Extremely slow settling behavior of particles in dilute wormlike micellar fluid with broad spectrum of relaxation times

We selected a 1.4 wt% Octadecyl trimethyl ammonium chloride (OTAC) concentration wormlike micellar fluid with broad relaxation time spectrum and another single relaxation Maxwell fluid of 4.0 wt% OTAC concentration with a constant weight ratio of OTAC to NaSal of 5:1. The shear viscosity, dynamic modulus as well as the extensional viscosity were tested. The steady and unsteady motion of spheres were investigated by high-speed video images in these two wormlike micellar fluids. Extremely slow settling velocity is found when smaller spheres with diameters of 3.2 and 4.0 mm settles in the 1.4 wt% OTAC solution. Stronger rebound of these spheres occurs in this fluid. Comparing with particle settling velocity in inelastic power law fluid, we concluded that this extremely slow sedimentation in the 1.4 wt% OTAC solution seems to be dominated by the slowest stress relaxation of this solution. The rheology test demonstrated that this wormlike micellar fluid has more tensile hardening, low viscosity and strong elasticity. The oscillation of sphere results from the elasticity of the entangled microstructures of wormlike micelles. For a Maxwell viscoelastic 4.0 wt% OTAC solution, settling behaviors of spheres agree with previous literatures that the flow becomes instability as the Wi number exceeds a critical value.