(396c) Effect of Counter-Ion Concentration on the Rheology of Shear-Thickening Surfactant Solutions

It is well known that certain wormlike surfactant solutions (e.g. CTAB, CTAT) can undergo a phase transition from a solution phase to a gel-like phase upon increasing the shear rate above a critical value [Wunderlich and co-workers (1987), Liu and Pine (1996), Hartmann and Cressely (1997,1998,2000), and Bandyopadhyay and Sood (2001)]. The apparent viscosity of the solution increases (i.e. the solution shear-thickens) as a result of such phase transitions. The critical shear rate and the extent of viscosity enhancement in such systems are very sensitive to salt concentration. In this study, the effect of salt (NaSal) concentration on surfactant (CTAB) solution rheology has been investigated in the shear thickening regime. Experimental data have been analyzed to obtain power-law relationships for the critical shear rate and apparent relaxation time of the shear-thickened solution as a function of salt concentration C_s. Secondly, a procedure has been developed to collapse viscosity vs. shear rate data at various values of C_s into a single master curve. Effect of C_s on the elastic properties of the shear-thickened solutions is discussed for different surfactant concentrations. Comparisons of experimental data for the time constants of the system with those predicted by linear stability analysis of "gel-fluid" models [Shankar and Kumar (2004), Kumaran and Muralikrishnan (2000), and Eggert and Kumar (2004)] are also provided.

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