Statistics of Colloidal Suspensions Stirred by Microswimmers

Area of Science:

• Physics
• Statistical Mechanics
• Biophysics

Background:

• Colloid dynamics in biological suspensions are complex.
• Microswimmers like Chlamydomonas reinhardtii introduce unique perturbations.
• Understanding particle displacement statistics is crucial for modeling active matter.

Purpose of the Study:

• To statistically analyze the trajectories of colloids in a Chlamydomonas reinhardtii suspension.
• To compare experimental data with various theoretical models of particle motion.
• To identify the most accurate model describing colloid diffusion influenced by microorganisms.

Main Methods:

• Experimental tracking of colloid trajectories in dilute algal suspensions.
• Statistical analysis of displacement probability density functions (pdfs).
• Systematic variation of tracer diameters, swimmer concentrations, and velocities.

Main Results:

• Measured pdfs span 7 orders of magnitude, showing non-Gaussian tails at intermediate times.
• Diffusive scaling of pdfs breaks down at longer intervals, transitioning to Gaussian.
• Mean squared displacements are linear across all measured time intervals.

Conclusions:

• Mean-field theories and effective temperature models are insufficient to describe the observed motion.
• The experimental data are best explained by a microscopic model of swimmer-induced displacements.
• This study provides a rigorous validation of theoretical frameworks for active Brownian motion.