The tackiness of smectite muds. 1. The dilute regime

The tackiness of bentonite or laponite muds, either neat or loaded with small granular particles, has been studied using a rate-controlled pull-off device made of two circular plates pulled apart at constant velocity. It was observed that the velocity dependence of the maximum pull-off force correlates satisfyingly with the shear-thinning parameters of the mud flow curves. This shows that, in the relatively dilute clay concentration range which was studied (5% w/w montmorillonite and less than 2% laponite), the force required for separation is determined by the viscous dissipation associated with the inwards flow of the mud and not by the cohesion (yield stress) of the mud itself. The decay of the pull-off force beyond the maximum can be fitted with a stretched exponential function. The decay time of this function correlates well with the characteristic time of the imposed flow rate, confirming the importance of viscous dissipation in tackiness. This complex behavior is likely to be related to the complex morphology of the air–mud interface, generated by the viscous fingering instability.