We present a critical assessment of the diffusing wave spectroscopy (DWS) technique for obtaining the characteristic lengths and for measuring the loss and storage moduli of a reasonable well-known wormlike micelle (WM) system. For this purpose, we tracked the Brownian motion of particles using DWS embedded in a Maxwellian fluid constituted by a wormlike micellar solution made of cetyltrimethylammonium bromide (CTAB), sodium salicylate (NaSal), and water. We found that the motion of particles was governed by the viscosity of the solvent at short times and by the stress relaxation mechanisms of the giant micelles at longer times. From the time evolution of the mean square displacement of particles, we could obtain for the WM solution the cage size where each particle is harmonically bound at short times, the long-time diffusion coefficient, and experimental values for the exponent that accounts for the broad spectrum of relaxation times at the plateau onset time found in the (deltar2(t)) vs. time curves. In addition, from the (deltar2(t)) vs. time curves, we obtained G'(omega) and G"(omega) for the WM solutions. All the DWS microreological information allowed us to estimate the characteristic lengths of the WM network. We compare our DWS microrheological results and characteristic lengths with those obtained with mechanical rheometers at different NaSal/CTAB concentration ratios and temperatures.
Localized oscillations present in the crystalline phases of the heneicosanoic acid Langmuir monolayer were studied in detail. They appear like blinking interference rings, when observed with Brewster angle microscopy. Monolayers with localized oscillations were transferred on mica to be characterized by atomic force microscopy. We found granules produced by the expulsion of matter from the monolayer. However, these granules are too short to produce Newton's rings; the common belief of the origin of the interference rings in the field of Langmuir monolayers. The analysis of the light intensity distribution and the sizes of the rings are consistent with Airy patterns produced by Fraunhofer diffraction due to the reflected light from the multilayer granules. The origin of the blinking of these patterns is still unclear.
Localized oscillations recently found in crystalline phases of
the heneicosanoic acid Langmuir monolayer were studied. They
appear like blinking interference rings, when observed with
Brewster angle microscopy. The optical analysis of the
interference rings is consistent with Fraunhofer diffraction
caused by light reflected from multilayer granules formed
by material expulsion from the monolayer.
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