Periodic compression/dilatation

Serrien et al. (1992) measured the damping of planar longitudinal waves, generated with a barrier on a Langmuir trough and detected with a Wilhelmy balance. The longitudinal waves correspond to slow periodic compressions/dilations of the protein adsorption layer (BSA, casein). The results for BSA are shown in Fig. 6.18. Further experiments using a stress... 

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. 

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. 

The gaseous bubbles suspended in liquid continuous phase suffer periodical compressions and dilatations, pulsations, characterised by specific frequencies [1106]. The bubbles amplitude and frequency become maximal whenever their own frequency coincides with the ultrasound frequency, therefore when together give rise to system in resonance. The resonance oscillations can generate local pressures that overtake up to 150,000 times the hydrostatic pressure. 

A new feature entering the picture is that fluids confined by substrates of the kind illustrated by the sketch in Fig. 5.7 can be sheared in addition to mere compression (dilatation). This is l)ecaus( the relative alignment of the substrates matters on account of the periodicity of their structure in the x-direction. In this case, the transformation matrix assumes the form... 

The viscoelastic properties of the surface layer are important parameters. The most useful technique for studying the viscoelastic properties of surfactant monolayers is surface scattering. When transversal ripples occur, a periodic dilation and compression of the monolayer occurs, and this can be accurately measured, enabling the viscoelastic behaviour of monolayers under equilibrium and nonequilibrium conditions, to be obtained, without disturbing the original sate of the adsorbed layer. Some correlations have been found between surface viscosity and elasticity and foam stability an example of this is the addition of lauryl alcohol to sodium lauryl sulphate, which tends to increase the surface viscosity and elasticity [10]. 

In solid-state physics, the mobility of delocalized electrons (or holes) is explained by the mechanism of scattering on lattice vibrations. The atoms or molecules comprising the lattice carry out vibrations around their equilibrium position. Compression zones alternate with dilation zones. This leads to periodic changes of the lattice constant and of the number density. As a consequence, the energy of the bottom of the conduction band, Vq, varies periodically, too. If the density of the liquid increases, the Vq level shifts upward (see Figure 31 of Chapter 6). An analog variation occurs with the top of the valence band. These conditions are depicted schematically in Figure 3. 

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