Collective motion of self-propelled colloids
Exploring universal properties of active matter with artificial active particles
Do collections of artificial moving objects obey universal laws common to groups of living matters? To answer such a question, active matter physics has dealt with not only living matters but also inanimate objects. Various types of artificial active matter systems have been devised so far, and among those systems we employ colloidal particle systems for our experimental study [A]. This choice is made because we can quite accurately control a large number of colloidal particles under a microscope, which makes the systems suitable for experimental statistical physics.
Janus particles: asymmetric colloidal particles fuelled by an AC electric field
We fabricate asymmetric colloidal particles with a diameter of about 3 µm, made of dielectric materials (e.g. silica), whose hemispheres are coated with metals (e.g. titanium) . This type of colloidal particles is called "Janus particles". When water suspensions of those Janus particles are sandwiched between two transparent electrodes and an electric field is applied in the z-direction, an asymmetric hydrodynamic flow is induced around each particle due to the particles' intrinsic asymmetry. As a result of this asymmetric flow, the Janus particles swim around in the xy-plane as if they were living microorganisms (Movie 1).
We can easily control the swimming speed of the Janus particles, just by tuning the amplitude of the applied AC electric field. In addition, the inter-particle interactions and the direction of motion can also be controlled by changing the AC frequency and/or the ion concentration in the water. This high controllability makes it possible to experimentally approach collective phenomena that are difficult to investigate quantitatively with living matters, such as bacteria, and systems reconstituted from biomaterials. As an example, the Janus paticle systems can be used to study beating motion that is analogous to flagellar dynamics in eukaryotic cells, with which we have successfully verified a theoretical prediction on scaling relation between the beating frequency and the driving force of the individual elements .
Currently, by using Janus particles as a model system, we continue investigating properties of interest from the statistical mechanics viewpoint, such as anomolous density fluctuations in various ordered phases and universal scaling in correlation functions.
[A] A review on recent experiments and theories： C. Bechinger, R. Di Leonardo, H. Löwen, C. Reichhardt, G. Volpe and G. Volpe, Rev. Mod. Phys. 88, 045006 (2016) [web].