Activity waves and freestanding vortices in populations of subcritical Quincke rollers.

Virtually all of the many active matter systems studied so far are made of units (biofilaments, cells, colloidal particles, robots, animals, etc.) that move even when they are alone or isolated. Their collective properties continue to fascinate, and we now understand better how they are unique to the bulk transduction of energy into work. Here we demonstrate that systems in which isolated but potentially active particles do not move can exhibit specific and remarkable collective properties. Combining experiments, theory, and numerical simulations, we show that such subcritical active matter can be realized with Quincke rollers, that is, dielectric colloidal particles immersed in a conducting fluid subjected to a vertical DC electric field. Working below the threshold field value marking the onset of motion for a single colloid, we find fast activity waves, reminiscent of excitable systems, and stable, arbitrarily large self-standing vortices made of thousands of particles moving at the same speed. Our theoretical model accounts for these phenomena and shows how they can arise in the absence of confining boundaries and individual chirality. We argue that our findings imply that a faithful description of the collective properties of Quincke rollers need to consider the fluid surrounding particles.

Incorporation of graphenes in nanostructured TiO(2) films via molecular grafting for dye-sensitized solar cell application.

This paper presents a systematic investigation on the incorporation of chemical exfoliation graphene sheets (GS) in TiO(2) nanoparticle films via a molecular grafting method for dye-sensitized solar cells (DSSCs). By controlling the oxidation time in the chemical exfoliation process, both high conductivity of reduced GS and good attachment of TiO(2) nanoparticles on the GS were achieved. Uniform GS/TiO(2) composite films with large areas on conductive glass were prepared by electrophoretic deposition, and the incorporation of GS significantly improved the conductivity of the TiO(2) nanoparticle film by more than 2 orders of magnitude. Moreover, the power conversion efficiency for DSSC based on GS/TiO(2) composite films is more than 5 times higher than that based on TiO(2) alone, indicating that the incorporation of GS is an efficient means for enhancing the photovoltaic (PV) performance. The better PV performance of GS/TiO(2) DSSC is also attributed to the better dye loading of GS/TiO(2) film than that of TiO(2) film. The effect of GS content on the PV performances was also investigated. It was found that the power conversion efficiency increased first and then decreased with the increasing of GS concentration due to the decrease in the transmittance at high GS content. Further improvements can be expected by fully optimizing fabrication conditions and device configuration, such as increasing dye loading via thicker films. The present synthetic strategy is expected to lead to a family of composites with designed properties.