Joel Stavans, WIS
Title: Anomalous Elasticity and Emergent Dipole Screening in Amorphous Solids: Kosterlitz-Thouless-like transition.
Abstract: In the absence of sex as in higher organisms such as mammals and plants, bacteria engage in horizontal gene transfer processes in order to acquire new beneficial traits and drive their genetic diversity in one stroke. Often, exogenous, imported DNA integrates into the genome of a bacterial host at unique sites tht are found within physiologically relevant times, among millions of possible sites on the host’s genome. Here we address the question of what are the search mechanisms that allow these unique sites to be found efficiently, within the complex interior of a bacterial cell in the case of conjugation. I will present experimental results on the search of integrative and conjugative elements (ICEs) for their integration site in the genome of B. subtilis cells. Our experiments were carried out by following in real time the search process at the level of individual molecules within single cells.
Ehud Meron, BGU
Title: Phenotypic plasticity – a missing element in the theory of vegetation pattern formation.
Abstract: Climate change and the development of drier climates threaten ecosystems’ health and the services they provide to humans. Understanding the response of ecosystems to drier climates may provide clues on how to improve their resilience. Two robust mechanisms that improve ecosystem resilience are phenotypic changes in individual plants and partial mortality of plant populations to form vegetation patterns. In nature, these mechanisms are likely to act in concert, but their interplay has escaped consideration. We demonstrate the need for a theory that integrates these plant- and population-level mechanisms by addressing the fascinating fairy-circle phenomenon in Namibia. We show that such an integration resolves two outstanding puzzles in the current theory: observations of multi-scale patterns and the absence of theoretically predicted large-scale stripe and spot patterns along the rainfall gradient. Importantly, we find that multi-level responses to stress unveil a wide variety of more effective stress relaxation pathways, compared to single-level responses, implying a previously underestimated resilience of dryland ecosystems.
Amir Erez, HUJI
Title: Generic flexibility in noisy cell-to-cell information dynamics
Abstract: Exchange of molecules allows cells to exchange information. How robust is the information to changes in cell parameters? We employ a mapping between the stochastic dynamics of two cells and parameters akin to Landau's equilibrium phase transition theory. We show that different single-cell dynamics, identical at steady state, also lead to the same dynamical response---a generic flexibility that cells can use to maintain the information level. The equivalent equilibrium Landau model behaves similarly, thereby describing the dynamics of information in a general class of models.
Yael Roichman, TAU
Title: What governs the thermalization process of two colloidal suspensions subjected to random driving forces in diffusive contact?
Abstract: Employing holographic optical tweezers, we induce random forces with a constant switching frequency on colloidal suspensions. Surprisingly, we discover that the effective temperature of the suspensions exhibits a non-monotonic behavior in relation to the switching frequency. Our findings reveal that the effective temperature, generated in this manner, adheres to the static fluctuation-dissipation relation only at high driving frequencies. To create two systems in diffusive contact, we drive the two sides of the sample independently, leading to observable particle flow between the subsystems. Interestingly, we observe the arrest of particle flow when the ratio of the chemical potential to the effective temperature becomes equal on both sides, in accordance with predictions for systems in thermal equilibrium.
Hillel Aharoni, WIS
Title: Ribbon Gridshells – Soft Elasticity and Inverse Design
Abstract: In this talk, we discuss the properties of a two-dimensional metamaterial composed of thin inextensible ribbons connected together to form a gridshell. We study both in-plane and out-of-plane deformations of such ribbon gridshells and point out their exotic geometric and mechanical properties. In particular, ribbon gridshells demonstrate anomalous softness with respect to some modes of deformation. At the same time, they are highly rigid with respect to other modes and conserve global geometric invariants. Shaping these objects constitutes a hyperbolic PDE system, and therefore properly imposing initial conditions on a one-dimensional subset rigidifies them and fully determines their shape. We further show that exploiting structural degrees of freedom in ribbon gridshells allows tuning their soft modes in various ways. In particular, we can encode shapes into them -- for example to make a flat object that can be accurately deformed into an arbitrary target surface shape. We demonstrate our results with experiments and suggest useful design and technology applications to our findings.
Tomer Markovich, TAU
Title: Chiral active fluids are both `odd’ and non-reciprocal
Abstract: Active materials are composed of many components that convert energy from the environment into directed mechanical motion, thus locally breaking time reversal
symmetry (TRS). Examples of active materials are abundant, from living systems such
as bacteria to colloidal rollers. A striking phenomenon of breaking TRS is the possible
appearance of odd viscosity. Onsager reciprocal relations require that when TRS holds
the viscosity tensor is symmetric for exchanging its first and last pair of indices.
However, when TRS is broken, Onsager relations predict an odd viscosity that is both
odd under TRS and under the change of indices. Such odd viscosity is non-dissipative
and should thus be derivable from a Hamiltonian theory. In this talk I will discuss chiral
active fluids in which both parity and TRS are broken at the microscale. This is usually
a result of continuous injection of energy and angular momentum through local torques,
which are abundant in living systems, and generically result in odd viscosity, even when
the constituents are non-interacting. I will show that the mere existence of spin angular
momentum density due to the local torques also breaks Onsager’s reciprocity relations
and leads to non-Hermitian dynamical matrix. When interactions are included
phenomenologically, we find regions in in the parameter space in which novel 3D
mechanical waves propagate in the bulk, and regions in which they are mechanically
unstable. The lines separating these regions are continuous lines of exceptional points,
suggesting of a non-reciprocal phase transition.
Ram Adar, Technion
Title: Active matter that modifies its environment: a framework for extra-cellular matrix remodeling
Abstract: Many exemplary active systems, including living cells and animals, can form a nematic (apolar) orientational order, while interacting with their environment. In this talk, I explain how a dynamic coupling of an active nematic with its environment results in novel physical behavior. Deposition of aligned environment segments modifies the isotropic-nematic phase diagram and allows for nematic order at arbitrarily low densities. The aligned environment acts as an external field that may also lead to arrested angular dynamics. We are motivated mainly by cells that remodel collagen fibers in the extra-cellular matrix (ECM), while being guided by the fibers during multicellular migration. Our predictions indicate that remodeling promotes macroscopic cellular order and ECM patterning, and possibly limits the coarsening of ordered ECM domains, in accordance with recent experiments.
Ilya Svetlizky, Technion
Title: Work hardening in colloidal crystals
Abstract: Colloidal crystals exhibit a rich behavior that is in many ways analogous to their atomic counterparts: they have the same crystal structures; they undergo the same phase transitions; and they possess the same crystallographic defects. In contrast to these structural properties, the mechanical properties of colloidal crystals are quite distinct from those of atomic systems. For example, unlike in atomic systems, the elasticity of hard-sphere colloidal crystals is purely entropic; as a result, they are so soft that they can be melted just by stirring. We use confocal microscopy to show that hard-sphere colloidal crystals exhibit work hardening, where they become stronger when subjected to increasing plastic deformation. Their strength increases with dislocation density, and, remarkably, ultimately follows the classic Taylor scaling behavior for atomic materials, even though hard-sphere interactions lack the complexity of atomic interactions. This striking resemblance between colloidal and atomic crystals, despite the many orders of magnitude difference in particle size and shear modulus, demonstrates the universality of work hardening.
Yariv Kafri, Technion
Title: The Anomalous Long-Ranged Influence of an Inclusion in Momentum-Conserving Active Fluids
Abstract: We show that an inclusion placed inside a dilute Stokesian suspension of microswimmers induces power-law number-density modulations and flows. These take a different form depending on whether the inclusion is held fixed by an external force, for example an optical tweezer, or if it is free. When the inclusion is held in place, the far-field fluid flow is a Stokeslet, while the microswimmer density decays as $1/r^{2+\epsilon}$, with $r$ the distance from the inclusion, and $\epsilon$ an anomalous exponent which depends on the symmetry of the inclusion and varies continuously as a function of a dimensionless number characterizing the relative amplitudes of the convective and diffusive effects. The angular dependence takes a non-trivial form which depends on the same dimensionless number. When the inclusion is free to move, the far-field fluid flow is a stresslet and the microswimmer density decays as $1/r^2$ with a simple angular dependence.
Baruch Meerson, HUJI
Title: Correlations of disorder strongly affect thermally activated particle motion in
disordered potentials.
Abstract: At low temperatures, the mean escape time of overdamped particles over potential barriers in quenched disorder potentials is very sensitive to the form of the tail of the probability distribution of the potential barriers. Assuming that the disorder statistics is Gaussian, we evaluate this tail by using the optimal fluctuation method. For monotone decreasing disorder correlations in one dimension the tail is determined by the potential disorder variance, and it is independent of the form of the correlation function. However, the tail becomes much higher when the disorder potential exhibits negative autocorrelations, and much lower when the autocorrelations are nonmonotonic but positive. This effect leads to an exponential increase or decrease, respectively, of the particle's escape time. Our theoretical predictions are fully supported by large-deviation simulations of the potential disorder using correlated random potential sampling based on the Wang-Landau algorithm and the circulant embedding method. In these simulations, we were able to probe the probability densities smaller than 10-1200
Baruch Barzel, BIU
Title: Constructing dynamically predictive networks
Abstract: The state of a complex system is often characterized by the dynamic activities of all its nodes, from the excitation of neurons in brain networks to the expression levels of genes in subcellular interactions. The dynamics around these states are then captured by the system's response to activity perturbations, e.g., a local spike in neuronal activity, an outbreak of an epidemic or a sudden hike in the expression of one or several genes. How then do networks respond to such perturbations? Will they remain stable and witness the perturbation decay, or will they lose stability and transition to an entirely new state? Will their response be rapid or slow? Will it be dispersed throughout the network or condense on specific nodes? Encoded within the system’s stability matrix, the Jacobian, the answer to all these questions is obscured by the scale and diversity of the relevant systems, their enormous parameter space, and their nonlinear interaction dynamics. To penetrate this complexity, we develop the dynamic Jacobian ensemble, which allows us to systematically investigate the fixed-point dynamics of a broad range of network-based models. We find that real-world Jacobians exhibit universal scaling patterns in which structure and dynamics are deeply intertwined. Once constructed, these unique - and most crucially, unexplored, Jacobians map each combination of topology and dynamics into an effective network, whose link weights adapt to capture the effect of the system's nonlinearity. Hence, identical networks will acquire distinct link weights, depending on the nature of their interaction dynamics - social, biological or technological. The result: effective network maps, whose weighted topology is designed to predict precisely whether the system is stable or unstable, rapid or slow, dispersed or condensed.
Relevant papers:
- Emergent stability in complex network dynamics, Nature Physics 19, 1033–1042 (2023)
- Reviving a failed network through microscopic interventions, Nature Physics 18, 338–349 (2022)
Haim Diamant, TAU
Title: Universal relation between entropy and diffusion
Abstract: We derive, based on first principles, a rigorous inequality relating the entropy of a system of particles and the dynamic propagator of particle configurations. The relation is applicable to steady states arbitrarily far from equilibrium. Applying it to diffusive dynamics gives an inequality relating the entropy and the late-time diffusion coefficient. The relation can be used to obtain useful bounds for the diffusion coefficient from the calculated steady-state entropy or, conversely, to bound the entropy based on measured diffusion coefficients. We demonstrate the validity and usefulness of the inequality through several examples and discuss cases where the bound becomes tight.
B. Sorkin, H. Diamant, G. Ariel, Phys. Rev. Lett. 131, 147101 (2023)
