Abstract
Nonlinear rheology of aqueous solutions of thread-like micelles of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) were investigated. The concentration of CTAB was fixed at 0.1 mol 1-1, and the concentration of NaSal was varied from 0.07 to 0.4 mol 1-1. For all test solutions, dynamic moduli were described with the Maxwell model having a single relaxation time, t. Time evolutions of the shear stress, σ, and the first normal stress difference, N1, after inception of the steady shear flow were measured. For all test solutions, strain-hardening was observed and a and N1 diverged at a certain strain when the shear rate, y, exceeded a critical rate being larger than T-1. A simple relationship for elastic solids, N1/σ = γ with γ being the strain imposed by shear flow, held for all the solutions in the strain-hardening regime. The strain-hardening was attributable to the strain dependent shear modulus and well described with the network theory considering finite extensibility of network strands. The segment size of network strand was successfully determined. Creep response of the solutions was also measured. At higher stress level than the plateau modulus, recoverable compliance was considerably increased, suggesting shear-induced structural changes.
