Glassy Dynamics in Dense Bacterial Population

With the growing fascination towards understanding the characteristics and properties of non-equilibrium soft matter, there is a quest to understand and inspect the dense emergent phases of active matter. It is known that the constituting entity in active matter can self-organize and can exhibit collective motion, which results in intriguing phases, such as active-fluid phase (with a low population). In recent years, researchers have expressed interest in exploring the dense phase of active matter with the hope of characterizing the glassy phase in active matter. Increasing the density or supercooling of the fluid can result in a dynamic transition from the fluid phase to an amorphous solid phase or glassy phase. The phenomenon is popularly known as glass transition [1]. The motion of the constituting entity in a glassy phase is kinetically arrested, while the system lacks the long-range structural order. In Takeuchi lab, we investigated the dense phase of active matter and inspect for the transition in the physical state from the active fluid to active glass using E. coli, RP437 [3]. The investigation was conducted within a microfluidic device specifically designed for this purpose, and we call it Extensive Micro Perfusion System (EMPS) [shown in Fig(a)]. 

As the density of bacterial cells increases, their motion slows down, leading to a transition from a fluid-like state to a kinetically arrested state. The system is akin to an amorphous solid wherein their neighbours arrest the motion of the constituting entities. The structural relaxation time a.k.a alpha-relaxation time, is a characteristic quantifier relevant to the system's dynamics. We estimate such relaxation time, associated with both translation and orientation motion for various area densities, as shown in [Fig. b]. As apparent from Fig. b, the magnitude of the relaxation time for both orientation and translation motion increases rapidly on increasing the area density of bacterial cells [Fig. b], indicating a rapid slowing down of bacterial motion and inferring the glass transition point or the critical area density (referred to with subscript 'c' and 'Q') at which the system motion rapidly slows down. They also display dynamical heterogeneity, fragility and ageing. Therefore, the suspension of bacteria vitrifies in a two-step: the orientation motion is suppressed first, whereas the translation motion is arrested at a higher area density [3].  The vitrification process involves the formation of dynamical microdomains comprising aligned bacterial cells, resulting in collective motion and unusual behaviour in characteristic quantifiers of glass. Our investigation may contribute to the general understanding of densely packed active rods, as well as the formation of dense microbial aggregates, including biofilms.

References

[1]  L. Berthier and G. Biroli, Rev. Mod. Phys. 83, 587 (2011).

[2]  M. C. Marchetti et al., Rev. Mod. Phys. 85, 1143 (2013).

[3]  H. Lama, M. J. Yamamoto, Y. Furuta, T. Shimaya, and K. A. Takeuchi, arXiv:2205.10436.

[4] T. Shimaya, R. Okura, Y. Wakamoto, and K. A. Takeuchi, Commun. Phys. 4, 238 (2021).