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Published on: October 25, 2012
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Pseudogiant Number Fluctuations and Nematic Order in Microswimmer Suspensions
Ismail El Korde1, Dóra Bárdfalvy1, Jason M Lewis1
1Lund University, Division of Physical Chemistry, P.O. Box 124, S-221 00 Lund, Sweden.
Physical Review Letters
|September 15, 2025
Summary
Giant number fluctuations (GNFs) were observed in wet active matter, but differ from dry systems. These pseudo-GNFs in microswimmers are transient and scale-dependent, questioning true GNFs in fluid flows.
Area of Science:
- Physics
- Soft Matter Physics
- Active Matter
Background:
- Giant number fluctuations (GNFs) characterize dry active matter with orientational order.
- Wet active matter systems, like bacterial suspensions, show enhanced fluctuations experimentally, but lack theoretical predictions for GNFs.
Purpose of the Study:
- To numerically investigate the emergence of super-Gaussian number fluctuations in a 3D suspension of pusher microswimmers.
- To determine the conditions under which these fluctuations appear and their relationship to collective motion and bacterial properties.
Main Methods:
- Numerical simulations of a three-dimensional suspension of pusher microswimmers.
- Analysis of particle number fluctuations and their dependence on bacterial persistence length and system length scales.
Main Results:
- Super-Gaussian number fluctuations emerge in microswimmer suspensions above the transition to collective motion.
- These fluctuations are dependent on the bacterial persistence length (ℓp) and occur on length scales smaller than the nematic patch size (ξ).
- The observed fluctuations are termed 'pseudo-GNFs' as they are transient and scale-dependent, unlike true GNFs.
Conclusions:
- Enhanced density fluctuations in biological active matter may be transient effects, decaying beyond mesoscopic scales.
- The findings suggest that 'true' GNFs with universal properties might be limited in fluid environments.
- This work raises questions about the existence and universality of GNFs in momentum-conserving active matter systems.
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