Paper Number
IN8 

Session
Flow Induced Instabilities and Non-Newtonian Fluids

Title
Interplay between shear banding and wall slip: Generalized lever rule

Presentation Date and Time
October 21, 2019 (Monday) 2:20

Track / Room
Track 4 / Room 305B

Authors (Click on name to view author profile)

1. Geri, Michela (MIT, Mechanical Engineering)
2. Saint-Michel, Brice (Imperial College)
3. Divoux, Thibaut (MIT,CNRS)
4. Manneville, Sebastien (ENS Lyon)
5. McKinley, Gareth H. (Massachusetts Institute of Technology, Department of Mechanical Engineering)

Author and Affiliation Lines (in printed abstract book)
Michela Geri¹, Brice Saint-Michel², Thibaut Divoux³, Sebastien Manneville⁴, and Gareth H. McKinley^1
1Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA; ²Imperial College, London, United Kingdom; ³MIT,CNRS, Cambridge, MA 02139; ⁴ENS Lyon, Lyon, France

Speaker / Presenter 
Geri, Michela

Text of Abstract
Shear banding has been shown to affect the flow of many complex fluids under simple viscometric conditions. A common feature of steady-state shear banding is the presence of a non-monotonic branch in the underlying flow curve. In thixotropic yield stress fluids banding usually occurs when the applied shear-rate is smaller than a critical value and is characterized by an arrested band adjacent to a steady sheared region; the stress measured during flow is usually constant and equal to the apparent yield stress. In this work, we show that steady-state shear banding in a thixotropic yield stress fluid with inherently different thixotropic time scales (for the plastic viscosity and yield stress) allows us to partially measure the non-monotonic branch of the flow curve in a rheometer. The test fluid consists of a suspension of microscopic paraffin platelets in mineral oil at different concentrations. Rheometric tests under decreasing shear rates are performed in a bespoke Couette cell with roughened walls while simultaneously measuring the local velocity field via ultrasonic velocimetry. Results for different concentrations show that, for a specific range of shear rates just below the critical value, the observed shear band develops together with a slip layer that guarantees the sample is always sheared at the globally-imposed shear rate. Using a generalized lever rule and accounting for the different thixotropic time-scales, we can explain not only the existence of this peculiar shear-banding scenario, but also derive a logarithmic scaling law for the slip velocity that is a direct consequence of the thixotropic bulk behavior. Finally, we show that the velocity field measured at the smallest shear rates accessible can be understood in terms of non-local effects related to the finite size of the particles relative to the dimension of the shear band.