  Motivated by the industrial processing of chocolate, we study experimentally
the fluidisation of a sessile drop of yield-stress fluid on a pre-existing
layer of the same fluid under vertical sinusoidal oscillations. We compare the
behaviours of molten chocolate and Carbopol which are both shear-thinning with
a similar yield stress but exhibit very different elastic properties. We find
that these materials spread when the forcing acceleration exceeds a threshold
which is determined by the initial deposition process. However, they exhibit
very different spreading behaviours: whereas chocolate exhibits slow long-term
spreading, the Carbopol drop rapidly relaxes its stress by spreading to a new
equilibrium shape with an enlarged footprint. This spreading is insensitive to
the history of the forcing. In addition, the Carbopol drop performs
large-amplitude oscillations with the forcing frequency, both above and below
the threshold. We investigate these viscoelastic oscillations and provide
evidence of complex nonlinear viscoelastic behaviour in the vicinity of the
spreading threshold. In fact, for forcing accelerations greater than the
spreading threshold, our drop automatically adjusts its shape to remain at the
yield stress. We discuss how our vibrated-drop experiment offers a new and
powerful approach to probing the yield transition in elastoviscoplastic fluids.
