Revealing the Nonequilibrium Nature of a Granular Intruder: The Crucial Role of Non-Gaussian Behavior.

The characterization of the distance from equilibrium is a debated problem in particular in the treatment of experimental signals. If the signal is a one-dimensional time series, such a goal becomes challenging. A paradigmatic example is the angular diffusion of a rotator immersed in a vibro-fluidized granular gas. Here, we experimentally observe that the rotator's angular velocity exhibits significant differences with respect to an equilibrium process. Exploiting the presence of two relevant timescales and non-Gaussian velocity increments, we quantify the breakdown of time-reversal asymmetry, which would vanish in the case of a 1D Gaussian process. We deduce a new model for the massive probe, with two linearly coupled variables, incorporating both Gaussian and Poissonian noise, the latter motivated by the rarefied collisions with the granular bath particles. Our model reproduces the experiment in a range of densities, from dilute to moderately dense, with a meaningful dependence of the parameters on the density. We believe the framework proposed here opens the way to a more consistent and meaningful treatment of out-of-equilibrium and dissipative systems.

Collective Drifts in Vibrated Granular Packings: The Interplay of Friction and Structure.

We simulate vertically shaken dense granular packings with horizontal periodic boundary conditions. A coordinated translating motion of the whole medium emerges when the horizontal symmetry is broken by disorder or defects in the packing and the shaking is weak enough to conserve the structure. We argue that such a drift originates in the interplay between structural symmetry breaking and frictional forces transmitted by the vibrating plate. A nonlinear ratchet model with stick slips reproduces many faces of the phenomenon. The collective motion discussed here underlies phenomena observed recently with vibrofluidized granular materials, such as persistent rotations and anomalous diffusion.

Spontaneous Velocity Alignment in Motility-Induced Phase Separation.

We study a system of purely repulsive spherical self-propelled particles in the minimal setup inducing motility-induced phase separation (MIPS). We show that, even if explicit alignment interactions are absent, a growing order in the velocities of the clustered particles accompanies MIPS. Particles arrange into aligned or vortexlike domains whose size increases as the persistence of the self-propulsion grows, an effect that is quantified studying the spatial correlation function of the velocities. We explain the velocity alignment by unveiling a hidden alignment interaction of the Vicsek-like form, induced by the interplay between steric interactions and self-propulsion.

Intra-operative enteroscopy for the identification of obscure bleeding source caused by gastrointestinal angiodysplasias: through a balloon-tip trocar is better.

AIM: Intra-operative enteroscopy (IE) is a valid diagnostic tool which can be adopted in order to identify the precise location of bleeding gastrointestinal angiodysplasias (GIADs) of the small bowel. We describe a novel IE technique that may increase the diagnostic yield by maintaining a steady distension of the bowel and that prevents microtrauma to the intestinal mucosa and spillage of enteric content into the operative field. METHODS: After laparotomy, a centimetric transverse enterotomy is performed approximately at the middle of the small bowel. A 12-mm trocar with balloon is then introduced and insufflated. The small bowel is gently distended by carbon dioxide insufflation. Through the trocar, a paediatric colonscope is then inserted and the enteroscopy is performed either retrogradely to the duodenum or anterogradely to the caecum. Once located, surgery is tailored to the precise site of bleeding, with a consequent sparing of intestinal resection. RESULTS: IE with 12-mm trocar with balloon was adopted in four elderly patients undergoing surgery for bleeding GIADs. The length of small bowel resection ranged from 10 to 200 cm, depending on the number of GIADs. Operating time ranged from 210 to 275 min. Intra-operative blood loss was nil. No patient developed organ-space or wound infections. There was no recurrence of bleeding from the midgut. CONCLUSION: Performing IE through a balloon trocar may increase the diagnostic accuracy of the procedure with the benefit of reducing the risk of traumatic injury to the bowel and the risk of surgical site infection.

Dynamical Collective Memory in Fluidized Granular Materials.

Recent experiments with rotational diffusion of a probe in a vibrated granular media revealed a rich scenario, ranging from a dilute gas to a dense liquid with cage effects and an unexpected superdiffusive behavior at large times. Here we set up a simulation that reproduces quantitatively the experimental observations and allows us to investigate the properties of the host granular medium, a task not feasible in the experiment. We discover a persistent collective rotational mode which emerges at a high density and a low granular temperature: a macroscopic fraction of the medium slowly rotates, randomly switching direction after very long times. Such a rotational mode of the host medium is the origin of the probe's superdiffusion. Collective motion is accompanied by a kind of dynamical heterogeneity at intermediate times (in the cage stage) followed by a strong reduction of fluctuations at late times, when superdiffusion sets in.

Controlled Viscosity in Dense Granular Materials.

We experimentally investigate the fluidization of a granular material subject to mechanical vibrations by monitoring the angular velocity of a vane suspended in the medium and driven by an external motor. On increasing the frequency, we observe a reentrant transition, as a jammed system first enters a fluidized state, where the vane rotates with high constant velocity, and then returns to a frictional state, where the vane velocity is much lower. While the fluidization frequency is material independent, the viscosity recovery frequency shows a clear dependence on the material that we rationalize by relating this frequency to the balance between dissipative and inertial forces in the system. Molecular dynamics simulations well reproduce the experimental data, confirming the suggested theoretical picture.

Lattice Model to Derive the Fluctuating Hydrodynamics of Active Particles with Inertia.

We derive the hydrodynamic equations with fluctuating currents for the density, momentum, and energy fields for an active system in the dilute limit. In our model, nonoverdamped self-propelled particles (such as grains or birds) move on a lattice, interacting by means of aligning dissipative forces and excluded volume repulsion. Our macroscopic equations, in a specific case, reproduce a transition line from a disordered phase to a swarming phase and a linear dispersion law accounting for underdamped wave propagation. Numerical simulations up to a packing fraction ∼10% are in fair agreement with the theory, including the macroscopic noise amplitudes. At a higher packing fraction, a dense-diluted coexistence emerges. We underline the analogies with the granular kinetic theories, elucidating the relation between the active swarming phase and granular shear instability.

Anomalous force-velocity relation of driven inertial tracers in steady laminar flows.

We study the nonlinear response to an external force of an inertial tracer advected by a two-dimensional incompressible laminar flow and subject to thermal noise. In addition to the driving external field F, the main parameters in the system are the noise amplitude [Formula: see text] and the characteristic Stokes time [Formula: see text] of the tracer. The relation velocity vs. force shows interesting effects, such as negative differential mobility (NDM), namely a non-monotonic behavior of the tracer velocity as a function of the applied force, and absolute negative mobility (ANM), i.e. a net motion against the bias. By extensive numerical simulations, we investigate the phase chart in the parameter space of the model, [Formula: see text], identifying the regions where NDM, ANM and more common monotonic behaviors of the force-velocity curve are observed.

Nonlinear Response of Inertial Tracers in Steady Laminar Flows: Differential and Absolute Negative Mobility.

We study the mobility and the diffusion coefficient of an inertial tracer advected by a two-dimensional incompressible laminar flow, in the presence of thermal noise and under the action of an external force. We show, with extensive numerical simulations, that the force-velocity relation for the tracer, in the nonlinear regime, displays complex and rich behaviors, including negative differential and absolute mobility. These effects rely upon a subtle coupling between inertia and applied force that induces the tracer to persist in particular regions of phase space with a velocity opposite to the force. The relevance of this coupling is revisited in the framework of nonequilibrium response theory, applying a generalized Einstein relation to our system. The possibility of experimental observation of these results is also discussed.

Thermodynamic bounds for diffusion in nonequilibrium systems with multiple timescales.

We derive a thermodynamic uncertainty relation bounding the mean squared displacement of a Gaussian process with memory, driven out of equilibrium by unbalanced thermal baths and/or by external forces. Our bound is tighter with respect to previous results and also holds at finite time. We apply our findings to experimental and numerical data for a vibrofluidized granular medium, characterized by regimes of anomalous diffusion. In some cases our relation can distinguish between equilibrium and nonequilibrium behavior, a nontrivial inference task, particularly for Gaussian processes.

Rescaling invariance and anomalous energy transport in a small vertical column of grains.

It is well known that energy dissipation and finite size can deeply affect the dynamics of granular matter, often making usual hydrodynamic approaches problematic. Here we report on the experimental investigation of a small model system, made of ten beads constrained into a 1D geometry by a narrow vertical pipe and shaken at the base by a piston excited by a periodic wave. Recording the beads motion with a high frame rate camera allows to investigate in detail the microscopic dynamics and test hydrodynamic and kinetic models. Varying the energy, we explore different regimes from fully fluidized to the edge of condensation, observing good hydrodynamic behavior down to the edge of fluidization, despite the small system size. Density and temperature fields for different system energies can be collapsed by suitable space and time rescaling, and the expected constitutive equation holds very well when the particle diameter is considered. At the same time, the balance between dissipated and fed energy is not well described by commonly adopted dependence due to the up-down symmetry breaking. Our observations, supported by the measured particle velocity distributions, show a different phenomenological temperature dependence, which yields equation solutions in agreement with experimental results.

Vicsek model by time-interlaced compression: A dynamical computable information density.

Collective behavior, both in real biological systems and in theoretical models, often displays a rich combination of different kinds of order. A clear-cut and unique definition of "phase" based on the standard concept of the order parameter may therefore be complicated, and made even trickier by the lack of thermodynamic equilibrium. Compression-based entropies have been proved useful in recent years in describing the different phases of out-of-equilibrium systems. Here, we investigate the performance of a compression-based entropy, namely, the computable information density, within the Vicsek model of collective motion. Our measure is defined through a coarse graining of the particle positions, in which the key role of velocities in the model only enters indirectly through the velocity-density coupling. We discover that such entropy is a valid tool in distinguishing the various noise regimes, including the crossover between an aligned and misaligned phase of the velocities, despite the fact that velocities are not explicitly used. Furthermore, we unveil the role of the time coordinate, through an encoding recipe, where space and time localities are both preserved on the same ground, and find that it enhances the signal, which may be particularly significant when working with partial and/or corrupted data, as is often the case in real biological experiments.

Engineered swift equilibration of a Brownian gyrator.

In the context of stochastic thermodynamics, a minimal model for nonequilibrium steady states has been recently proposed: the Brownian gyrator (BG). It describes the stochastic overdamped motion of a particle in a two-dimensional harmonic potential, as in the classic Ornstein-Uhlenbeck process, but considering the simultaneous presence of two independent thermal baths. When the two baths have different temperatures, the steady BG exhibits a rotating current, a clear signature of nonequilibrium dynamics. Here, we consider a time-dependent potential, and we apply a reverse-engineering approach to derive exactly the required protocol to switch from an initial steady state to a final steady state in a finite time τ. The protocol can be built by first choosing an arbitrary quasistatic counterpart, with few constraints, and then adding a finite-time contribution which only depends upon the chosen quasistatic form and which is of order 1/τ. We also get a condition for transformations which, in finite time, conserve internal energy, useful for applications such as the design of microscopic thermal engines. Our study extends finite-time stochastic thermodynamics to transformations connecting nonequilibrium steady states.

Front pinning in capillary filling of chemically coated channels.

The dynamics of capillary filling in the presence of chemically coated heterogeneous boundaries is investigated both theoretically and numerically. In particular, by mapping the equations of front motion onto the dynamics of a dissipative driven oscillator, an analytical criterion for front pinning is derived under the condition of diluteness of the coating spots. The criterion is tested against two-dimensional lattice Boltzmann simulations and found to provide satisfactory agreement as long as the width of the front interface remains much thinner than the typical heterogeneity scale of the chemical coating.

Anomalous mobility of a driven active particle in a steady laminar flow.

We study, via extensive numerical simulations, the force-velocity curve of an active particle advected by a steady laminar flow, in the nonlinear response regime. Our model for an active particle relies on a colored noise term that mimics its persistent motion over a time scale [Formula: see text]. We find that the active particle dynamics shows non-trivial effects, such as negative differential and absolute mobility (NDM and ANM, respectively). We explore the space of the model parameters and compare the observed behaviors with those obtained for a passive particle ([Formula: see text]) advected by the same laminar flow. Our results show that the phenomena of NDM and ANM are quite robust with respect to the details of the considered noise: in particular for finite [Formula: see text] a more complex force-velocity relation can be observed.

Self-propulsion against a moving membrane: Enhanced accumulation and drag force.

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 cτ/ξ, where τ is the AP's reorientation time and ξ the width characterizing the surface's smoothness (ξ→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.

Laboratory MCAO Test-Bed for Developing Wavefront Sensing Concepts.

An experimental optical bench test-bed for developing new wavefront sensing concepts for Multi-Conjugate Adaptive Optics (MCAO) systems is described. The main objective is to resolve imaging problems associated with wavefront sensing of the atmospheric turbulence for future MCAO systems on Extremely Large Telescopes (ELTs). The test-bed incorporates five reference sources, two deformable mirrors (DMs) and atmospheric phase screens to simulate a scaled version of a 10-m adaptive telescope operating at the K band. A recently proposed compact tomographic wavefront sensor is employed for star-oriented DMs control in the MCAO system. The MCAO test-bed is used to verify the feasibility of the wavefront sensing concept utilizing a field lenslet array for multi-pupil imaging on a single detector. First experimental results of MCAO correction with the proposed tomographic wavefront sensor are presented and compared to the theoretical prediction based on the characteristics of the phase screens, actuator density of the DMs and the guide star configuration.

Tuning somatic hypermutation by transcription.

The dependence of somatic hypermutation on transcription was studied in three mutant immunoglobulin heavy chain (IgH) insertion mice in which a targeted non-functional VHB1-8 passenger transgene was either placed under the transcriptional control of a truncated DQ52 promoter (p delta), its own RNA polymerase II dependent IgH promoter (pII) or a RNA polymerase I dependent promoter (pI). The relative mutation-frequency of the VHB1-8 passenger transgene in memory B cells of p delta, pI and pII mice (7%, 60% and 100%) correlated with the relative levels of transgene-specific pre-mRNA expressed in germinal center B cells isolated from the mutant mice (8%, 72% and 100%, respectively). These data indicate that the mutation load of rearranged Ig genes can be tuned by transcription. The question, whether somatic hypermutation requires transcription per se or a specific component of the RNA polymerase II complex, is under investigation.

The biomolecular corona of nanoparticles in circulating biological media.

When nanoparticles come into contact with biological media, they are covered by a biomolecular 'corona', which confers a new identity to the particles. In all the studies reported so far nanoparticles are incubated with isolated plasma or serum that are used as a model for protein adsorption. Anyway, bodily fluids are dynamic in nature so the question arises on whether the incubation protocol, i.e. dynamic vs. static incubation, could affect the composition and structure of the biomolecular corona. Here we let multicomponent liposomes interact with fetal bovine serum (FBS) both statically and dynamically, i.e. in contact with circulating FBS (≈40 cm s(-1)). The structure and composition of the liposome-protein corona, as determined by dynamic light scattering, electrophoretic light scattering and liquid chromatography tandem mass spectrometry, were found to be dependent on the incubation protocol. Specifically, following dynamic exposure to FBS, multicomponent liposomes were less enriched in complement proteins and appreciably more enriched in apolipoproteins and acute phase proteins (e.g. alpha-1-antitrypsin and inter-alpha-trypsin inhibitor heavy chain H3) that are involved in relevant interactions between nanoparticles and living systems. Supported by our results, we speculate that efficient predictive modeling of nanoparticle behavior in vivo will require accurate knowledge of nanoparticle-specific protein fingerprints in circulating biological media.

Randomized crossover comparison of right atrial appendage pacing versus interatrial septum pacing for prevention of paroxysmal atrial fibrillation in patients with sinus bradycardia.

BACKGROUND: New atrial pacing techniques and overdrive pacing algorithms have been introduced to prevent atrial fibrillation. This study was designed to test the hypotheses that (1) interatrial septum pacing (IASP) at the triangle of Koch would be more effective than right atrial appendage pacing (RAAP) in preventing paroxysmal atrial fibrillation (PAF) in patients with sinus bradycardia and (2) an algorithm (CAP) designed to achieve constant atrial capture would increase the efficacy of rate-responsive atrial pacing. METHODS: We studied 46 patients with PAF and sinus bradycardia implanted with a DDD(R) (Medtronic Thera) pacemaker. Twenty-four patients (6.0 +/- 10.1 PAF episodes/month within 3 months before study) were randomized to RAAP and 22 patients (5.4 +/- 7.1, not significant) to IASP. Within each arm 2 randomized crossover periods of CAP-OFF and CAP-ON function were programed. RESULTS: The PAF episodes per month significantly decreased in the RAAP (CAP-OFF: 2.1 +/- 4.2, P <.05; CAP-ON: 1.9 +/- 3.8, P <.05) and in the IASP group (CAP-OFF: 0.2 +/- 0.5, P <.05; CAP-ON: 0.2 +/- 0.5, P <.05). Values were significantly lower in the IASP group than in the RAAP group in both CAP-OFF (0.2 +/- 0.5 vs 2.1 +/- 4.2, P <.05) and CAP-ON (0.2 +/- 0.5 vs 1.9 +/- 3.8, P <.05) conditions. PAF burden was significantly lower in the IASP than in the RAAP group in CAP-OFF (47 +/- 84 min/d vs 140 +/- 217, P <.05) and in CAP-ON (41 +/- 72 vs 193 +/- 266, P <.05) conditions. No differences were observed within each arm in PAF burden between the 2 crossover CAP programing periods. CONCLUSIONS: Rate-adaptive IASP at the triangle of Koch is more effective than RAAP in preventing PAF in patients with sinus bradycardia. In our sample of patients no additional clinical benefit is furnished by the CAP algorithm.