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A experimental study of low Reynolds numbers unsteady pipe flows of a yieldstress shear thinning fluid (Carbopol- 980) is presented. The investigation ofthe solid-fluid transition in a rheometric flow in the presence and in the ab-sence of the wall slip reveals a coupling between the irreversible deformationstates and the wall slip phenomenon. Particularly, the presence of wall slipnearly suppresses the scaling of the deformation power deficit associated tothe rheological hysteresis with the rate at which the material is forced. Theirreversible solid-fluid transition and the wall slip behaviour emerge in thesame range of the applied stresses and thus, the two phenomena appear to becoupled to each other. In-situ measurements of the flow fields performed duringan increasing/decreasing stepped pressure ramp reveal three distinct flowregimes: solid (pluglike), solid-fluid and fluid. The deformation power deficitassociated with the hysteresis observed during the increasing/decreasingbranches of the pressure ramps reveals a dependence on the rate at which theunsteady flow is driven consistent with that observed during the rheologicalmeasurements in the presence of slip. The dependence of the slip velocity onthe wall shear stresses reveals a Navier-type slip behaviour only within thefluid flow regime, which indicates that the wall slip phenomenon is directlycoupled to the solid-fluid transition. A universal scaling of the slip velocitywith the wall velocity gradients is found and the slip length is independent onthe characteristic time of forcing t0. The paper closes with a discussion ofthe main findings, their possible impact on our current understanding of theyielding and slip behaviour of Carbopol gels. Several steps worth being pursuedby future experimental/theoretical studies are proposed.

Unsteady laminar pipe flow of a Carbopol gel. Part I: experiment