Delayed flow in thermo-reversible colloidal gels

In this study we present a systematic investigation of the highly nonlinear creep behavior of thermo-reversible gels composed of octadecyl coated silica particles suspended in decalin. These suspensions display a gelation transition below a volume fraction dependent critical temperature. The mechanical response of the resulting gels is characterized by a time for the elastic modulus to recover after preshear that can take several hours. Once steady state is reached, upon application of a constant stress, sigma, the compliance of the gel falls into two regimes. Below a critical stress, sigma(crit), the strain produced in the gel increases slowly with time where the rate of increase decreases with time. Above sigma(crit), at short times, the strain response is nearly identical to that observed when sigma < sigma(crit). However, at a stress dependent characteristic time, T-break, the gel yields under the shear stress and begins to flow similar to a liquid leading to a rapid increase in the strain by several orders in magnitude. T-break decreases with increasing stress and above a certain stress falls below the measurable time windows and the gel appears to flow at the instant that the stress is applied. T-break is also found to be a strong function of volume fraction and temperature. We develop a simplified model built on the hypothesis that the phenomenon is the result of a competition between the rate of stress-induced bond-breakage events and the rate at which these broken bonds are reformed. Below the critical stress, bond-reformation rates can match the rate at which bonds are broken thereby retaining connectivity within the gel network to support the applied stress and permitting a slow increase in compliance with time. However, above the critical stress, the bond-breakage rates overwhelm the rate at which the gel can heat itself thereby resulting in the eventual degradation of the gel structure and the generation of liquidlike behavior. (c) 2007 The Society of Rheology.