Paper Number
SC29 

Session
Suspensions and Colloids

Title
Modeling thixotropic colloidal dispersions in Large Amplitude Oscillatory Shear (LAOS) experiments

Presentation Date and Time
October 7, 2014 (Tuesday) 5:15

Track / Room
Track 1 / Commonwealth A

Authors (Click on name to view author profile)

1. Armstrong, Matthew J. (University of Delaware, Chemical and Biomolecular Engineering)
2. Beris, Antony N. (University of Delaware, Chemical and Biomolecular Engineering)
3. Wagner, Norman J. (University of Delaware, Department of Chemical and Biomolecular Engineering)

Author and Affiliation Lines (in printed abstract book)
Matthew J. Armstrong, Antony N. Beris, and Norman J. Wagner
Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE

Speaker / Presenter 
Armstrong, Matthew J.

Text of Abstract
Applying large amplitude oscillatory shear (LAOS) to complex fluids induces nonlinear rheological responses that, with proper modeling, can be used to sensitively probe the underlying microstructure and its dynamics. We demonstrate this for prototype concentrated colloidal suspensions using three semi-empirical thixotropic models from the literature [1,2], and one that we developed as an evolution of the model described in [3], all based on a scalar internal structural parameter. In addition to experimental data from literature we also used our own data. A 2.9vol% fumed silica in paraffin oil and poly-isobutylene (31wt%) [4] was formulated and its rheological response to steady shear, transient step-up and step-down, and LAOS, deformations measured. The experiments were performed on a strain-controlled TA Instruments ARES-G2 rheometer. Modeling of this stress response with several different thixotropic models described above, enables extracting also information about the structural order parameter [1,2,3]. The respective model parameters were found using a recently developed robust least squares stochastic global optimization. While all the models were found to be able to quantitatively fit the steady state experimental data, important quantitative and qualitative differences were observed regarding the transient LAOS data. A critical comparison of the predictions of selected LAOS data is made based on model parameters obtained from steady state and other LAOS data or, alternatively step-up and step-down experiments. The results are used to identify areas for improvement in thixotropic suspension modeling.
References
[1] J. Mewis and N.J. Wagner, Colloidal Suspension Rheology, Cambridge Univ. Press, 2012.
[2] P. de Souza Mendes and R. Thompson. Rheol. Acta (2013) 52:673-694.
[3] A. Mujumbdar, A.N. Beris and A.B. Metzner. J. Non-Newtonian Fluid Mech. (2002) 102:157-178.
[4] K. Dullaert and J. Mewis. Rheol. Acta (2005) 45:23-32.