Thinning and thickening in active microrheology

Area of Science:

• Physics
• Statistical Mechanics
• Soft Matter

Background:

• Effective friction of a probe particle in a many-particle system is crucial for understanding transport phenomena.
• The probe's friction can vary with velocity, showing complex behaviors like thinning and thickening.
• Existing models may not fully capture the microscopic origins of these velocity-dependent friction regimes.

Purpose of the Study:

• To propose a new theoretical framework explaining the thinning and thickening of effective friction.
• To identify the key time scales governing these friction behaviors.
• To investigate the validity of equilibrium velocity distributions in non-equilibrium scenarios.

Main Methods:

• Developing a three-time-scale picture based on diffusion, damping, and collision timescales.
• Conducting Langevin dynamics simulations to test the proposed model.
• Analyzing the microscopic origins of effective friction, particularly at high Reynolds and Peclet numbers.

Main Results:

• A three-time-scale model successfully describes the thinning and thickening of effective friction.
• The dominant timescales, influenced by pulling velocity, dictate the friction behavior.
• The Maxwellian velocity distribution of bath particles is inadequate for high Reynolds and Peclet number regimes.
• A microscopic mechanism for thickening at T=0 is identified.

Conclusions:

• The proposed three-time-scale picture provides a comprehensive explanation for probe particle friction dynamics.
• Pulling velocity and the interplay of different timescales are key determinants of friction regimes.
• The study highlights limitations of equilibrium assumptions in non-equilibrium systems and offers insights into observed phenomena in literature.