Stress overshoot in a simple yield stress fluid: An extensive study combining rheology and velocimetry
Abstract
We report a large amount of . (i) At small strains (γ < 1), σ(t) increases linearly and the microgel undergoes homogeneous deformation. (ii) At a time tm, the stress reaches a maximum value σm which corresponds to the failure of the microgel and to the nucleation of a thin lubrication layer at the moving wall. (iii) The microgel then experiences a strong elastic recoil and enters a regime of total wall slip while the stress slowly decreases. (iv) Total wall slip gives way to a transient shear-banding phenomenon, which occurs on timescales much longer than that of the stress overshoot and has been described elsewhere [Divoux et al., Phys. Rev. Lett., 2010, 104, 208301]. This whole sequence is very robust to concentration changes in the explored range (0.5 ≤ C ≤ 3%w/w). We further demonstrate that the maximum stress σm and the corresponding strain γm =
tm both depend on the applied shear rate
and on the waiting time tw between preshear and shear start-up: they remain roughly constant as long as
is smaller than some critical shear rate
w ∼ 1/tw and they increase as weak power laws of
for
>
w. Finally, by changing the boundary conditions from rough to smooth, we show that there exists a critical shear rate
s fixed by the wall surface roughness below which slip at both walls allows for faster stress relaxation and for stress fluctuations strongly reminiscent of stick-slip. Interestingly, the value of
s is observed to coincide with the shear rate below which the flow curve displays a kink attributed to wall slip.