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Speaker: Yoshihiko Nishikawa (Kitasato University)
Time & Date: 14:00 – 15:00, May 7 (Wednesday), 2025 (JST)
Place: Hybrid (ISSP Room 615 & zoom)
Title:
*Orientational ordering in a two-dimensional active model for dense
bacterial suspension*
Abstract:
Active matter systems often exhibit dynamic and static properties that are
highly distinct from thermal equilibrium, including motility-induced phase
separation, active turbulence, and crystallization in low dimensions.
Bacterial suspension is one typical example, in which self-propelled
bacteria move and interact with others, leading to complex, emergent
behaviors. Recently, H. Lama et al. (2024) experimentally showed that
two-dimensional dense suspension of E. Coli has two glassy transitions at
different densities, where the orientational and translational degrees of
freedom become dynamically arrested, respectively. They further found that
the exponent for the critical divergence of the relaxation time is smaller
than the lower-bound of the mode-coupling theory for equilibrium glassy
systems. While this suggests the glassy transition to be qualitatively
different from the equilibrium counterpart, the origin of the small
exponent remains unclear.
Here, we propose a minimal active model for dense bacterial suspension in
two dimensions, and numerically study its dynamics and statics. In our
model, each bacterium is represented by a spherocylinder of fixed length,
with its state specified by position of the center of mass and orientation.
Bacteria interact with each other via a short-range repulsive interaction
and actively move in the direction of its orientation. To mimic the
tumbling motion of bacteria in a crowded environment, we further
incorporate in the model stochastic velocity reversal with a fixed rate per
unit time.
With increasing density, the orientational dynamics of the system
drastically slows down and its relaxation time shows a rapid growth,
suggesting the critical divergence at a finite density \phi_c. On the other
hand, even at \phi_c, the translational dynamics has a short relaxation
time and bacteria can easily change their positions, with their
orientations virtually fixed for a very long time. These dynamical
properties are consistent with the experiment. Despite the long time scale
for the bacterial orientation, the nematic order remains short ranged,
again consistent with the experimental results. However, we find that the
length scale of the ‘tetratic’ fourfold orientational order grows much
faster than that of the nematic order when approaching \phi_c, indicating
that the slow orientational dynamics is controlled by this orientational
order. We will also discuss the collective orientational and translational
dynamics near \phi_c.
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