Self-propelling bacteria are a nanotechnology dream. These unicellular organisms are not just capable of living and reproducing, but they can swim very efficiently, sense the environment, and look for food, all packaged in a body measuring a few microns. Before such perfect machines can be artificially assembled, researchers are beginning to explore new ways to harness bacteria as propelling units for microdevices. Proposed strategies require the careful task of aligning and binding bacterial cells on synthetic surfaces in order to have them work cooperatively. Here we show that asymmetric environments can produce a spontaneous and unidirectional rotation of nanofabricated objects immersed in an active bacterial bath. The propulsion mechanism is provided by the self-assembly of motile Escherichia coli cells along the rotor boundaries. Our results highlight the technological implications of active matter's ability to overcome the restrictions imposed by the second law of thermodynamics on equilibrium passive fluids.biological motors | self-propulsion | ratchet effect

We numerically investigate the supercooled dynamics of two simple model liquids exploiting the partition of the multidimensional configuration space in basins of attraction of the stationary points (inherent saddles) of the potential energy surface. We find that the inherent saddle order and potential energy are well-defined functions of the temperature T. Moreover, by decreasing T, the saddle order vanishes at the same temperature (T(MCT)) where the inverse diffusivity appears to diverge as a power law. This allows a topological interpretation of T(MCT): it marks the transition from a dynamics between basins of saddles (T > T(MCT)) to a dynamics between basins of minima (T < T(MCT)).

Micromotors pushed by biological entities, like motile bacteria, constitute a fascinating way to convert chemical energy into mechanical work at the micrometer scale. Here we show, by using numerical simulations, that a properly designed asymmetric object can be spontaneously set into the desired motion when immersed in a chaotic bacterial bath. Our findings open the way to conceive new hybrid microdevices exploiting the mechanical power production of bacterial organisms. Moreover, the system provides an example of how, in contrast with equilibrium thermal baths, the irreversible chaotic motion of active particles can be rectified by asymmetric environments.Ensembles of animate organisms behave in a very rich and surprising way if compared to inanimate objects, as atoms or molecules in a gas or a liquid. Everyone has been amazed by the cooperative motion of birds in a flock, fishes in a school or wildebeest in a herd [1,2]. Also at the micrometer scale elementary living organisms, like bacterial cells, show an extraordinary variety of behaviors, such as collective motions [3,4,5,6], complex chemical-mediated motility or chemotaxis [7], spatiotemporal patterns [8], self-organized structures [9], biofilms formation [10]. An important peculiarity of animate organisms is the fact that they can be self-propelled, using a variety of different mechanisms for this purpose [11]. Motile cilia and turned flagella are two example of evolutionary tricks adopted by living organisms to accomplish the hard task of swimming at low Reynolds number [12]. One can think about such ensembles of organisms as open systems, with a net incoming flux of energy (provided by nutrients) stored and converted into mechanical motion by irreversible processes happening inside the cell body. The resulting dynamics breaks time inversion symmetry so that asymmetric environments can result in directed motions which, in equilibrated Hamiltonian systems, would be forbidden by detailed balance [13,14]. A natural question then arises: is it possible to rectify such a non equilibrium dynamics to propel microdevices?Biological molecular motors constitute a fascinating mechanism to generate motion at the nanoscale [15,16]. When larger, micron sized, structures need propulsion the preassembled motor units found in unicellular motile organism may offer several advantages over isolated proteins. In a recent experiment [17,18] bacterial driven micromotors have been assembled by biochemically attaching motile bacteria to a microrotary motor. Such procedures require the construction of narrow tracks to induce a unidirectional binding of bacterial cells on to the moving rotor with a consequent increased complexity in designs and limited number of working bacteria.Here we numerically show that a properly designed asymmetric motor immersed in a chaotic bacterial bath can be spontaneously set into the desired motion. Our numerical findings suggest the possibility to construct new opportunely shaped microdevices able to exploit the propelling power of motile bacteria...

We study experimentally and numerically the dynamics of colloidal beads confined by a harmonic potential in a bath of swimming E. coli bacteria. The resulting dynamics is well approximated by a Langevin equation for an overdamped oscillator driven by the combination of a white thermal noise and an exponentially correlated active noise. This scenario leads to a simple generalization of the equipartition theorem resulting in the coexistence of two different effective temperatures that govern dynamics along the flat and the curved directions in the potential landscape.

The hydrodynamic interactions of a swimming bacterium with a neighboring surface can cause it to swim in circles. For example, when E. coli is above a solid surface it had been observed to swim in a clockwise direction. By contrast we observe that, when swimming near a liquid-air interface, the sense of rotation is reversed. We quantitatively account for this through the hydrodynamic interaction of the bacterium with its own mirror image swimming on the opposite side of a perfect-slip boundary. The strength of the coupling is reduced for longer cells, where the torque is spread over a larger length, resulting in longer bacteria swimming in larger circles. We confirm this through precise video measurements of bacterial trajectories and orientations.

A simple model to investigate the long time dynamics of glass-formers is presented and applied to study a Lennard-Jones system in supercooled and glassy phases. According to our model, the point representing the system in the configurational phase space performs harmonic vibrations around (and activated jumps between) minima pertaining to a connected network. Exploiting the model, in agreement with the experimental results, we find evidence for: i) stretched relaxational dynamics; ii) a strong T-dependence of the stretching parameter; iii) breakdown of the Stokes-Einstein law.Comment: 4 pages (Latex), 4 eps figure

The dynamics of passive colloidal tracers in a bath of self-propelled particles is receiving a lot of attention in the context of nonequilibrium statistical mechanics. Here we demonstrate that active baths are also capable of mediating effective interactions between suspended bodies. In particular we observe that a bath of swimming bacteria gives rise to a short range attraction similar to depletion forces in equilibrium colloidal suspensions. Using numerical simulations and experiments we show how the features of this interaction arise from the combination of nonequilibrium dynamics (peculiar of bacterial baths) and excluded volume effects.

We study the nonlinear dynamics of a multimode random laser using the methods of statistical physics of disordered systems. A replica-symmetry breaking phase transition is predicted as a function of the pump intensity. We thus show that light propagating in a random nonlinear medium displays glassy behavior; i.e., the photon gas has a multitude of metastable states and a nonvanishing complexity, corresponding to mode-locking processes in random lasers. The present work reveals the existence of new physical phenomena, and demonstrates how nonlinear optics and random lasers can be a benchmark for the modern theory of complex systems and glasses.