Fluid Flows Prevent Microswimmer Clumps
Groups of self-propelled microscopic swimmers—ranging from bacteria to tiny robots—can sometimes spontaneously form dense clumps, leaving dilute regions between them, even in the absence of attractive forces. This “motility-induced phase separation” is well established in systems where the swimmers do not interact with one another through flows in the fluid. But a long-standing question has been whether it can persist in more common systems that include such hydrodynamic interactions. Now Tingtao Zhou and John Brady at Caltech have used a combination of theoretical work and simulations to show that these interactions always destroy any developing clusters [1]. The team’s findings could help scientists design systems of swimmers for biochemical sensing, targeted drug delivery, and other applications.
Zhou and Brady identified two independent reasons why hydrodynamic interactions impede motility-induced phase separation. First, linear fluid flows can sweep swimmers out of clusters as they are forming. And second, rotational fluid flows can make swimmers switch direction more quickly than they otherwise would. This difference means that swimmers don’t have enough time to push into crowds to produce stable clumps.
In earlier experiments and simulations, researchers uncovered what appeared to be motility-induced clustering of microscopic swimmers in the presence of hydrodynamic interactions. However, Zhou and Brady found that this effect was instead caused by the swimmers interacting with nearby walls and producing attractive fluid flows that mimicked motility-induced clumping. The team proposes that, by adding brush-like surface coatings to swimmers, hydrodynamic effects could be reduced, and motility-induced phase separation could be recovered.
–Ryan Wilkinson
Ryan Wilkinson is a Corresponding Editor for Physics Magazine based in Durham, UK.
References
- T. Zhou and J. F. Brady, “Hydrodynamic interactions destroy motility-induced phase separation in active suspensions,” Phys. Rev. Lett. 136, 088301 (2026).