Thermal instabilities in a yield stress fluid: Existence and morphology

► We study the development of thermal plumes out of a localized heat source in yield-stress fluids. ► The yield number Y0, comparing thermal and yield stresses, determines the convective regime. ► Hot plumes develops for Y0>260. ► Their finger-shape is characteristic of yield-stress fluids. ► A damaged zone precedes and surrounds the plume. We present new laboratory experiments on the development of thermal plumes out of a localized heat source in Carbopol, a yield-stress and shear-thinning fluid. Depending on the yield number Y0, which compares the thermally-induced stress to the yield stress, three different regimes obtain. For low Y0 (260), thermal plumes develop. Their morphology differs from the mushroom-shape typically encountered in Newtonian fluids. Combined temperature and velocity field measurements show that a plug flow develops within the plume thermal anomaly, therefore producing a rising finger-shape. Moreover, light scattering highlights the development of a damaged zone prior to the plume onset, and the peculiar structure of the gel around the plume as it rises. This brings new insights into the solid–liquid transition of soft gels.