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It is still a debated issue whether all critical active particles belong to the same universality class. Here we numerically study the critical behavior of quorum sensing active particles that represents the archetypal model for interpreting motility-induced phase separation. Mean-field theory predicts that this model should undergo a full phase separation if particles slow-down enough when sensing the presence of their neighbours and that the coexistence line terminates in a critical point. By performing large-scale numerical simulations we confirm this scenario, locate the critical point and use finite-size scaling analysis to show that the static and dynamic critical exponents of this active system agree with the Ising universality class, Comment: 7 pages, 4 figures

Sperm motility is a prerequisite for male fertility. Enhancing the concentration of motile sperms in assisted reproductive technologies - for human and animal reproduction - is typically achieved through aggressive methods such as centrifugation. Here we propose a passive technique for the amplification of motile sperm concentration, with no externally imposed forces or flows. The technique is based upon the disparity between probability rates, for motile cells, of entering in and escaping from complex structures. The effectiveness of the technique is demonstrated in microfluidic experiments with microstructured devices, comparing the trapping power in different geometries. In these micro-traps we observe an enhancement of cells concentration close to 10, with a contrast between motile and non-motile increased by a similar factor. Simulations of suitable interacting model sperms in realistic geometries reproduce quantitatively the experimental results, extend the range of observations and highlight the ingredients that are key to optimal trap design.

We investigate a two-dimensional system of active particles confined to a narrow annular domain. Despite the absence of explicit interactions among the velocities or the active forces of different particles, the system displays a transition from a disordered and stuck state to an ordered state of global collective motion where the particles rotate persistently clockwise or anticlockwise. We describe this behavior by introducing a suitable order parameter, the velocity polarization, measuring the global alignment of the particles' velocities along the tangential direction of the ring. We also measure the spatial velocity correlation function and its correlation length to characterize the two states. In the rotating phase, the velocity correlation displays an algebraic decay that is analytically predicted together with its correlation length, while in the stuck regime, the velocity correlation decays exponentially with a correlation length that increases with the persistence time. In the first case, the correlation (and, in particular, its correlation length) does not depend on the active force but the system size only. The global collective motion, an effect caused by the interplay between finite-size, periodicity, and persistent active forces, disappears as the size of the ring becomes infinite, suggesting that this phenomenon does not correspond to a phase transition in the usual thermodynamic sense.

We perform large-scale computer simulations of an off-lattice two-dimensional model of active particles undergoing a motility-induced phase separation (MIPS) to investigate the systems critical behaviour close to the critical point of the MIPS curve. By sampling steady-state configurations for large system sizes and performing finite size scaling analysis we provide exhaustive evidence that the critical behaviour of this active system belongs to the Ising universality class. In addition to the scaling observables that are also typical of passive systems, we study the critical behaviour of the kinetic temperature difference between the two active phases. This quantity, which is always zero in equilibrium, displays instead a critical behavior in the active system which is well described by the same exponent of the order parameter in agreement with mean-field theory., 14 pages, 13 figures

Over the last decade we have witnessed an unprecedented growth in the number of cross-border R&D investments towards large emerging countries such as China and India. However, small middle-income countries have played a marginal role as recipients of R&D-related FDI despite increasing policy efforts. In particular, several Latin American countries have recently launched new policy programs with the aim of attracting this kind of investments, but it remains uncertain whether public incentives can be useful to compensate for other locational disadvantages. The case of Chile provides an interesting empirical setting to explore these issues because during the last decade its government has been actively promoting R&D-related FDI through a new policy mix. This article suggests that for national innovation systems to benefit from the attraction of internationally-mobile R&D it is critical for public policies to ensure that appropriate linkages are established with local actors that hold absorptive capacities. Equally important for a small emerging economy like Chile is to prioritize R&D-related FDI in strategic technology niches where the country can realistically attain critical mass.

Many active particles, both of biological and synthetic origin, can have a light controllable propulsion speed, a property that in biology is commonly referred to as photokinesis. Here we investigate directed transport of photokinetic particles by traveling light patterns. We find general expressions for the current in the cases where the motility wave, induced by light, shifts very slowly or very quickly. These asymptotic formulas are independent of the shape of the wave and are valid for a wide class of active particle models. Moreover we derive an exact solution for the one-dimensional "run and tumble" model. Our results could be used to design time-varying illumination patterns for fast and efficient spatial reconfiguration of photokinetic colloids or bacteria.

We investigate experimentally and numerically the stochastic dynamics and the time-dependent response of colloids subject to a small external perturbation in a dense bath of motile E. coli bacteria. The external field is a magnetic field acting on a superparamagnetic microbead suspended in an active medium. The measured linear response reveals an instantaneous friction kernel despite the complexity of the bacterial bath. By comparing the mean squared displacement and the response function we detect a clear violation of the fluctuation dissipation theorem.

We propose a mechanism to control the formation of stable obstructions in two-dimensional micro-channels of variable sections taking advantage of the peculiar clustering property of active systems. Under the activation of the self-propulsion by external stimuli, the system behaves as a switch according to the following principle: by turning-on the self-propulsion the particles become active and even at very low densities stick to the walls and form growing layers eventually blocking the channel bottleneck, while the obstruction dissolves when the self-propulsion is turned off. We construct the phase diagram distinguishing clogged and open states in terms of density and bottleneck width. The study of the average clogging time, as a function of density and bottleneck width, reveals the marked efficiency of the active clogging that swiftly responds to the self-propulsion turning on. The resulting picture shows a profound difference with respect to the clogging obtained through the slow diffusive dynamics of attractive passive Brownian disks. This numerical work suggests a novel method to use particles with externally tunable self-propulsion to create or destroy plugs in micro-channels.

The project has evaluated the effectiveness of interventions for nutritional counseling through the use of techniques of motivational counseling, aimed at increasing motivation to change unhealthy lifestyles. The study was carried out almost experimentally with three nutritional outpatients, that enrolled subjects to be submitted to nutritional counseling using techniques of motivational counseling representing the "experimental group" and one outpatient which recruited subjects submitted only to traditional Nutritional Counseling representing the "control group". The trial, conducted between September 2009 to December 2011, was structured in 5 stages: Visit, First Control, Second, Third and Fourth Control. Aspects investigated were: 1) Willingness to change, 2) Motivational factors, 3) Body Mass Index and 4) Lifestyles and physical activity. The work of NO was considered effective if it had obtained following results: not gaining weight for normal weight people, a reduction of at least 5% for overweight in the period that went from the first visit to the fourth control. This definition allowed us to distinguish outcomes in "Success" and "Failures". For the four motivational factors analyzed in the "Pilot" and "Controls", it was found that the Inner Fracture and the Temptation decreased, while the Self-efficacy and Importance increased. The increase of scores recorded in the evaluation of the Availability to change has increased the effectiveness of treatment. From the analysis of results for the Willingness to change and the four motivational factors, it was observed that the increase of average scores recorded also an increase of the effectiveness of the treatment.

Motile cells often explore natural environments characterized by a high degree of structural complexity. Moreover cell motility is also intrinsically noisy due to spontaneous random reorientations and speed fluctuations. This interplay of internal and external noise sources gives rise to a complex dynamical behavior that can be strongly sensitive to details and hard to model quantitatively. In striking contrast to this general picture we show that the mean residence time of swimming bacteria inside artificial complex microstructures is quantitatively predicted by a generic invariance property of random walks. We find that while external shape and internal disorder have dramatic effects on the distributions of path lengths and residence times, the corresponding mean values are constrained by the sole free surface to perimeter ratio. As a counterintuitive consequence, bacteria escape faster from structures with higher density of obstacles due to the lower accessible surface., It has been previously shown theoretically that the average path length of random walks inside a closed domain is invariant. Here the authors demonstrate that this invariance property can be used to predict the mean residence time of swimming bacteria exploring structured micro-environments.

We employ Statistical Field Theory techniques for coarse-graining the steady-state properties of Active Ornstein-Uhlenbeck particles. The computation is carried on in the framework of the Unified Colored Noise approximation that allows an effective equilibrium picture. We thus develop a mean-field theory that allows to describe in a unified framework the phenomenology of scalar Active Matter. In particular, we are able to describe through spontaneous symmetry breaking mechanism two peculiar features of Active Systems that are (i) The accumulation of active particles at the boundaries of a confining container, and (ii) Motility-Induced Phase Separation (MIPS). \textcolor{black}{We develop a mean-field theory for steric interacting active particles undergoing to MIPS and for Active Lennard-Jones (ALJ) fluids.} \textcolor{black}{Within this framework}, we discuss the universality class of MIPS and ALJ \textcolor{black}{showing that it falls into Ising universality class.} We \textcolor{black}{thus} compute analytically the critical line $T_c(\tau)$ for both models. In the case of MIPS, $T_c(\tau)$ gives rise to a reentrant phase diagram compatible with an inverse transition from liquid to gas as the strength of the noise decreases. \textcolor{black}{However, in the case of particles interacting through anisotropic potentials, } the field theory acquires a $\varphi^3$ term that, \textcolor{black}{in general, cannot be canceled performing the expansion around the critical point.} In this case, the \textcolor{black}{Ising} critical point might \textcolor{black}{be replaced} by a first-order phase transition \textcolor{black}{region}.

Active matter is made of active particles which are able to convert energy from the environment into directed persistent motion. They can be modelled by stochastic differential equations subject to persistent noise. Run and tumble and active Brownian particle (ABP) models have been first proposed and still are considered closer to experimental observations but do not allow for much analytical progress. The Gaussian coloured noise (GCN) model, introduced as a time coarse-grained version of the ABP can be tuned to have the same variance of the active force as the ABP, which leads to a simpler analytical treatment. Finally, the UCNA can be considered as a Markovian reduction of the GCN. We give a simple derivation of the governing equation and analyse some of its recent applications ranging from the study of the swim pressure, its relation to the mobility, to the state induced by a moving object.

Scientific Reports 7: Article number: 46496; published online: 21 April 2017; updated: 08 June 2018 The original version of this Article contained an error in the Accepted date ‘17 March 2017’ which was incorrectly given as ‘17 March 2015’. This error has now been corrected in the PDF and HTML versions of the Article.

We study the effect of exponentially correlated noise on the $xy$ model in the limit of small correlation time, discussing the order-disorder transition in the mean field and the topological transition in two dimensions. We map the steady states of the nonequilibrium dynamics into an effective equilibrium theory. In the mean field, the critical temperature increases with the noise correlation time $\ensuremath{\tau}$, indicating that memory effects promote ordering. This finding is confirmed by numerical simulations. The topological transition temperature in two dimensions remains untouched. However, finite-size effects induce a crossover in the vortices proliferation that is confirmed by numerical simulations.

Methods of stochastic thermodynamics and hydrodynamics are applied to the a recently introduced model of active particles. The model consists of an overdamped particle subject to Gaussian coloured noise. Inspired by stochastic thermodynamics, we derive from the system's Fokker-Planck equation the average exchanges of heat and work with the active bath and the associated entropy production. We show that a Clausius inequality holds, with the local (non-uniform) temperature of the active bath replacing the uniform temperature usually encountered in equilibrium systems. Furthermore, by restricting the dynamical space to the first velocity moments of the local distribution function we derive a hydrodynamic description where local pressure, kinetic temperature and internal heat fluxes appear and are consistent with the previous thermodynamic analysis. The procedure also shows under which conditions one obtains the unified coloured noise approximation (UCNA): such an approximation neglects the fast relaxation to the active bath and therefore yields detailed balance and zero entropy production. In the last part, by using multiple time-scale analysis, we provide a constructive method (alternative to UCNA) to determine the solution of the Kramers equation and go beyond the detailed balance condition determining negative entropy production., Comment: 19 pages, 1 figure. Major changes in the text. 1 figure has been replaced

Self-propelled bacteria can be integrated into synthetic micromachines and act as biological propellers. So far, proposed designs suffer from low reproducibility, large noise levels or lack of tunability. Here we demonstrate that fast, reliable and tunable bio-hybrid micromotors can be obtained by the self-assembly of synthetic structures with genetically engineered biological propellers. The synthetic components consist of 3D interconnected structures having a rotating unit that can capture individual bacteria into an array of microchambers so that cells contribute maximally to the applied torque. Bacterial cells are smooth swimmers expressing a light-driven proton pump that allows to optically control their swimming speed. Using a spatial light modulator, we can address individual motors with tunable light intensities allowing the dynamic control of their rotational speeds. Applying a real-time feedback control loop, we can also command a set of micromotors to rotate in unison with a prescribed angular speed., Bacteria are able to propel themselves and thus drive systems out of equilibrium. Here the authors aim to control this motion and exploit it in microengineered motors which are powered by genetically modified bacteria and driven by light.

We consider the pressure in the steady-state regime of three stochastic models characterized by self-propulsion and persistent motion and widely employed to describe the behavior of active particles, namely the Active Brownian particle (ABP) model, the Gaussian colored noise (GCN) model and the unified colored noise model (UCNA). Whereas in the limit of short but finite persistence time the pressure in the UCNA model can be obtained by different methods which have an analog in equilibrium systems, in the remaining two models only the virial route is, in general, possible. According to this method, notwithstanding each model obeys its own specific microscopic law of evolution, the pressure displays a certain universal behavior. For generic interparticle and confining potentials, we derive a formula which establishes a correspondence between the GCN and the UCNA pressures. In order to provide explicit formulas and examples, we specialize the discussion to the case of an assembly of elastic dumbbells confined to a parabolic well. By employing the UCNA we find that, for this model, the pressure determined by the thermodynamic method coincides with the pressures obtained by the virial and mechanical methods. The three methods when applied to the GCN give a pressure identical to that obtained via the UCNA. Finally, we find that the ABP virial pressure exactly agrees with the UCNA and GCN result., Comment: 12 pages, 1 figure Submitted for publication 23rd of January 2017 The introduction has been modified

Self-propulsion (SP) is a main feature of active particles (AP), such as bacteria or biological micromotors, distinguishing them from passive colloids. A renowned consequence of SP is accumulation at static interfaces, even in the absence of hydrodynamic interactions. Here we address the role of SP in the interaction between AP and a moving semipermeable membrane. In particular, we implement a model of noninteracting AP in a channel crossed by a partially penetrable wall, moving at a constant velocity $c$. With respect to both the cases of passive colloids with $c>0$ and AP with $c=0$, the AP with finite $c$ show enhancement of accumulation in front of the obstacle and experience a largely increased drag force. This effect is understood in terms of an effective potential localised at the interface between particles and membrane, of height proportional to $c\tau/\xi$, where $\tau$ is the AP's re-orientation time and $\xi$ the width characterising the surface's smoothness ($\xi\to 0$ for hard core obstacles). An approximate analytical scheme is able to reproduce the observed density profiles and the measured drag force, in very good agreement with numerical simulations. The effects discussed here can be exploited for automatic selection and filtering of AP with desired parameters., Comment: 13 pages, 3 figures

Active particles driven by colored noise can be approximately mapped onto a system that obeys detailed balance. The effective interactions which can be derived for such a system allow the description of the structure and phase behavior of the active fluid by means of an effective free energy. In this paper we explain why the related thermodynamic results for pressure and interfacial tension do not represent the results one would measure mechanically. We derive a dynamical density functional theory, which in the steady state simultaneously validates the use of effective interactions and provides access to mechanical quantities. Our calculations suggest that in the colored-noise model the mechanical pressure in the coexisting phases might be unequal and the interfacial tension can become negative.

Swimming bacteria exploit viscous drag forces to generate propulsion in low Reynolds number environments. A rotating helical flagellar bundle can propel the cell body at typical speeds of ten body lengths per second. Not surprisingly, this ability to efficiently swim is preserved even in confining micro-environments which constitute their typical habitat. Quantitative studies would require the ability of fabricating complex environments with controlled geometrical properties. Experimental studies so far were limited to large diameter micro capillaries or 2D confinement. In this last case, E.coli has been shown to swim with an unaltered speed even when the gap size is slightly larger than the cell body thickness. The case of tight 1D confinement is however more challenging requiring 3D fabrication capabilities. Using two-photon polymerization we fabricate 3D microstructures that can confine swimming bacteria in quasi 1D geometries. We observe individual E.coli cells swimming through a sequence of micro-tunnels with progressively decreasing diameters. We demonstrate that E.coli motility is preserved also in tight 1D confinement. Moreover we find that there's an optimal channel diameter for which the increase in flagellar thrust due to 1D confinement can even overcome the increased drag on the cell body resulting in swimming speeds that can be up to 15% larger then the bulk speed.

Scientific Reports 6: Article number: 2329710.1038/srep23297; published online: March222016; updated: May312016 In this Article, Roberto Di Leonardo is incorrectly affiliated with ‘CNR-IMIP, UOS Roma, Dipartimento di Fisica Universita Sapienza, I-00185, Roma, Italy’. The correct affiliation is listed below: NANOTEC-CNR, Institute of Nanotechnology, Soft and Living Matter Laboratory, Roma, Italy.

We investigate the steady state properties of an active fluid modeled as an assembly of soft repulsive spheres subjected to Gaussian colored noise. Such a noise captures one of the salient aspects of active particles, namely the persistence of their motion and determines a variety of novel features with respect to familiar passive fluids. We show that within the so-called multidimensional unified colored noise approximation, recently introduced in the field of active matter, the model can be treated by methods similar to those employed in the study of standard molecular fluids. The system shows a tendency of the particles to aggregate even in the presence of purely repulsive forces because the combined action of colored noise and interactions enhances the the effective friction between nearby particles. We also discuss whether an effective two-body potential approach, which would allow to employ methods similar to those of density functional theory, is appropriate. The limits of such an approximation are discussed., 14 pages, 6 figures in Molecular Physics, 11 march 2016. arXiv admin note: text overlap with arXiv:cond-mat/0605094 by other authors

Este artículo presenta la experiencia de los últimos tres años de desarrollo del cluster basado de innovación en torno a la minería metálica de Chile. Se hace un especial énfasis en la importancia de la innovación, sus movilizadores y las distintas maneras para innovar. Además, se exponen los principales lineamientos de los programas que se desarrollan actualmente en Chile, con sus respectivas estrategias empresariales y su política pública y privada.//This article details the experience of the last three years in regards to the development of innovation- based clusters within Chilean metal mining. Special emphasis is placed on the importance of innovation, on mobilizing innovation, and the distinct ways to innovate. In addition, it discusses the main aspects of programs currently being developed in Chile, with their respective business strategies and both public and private policies.

Inelastic time history analyzes (ITHA) are recognized as the most suitable tool for describing the behavior of structures under seismic actions. It is clear that the reliability of ITHA results is strongly dependent on the modeling choices. One of the aspects still debated, in spite of a large amount of research, is the modeling of viscous damping forces. In the case of a single degree of freedom (SDOF) structure, the damping not represented by hysteretic response is commonly assumed proportional to the square root of the system stiffness: typically in the research papers it is not declared whether the initial or tangent-stiffness is considered. Recent research has shown that initial or tangent-stiffness proportional damping can give very different results in terms of energy dissipation, and peak response displacement. In this work, experimental and numerical tests performed to detect the most appropriate model are described and discussed.

We investigate the effect of self-propulsion on a mean-field order-disorder transition. Starting from a $\varphi^4$ scalar field theory subject to an exponentially correlated noise, we exploit the Unified Colored Noise Approximation to map the non-equilibrium active dynamics onto an effective equilibrium one. This allows us to follow the evolution of the second-order critical point as a function of the noise parameters: the correlation time $\tau$ and the noise strength $D$. Our results suggest that $\tau$ is a crucial ingredient that changes the location of the critical point but, remarkably, not the universality class of the model. We also estimate the effect of Gaussian fluctuations on the mean-field approximation finding an Ornstein-Zernike like expression for the static structure factor at long wave lengths. Finally, to assess the validity of our predictions, we compare the mean-field theoretical results with numerical simulations of active Lennard-Jones particles in two and three dimensions, finding a good qualitative agreement at small $\tau$ values.

Janus particles can self-assemble around microfabricated gears in reproducible configurations with a high degree of spatial and orientational order. The final configuration maximizes the torque applied on the rotor leading to a unidirectional and steady rotating motion. The interplay between geometry and dynamical behavior leads to the self-assembly of Janus micromotors starting from randomly distributed particles.

Active granular particles can harness unbiased mechanical vibrations in the environment to generate directed motion. We provide a theoretical framework that connects the geometrical shape of a three dimensional object to its self-propulsion characteristics over a vertically vibrated plate. We find that a maximally efficient propulsion is achieved for structures having tilted flexible legs forming a characteristic angle with the vertical. Our predictions are verified by experimental observations on a class of 3D printed structures with smoothly varying geometrical features.

We derive an analytic expression for the distribution of velocities of multiple interacting active particles which we test by numerical simulations. In clear contrast with equilibrium we find that the velocities are coupled to positions. Our model shows that, even for two particles only, the individual velocities display a variance depending on the interparticle separation and the emergence of correlations between the velocities of the particles. When considering systems composed of many particles we find an analytic expression connecting the overall velocity variance to density, at the mean-field level, and to the pair distribution function valid in the limit of small noise correlation times. Finally we discuss the intriguing analogies and main differences between our effective free energy functional and the theoretical scenario proposed so far for phase-separating active particles.

We discuss different definitions of pressure for a system of active spherical particles driven by a non-thermal coloured noise. We show that mechanical, kinetic and free-energy based approaches lead to the same result up to first order in the non-equilibrium expansion parameter. The first prescription is based on a generalisation of the kinetic mesoscopic virial equation and expresses the pressure exerted on the walls in terms of the average of the virial of the inter-particle forces. In the second approach, the pressure and the surface tension are identified with the volume and area derivatives, respectively, of the partition function associated with the known stationary non-equilibrium distribution of the model. The third method is a mechanical approach and is related to the work necessary to deform the system. The pressure is obtained by comparing the expression of the work in terms of local stress and strain with the corresponding expression in terms of microscopic distribution. This is determined from the force balance encoded in the Born-Green-Yvon equation. Such a method has the advantage of giving a formula for the local pressure tensor and the surface tension even in inhomogeneous situations. By direct inspection, we show that the three procedures lead to the same values of the pressure, and give support to the idea that the partition function, obtained via the unified coloured noise approximation, is more than a formal property of the system, but determines the stationary non-equilibrium thermodynamics of the model.

Colloidal particles immersed in a dynamic speckle pattern experience an optical force that fluctuates both in space and time. The resulting dynamics presents many interesting analogies with a broad class of non-equilibrium systems like: active colloids, self propelled microorganisms, transport in dynamical intracellular environments. Here we show that the use of a spatial light modulator allows to generate light fields that fluctuate with controllable space and time correlations and a prescribed average intensity profile. In particular we generate ring-shaped random patterns that can confine a colloidal particle over a quasi one-dimensional random energy landscape. We find a mean square displacement that is diffusive at both short and long times, while a superdiffusive or subdiffusive behavior is observed at intermediate times depending on the value of the speckles correlation time. We propose two alternative models for the mean square displacement in the two limiting cases of a short or long speckles correlation time. A simple interpolation formula is shown to account for the full phenomenology observed in the mean square displacement across the entire range from fast to slow fluctuating speckles.

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., 5 pages, 2 figures

The direct conversion of light into work allows the driving of micron-sized motors in a contactless, controllable and continuous way. Light-to-work conversion can involve either direct transfer of optical momentum or indirect opto-thermal effects. Both strategies have been implemented using different coupling mechanisms. However, the resulting efficiencies are always very low, and high power densities, generally obtained by focused laser beams, are required. Here we show that microfabricated gears, sitting on a liquid–air interface, can efficiently convert absorbed light into rotational motion through a thermocapillary effect. We demonstrate rotation rates up to 300 r.p.m. under wide-field illumination with incoherent light. Our analysis shows that thermocapillary propulsion is one of the strongest mechanisms for light actuation at the micron- and nanoscale., The direct conversion of light into work allows the control of micromotors, but typically with low efficiencies and high power density requirements. Here, Maggi et al. demonstrate efficient thermocapillary propulsion of microgears on a liquid–air interface with wide-field, incoherent illumination.

The equations of motion of active systems can be modeled in terms of Ornstein– Uhlenbeck processes (OUPs) with appropriate correlators. For further theoretical studies, these should be approximated to yield a Markovian picture for the dynamics and a simplified steady-state condition. We perform a comparative study of the unified colored noise approximation (UCNA) and the approximation scheme by Fox recently employed within this context. We review the approximations necessary to define effective interaction potentials in the low-density limit and study the conditions for which these represent the behavior observed in two-body simulations for the OUPs model and active Brownian particles. The demonstrated limitations of the theory for potentials with a negative slope or curvature can be qualitatively corrected by a new empirical modification. In general, we find that in the presence of translational white noise the Fox approach is more accurate. Finally, we examine an alternative way to define a force-balance condition in the limit of small activity.

Italian legislation (Article 13 of Legislative Decree 31/2001) provides for the possibility of establishing derogations from chemical parametric values for drinking water set by EU legislation (Council Directive 98/83/EC), if the supply of drinking water cannot be maintained by any other reasonable means. A derogation is possible only after obtaining a specific authorization and must be limited to the shortest time possible. This paper presents the history of derogations granted in Italy and the case of arsenic and nickel in Piedmont. From 2003 to 2009, 13 regions requested a derogation (Campania, Emilia Romagna, Latium, Lombardy, Marche, Piedmont, Apulia, Sardinia, Sicily, Tuscany, Trentino Alto Adige, Umbria, Venetia) at different times and for a total of 13 parameters. In 2012, tap water provided to almost one million Italians in 112 municipalities of three regions (Latium, Tuscany and Campania), did not comply with the legal limits for the following parameters: arsenic, boron and fluorides. Currently, in the Piedmont region there are no derogations from the maximum permitted concentrations of contaminants in drinking water. In the past derogations have been applied for the nickel parameter (years 2006-2007) and from the arsenic parameter (years 2006-2008).

We theoretically and numerically investigate the transport of active colloids to target regions, delimited by asymmetric energy barriers. We show that it is possible to introduce a generalized effective temperature that is related to the local variance of particle velocities. The stationary probability distributions can be derived from a simple diffusion equation in the presence of an inhomogeneous effective temperature resulting from the action of external force fields. In particular, transition rates over asymmetric energy barriers can be unbalanced by having different effective temperatures over the two slopes of the barrier. By varying the type of active noise, we find that equal values of diffusivity and persistence time may produce strongly varied effective temperatures and thus stationary distributions. © the Partner Organisations 2014.

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., 4 pages, 3 figures

The possibility of exploiting motile microorganisms as tiny propellers represents a fascinating strategy for the transport of colloidal cargoes. However, delivery on target sites usually requires external control fields to steer propellers and trigger cargo release. The need for a constant feedback mechanism prevents the design of compact devices where biopropellers could perform their tasks autonomously. Here we show that properly designed three-dimensional (3D) microstructures can define accumulation areas where bacteria spontaneously and efficiently store colloidal beads. The process is stochastic in nature and results from the rectifying action of an asymmetric energy landscape over the fluctuating forces arising from collisions with swimming bacteria. As a result, the concentration of colloids over target areas can be strongly increased or depleted according to the topography of the underlying structures. Besides the significance to technological applications, our experiments pose some important questions regarding the structure of stationary probability distributions in non-equilibrium systems., Transport of colloidal cargoes to target sites can be done by controlling active carriers, like self-propelled bacteria, under external stimuli. Koumakis et al. show that bacteria can autonomously achieve the same goal when moving over pre-designed asymmetric microstructures.

We show that a generalized fluctuation-dissipation relation applies upon instantaneously increasing the temperature of a deeply supercooled liquid. This has the same two-step shape of the relation found upon cooling the liquid, but with opposite violation, indicating an effective temperature that is lower than bath temperature. We show that the effective temperature exhibits some sensible time-dependence and that it retains its connection with the partitioned phase space visited in ageing. We underline the potential relevance of our numerical results for experimental studies of the fluctuation-dissipation relation in glassy systems., Comment: 5 pages, 4 figures