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Colloids in active fluids: anomalous microrheology and negative drag
G Foffano1, J S Lintuvuori, K Stratford
1SUPA, School of Physics and Astronomy, University of Edinburgh, Mayfield Road, Edinburgh, EH9 3JZ, United Kingdom.
Physical Review Letters
|October 4, 2012
Summary
Active nematic fluids exhibit non-Stokesian drag on colloidal probes. Large particles in contractile fluids can experience negative viscous drag, moving against applied forces due to disrupted orientational order.
Area of Science:
- Soft matter physics
- Fluid dynamics
- Biophysics
Background:
- Active nematic fluids are complex materials with self-propelling elements.
- Understanding particle dynamics in these fluids is crucial for applications in micro-robotics and biological systems.
- Colloidal probe experiments are standard for measuring forces in complex fluids.
Purpose of the Study:
- To investigate the hydrodynamic interactions between a colloidal probe and an active nematic fluid.
- To determine if the drag force on the probe is dependent on its size and the fluid's properties.
- To explore the phenomenon of negative viscous drag in active matter systems.
Main Methods:
- Numerical simulations of a colloidal probe pulled through an active nematic fluid.
- Analysis of the forces acting on the probe and the fluid's orientational order parameter.
- Varying probe size and fluid contractility to observe drag effects.
Main Results:
- Observed non-Stokesian drag, meaning drag is not simply proportional to particle radius.
- Demonstrated negative viscous drag for large particles in contractile active nematics (e.g., actomyosin gels).
- Correlated negative drag with the disruption of orientational order around the probe.
Conclusions:
- The drag experienced by a probe in an active nematic is complex and deviates from classical fluid dynamics.
- Negative viscous drag is a consequence of active stresses and altered orientational order, not just passive hydrodynamics.
- Simulations provide a framework for understanding particle manipulation in active soft matter.
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