[Paper Review] Flow behavior of colloidal rod-like viruses in the nematic phase

M. P. Lettinga, Zvonimir Dogic+2 more|arXiv (Cornell University)|Jun 7, 2005

Liquid Crystal Research Advancements4 citations

TL;DR

This study investigates the flow behavior of monodisperse, colloidal fd viruses in the nematic phase under shear, using transient rheology and in situ microscopy to probe transitions between tumbling, wagging, and flow-aligning states. The results show excellent quantitative agreement with the Doi-Edwards-Hess theory when scaled by the order parameter, confirming its predictive power for hard rod systems despite the rods' semi-flexibility and surface charge.

ABSTRACT

The behavior of a colloidal suspension of rod-like {\it fd} viruses in the nematic phase, subjected to steady state and transient shear flows is studied. The monodisperse nature of these rods combined with relatively small textural contribution to the overall stress make this a suitable model system to investigate the effects of flow on the non-equilibrium phase diagram. Transient rheological experiments are used to determine the critical shear rates at which director tumbling, wagging and flow-aligning occurs. The present model system enables us to study the effect of rod concentration on these transitions. The results are in quantitatively agreement with the Doi-Edwards-Hess model. Moreover, we observe that there is a strong connection between the dynamic transitions and structure formation, which is not incorporated in theory.

Motivation & Objective

• To investigate the concentration and shear rate dependence of dynamic flow transitions (tumbling, wagging, flow-aligning) in a colloidal nematic phase.
• To assess the contribution of textural stress to the overall rheological response in rod-like virus suspensions.
• To test the predictive power of the Doi-Edwards-Hess (DEH) theory for non-equilibrium behavior in a well-characterized model system.
• To explore the connection between dynamic transitions and microstructural evolution, including shear banding.
• To determine whether the DEH theory can quantitatively describe experimental results when accounting for rod flexibility and charge via the order parameter.

Proposed method

• Conduct transient rheological experiments with flow reversal to probe the time-dependent response and identify critical shear rates for dynamic transitions.
• Use in situ microscopy under shear to visualize microstructural changes, including shear banding and director reorientation.
• Measure steady-state viscosity and relaxation after flow cessation to assess stress contributions and structural recovery.
• Apply the Doi-Edwards-Hess (DEH) theory to model the time evolution of the orientational distribution function of rods under shear.
• Scale experimental data using the order parameter from the isotropic-nematic transition to compare with theory without fitting parameters.
• Analyze the stress tensor decomposition to isolate molecular and textural contributions, particularly at high concentrations.

Experimental results

Research questions

• RQ1 How does rod concentration influence the critical shear rates for transitions between tumbling, wagging, and flow-aligning in fd virus suspensions?
• RQ2 To what extent do textural contributions (e.g., defects, domain boundaries) affect the rheological response in fd virus nematic suspensions?
• RQ3 How well does the Doi-Edwards-Hess theory predict the observed dynamic transitions in a system of semi-flexible, charged rods?
• RQ4 What is the relationship between dynamic transitions and microstructural features such as shear banding and orientational order?
• RQ5 Can the experimental data be quantitatively matched to the DEH theory using the order parameter as a scaling variable without adjustable parameters?

Key findings

• The experimental critical shear rates for the tumbling-to-wagging and wagging-to-flow-aligning transitions show excellent quantitative agreement with the Doi-Edwards-Hess theory when scaled by the order parameter.
• The contribution of textural stress to the total stress is minimal, as evidenced by the absence of strain scaling and the dominance of Péclet number scaling in transient responses.
• A local maximum in viscosity is observed at the tumbling-to-wagging transition, which is unexplained by the DEH theory and may be linked to structural reorganization.
• Microscopy reveals the formation of macroscopic shear bands at high concentrations, coinciding with the disappearance of oscillatory features in the flow reversal response.
• The dynamic transitions are strongly correlated with structural evolution, suggesting that microstructural changes play a key role beyond the scope of current homogeneous DEH theory.
• Despite the rods' semi-flexibility and surface charge, the DEH theory accurately captures the dynamic behavior when the order parameter is used as a scaling variable, indicating robustness of the model for hard rod systems.