Bipolar & periodic active matter: "AC Quincke rollers"
I worked on the Quincke rollers under an AC electric field as a study in active matter. The characteristic of our system is bipolar and periodic: back-and-forth motion at an intended 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, a particle also rolls on an electrode but shows back-and-forth motion periodically with active Brownian particle (ABP)-like long-time persistent motions experimentally. 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: roughness of electrode surface.
We also found the formation of small clusters and characterized the cooperated motion.
Rough particles at a liquid interface
Shapes of colloidal particles matter the 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.
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.)
