Trapped-particle microrheology of active suspensions

Area of Science:

• Soft Matter Physics
• Rheology
• Colloidal Science

Background:

• Microrheology infers fluid properties from probe particle motion.
• Existing models often use constant-force (CF) or constant-velocity (CV) probes, which have theoretical limitations.
• CF/CV constraints conflict with thermodynamic uncertainty relations for probe fluctuations.

Purpose of the Study:

• Develop a generalized microrheology model allowing probe position and trap force to fluctuate.
• Overcome limitations of CF/CV models in quantifying probe dynamics.
• Provide a theoretical framework applicable to experimental microrheology.

Main Methods:

• Derived a pair Smoluchowski equation from the N-particle Smoluchowski equation for active Brownian particles.
• Modeled probe dynamics interacting with a single bath particle, neglecting hydrodynamic interactions.
• Determined probe position mean and variance from the pair probability distribution.

Main Results:

• Characterized system behavior in weak and strong trap limits.
• Demonstrated that the generalized model reduces to CF and CV models in appropriate limits.
• Quantified probe position and force fluctuations within a unified theoretical framework.

Conclusions:

• The proposed model offers a more realistic approach to microrheology by allowing natural fluctuations.
• This work bridges the gap between theoretical constraints and experimental realities in probe-based rheology.
• The generalized framework enhances the accurate measurement of complex fluid viscoelasticity.