Airi N. KATO website

My current interests: Active matter in Soft Boundaries, especially interfaces

Soft boundaries such as droplet interfaces provide deformable confinement for active particles. The bulk behavior of self-propelled particles has been extensively studied for many years. More recently, it has become clear that geometrical confinements can strongly modify not only their collective dynamics but even the motion of individual active particles.

Among different types of confinement, deformable boundaries are particularly intriguing because they can dynamically couple to the motion of active particles, giving rise to rich behaviors. They are also important for understanding how the properties of bulk active matter are manifested under confinement.
In addition, liquid interfaces are soft boundaries that are ubiquitous in soft matter systems. I believe that active systems involving fluid interfaces will play an important role in the development of active matter as a broader class of nonequilibrium soft materials.

Active particle in a thin film droplet

We investigate a light-driven Janus particle in a surfactant-free thin oil droplet stabilized at an air–water interface. The particle motion strongly couples to changes in the droplet thickness profile and is driven by thermal Marangoni flows at speeds ranging from 1 mm/s to 1 cm/s. Depending on the conditions, the particle exhibits motions from periodic circular trajectories to intermittent irregular dynamics. By incorporating capillary effects, we show that the periodic motion arises from the interplay between activity, a capillary centripetal force associated with the lens-shaped droplet geometry, and a capillary torque generated by a nonequilibrium interfacial profile.

For more, see our PRResearch paper!

Bipolar & periodic active matter: "AC Quincke rollers"

I worked on the Quincke rollers under an AC electric field as a study in active matter. Our system is bipolar and periodic: back-and-forth motion at a set frequency.

Quincke rollers are dielectric spheres suspended in a conducting liquid and driven by usually a DC electric field. DC Quincke roller rolls on an electrode unidirectionally at a constant speed by an ambient charge-driven flow; the dynamics is described as electro-rotation by emerging a constant effective polarization.

In our system, AC Quincke rollers also roll on an electrode but exhibit periodic back-and-forth motion with experimentally observed active Brownian particle (ABP)-like long-time persistent motion. Importantly, the periodic nature can be explained by the simple generalization of the DC Quincke rollers model, but the ABP-like long-time motions are not. By introducing the top-bottom asymmetry due to the lower electrode, we could explain the persistent motion.

Moreover, we pointed out the reason that the diffusion coefficient of Quincke rollers is much larger than the thermal one: the roughness of the electrode surface.

We also found the formation of small clusters and characterized the cooperative motion.

Paper: Soft Matter, 2022,18, 5435-5445

Rough particles at a liquid interface

Shapes of colloidal particles matter for interparticle interaction at liquid interfaces. In this study, the effect of surface roughness was investigated experimentally and numerically in terms of isotherms and particle configuration changes upon compression. Sufficiently rough particles exhibit an intermediate state between gas-like and solid-like states due to roughness-induced capillary attraction, forming a percolated network. Moreover, the surface roughness decreases the jamming point, attributed to the friction and interlocking due to the particles’ surface asperities. Furthermore, the tangential contact force owing to surface asperities can cause a gradual off-plane collapse of the compressed monolayer. Our study on rough colloids at interfaces will also benefit the development of functional materials.

Paper: JCIS 641 492–498 , review article in Japanese: JJACG 50 2.

In addition, we used the rough-surfaced colloids to verify the stabilization and mechanism of emulsions (Pickering emulsions) without surfactants. We systematically investigated the differences in morphology depending on the ratio of water, ethanol, and oil, and discovered that at a certain ratio, stable Pickering emulsions with continuous channels due to rough particles were formed. In the process of channel structure formation, it is thought that the large friction due to the roughness of the particle surface hinders rearrangement, resulting in slow dynamics. In fact, the relaxation time increased due to roughness in the fracture under a constant stress. It is thought that the entanglement by the protrusions on the particle surface creates a structure that mechanically supports the emulsion. Ethanol, which forms part of the aqueous phase, is thought to increase the amphiphilicity of the particles, and the movement of ethanol to the interface was also verified by simulation. Since it is possible to create stable structures such as water-in-oil or oil-in-water droplets depending on the composition, this technology has potential applications in drug delivery of components whose solubility changes depending on the composition, such as curcumin.
Paper: Advanced Functional Materials 2308608

Quantum estimation, theory

At the very beginning of my carrier, I joined the project of theoretical study on quantum estimation via sequential measurement.

In quantum estimation, people estimate non-observable physical values (e.g. coupling constant, dissipating rate, temperature, etc.) via the result of repeated measurements. Measurements in quantum mechanics are usually assumed to prepare a lot of identical copies of an initial state, measure respectively, then average each result. However, especially when it is difficult to prepare identical initial states, we can sequentially measure with a time interval and estimate parameters from the non-i.i.d. (non-identical) results.

My contribution part in the paper is Sec. . The imperfectness of each measurement sometimes can be beneficial for parameters’ estimation. We consider the example that a qubit inserted as a thermometer into a thermal reservoir. We can estimate the temperature by this scheme, for example.

For more detail See "Quantum Estimation via Sequential Measurements"

New Journal of Physics 17, 113055 (2015)

The right figure is the test image of keywords. (https://tagul.com/create was used.)