arXiv subjects

Oded Agam

Publications and source records attributed to Oded Agam.

At least 19 recordsLinked to original sources

arXiv2026

Dynamical noisy canalization in morphogenesis: lessons from Hydra regeneration

Developmental robustness is framed as progress through a fixed Waddington-type landscape. We argue that in morphogenesis this landscape evolves through coupled bio-signaling, mechanical, and physiological processes, while fluctuations aid exploration. In Hydra regeneration, stochastic Ca activity plays a major role in reshaping the landscape of accessible morphologies as regeneration unfolds, including the early progressive confinement of tissue fluctuations. We propose testing this framework of dynamical noisy canalization in developmental systems.

physics.bio-ph↗

arXiv2026

Rupture-Repair Cycles in Regenerating Hydra Tissues

Destructive mechanical breakdowns and fractures are ubiquitous events in driven physical matter; living tissues, by contrast, can rupture repeatedly while restoring integrity. Here we study rupture repair interplay in regenerating Hydra tissues, which cycle through osmotic inflation, pressure release by rupture, and resealing. We utilize bright field imaging of the tissue projected area as a readout of the rupture magnitude before it is arrested. Analyzing these event statistics, we find that the tail of the area-drop distribution is controlled by Ca2+-dependent repair efficiency. When the Ca2+ response is weakened, either by partially blocking gap-junctions mediating the intercellular communication, or by inhibiting stretch-activated Ca2+ channels, the actomyosin force that arrests the rupture process is delayed or reduced. Under these conditions, rare large pressure releases become more likely, and the tail of the distribution crosses over from an exponential behavior, exhibiting a characteristic scale, to a power-law one consistent with a critical-like regime reflecting intermittent rupture propagation. These results identify mechanically evoked Ca2+ activity as a control axis linking repair to rupture statistics in a living tissue. It supports a picture of rupture front advancing by stick-slip-like dynamics as it encounters a heterogeneous mechanical landscape, akin to failure-front propagation in disordered materials.

physics.bio-ph↗

arXiv2025

The Dual Nature of Body-Axis Formation in Hydra Regeneration: Polarity-Morphology Concurrency

The formation of a body-axis is central to animal development and involves both polarity and morphology. While polarity is traditionally associated with biochemical patterning, the morphological aspect of axis formation remains elusive. In regenerating Hydra tissues, we find that morphological evolution in all tissue samples depends on inherited positional information from the donor axis, and a foot precursor emerges early in the process. From the onset of regeneration, the Ca2+ excitations that drive actomyosin forces for tissue reshaping follow a gradient aligned with the head-foot polarity direction. We conclude that polarity and morphological axis progression occur concurrently through interlinked processes, and that the foot plays a dominant role in this process, a role usually attributed to the head organizer. A simple toy model accounts for the observed regeneration dynamics and illustrates the mechanochemical integration of polarity and morphogenesis. We expect the insights from Hydra to be relevant to broader developmental systems.

physics.bio-ph↗

arXiv2025

Anomalous Thickness Dependence of the Vortex Pearl Length in Few-Layer NbSe2

The coexistence of multiple types of orders is a common thread in condensed matter physics and unconventional superconductors. The nature of superconducting orders may be unveiled by analyzing local perturbations such as vortices. For thin films, the vortex magnetic profile is characterized by the Pearl-length {\Lambda}, which is inversely proportional to the 2D superfluid density; hence, normally, also inversely proportional to the film thickness, d. Here we employ the scanning SQUID-on-tip microscopy to measure {\Lambda} in NbSe2 flakes with thicknesses ranging from N=3 to 53 layers. For N>10, we find the expected dependence {\Lambda}{\varpropto}1/d. However, six-layer films show a sharp increase of {\Lambda} deviating by a factor of three from the expected value. This value remains fixed for N=3 to 6. This unexpected behavior suggests the competition between two orders; one residing only on the first and last layers of the film while the other prevails in all layers.

cond-mat.supr-con↗

arXiv2024

Pattern formation in e. coli through negative chemotaxis: instability, condensation, and merging

Motile bacteria can migrate along chemical gradients in a process known as chemotaxis. When exposed to uniform environmental stress, Escherichia coli cells coordinate their chemotactic responses to form millimeter-sized condensates containing hundreds of thousands of motile cells. In this study, we combined experiments with mathematical modeling based on modified Keller-Segel equations to investigate the dynamics of this collective behavior across three distinct time scales: the shortest time scale, where spatial instability emerges; an intermediate time scale, where quasi-stationary bacterial condensates form; and finally, a longer time scale, during which neighboring bacterial accumulations coalesce. The model closely agrees with experimental results, quantitatively capturing the observed instability, the shape of mature condensates, and their coalescence dynamics. Specifically, we found that the force between neighboring bacterial accumulations decays exponentially with distance due to screening effects. We suggest that the model presented here could describe more broadly the dynamics of stress-induced condensation mediated by bacterial chemotaxis.

physics.bio-ph↗

arXiv2024

Fluctuation-Driven Morphological Patterning: A Novel Approach to Morphogenesis

Recent experimental investigations into Hydra regeneration revealed a remarkable phenomenon: the morphological transformation of a tissue fragment from the incipient spherical configuration to a tube-like structure - the hallmark of a mature Hydra - has the dynamical characteristics of a first-order phase-transition, with calcium field fluctuations within the tissue playing an essential role. This morphological transition was shown to be generated by activation over an energy barrier within an effective potential that underlies morphogenesis. Inspired by this intriguing insight, we propose a novel mechanism where stochastic fluctuations drive the emergence of morphological patterns. Thus, the inherent fluctuations determine the nature of the dynamics and are not incidental noise in the background of the otherwise deterministic dynamics. Instead, they play an important role as a driving force that defines the attributes of the pattern formation dynamics and the nature of the transition itself. Here, we present a simple model that captures the essence of this novel mechanism for morphological pattern formation. Specifically, we consider a one-dimensional tissue arranged as a closed contour embedded in a two-dimensional space, where the local curvature of the contour is coupled to a non-negative scalar field. An effective temperature parameter regulates the strength of the fluctuations in the system. The tissue exhibits fluctuations near a circular shape at sufficiently low coupling strengths, but as the coupling strength exceeds some critical value, the circular state becomes unstable. The nature of the transition, namely whether it is a first or a second-order-like transition, depends on the temperature and the effective cutoff on the wavelength of the spatial variations in the system. It is also found that entropic barriers separate the various metastable states of the system.

physics.bio-ph↗

arXiv2023

Hydra morphogenesis as phase-transition dynamics

We utilize whole-body Hydra regeneration from a small tissue segment to develop a physics framework for animal morphogenesis. Introducing experimental controls over this process, an external electric field and a drug that blocks gap junctions, allows us to characterize the essential step in the morphological transition - from a spherical shape to an elongated spheroid. We find that spatial fluctuations of the Ca2+ distribution in the Hydra's tissue drive this transition and construct a field-theoretic model that explains the morphological transition as a first-order-like phase transition resulting from the coupling of the Ca2+ field and the tissue's local curvature. Various predictions of this model are verified experimentally.

physics.bio-ph↗

arXiv2023

Universal Calcium fluctuations in Hydra morphogenesis

Understanding how the collective physical processes drive robust morphological transitions in animal development requires the characterization of the relevant fields underlying morphogenesis. Calcium (Ca2+) is known to be such a field. Here we show that the Ca2+ spatial fluctuations, in whole-body Hydra regeneration, exhibit universal properties captured by a field-theoretic model describing fluctuations in a tilted double-well potential. We utilize an external electric field and Heptanol, a drug blocking gap junctions, as two separate controls affecting the Ca2+ activity and pausing the regeneration process in a reversible way. Subjecting the Hydra tissue to an electric field increases the calcium activity and its spatial correlations, while applying Heptanol inhibits the activity and weakens the spatial correlations. The statistical characteristics of the Ca2+ spatial fluctuations, i.e., the coefficient of variation and the skewness - exhibit universal shape distributions across tissue samples and conditions, demonstrating the existence of global constraints over this field. Our analysis shows that the Hydra's tissue resides near the onset of bistability; the local Ca2+ activity in different regions fluctuates between low and high excited states. The controls modulate the dynamics near that onset, preserving the universal characteristics of the Ca2+ fluctuations and, by that, maintaining the tissue's ability to regenerate.

physics.bio-ph↗

arXiv2019

New exact results for the two-phase model with several conserved currents

We consider the macroscopic transport properties of two-dimensional random binary mixtures with identical spatial distributions of the two phases. Previous studies have obtained exact analytical results for the electrical conductivity of a single layer with and without a magnetic field, as well as for the thermoelectric response of a magnetic field-free double-layer. Here, we generalize these exact solutions to the magneto-thermoelectric response of a single layer and to the thermoelectric response of a double-layer. The magneto-thermoelectric transport coefficients of the double-layer are calculated perturbatively for weak magnetic field.

cond-mat.mes-hall↗

arXiv2018

Viscous fingering in the presence of weak disorder

We consider the problem of viscous fingering in the presence of quenched disorder that is both weak and short-range correlated. The two point correlation function of the harmonic measure is calculated perturbatively, and is used in order to calculate the correction the the box-counting fractal dimension. We show that the disorder increases the fractal dimension, and that its effect decreases logarithmically with the size of the fractal.

cond-mat.soft↗

arXiv2017

Saturation of Strong Electron-Electron Umklapp Scattering at High Temperature

We consider clean metals, at finite temperature, in which the inelastic rate, $\hbar/ τ_{ee}$, can become of the order of, or larger, than the band splitting energy. We show that in suchsystems, contrary to the common knowledge, the umklapp scattering rate becomes independent of both $τ_{ee}$ and the temperature $T,$ in three dimensional systems. We discuss the relation of this phenomenon to the saturation of resistivity at high temperature.

cond-mat.str-el↗

arXiv2016

Instability of Abrikosov lattice due to nonanalytic core reconstruction of vortices in Bosonic superfluids

We study the impact of the non-analytic reconstruction of vortex cores on static vortex structures in weakly coupled superfluids. We show that in rotating two-dimensional systems, the Abrikosov vortex lattice is unstable to vortex core deformation: Each zero of the wave function becomes a cut of finite length. The directors characterizing the orientations of the cuts are themselves ordered in superstructures due either to surface effects or to interaction with shear deformations of the lattice (spiral structure). Similar instability may be also observable in clean superconducting films.

cond-mat.other↗

arXiv2016

Speckle statistics of entangled photons

We consider the propagation of several entangled photons through an elastically scattering medium and study statistical properties of their speckle patterns. We find the spatial correlations of multiphoton speckles and their sensitivity to changes of system parameters. Our analysis covers both the directed-wave regime, where rays propagate almost ballistically while experiencing small-angle diffusion, and the real-space diffusive regime. We demonstrate that long-range correlations of the speckle patterns dominate experimental signatures for large-aperture photon detectors. We also show that speckle sensitivity depends strongly on the number of photons $N$ in the incoming beam, increasing as $\sqrt{N}$ in the directed-wave regime and as $N$ in the diffusive regime.

physics.optics↗

arXiv2015

Non-analytic Vortex Core and a Nonlinear Vortex Flow in Bosonic Superfluids

We analyze the disorder limited motion of quantum vortices in a two-dimensional bosonic superfluid with a large healing length. It is shown that the excitations of low-energy degrees of freedom associated with the non-analytic reconstruction of the vortex core [Ann. Phys. {\bf 346}, 195 (2014)] determine strong non-linear effects in the vortex transport at velocities much smaller than Landau's critical velocity. Experiments are suggested to verify our predictions.

cond-mat.other↗

arXiv2014

Memory effects, two color percolation, and the temperature dependence of Mott's variable range hopping

There are three basic processes that determine hopping transport: (a) hopping between normally empty sites (i.e. having exponentially small occupation numbers at equilibrium); (b) hopping between normally occupied sites, and (c) transitions between normally occupied and unoccupied sites. In conventional theories all these processes are considered Markovian and the correlations of occupation numbers of different sites are believed to be small(i.e. not exponential in temperature). We show that, contrary to this belief, memory effects suppress the processes of type (c), and manifest themselves in a subleading {\em exponential} temperature dependence of the variable range hopping conductivity. This temperature dependence originates from the property that sites of type (a) and (b) form two independent resistor networks that are weakly coupled to each other by processes of type (c). This leads to a two-color percolation problem which we solve in the critical region.

cond-mat.dis-nn↗

arXiv2014

Spin-memory effect and negative magnetoresistance in hopping conductivity

We propose a mechanism for negative isotropic magnetoresistance in the hopping regime. It results from a memory effect encrypted into spin correlations that are not taken into account by the conventional theory of hopping conductivity. The spin correlations are generated by the nonequilibrium electric currents and lead to the decrease of the conductivity. The application of the magnetic field destroys the correlations thus enhancing the conductance. This effect can occur even at magnetic fields as small as a few gauss.

cond-mat.dis-nn↗

arXiv2012

Fluctuations in the relaxation dynamics of mixed chaotic systems

The relaxation dynamics in mixed chaotic systems are believed to decay algebraically with a universal decay exponent that emerges from the hierarchical structure of the phase space. Numerical studies, however, yield a variety of values for this exponent. In order to reconcile these result we consider an ensemble of mixed chaotic systems approximated by rate equations, and analyze the fluctuations in the distribution of Poincare recurrence times. Our analysis shows that the behavior of these fluctuations, as function of time, implies a very slow convergence of the decay exponent of the relaxation.

cond-mat.stat-mech↗

arXiv2012

Topological transitions in evaporating thin films

A thin water film evaporating from a cleaved mica substrate undergoes a first-order phase transition between two values of film thickness. During evaporation, the interface between the two phases develops a fingering instability similar to that observed in the Saffman-Taylor problem. The dynamics of the droplet interface is dictated by an infinite number of conserved quantities: all harmonic moments decay exponentially at the same rate. A typical scenario is the nucleation of a dry patch within the droplet domain. We construct solutions of this problem and analyze the toplogical transition occuring when the boundary of the dry patch meets the outer boundary. We show a duality between Laplacian growth and evaporation, and utilize it to explain the behaviour near the transition. We construct a family of problems for which evaporation and Laplacian growth are limiting cases and show that a necessary condition for a smooth topological transition, in this family, is that all boundaries share the same pressure.

cond-mat.soft↗