FLUIDISATION OF YIELD STRESS MATERIALS UNDER VIBRATION

Abstract

Yield stress fluids are routinely fluidised for use in several industries. However, the mechanics of fluidisation of such materials due to external forcing remains poorly understood. Using a combination of experiments and theory, we study the fluidisation of a sessile drop of yield-stress materials, molten tempered chocolate and Carbopol microgel, on a uniform layer of the same material under vertical sinusoidal oscillations. Both these materials are shear-thinning with the same yield stress but possessed different elastic moduli. We find that molten tempered chocolate and Carbopol microgel exhibit quite different fluidisation behaviours. While chocolate spreads slowly over a long duration of time, Carbopol is seen to rapidly spread to a new equilibrium shape. We find that for a given extent of spreading Carbopol requires 3-4 times larger forcing acceleration. The Carbopol drop oscillates with large-amplitude, both above and below the yield threshold. We investigate these viscoelastic oscillations and provide evidence of complex nonlinear viscoelastic behaviour in the vicinity of the spreading threshold. In fact, for each subsequent increase in forcing acceleration greater than the spreading threshold, the vertically oscillated drop spreads to newer shape such that the stresses remains at the yield threshold. We discuss how our vibrated-drop experiment offers a new and powerful approach to probing the yield transition in elastoviscoplastic fluids. Informed by the rheological properties, we model chocolate and Carbopol as a viscoplastic and an elastoviscoplastic fluid, respectively. Based on the evidence of decreasing elastic modulus with increasing forcing from our experimental studies on Carbopol, we propose a Saramito-like model with nonlinear elasticity. We parameterise the proposed model using the steady shear and oscillatory rheological measurements. We find that the non-linear elastic model fits the oscillatory rheological measurements better than the constant modulus Saramito model. Using the proposed model, we derive a depth-averaged model for the dynamics of the drop in the limit of a shallow axisymmetric drop and compute solutions using finite-difference methods. We find that the Carbopol drop deformations are associated with viscoelastic deformations and that the yield-stress controls the critical acceleration at which spreading is initiated. The extent of the spreading for given acceleration results from the interaction between elastic and viscous forces, and the yield-stress alone does not determine fluidisation. Furthermore, the spreading process is shown to be insensitive to the history of forcing, and nonlinear elasticity suppresses spreading in Carbopol.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Matthias Heil (Supervisor) & Anne Juel (Supervisor)