Flow of ordered latex suspensions: yielding and catastrophic shear thinning

Abstract

The flow properties of ordered latex suspensions are investigated using a constant-stress rheometer. The suspensions, studied over a volume-fraction range 0.38–0.62 and at ionic strength 10^–2, 10^–3 and 10^–4 mol dm^–3, show four distinct responses to increasing stress. At low stresses the suspensions behave like elastic solids holding a strain for times greater than 10⁴ s and displaying complete strain recovery upon release of stress. At higher stresses, the suspensions show non-recoverable strain upon application of the stress, but steady-state rates of deformation are not reached for times greater than 10⁴ s. Over a narrow stress range, the suspensions begin to flow reaching a steady-state rate of deformation in times less than 500 s. The stress becomes independent of steady-state shear rate in this region suggesting the onset of flow occurs at a dynamic yield stress which marks the onset of the low-stress, shear-thinning region. The high-stress, shear-thinning region is entered at a critical stress where the steady-state viscosity drops 1–3 orders of magnitude. Scaling of the long-time zero-stress modulus, yield stress and critical stress on volume fraction and suspending medium ionic strength are discussed in light of structural studies reported in the literature.