Wednesday, October 5, 2016

High frequency viscoelastic measurements using optical tweezers on wormlike micelles of nonionic and cationic surfactants in aqueous solutions

Ken Morishima and Tadashi Inoue

Linear viscoelasticity of two kinds of wormlike micelles in aqueous solutions, one is nonionic surfactants and the other one is cationic surfactant with organic salt, was measured over a wide frequency range with Brownian motion tracking microrheology (BMTR) using optical tweezers and a conventional rheometer. In BMTR measurements, the Brownian motion of a small particle embodied in the sample is traced and the complex modulus is calculated from the trajectory. The wideband linear viscoelastic spectra thus obtained for each of wormlike micelles were classified into the following three relaxation types as already known. Type A is similar to the spectrum of the nonentangle polymer solutions, type B similar to that of the entanglepolymer systems, and type C has a single Maxwell relaxation at low frequencies. In the high-frequency region, spectra of all types showed a common power-law relaxation, which reflects the reorientation of the viscoelastic segment of the wormlike micelles, indicating that the dynamics of wormlike micelles is identical with that of the ordinary polymeric systems. For type A solutions, the molar mass of wormlike micelles was estimated by fitting the beads-spring models to the viscoelastic spectra. For type B solutions, the molar mass was estimated by using the universality of entangled system. For the case of nonionic micelles, thus determined molar mass is in good agreement with the reported result with the light scattering measurement.

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