Differently from passive Brownian particles, active particles, also known as self-propelled Brownian particles or microswimmers and nanoswimmers, are capable of taking up energy from their environment and converting it into directed motion. Because of this constant flow of energy, their behavior can be explained and understood only within the framework of nonequilibrium physics. In the biological realm, many cells perform directed motion, for example, as a way to browse for nutrients or to avoid toxins. Inspired by these motile microorganisms, researchers have been developing artificial particles that feature similar swimming behaviors based on different mechanisms. These man-made micromachines and nanomachines hold a great potential as autonomous agents for health care, sustainability, and security applications. With a focus on the basic physical features of the interactions of self-propelled Brownian particles with a crowded and complex environment, this comprehensive review will provide a guided tour through its basic principles, the development of artificial self-propelling microparticles and nanoparticles, and their application to the study of nonequilibrium phenomena, as well as the open challenges that the field is currently facing.

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 derive the stationary probability distribution for a non-equilibrium system composed by an arbitrary number of degrees of freedom that are subject to Gaussian colored noise and a conservative potential. This is based on a multidimensional version of the Unified Colored Noise Approximation. By comparing theory with numerical simulations we demonstrate that the theoretical probability density quantitatively describes the accumulation of active particles around repulsive obstacles. In particular, for two particles with repulsive interactions, the probability of close contact decreases when one of the two particle is pinned. Moreover, in the case of isotropic confining potentials, the radial density profile shows a non trivial scaling with radius. Finally we show that the theory well approximates the “pressure” generated by the active particles allowing to derive an equation of state for a system of non-interacting colored noise-driven particles.

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...