Self-similarity of pressure profiles during forced granular flows.

We present measurements of the vertical stress profile σ on the base of flat-bottomed cylindrical silos discharged through an orifice centered on its base. An overweight forces the material on top of the free surface. The mean bottom pressure σ(z,D,W), with z the height of the granular column, D the silo diameter, and W the mass of the overweight, increases significantly at the end of the discharge. Inspired by early models of stress distribution, we show that σ measured at z=0 can be rescaled to yield a collapse of the data, as a function of z/D, for all D and W explored. We also show that the profile σ(r) is self-similar as a function of the radial coordinate r and can be rescaled to collapse the data for different z,D, and W. Although the model correctly predicts the functional dependences, it fails in quantitative terms. These results challenge our understanding of free and forced granular flows through orifices.

Cell-cycle exit and stem cell differentiation are coupled through regulation of mitochondrial activity in the Drosophila testis.

Whereas stem and progenitor cells proliferate to maintain tissue homeostasis, fully differentiated cells exit the cell cycle. How cell identity and cell-cycle state are coordinated during differentiation is still poorly understood. The Drosophila testis niche supports germline stem cells and somatic cyst stem cells (CySCs). CySCs give rise to post-mitotic cyst cells, providing a tractable model to study the links between stem cell identity and proliferation. We show that, while cell-cycle progression is required for CySC self-renewal, the E2f1/Dp transcription factor is dispensable for self-renewal but instead must be silenced by the Drosophila retinoblastoma homolog, Rbf, to permit differentiation. Continued E2f1/Dp activity inhibits the expression of genes important for mitochondrial activity. Furthermore, promoting mitochondrial biogenesis rescues the differentiation of CySCs with ectopic E2f1/Dp activity but not their cell-cycle exit. In sum, E2f1/Dp coordinates cell-cycle progression with stem cell identity by regulating the metabolic state of CySCs.

Proliferative stem cells maintain quiescence of their niche by secreting the Activin inhibitor Follistatin.

Aging causes stem cell dysfunction as a result of extrinsic and intrinsic changes. Decreased function of the stem cell niche is an important contributor to this dysfunction. We use the Drosophila testis to investigate what factors maintain niche cells. The testis niche comprises quiescent "hub" cells and supports two mitotic stem cell pools: germline stem cells and somatic cyst stem cells (CySCs). We identify the cell-cycle-responsive Dp/E2f1 transcription factor as a crucial non-autonomous regulator required in CySCs to maintain hub cell quiescence. Dp/E2f1 inhibits local Activin ligands through production of the Activin antagonist Follistatin (Fs). Inactivation of Dp/E2f1 or Fs in CySCs or promoting Activin receptor signaling in hub cells causes transdifferentiation of hub cells into fully functional CySCs. This Activin-dependent communication between CySCs and hub regulates the physiological decay of the niche with age and demonstrates that hub cell quiescence results from signals from surrounding stem cells.

Inhibition of PKCε induces primordial germ cell reprogramming into pluripotency by HIF1&2 upregulation and histone acetylation.

Historically, primordial germ cells (PGCs) have been a good model to study pluripotency. Despite their low numbers and limited accessibility in the mouse embryo, they can be easily and rapidly reprogrammed at high efficiency with external physicochemical factors and do not require transcription factor transfection. Employing this model to deepen our understanding of cell reprogramming, we specifically aimed to determine the relevance of Ca2+ signal transduction pathway components in the reprogramming process. Our results showed that PGC reprogramming requires a normal extracellular [Ca2+] range, in contrast to neoplastic or transformed cells, which can continue to proliferate in Ca2+-deficient media, differentiating normal reprogramming from neoplastic transformation. Our results also showed that a spike in extracellular [Ca2+] of 1-3 mM can directly reprogram PGC. Intracellular manipulation of Ca2+ signal transduction pathway components revealed that inhibition of classical Ca2+ and diacylglycerol (DAG)-dependent PKCs, or intriguingly, of only the novel DAG-dependent PKC, PKCε, were able to induce reprogramming. PKCε inhibition changed the metabolism of PGCs toward glycolysis, increasing the proportion of inactive mitochondria. This metabolic switch from oxidative phosphorylation to glycolysis is mediated by hypoxia-inducible factors (HIFs), given we found upregulation of both HIF1α and HIF2α in the first 48 hours of culturing. PKCε inhibition did not change the classical pluripotency gene expression of PGCs, Oct4, or Nanog. PKCε inhibition changed the histone acetylation of PGCs, with histones H2B, H3, and H4 becoming acetylated in PKCε-inhibited cultures (markers were H2BacK20, H3acK9, and H4acK5K8, K12, K16), suggesting that reprogramming by PKCε inhibition is mediated by histone acetylation.

Systematic experimental investigation of the obstacle effect during non-competitive and extremely competitive evacuations.

Although some experimental evidence showed that an obstacle placed in front of a door allows making people's evacuations faster, the efficacy of such a solution has been debated for over 15 years. Researchers are split between those who found the obstacle beneficial and those who could not find a significant difference without it. One of the reasons for the several conclusions lies in the variety of the experiments performed so far, both in terms of competitiveness among participants, geometrical configuration and number of participants. In this work, two unique datasets relative to evacuations with/without obstacle and comprising low and high competitiveness are analyzed using state-of-the-art definitions for crowd dynamics. In particular, the so-called congestion level is employed to measure the smoothness of collective motion. Results for extreme conditions show that, on the overall, the obstacle does not reduce density and congestion level and it could rather slightly increase it. From this perspective, the obstacle was found simply shifting the dangerous spots from the area in front of the exit to the regions between the obstacle and the wall. On the other side, it was however confirmed, that the obstacle can stabilize longitudinal crowd waves, thus reducing the risk of trampling, which could be as important (in terms of safety) as improving the evacuation time. However, under urgent, competitive, but non-extreme conditions, the obstacle generally had a positive effect, helping channeling the flow of pedestrians through the exit while facilitating their interactions.

Clogging-jamming connection in narrow vertical pipes.

We report experimental evidence of clogging due to the spontaneous development of hanging arches when a granular sample composed of spherical particles flows down a narrow vertical pipe. These arches, akin to the ones responsible for silo clogging, can only be possible due to the role of frictional forces; otherwise they will be unstable. We find that, contrary to the silo case, the probability of clogging in vertical narrow tubes does not decrease monotonically with the ratio of the pipe-to-particle diameters. This behavior is related to the clogging prevention caused by the spontaneous ordering of particles apparent in certain aspect ratios. More importantly, by means of numerical simulations, we discover that the interparticle normal force distributions broaden in systems with higher probability of clogging. This feature, which has been proposed before as a distinctive feature of jamming in sheared granular samples, suggests that clogging and jamming are connected in pipe flow.

Effect of hopper angle on granular clogging.

We present experimental results of the effect of the hopper angle on the clogging of grains discharged from a two-dimensional silo under gravity action. We observe that the probability of clogging can be reduced by three orders of magnitude by increasing the hopper angle. In addition, we find that for very large hopper angles, the avalanche size (〈s〉) grows with the outlet size (D) stepwise, in contrast to the case of a flat-bottom silo for which 〈s〉 grows smoothly with D. This surprising effect is originated from the static equilibrium requirement imposed by the hopper geometry to the arch that arrests the flow. The hopper angle sets the bounds of the possible angles of the vectors connecting consecutive beads in the arch. As a consequence, only a small and specific portion of the arches that jam a flat-bottom silo can survive in hoppers.

Role of particle size in the kinematic properties of silo flow.

We experimentally analyze the effect that particle size has on the mass flow rate of a quasi two-dimensional silo discharged by gravity. In a previous work, Janda et al. [Phys. Rev. Lett. 108, 248001 (2012)PRLTAO0031-900710.1103/PhysRevLett.108.248001] introduced a new expression for the mass flow rate based on a detailed experimental analysis of the flow for 1-mm diameter beads. Here, we aim to extend these results by using particles of larger sizes and a variable that was not explicitly included in the proposed expression. We show that the velocity and density profiles at the outlet are self-similar and scale with the outlet size with the same functionalities as in the case of 1-mm particles. Nevertheless, some discrepancies are evidenced in the values of the fitting parameters. In particular, we observe that larger particles lead to higher velocities and lower packing fractions at the orifice. Intriguingly, both magnitudes seem to compensate giving rise to very similar flow rates. In order to shed light on the origin of this behavior we have computed fields of a solid fraction, velocity, and a kinetic-stress like variable in the region above the orifice.

Relevance of system size to the steady-state properties of tapped granular systems.

We investigate the steady-state packing fraction ϕ and force moment tensor Σ of quasi-two-dimensional granular columns subjected to tapping. Systems of different height h and width L are considered. We find that ϕ and Σ, which describe the macroscopic state of the system, are insensitive to L for L>50d (with d the grain diameter). However, results for granular columns of different heights cannot be conciliated. This suggests that comparison between results of different laboratories on this type of experiments can be done only for systems of same height. We show that a parameter ɛ=1+(Aω)2/(2gh), with A and ω the amplitude and frequency of the tap and g the acceleration of gravity, can be defined to characterize the tap intensity. This parameter is based on the effective flight of the granular bed, which takes into account the h dependency. When ϕ is plotted as a function of ɛ, the data collapses for systems of different h. However, this parameter alone is unable to determine the steady state to be reached since different Σ can be observed for a given ɛ if different column heights are considered.

Hypoxia induces pluripotency in primordial germ cells by HIF1α stabilization and Oct4 deregulation.

AIMS: To study the mechanisms of pluripotency induction, we compared gene expression in pluripotent embryonic germ cells (EGCs) and unipotent primordial germ cells (PGCs). RESULTS: We found 11 genes ≥1.5-fold overexpressed in EGCs. None of the genes identified was the Yamanaka genes but instead related to glycolytic metabolism. The prospect of pluripotency induction by cell metabolism manipulation was investigated by hypoxic culturing. Hypoxia induced a glycolytic program in PGCs in detriment of mitochondrial oxidative phosphorylation. We demonstrate that hypoxia alone induces reprogramming in PGCs, giving rise to hypoxia-induced EGC-like cells (hiEGLs), which differentiate into cells of the three germ layers in vitro and contribute to the internal cell mass of the blastocyst in vivo, demonstrating pluripotency. The mechanism of hypoxia induction involves HIF1α stabilization and Oct4 deregulation. However, hiEGL cannot be passaged long term. Self-renewal capacity is not achieved by hypoxia likely due to the lack of upregulation of c-Myc and Klf4. Gene expression analysis of hypoxia signaling suggests that hiEGLs have not reached the stabilization phase of cell reprogramming. INNOVATION AND CONCLUSION: Our data suggest that the two main properties of stemness, pluripotency and self-renewal, are differentially regulated in PGC reprogramming induced by hypoxia.

Experimental proof of faster-is-slower in systems of frictional particles flowing through constrictions.

The "faster-is-slower" (FIS) effect was first predicted by computer simulations of the egress of pedestrians through a narrow exit [D. Helbing, I. J. Farkas, and T. Vicsek, Nature (London) 407, 487 (2000)]. FIS refers to the finding that, under certain conditions, an excess of the individuals' vigor in the attempt to exit causes a decrease in the flow rate. In general, this effect is identified by the appearance of a minimum when plotting the total evacuation time of a crowd as a function of the pedestrian desired velocity. Here, we experimentally show that the FIS effect indeed occurs in three different systems of discrete particles flowing through a constriction: (a) humans evacuating a room, (b) a herd of sheep entering a barn, and (c) grains flowing out a 2D hopper over a vibrated incline. This finding suggests that FIS is a universal phenomenon for active matter passing through a narrowing.

Clogging transition of many-particle systems flowing through bottlenecks.

When a large set of discrete bodies passes through a bottleneck, the flow may become intermittent due to the development of clogs that obstruct the constriction. Clogging is observed, for instance, in colloidal suspensions, granular materials and crowd swarming, where consequences may be dramatic. Despite its ubiquity, a general framework embracing research in such a wide variety of scenarios is still lacking. We show that in systems of very different nature and scale -including sheep herds, pedestrian crowds, assemblies of grains, and colloids- the probability distribution of time lapses between the passages of consecutive bodies exhibits a power-law tail with an exponent that depends on the system condition. Consequently, we identify the transition to clogging in terms of the divergence of the average time lapse. Such a unified description allows us to put forward a qualitative clogging state diagram whose most conspicuous feature is the presence of a length scale qualitatively related to the presence of a finite size orifice. This approach helps to understand paradoxical phenomena, such as the faster-is-slower effect predicted for pedestrians evacuating a room and might become a starting point for researchers working in a wide variety of situations where clogging represents a hindrance.

Topological analysis of tapped granular media using persistent homology.

We use the first Betti number of a complex to analyze the morphological structure of granular samples in mechanical equilibrium. We investigate two-dimensional granular packings after a tapping process by means of both simulations and experiments. States with equal packing fraction obtained with different tapping intensities are distinguished after the introduction of a filtration parameter which determines the particles (nodes in the network) that are joined by an edge. This is accomplished by just using the position of the particles obtained experimentally and no other information about the possible contacts, or magnitude of forces.

Role of driving force on the clogging of inert particles in a bottleneck.

We present numerical results of the effect that the driving force has on the clogging probability of inert particles passing through a bottleneck. When the driving force is increased by four orders of magnitude, the mean avalanche size remains almost unaltered (increases 1.6 times) while the flow rate and the avalanche duration display strong dependence on this magnitude. This indicates that in order to characterize the ability of a system to clog, the right variable to consider is the number of particles that pass through the outlet. The weak dependence of this magnitude on the driving force is explained in terms of the average kinetic energy of the flowing grains that has to be dissipated in order to get an arch stabilized.

Vibrot, a simple device for the conversion of vibration into rotation mediated by friction: preliminary evaluation.

While "vibrational noise" induced by rotating components of machinery is a common problem constantly faced by engineers, the controlled conversion of translational into rotational motion or vice-versa is a desirable goal in many scenarios ranging from internal combustion engines to ultrasonic motors. In this work, we describe the underlying physics after isolating a single degree of freedom, focusing on devices that convert a vibration along the vertical axis into a rotation around this axis. A typical Vibrot (as we label these devices) consists of a rigid body with three or more cantilevered elastic legs attached to its bottom at an angle. We show that these legs are capable of transforming vibration into rotation by a "ratchet effect", which is caused by the anisotropic stick-slip-flight motion of the leg tips against the ground. Drawing an analogy with the Froude number used to classify the locomotion dynamics of legged animals, we discuss the walking regime of these robots. We are able to control the rotation frequency of the Vibrot by manipulating the shaking amplitude, frequency or waveform. Furthermore, we have been able to excite Vibrots with acoustic waves, which allows speculating about the possibility of reducing the size of the devices so they can perform tasks into the human body, excited by ultrasound waves from the outside.

Contact network topology in tapped granular media.

We analyze the contact network of simulated two-dimensional granular packings in different states of mechanical equilibrium obtained by tapping. We show that topological descriptors of the contact network allow one to distinguish steady states of the same mean density obtained with different tap intensities. These equal-density states were recently proven to be distinguishable through the mean force moment tensor. In contrast, geometrical descriptors, such as radial distribution functions, bond order parameters, and Voronoi cell distributions, can hardly discriminate among these states. We find that small-order loops of contacts-the polygons of the network-are especially sensitive probes for the contact structure.

Flow and clogging in a silo with an obstacle above the orifice.

In a recent paper [Zuriguel et al., Phys. Rev. Lett. 107, 278001 (2011)] it has been shown that the presence of an obstacle above the outlet can significatively reduce the clogging probability of granular matter pouring from a silo. The amount of this reduction strongly depends on the obstacle position. In this work, we present new measurements to analyze different outlet sizes, extending foregoing results and revealing that the effect of the obstacle is enhanced as the outlet size is increased. In addition, the effect of the obstacle position on the flow rate properties and in the geometrical features of arches is studied. These results reinforce previous evidence of the pressure reduction induced by the obstacle. In addition, it is shown how the mean avalanche size and the average flow rate are not necessarily linked. On the other hand, a close relationship is suggested between the mean avalanche size and the flow rate fluctuations.

Flow rate of particles through apertures obtained from self-similar density and velocity profiles.

"Beverloo's law" is considered as the standard expression to estimate the flow rate of particles through apertures. This relation was obtained by simple dimensional analysis and includes empirical parameters whose physical meaning is poorly justified. In this Letter, we study the density and velocity profiles in the flow of particles through an aperture. We find that, for the whole range of apertures studied, both profiles are self-similar. Hence, by means of the functionality obtained for them the mass flow rate is calculated. The comparison of this expression with the Beverloo's one reveals some differences which are crucial to understanding the mechanism that governs the flow of particles through orifices.

Fluctuations of grains inside a discharging two-dimensional silo.

We present experimental data corresponding to a two-dimensional dense granular flow, namely, the gravity-driven discharge of grains from a small opening in a silo. We study the local velocity field at the scale of single grains at different places with the help of particle-tracking techniques. From these data, the velocity profiles can be obtained and the validity of some long-standing approaches can be assessed. Moreover, the fluctuations of the velocities are taken into consideration to characterize the features of the advective motion (due to the gravity force) and the diffusive motion, which shows nontrivial behavior.

Silo clogging reduction by the presence of an obstacle.

We present experimental results on the effect that inserting an obstacle just above the outlet of a silo has on the clogging process. We find that, if the obstacle position is properly selected, the probability that the granular flow is arrested can be reduced by a factor of 100. This dramatic effect occurs without any remarkable modification of the flow rate or the packing fraction above the outlet, which are discarded as the cause of the change in the clogging probability. Hence, inspired by previous results of pedestrian crowd dynamics, we propose that the physical mechanism behind the clogging reduction is a pressure decrease in the region of arch formation.