research
(not up-to-date...sorry! I wrote this page during my PhD before publishing my 1st bacterial paper...)
Please look at the Japanese version of this page with Google translate etc., which is more up-to-date!
For recent studies, please have a look at our lab web pages: about bacterial turbulence and Janus particles
I'm interested in physics of active matter, especially their collective behavior.
In order to experimentally study collective behavior of self-propelled particles,
I use two kinds of particles: Janus particles and bacteria.
By using these artificial and biological swimmers, I'm trying to bridge the gap between theoretical works, real natural phenomena, and biological significance of collective motion.
In order to clarify whether the phenomena seen in active matter result purely from their motility itself or from biological activities, experimental approaches with non-biological self-propelled particles are of great importance.
In the case of living things, they need food to gain energy required for motility. Similarly, non-biological self-propelled particles need some energy injection from outside to convert the energy into kinetic energy, so we need to prepare some external energy source such as chemicals, electric fields, vibration, etc.
We have made micrometer-scale asymmetrical colloidal particles (Janus particles) and investigated the individual motions and collective motions in pure water. When we apply a homogeneous AC electric field, the Janus particles swim perpendicular to the electric field, which is different from usual electrophoresis whose direction of motion is parallel to the electric field. The applied electric field drives asymmetric flow around the particles due to the difference of the dielectric constants of the both hemispheres. The counteractions of this flow causes the Janus particles to move around in a quasi-two-dimensional plane perpendicular to the electric field.
The Janus particles driven by an electric field are suitable for studying collective behavior, because we can control their driving force, speed, interactions and even their directions of motion by changing the amplitude and the frequency of the electric field. We investigate the properties of their collective behavior such as velocity correlations and local order parameters. We have revealed that the Janus particles can form mesoscopic turbulence, or active turbulence, similar to bacterial turbulence.
recent work on Janus particles:
Collective motion of bacteria (ref: arXiv)
I'm also working on experiments using bacteria to demonstrate that what has been observed in numerical and theoretical studies on active matter can really be observed in biological systems.
At the moment, I elongate Escherichia coli bacteria to obtain filamentous cells and investigating large-scale properties of their collective behavior. These bacteria exhibit nematic phase at high density, and it has turned out that this nematic phase has true long-range order and exhibit "giant number fluctuations (GNF)", which is predicted by seminal works on hydrodynamic theories by Toner, Tu and Ramaswamy et al. and confirmed by large-scale simulations on Vicsek-style models. Hence our experimental system falls into Vicsek universality class, although actual interactions of bacteria are much more complicated than Vicsek-style models. This is the first experimental realization of Vicsek-class phenomena, especially the Toner-Tu-Ramaswamy phase.
[See my publications]
Apart from beautiful mathematical properties behind the collective motion of our bacteria, I'm also trying to figure out how biologically significant their usual shapes are and whether their collective motion is optimal or not from the viewpoint of active matter physics.