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Unsteady flow of yield stress materials

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Viscoplastic materials such as Bingham or Heschel Bulkley fluids have a yield stress which must be exceeded before they flow. Their behaviour lies somewhere between liquids and solids. In this talk I will present experimental and numerical results for two unsteady flow problems involving a viscoplastic material.

In the first, we look at laminar unsteady pipe flow of a Carbopol gel, which is a model viscoplastic fluid. By looking in detail at the solid-fluid transition of this material, we found a strong coupling between the irreversible deformation states and the phenomenon of wall slip (where the fluid behaves as if it is sliding along the wall). In addition, the dependence of the slip velocity on the wall shear stress reveals a Navier-type behaviour only within the fluid regime, which provides strong evidence that the wall slip phenomenon is directly coupled to the solid-fluid transition.

The second is the Landau-Levich ‘chocolate dipping’ problem, whereby a plate is raised out of bath of viscous (in this case viscoplastic) fluid. The fluid drains off the plates as it is raised. The validity of the classical Landau-Levich scaling result is investigated via experimental measurement of the width of the coating film for different values of the yield stress for the slip and no-slip cases.

This talk is part of the Chemical Engineering and Biotechnology Departmental Seminars series.

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