L. Angelani

Bacterial ratchet motors

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.

Self-starting micromotors in a bacterial bath.

Micromotors pushed by biological entities, such as 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.

Condensation in disordered lasers: Theory, 3D+1 simulations, and experiments

The complex processes underlying the generation of a coherent emission from the multiple scattering of photons and wave localization in the presence of structural disorder are still mostly unexplored. Here we show that a single nonlinear Schrödinger equation, playing the role of the Schwalow-Townes law for standard lasers, quantitatively reproduces experimental results and three-dimensional time-domain parallel simulations of a colloidal laser system.

Configurational entropy of hard spheres

We numerically calculate the configurational entropy Sconf of a binary mixture of hard spheres, by using a perturbed Hamiltonian method trapping the system inside a given state, which requires fewer assumptions than the previous methods (Speedy 1998 Mol.Phys. 95 169). We find that Sconf is a decreasing function of the packing fraction φ and extrapolates to zero at the Kauzmann packing fraction , suggesting the possibility of an ideal glass transition for the hard-sphere system. Finally, the Adam-Gibbs relation is found to hold.

The low energy excess of vibrational states in v-SiO2: the role of transverse dynamics

A numerical simulation study of the density dependence (ρ = 2.2–4.0 g cm−3) of the high energy collective dynamics in vitreous silica at mesoscopic wavevectors (Q = 1–18 nm−1) is reported. The dynamic structure factor, S(Q,ω), and the density of states, ρ(E), have been determined in the harmonic approximation via the system eigenvalues and the eigenvectors, in turn obtained by the direct diagonalization of the dynamical matrix. The BKS interaction potential employed is capable of reproducing the experimentally observed excess of states (boson peak), and its density dependence. The numerical simulation also indicates a strong density dependence of the transverse excitation dispersion relation, ΩT(Q), at large Q. Specifically, ΩT(Q) is found to flatten at high Q to a value that increases with increasing density. The parallel between the density dependent flattening of ΩT(Q) and the density dependence of the boson peak suggests that the latter feature arises from the high Q portion of the transverse branch. This hypothesis is in line with both the interpretation by Elliott and co-workers (Taraskin et al 2001 Phys. Rev. Lett. 86 1255), who assign the boson peak to a phenomenon in glass reminiscent of the lowest energy Van Hove singularity in the companion crystal, and the Buchenau et al (1986 Phys. Rev. B 34 5665) assignment of the boson peak to the localized hindered rotation of SiO2 tetrahedra.The study of the effects of the density variations on the vibrational dynamics in vitreous silica is presented. A detailed analysis of the dynamical structure factor, as well as of the current spectra, allows the identification of a flattened transverse branch which is highly sensitive to the density variations. The experimental variations on the intensity and position of the Boson Peak (BP) in v-SiO2 as a function of density are reproduced and interpreted as being due to the shift and disappearance of the latter band. The BP itself is found to correspond to the lower energy tail of the excess states due to the piling up of vibrational modes at energies corresponding to the flattening of the transverse branch.

Glassy Behavior of Light

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.

Geometrically biased random walks in bacteria-driven micro-shuttles

Micron-sized objects having asymmetric boundaries can rectify the chaotic motions of an active bacterial suspension and perform geometrically biased random walks. Using numerical simulations in a planar geometry, we show that arrow-shaped micro-shuttles, constrained to move in one dimension (1D) in a bath of self-propelled micro-organisms, spontaneously perform unidirectional translational motions with a strongly shape-dependent speed. Relaxing the 1D constraint, a random motion in the whole plane sets in at long times, due to random changes in shuttle orientation caused by bacterial collisions. The complex dynamics arising from the mechanical interactions between bacteria and the object boundaries can be described by a Gaussian stochastic force with a shape-dependent mean and a self-correlation decaying exponentially on the timescale of seconds.

Phase diagram and complexity of mode-locked lasers: from order to disorder.

We investigate mode-locking processes in lasers displaying a variable degree of structural randomness. By a spin-glass theoretic approach, we analyze the mean-field Hamiltonian and derive a phase diagram in terms of pumping rate and degree of disorder. Paramagnetic (noisy continuous wave emission), ferromagnetic (standard passive mode locking), and spin-glass phases with an exponentially large number of configurations are identified. The results are also relevant for other physical systems displaying a random Hamiltonian, such as Bose-condensed gases and nonlinear optics.

Structural and dynamical consequences of density variation in vitreous silica

We present structural and dynamical results of molecular dynamics simulation of vitreous silica undergoing a hydrostatic compression and decompression cycle at room temperature. Structural results as a function of density compare fairly well with experiments as well as with the longitudinal and transverse sound velocity pressure dependence. A shift of the boson peak (BP) toward higher energies and its simultaneous weakening is observed as in experiments. A detailed study of the dispersion of the glass vibration is presented at several densities and for the densified state. Evidence of phonon-like character with two distinct pseudo-periods is shown for longitudinal and transverse dynamics. The relationship between the BP vibrations and the correlation length scale indicated by the first sharp diffraction peak is discussed.

Quasisaddles as relevant points of the potential energy surface in the dynamics of supercooled liquids

we describe and revisit in a detailed way an additional approach, that has been very useful to give new insight in the analysis of the relevant processes taking place in the supercooled liquid regime. This approach focuses on the minima of the square gradient of the total potential energy, ‘‘closest’’ to the instantaneous points of the molecular dynamics trajectory. This approach allows one to obtain a microscopic interpretation of the relevant processes, the main result being the characterization of the dynamics above and below the mode coupling temperature Tc . 14 Moreover, the analysis of these points allows one to obtain information about some relevant characteristics of the PES, of great importance to construct simplified models of the landscape. Let us briefly resume the two main PES approaches, the INM and the IS approach. The INM method is based on the investigation of the PES around the instantaneous configurations r ~r represents the 3 N-dimensional vector of the representative point in the configuration space! during the molecular dynamics evolution of the system. The diffusive quantities are supposed to be related to the shape of the energy surface at r, that is to say the eigenvalues and eigenvectors of the Hessian matrix ~the second derivative of the potential energy!. The main hypothesis of the INM approach is that the relevant diffusive directions have to be searched among the downward curvatures ~eigenvectors with negative eigenvalues!. Many attempts have been devoted to extend this approach to different liquid systems and to develop a theory of the supercooled liquid state based on INM concepts. Moreover many efforts have been spent to recognize the true diffusive directions among all the downward ones, as it is known there are downward curvatures that do not correspond to diffusive directions, notably in the crystalline state. The INM negative curvature directions are classified as shoulder modes ~related to anharmonicities of the PES! and double wells ~with a double well shaped one-dimensional profile!. Diffusive directions are finally identified as those leading to different minima 15 ~this analysis involves the