Electric powered microrotors roll and hover

The Quincke effect, i.e. the spontaneous spinning of a dielectric sphere in a uniform DC electric field, arises from particle electric polarization. Discovered over a century ago, it has had its fair share of research attention, nonetheless, it has recently started attracting increasing interest. Such research spark has been ignited by the recently reported Lorenz chaotic rotations. In addition, pairs of Quincke rotating spheres have been found to display intricate trajectories. Ideally, Quincke rotation in complex media is affected by the medium structure. In fact, Quincke rotors initially resting on a surface roll with steady velocity. In a uniform field, if an induced dipole is antiparallel to the applied field, a spontaneous symmetry breaking occurs in strong fields. The theoretical analysis of this instability for a sphere in an unbounded domain predicts that the dipole adopts a steady tilt angle relative to the applied field direction above a threshold electric field. The resulting electric torque drives rotation. Previous studies have reported that a sphere initially resting on the electrode rolls with steady velocity. Regardless, more research is necessitated to fully comprehend the dynamics of Quincke rollers.

Researchers at the Northwestern University: Gerardo Pradillo, Dr. Hamid Karani (postdoctoral fellow) and led by Professor Petia Vlahovska studied the effect of confinement on the Quincke rotor dynamics. Their work was motivated by the fact that in most experiments with Quincke rollers, the particles used are often sandwiched between electrodes and are rolling on the bottom surface. Their work is currently published in the research journal, Soft Matter.

Technically, their empirical setup was generally comprised of two indium-tin-oxide (ITO) coated glass slides separated by a Teflon tape of specific thickness. Owing to the fact that the Quincke effect is very sensitive to the suspending fluid conductivity, the researchers opted to control it by adding surfactant. To be specific, the conductivity of the suspending fluid was controlled by adding dioctyl sulfosuccinate sodium salt (AOT) to hexadecane.

The research team reported that the additive strongly influenced the Quincke dynamics and in addition to rolling, a new regime of hovering, where the sphere lifted off the bottom surface and spun in the space between the electrodes, was observed. The research trio also reported that the Quincke effect in confinement was very sensitive to the additive used to control the conductivity of the suspending oil. In other words, for their system of hexadecane with added AOT, moisture in the AOT dramatically changed the Quincke behavior.

In summary, the study by Professor Petia Vlahovska and her group experimentally investigated the Quincke effect in strong confinement with particle diameter to gap ratio d/h ranging between 0.17 and 0.83. Generally, they found out that in strong fields, the rotation becomes unsteady, with time-periodic or chaotic rate. Overall, their experimental results showed that the onset of Quincke rotation strongly depended on particle confinement and the threshold for rolling was higher compared to rotation in the hovering state. The newly-discovered hovering regime opens opportunities to “propel” freely suspended colloidal particles with shape anisotropy.