Whereas the laws of thermodynamics prohibit extraction of useful work from the Brownian motion of particles in equilibrium, these motions can be "rectified" under nonequilibrium conditions, for example, in the presence of asymmetric geometrical obstacles. Here, we describe a class of systems in which aerobic bacteria Bacillus subtilis moving randomly in a fluid film power submillimeter gears and primitive systems of gears decorated with asymmetric teeth. The directional rotation is observed only in the regime of collective bacterial swimming and the gears' angular velocities depend on and can be controlled by the amount of oxygen available to the bacteria. The ability to harness and control the power of collective motions appears an important requirement for further development of mechanical systems driven by microorganisms.T he laws of thermodynamics prohibit extraction of useful work from the Brownian motion of molecules or particles in systems at equilibrium (nonexistence of a perpetuum mobile of the second kind or Maxwell demon) (1, 2). When, however, such randomly moving objects interact with certain types of timevarying external potentials (3-5) or with asymmetric geometrical obstacles under nonequilibrium conditions (6-10), their motions can be "rectified" and made directional. This phenomenon, first considered by Smoluchowski (11) and then analyzed in detail by Feynman (1), underlies the operation of so-called Brownian ratchets and motors (12)(13)(14)(15). The examples of biological "Brownian motors" include kinesin and myosin proteins converting chemical energy into directed motion on microtubules (16), and bacteria propelling themselves in viscous fluid owing to the "asymmetry"/chirality of flagellar rotation (14, 17). In man-made systems, ratcheting in asymmetric, funnel-like microchannels has been used to guide bacteria (18) and to sort cancerous from noncancerous cells (18,19). Recently, there has been interest in using randomly moving bacteria (20), cells (21), or even extracts of cellular cytoskeleton (22, 23) to serve as "biological fuel" powering mechanical micromachines-for instance, systems of microscopic gears. Although recent theoretical work (24) indicates that the vision of such machines is within the realm of possibility, there have been no experimental demonstrations, save the arrangements in which the motions of the bacteria/cells have been preorganized using microfluidic channels (9,20,(25)(26)(27)(28).In this paper we describe a class of systems in which common aerobic motile bacteria Bacillus subtilis moving randomly in a thin fluid film power submillimeter gears and primitive systems of gears decorated with asymmetric teeth (Fig. 1A and B). Whereas the gear's center of mass exhibits apparent random motion, the gears are spun in the direction determined by the gear's asymmetry, i.e. orientation of the teeth's slanted edges. Remarkably, directional rotation of the gears is observed only in the regime of collective bacterial swimming and the gears' angular velocities depend on and can ...

In traditional photoconductors, the impinging light generates mobile charge carriers in the valence and/or conduction bands, causing the material's conductivity to increase. Such positive photoconductance is observed in both bulk and nanostructured photoconductors. Here we describe a class of nanoparticle-based materials whose conductivity can either increase or decrease on irradiation with visible light of wavelengths close to the particles' surface plasmon resonance. The remarkable feature of these plasmonic materials is that the sign of the conductivity change and the nature of the electron transport between the nanoparticles depend on the molecules comprising the self-assembled monolayers (SAMs) stabilizing the nanoparticles. For SAMs made of electrically neutral (polar and non-polar) molecules, conductivity increases on irradiation. If, however, the SAMs contain electrically charged (either negatively or positively) groups, conductivity decreases. The optical and electrical characteristics of these previously undescribed inverse photoconductors can be engineered flexibly by adjusting the material properties of the nanoparticles and of the coating SAMs. In particular, in films comprising mixtures of different nanoparticles or nanoparticles coated with mixed SAMs, the overall photoconductance is a weighted average of the changes induced by the individual components. These and other observations can be rationalized in terms of light-induced creation of mobile charge carriers whose transport through the charged SAMs is inhibited by carrier trapping in transient polaron-like states. The nanoparticle-based photoconductors we describe could have uses in chemical sensors and/or in conjunction with flexible substrates.

Elektrisierender Kontakt: Gleichartige Stücke von Isolatoren mit Dicken in der Größe von Atomen trennen eine Ladung Q, wenn sie in Kontakt gebracht und wieder getrennt werden. Bei wiederholten Kontakten steigt die Größe der getrennten Ladung monoton (siehe Bild). Ein Modell erklärt diese Phänomene anhand von Fluktuationen in der Zusammensetzung der scheinbar identischen Kontaktoberflächen auf molekularer Ebene.

Ch‐ch‐ch‐charges: Pieces of identical, atomically flat insulators separate a charge Q when brought into contact and then parted. Repeated contacts cause the magnitudes of the separated charges to increase monotonically (see picture). A theoretical model is presented that explains these phenomena by the inherent, molecular‐scale fluctuations in the composition of the seemingly identical contacting surfaces.

Millimeter-sized reactor particles made of permeable polymer doped with catalysts arranged in a core/shell fashion direct sequences of chemical reactions (e.g., alkyne coupling followed by hydrogenation or hydrosilylation followed by hydrogenation). Spatial compartmentalization of catalysts coupled with the diffusion of substrates controls reaction order and avoids formation of byproducts. The experimentally observed yields of reaction sequences are reproduced by a theoretical model, which accounts for the reaction kinetics and the diffusion of the species involved.