Fluorescence drainage profiles of thin liquid films.
Fluorescence drainage-profiles of thin liquid films formed from Rhodamine solutions containing anionic, cationic and non-ionic surfactants have been investigated. It is found that the influence of system variables such as electrolyte concentration, dye concentration, film environment and solution viscosity can be evaluated by means of the profiles. The addition of sodium chloride leads to the expansion (non-ionic surfactant), or contraction (anionic surfactant) of the profiles. In the case of the cationic surfactant, it reduces the number of fringes in the profiles. The height of selected fringes changes linearly with dye concentration, and no fringes are observed when the films are submerged in non-polar solvents. The influence of solution viscosity on film thickness and drainage rate is demonstrated by the number and frequency of fringes in the profile. The formation of first and second "black films" from solutions containing varying concentrations of sodium chloride can be shown.
FTIR and UV-Vis Spectroscopic Studies of Black Soap Film.
FTIR and UV-vis spectroscopies have been employed to investigate the self-assembly features of a black soap film (BSF) prepared from aqueous solution consisting of 10(-2) M cetyltrimethylammonium bromide (CTAB) and 10(-3) M thiazole yellow (TY) in the liquid-crystalline and gel states, respectively. The construction of two surfactant monolayers of the soap film is independent of the thickness of aqueous core, and the thickness is approximately 1.7 nm at equilibrium. In the liquid-crystalline state, the methylene segments of the hydrocarbon chains are averagely oriented at an angle of 70 degrees with respect to the film normal, and the alkyl chains take a long-range interaction to improve the film elasticity. The TY molecules in the film are horizontally aligned at the monolayer interface in the J-aggregate form. The double-negatively charged TY aggregate greatly enhances the film stability. Dye neutral red is further introduced into the CTAB soap film, the film lifetime is related to the type of the added dyes, in the order of cationic < nonionic < anionic. Copyright 1998 Academic Press.
Turbulent boundary layer wall‐pressure fluctuations downstream from an abrupt change in surface roughness
Spectra of wall‐pressure fluctuations beneath a turbulent boundary layer have been investigated immediately downstream from an abrupt change in surface roughness, from slightly rough to very rough. Over the measurable frequency band (400 Hz to 7 kHz), the spectra in the first 150 mm scale on wall variables. This indicates that the motions responsible for the pressure fluctuations measured in this region are very close to the wall. Farther downstream this scaling procedure breaks down. It is suggested that the initially thin internal layer behaves independently of the flow external to it until it has grown appreciably and only then interacts with the outer boundary layer. It was found that, for all frequencies measured, the dimensional spectral level decreases initially then rises to a maximum at 250 mm from the roughness change before decreasing once more. The position of the maximum is close to where the internal layer ceases to have a distinct edge.