How are mobility and friction related in viscoelastic fluids?
Juliana Caspers1, Nikolas Ditz2, Karthika Krishna Kumar2
1Institute for Theoretical Physics, Georg-August Universität Göttingen, 37077 Göttingen, Germany.
The Journal of Chemical Physics
|January 14, 2023
Summary
Researchers explored colloidal probe motion in viscoelastic fluids, generalizing the Einstein relation. They found distinct behaviors in recoil and mean squared displacement (MSD) at higher concentrations, revealing a Volterra relation between friction and mobility kernels.
Area of Science:
- Soft matter physics
- Rheology
- Colloidal science
Background:
- Colloidal probe motion in Newtonian fluids follows the Einstein relation.
- Viscoelastic fluids introduce time-dependent memory effects, complicating this relationship.
- Friction and mobility describe probe response to force or velocity, respectively.
Purpose of the Study:
- To generalize the Einstein relation for colloidal probes in viscoelastic fluids.
- To investigate the relationship between probe recoil and mean squared displacement (MSD).
- To understand how concentration affects friction and mobility kernels.
Main Methods:
- Theoretical investigation using linear response theory.
- Experimental measurements of probe recoil and equilibrium MSD.
- Application of two theoretical models: linear two-bath particle model and mode coupling theory for hard spheres.
Main Results:
- A generalized Einstein relation was derived, connecting recoil and MSD to the mobility kernel.
- Distinct behaviors of MSD and recoil were observed with increasing particle concentration.
- A Volterra relation was identified between friction and mobility kernels, explaining timescale differences.
Conclusions:
- The generalized Einstein relation provides a unified framework for analyzing probe dynamics in viscoelastic media.
- Concentration-dependent variations in kernels explain the divergent behaviors of recoil and MSD.
- The Volterra relation offers insights into the complex interplay of friction and mobility in concentrated colloidal systems.
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