Activity-crowding coupling effect on the diffusion dynamics of a self-propelled particle in polymer solutions

Area of Science:

• Soft Matter Physics
• Polymer Science
• Statistical Mechanics

Background:

• Anomalous diffusion is prevalent in complex fluids like polymer solutions.
• Active particles introduce self-propulsion, altering standard diffusion behaviors.
• Understanding particle dynamics in crowded environments is crucial for various applications.

Purpose of the Study:

• To investigate the anomalous diffusion dynamics of an active particle in polymer solutions.
• To explore the transition between superdiffusion and subdiffusion regimes.
• To elucidate the interplay between particle activity and polymer crowding effects on diffusion.

Main Methods:

• Langevin Brownian dynamics simulations were employed.
• Mean-square displacement (MSD) was analyzed across various system parameters (active force, probe size, polymer volume fraction, chain length).
• Long-time diffusion coefficients were evaluated in pure solvent and polymer solutions.

Main Results:

• A novel transition between superdiffusion and subdiffusion was observed, driven by activity-crowding competition.
• The mean-square displacement (MSD) exhibited a power-law relation with dynamical persistence length (ΔMSD = 2L^m), with exponent m decreasing with polymer volume fraction.
• Relative diffusivity (D_a/D) followed Phillies' equation, with a fitting parameter κ increasing with activity.
• A multi-length scaling relation was justified, similar to passive probes.
• Activity was found to enhance effective viscosity, strengthening diffusion slowing due to crowding.

Conclusions:

• Particle activity and polymer crowding exhibit a complex coupling effect on anomalous diffusion.
• Activity can paradoxically enhance the slowing effect of crowding by increasing effective viscosity.
• The study provides insights into the fundamental mechanisms governing active particle transport in polymeric media.