Active and passive microrheology with large tracers in hard colloids

Area of Science:

• Soft Matter Physics
• Colloidal Science
• Computational Physics

Background:

• Understanding tracer particle dynamics is crucial for characterizing complex fluids.
• Microrheology techniques probe fluid properties at the microscale.

Purpose of the Study:

• To investigate the dynamics of a tracer particle in a quasi-hard colloidal sphere bath.
• To explore the influence of tracer size on passive and active microrheology.
• To connect simulation results with theoretical predictions from mode coupling theory (MCT).

Main Methods:

• Langevin dynamics simulations were employed.
• Mode Coupling Theory (MCT) was used for theoretical analysis.
• Tracer radius was systematically varied relative to bath particle radius.

Main Results:

• Tracer diffusion and friction coefficients were determined for various tracer sizes.
• Linear response theory successfully connected passive and active microrheology regimes.
• MCT provided insights into short-time rattling dynamics and long-time diffusion mechanisms.

Conclusions:

• Tracer size significantly impacts dynamics, with diffusion arising from transverse momentum transport.
• The Brinkman equation effectively describes flow fields around pulled tracers.
• MCT, extended with flow fields, offers a more complete picture than density-only couplings.