In solution. This represents a serious limitation to the study of certain phenomena It allows, however, the Investigation of films or adsorbed molecules on solid surfaces without Interference from other species In solution. This factor In conjunction with the low attenuation of Y-rays by thin layers of liquids, metals or other materials makes Mossbauer spectroscopy particularly attractive for situ studies of a variety of electrochemical systems. These advantages, however, have not apparently been fully realized, as evidenced by the relatively small number of reports In the literature (17). 

The first crystallization nucleus forms in the subsurface approximately at 45 ns. The second subsurface nucleus occurs 110 ns after the beginning of the simulation. Freezing proceeds independently from both nuclei and the freezing fronts meet at 160 ns, forming a somewhat disordered contact zone. Cubic ice Ic forms predominantly in the simulations and many defects can be identified in the ice lattice. Freezing to cubic ice is consistent with the recent findings that small droplets and thin water films prefer this crystal modification over hexagonal ice Ih." ... 

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