Diffusing wave spectroscopy in Maxwellian fluids |
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Authors: | J Galvan-Miyoshi J Delgado R Castillo |
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Institution: | (1) Instituto de Fisica, Universidad Nacional Autonoma de Mexico, P. O. Box 20-364, Mexico, D. F., 01000, Mexico |
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Abstract: | 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 cetyltrimethylam-monium 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 〈Δr
2(t)〉 vs. time curves. In addition, from the 〈Δr
2(t) vs. time curves, we obtained G′(ω) and G″(ω) 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. |
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Keywords: | PACS" target="_blank">PACS 82 70 Uv Surfactants micellar solutions vesicles lamellae amphiphilic systems (hydrophilic and hydrophobic interactions) 82 70 Dd Colloids 83 10 Pp Particle dynamics 83 60 Bc Linear viscoelasticity |
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