Periodic compression-expansion

In a second technique, a freshly swept surface of a dilute surfactant solution is subjected to periodic compression-expansion cycles. With the... 

The film elasticity was derived from tt-A isotherms as E = — A (d7r/dA). The surface dilatational modulus (E) of films with its elastic and viscous components (Ed and Ev) and loss angle tangent (tan 8) were obtained by sinusoidal periodic compressions and expansions. 

Analyser systems are expensive and should therefore be adequately protected from their environment and the process streams that they control. Every analyser is liable to malfunction upon contact with rain, snow, Ice, wind, sand, dust and so forth. After some time, alternate hot-cold or humid-dry periods cause expansions and compressions that results In erosion and corrosion of the analyser. On the other hand, industrial environments are particularly severe as the rain and atmospheric humidity react with traces of hydrocarbons, sulphurized products and nitrogen oxides to form acids which accelerate corrosion. All these reasons recommend protecting the analyser to an extent depending on the potential hazards of the area where the analyser is... 

In the drop shape technique sinusoidal area changes can be easily generated via changes of the drop volume in a very accurate way. The Fourier analysis of the surface tension response however shows that besides the main mode with the period T of the generated oscillation there are also modes with periods of 3T/2, T/2, T/4 and T/8. The origin of these modes is not yet fully understood but certainly caused by deviation of the area changes from harmonicity and surface layer compression/expansion beyond the limits of a linear theory. 

The kinetics of formation and disintegration of micelles has been studied for about thirty years [106-130] mainly by means of special experimental methods, which have been proposed for investigation of fast chemical reaction in liquids [131]. Most of the experimental methods for micellar solutions study the relaxation of small perturbations of the aggregation equilibrium in the system. Small perturbations of the micellar concentration can be generated by either fast mixing of two solutions when one of them does not contain micelles (method of stopped flow [112]), or by a sudden shift of the equilibrium by instantaneous changes of the temperature (temperature jump method [108, 124, 129, 130]) or pressure (pressure jump method [1, 107, 116, 122, 126]). The shift of the equilibrium can be induced also by periodic compressions or expansions of a liquid element caused by ultrasound (methods of ultrasound spectrometry [109-111, 121, 125, 127]). All experimental techniques can be described by the term relaxation spectrometry [132] and are characterised by small deviations from equilibrium. Therefore, linearised equations can be used to describe various processes in the system. 

Here A is the amplitude, cp the initial phase, and coo the frequency of free vibrations. Thus, in the absence of attenuation free vibrations are sinusoidal functions and this result can be easily predicted since mass is subjected to the action of the elastic force only. In other words, the sum of the kinetic and potential energy of the system remains the same at all times and the mass performs a periodic motion with respect to the origin that is accompanied by periodic expansion and compression of the spring. As follows from Equation (3.105) the period of free vibrations is... 

The amphiphilic material to be studied is dissolved at a known concentration in a volatile solvent which is not miscible with water and a known quantity is spread at the water surface using a micropipette. In order to study the physical properties of the film thus formed, one needs to be able to confine the film to a definite area and to be able to vary this area at will. It might appear that it would be equally possible to maintain a constant area and vary the amount of material which is spread. For the majority of materials this latter procedure is not satisfactory as equilibrium is not arrived at in a reasonable period of time and this method would not allow one to take the material through successive cycles of compression and expansion. We thus turn to a discussion of the various ways in which a film can be confined and its area varied in a systematic manner. Leaving aside methods which are really only of historical interest, for which reference should be made to the book by... 

The trough itself measured 20 x 12 cm, was milled from a block of Teflon, and held approximately 750 ml of liquid. Monomolecular films were prepared on a subphase of (ultrapure) distilled water or on aqueous subsolutions containing varying concentrations of either NaF, HC1, NaOH, thiourea, or dimethyl sulfoxide (DMSO). Aliquots, between 50 and 250 pi, of the ethanol-solubilized microbubble-surfactant mixture were applied slowly to the surface of the subsolution from a Hamilton microsyringe. It was found unnecessary to allow the films to stand for more than 2 min after spreading before taking measurements. Furthermore, following compression or expansion, the surface pressure was observed to remain constant for periods of up to at least 10 min. All measurements were made at 20.0 0.5°C. 

The word cyclic requires that the system be restored periodically to its original state. In the case of a gas in a piston-and-cylinder assembly the expansion and compression back to the original state constitute a complete cycle. If the process... 

Before reviewing the specific results of these experiments, two observations made during this study should be reported. To remove the books from the compressed Plexiglas plates at the end of the submersion period, a reverse torque of 15-20 kg-cm for the coated and 11-14 kg-cm for the uncoated books had to be applied to the C-clamps. This increased amount of torque required to release the clamps was a result of pressure from the expansion of the paper while it was wet. In the case of the coated-paper book, an actual bowing of the center of the Plexiglas plates clamped against this book was observed. 

Over a long period of time experimental results on amphiphilic monolayers were limited to surface pressure-area ( r-A) isotherms only. As described in sections 3.3 and 4, from tc[A) Isotherms, measured under various conditions, it is possible to obtain 2D-compressibilities, dilation moduli, thermal expansivities, and several thermodynamic characteristics, like the Gibbs and Helmholtz energy, the energy cmd entropy per unit area. In addition, from breaks in the r(A) curves phase transitions can in principle be localized. All this information has a phenomenological nature. For Instance, notions as common as liquid-expanded or liquid-condensed cannot be given a molecular Interpretation. To penetrate further into understanding monolayers at the molecular level a variety of additional experimental techniques is now available. We will discuss these in this section. 

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