arXiv subjects

Tal Cohen

Publications and source records attributed to Tal Cohen.

At least 19 recordsLinked to original sources

arXiv2026

Virtual Surjection and the $n$-$(n+1)$-$(n+2)$ Theorem for Discrete Groups

We prove the Virtual Surjection Conjecture for discrete groups, both for property $F_{n}$ and for property $FP_{n}$: given a product of groups of type $F_k$ (respectively $FP_k$), a subgroup that virtually surjects onto $k$-tuples must be $F_k$ (respectively $FP_k$) as well. We prove the homological $n$-$(n+1)$-$(n+2)$ Conjecture for discrete groups under the assumption the common quotient is finitely presented, and prove that this assumption cannot be dropped. We deduce the $n$-$(n+1)$-$(n+2)$ Conjecture for property $F_{n}$.

math.GR↗

arXiv2026

Virtual Surjection and the $n$-$(n+1)$-$(n+2)$ Theorem for Profinite Groups

We prove the Virtual Surjection Conjecture for profinite groups. Namely, given a product of $n$ profinite $\mathrm{FP}_{k}$ groups, a subgroup that virtually surjects onto $k$-tuples must be $\mathrm{FP}_{k}$ as well. We also prove the $n$-$(n+1)$-$(n+2)$ Conjecture for profinite groups, as well as a few other $\mathrm{FP}_{n}$ permanence results for fibre products. Our main tool is a numerical criterion for property $\mathrm{FP}_{n}$ of modules of profinite groups. Our work suggests a new finiteness property to investigate.

math.GR↗

arXiv2026

Compact Invariant Random Subgroups

We study ergodic invariant random subgroups that give full measure to the subset of compact subgroups. We show that in real Lie groups, compactly generated $p$-adic Lie groups, locally compact hyperbolic groups and infinitely ended groups they are always contained in a compact normal subgroup. In general $p$-adic Lie groups, we show they are contained in the locally elliptic radical. In totally disconnected locally compact groups, we show they are contained in the intersection of all Levi subgroups of inner automorphisms.

math.GR↗

arXiv2026

On the Maximal Size of Irredundant Generating Sets in Lie Groups and Algebraic Groups

We show the following dichotomy for a connected Lie group $G$: If $G$ is amenable, then any topologically generating set $X\subset G$ of size larger than a fixed polynomial in the dimension of $G$ must be redundant (i.e., a proper subset of $X$ still generates $G$). If $G$ is non-amenable, then it admits arbitrarily large topologically generating sets that are irredundant, and remain irredundant even after applying Nielsen transformations. The polynomial bound for amenable groups is obtained by reduction to finite simple groups of Lie type via strong approximation. This partially answers two conjectures by Gelander on generation in compact Lie groups and simple algebraic groups, and moreover shows that these conjectures are implied by the Wiegold conjecture. The construction of large Nielsen irredundant generating sets in non-amenable groups is done by extending Minsky's work to higher rank Lie groups, exhibiting dense representations in the domain of discontinuity of the $\mathrm{Out}(F_{n})$-action on the character variety.

math.GR↗

arXiv2025

A Unified Thermo-Chemo-Mechanical Framework for Bulk and Frontal Polymerization: Coupled Kinetics and Front Stability

Polymerization is a fundamental chemical process enabling large-scale production of material components across modern industries. By transforming a monomer mixture into a cross-linked polymer network, polymerization induces changes in temperature and material properties such as density and stiffness, which can generate residual stress and warping through coupled mechanisms that remain incompletely understood. Depending on processing conditions, polymerization may occur either in the bulk, sustained by continuous external energy input, or as a self-sustaining exothermic reaction front, commonly referred to as frontal polymerization. While frontal polymerization offers rapid and energy-efficient curing, its localized reaction zone produces sharp spatial gradients that amplify thermo-chemo-mechanical coupling effects. In this work, we develop a thermodynamically consistent framework that captures both bulk and frontal polymerization, incorporating stress-dependent reaction kinetics and the evolution of the stress-free configuration during curing. Using a narrow reaction-zone approximation in a uniaxial setting, we derive analytical predictions for propagation velocity, residual stress development, and stability. A perturbation analysis yields a stability criterion that generalizes the classical Zeldovich number by accounting for heat loss and mechanical loading, and enables construction of a phase diagram distinguishing stable, unstable, and quenched propagation regimes.

cond-mat.soft↗

arXiv2025

Interfacial Cavitation with Surface Tension: New Insights into Failure of Particle Reinforced Polymers

Understanding and mitigating the failure of reinforced elastomers has been a long-standing challenge in many industrial applications. In an early attempt to shed light on the fundamental mechanisms of failure, Gent and Park presented a systematic experimental study examining the field that develops near rigid beads that are embedded in the material and describe two distinct failure phenomena: cavitation that occurs near the bead in the bulk of the material, and debonding at the bead--rubber interface [Gent, A.N. and Park, B., 1984. Journal of Materials Science, 19, pp.1947-1956]. Although the interpretation of their results has not been challenged, several questions stemming from their work remain unresolved. Specifically, the reported dependence of the cavitation stress on the diameter of the bead and the counterintuitive relationship between the delamination threshold and the material stiffness. In this work, we revisit the work of Gent and Park and consider an alternative explanation of their observations, interfacial cavitation. A numerically validated semi-analytical model shows that in {the} presence of surface tension, defects at the bead-rubber interface may be prone to cavitate at lower pressures compared to bulk cavitation, and that surface tension can explain the reported length-scale effects. A phase-map portrays the distinct regions of `cavitation dominated' and `delamination dominated' failure and confirms that for the expected range of material properties of the rubbers used by Gent and Park, interfacial cavitation is a likely explanation. Crucially, this result offers a new avenue to tune and optimize the performance of reinforced polymers and other multi-material systems.

cond-mat.soft↗

arXiv2024

Lifting Generators in Connected Lie Groups

Given an epimorphism between topological groups $f:G\to H$, when can a generating set of $H$ be lifted to a generating set of $G$? We show that for connected Lie groups the problem is fundamentally abelian: generators can be lifted if and only if they can be lifted in the induced map between the abelianisations (assuming the number of generators is at least the minimal number of generators of $G$). As a consequence, we deduce that connected perfect Lie groups satisfy the Gasch\"utz lemma: generating sets of quotients can always be lifted. If the Lie group is not perfect, this may fail. The extent to which a group fails to satisfy the Gasch\"utz lemma is measured by its \emph{Gasch\"utz rank}, which we bound for all connected Lie groups, and compute exactly in most cases. Additionally, we compute the maximal size of an irredundant generating set of connected abelian Lie groups, and discuss connections between such generation problems with the Wiegold conjecture.

math.GR↗

arXiv2024

Cylindrical Cavity Expansion: A Novel Method for Characterizing the Mechanical Properties of Soft Materials

The low elastic modulus of soft materials, combined with geometric nonlinearity and rate dependence, presents significant challenges in the characterization of their mechanical response. We introduce a novel method for measuring the mechanical properties of soft materials under large deformations via cylindrical cavity expansion. In this method, a cylindrical cavity is fabricated in the material and expanded by volume-controlled injection of an incompressible fluid with simultaneous measurement of the applied pressure at the cavity wall. The relationship between applied pressure and deformation at the cavity wall is then employed to characterize the nonlinear mechanical properties. We demonstrate the feasibility of the proposed method and validate it by measuring the mechanical properties of synthetic polydimethylsiloxane (PDMS) and comparing with reported values in the literature. Results indicate that the cylindrical cavitation method effectively captures the response of PDMS over a wide range of stiffness (shear modulus ranging from 5 kPa to 300 kPa) and exhibit high repeatability. The proposed method overcomes limitations in characterization of ultra-soft materials using traditional testing methods, such as challenges with fabrication and clamping in unaxial tension testing and friction and adhesion effects in compression and indentation testing, thus enabling accurate and precise characterization. It also offers improved accuracy and repeatability over other needle induced cavity expansion methods due to precise control over the initial cavity dimension and shape at the cost of increased invasiveness of testing.

cond-mat.mtrl-sci↗

arXiv2024

Explaining the spread in measurement of PDMS elastic properties: influence of test method and curing protocol

Accuracy in the measurement of mechanical properties is essential for precision engineering and for the interrogation of composition-property relationships. Conventional methods of mechanical testing, such as uniaxial tension, compression, and nanoindentation, provide highly repeatable and reliable results for stiff materials, for which they were originally developed. However, when applied to the characterization of soft and biological materials, the same cannot be said, and the spread of reported properties of similar materials is vast. Polydimethylsiloxane (PDMS), commonly obtained from Dow as SYLGARD 184, is a ubiquitous such material, which has been integral to the rapid development of biocompatible microfluidic devices and flexible electronics in recent decades. However, reported shear moduli of this material range over 2 orders of magnitude for similar chemical compositions. Taking advantage of the increased mechanical scrutiny afforded to SYLGARD 184 in recent years, we combine both published and new experimental data obtained using 9 mechanical test methods. A statistical analysis then elucidates the significant bias induced by the test method itself, and distinguishes this bias from the influence of curing protocols on the mechanical properties. The goal of this work is thus two-fold: (i) it provides a quantitative understanding of the different factors that influence reported properties of this particular material, and (ii) it serves as a cautionary tale. As researchers in the field of mechanics strive to quantify the properties of increasingly complex soft and biological materials, converging on a standardized measurement of PDMS is a necessary first step.

cond-mat.soft↗

arXiv2024

An Invitation to Analytic Group Theory

This book is concerned with analytic approaches of studying groups and their actions. Much attention is devoted to the study of amenability and Kazhdan's property (T), which are perhaps the most important analytic properties of a group, but we also discuss other analytic notions. We tried to introduce tricks, ideas and lemmas that repeatedly turn out to be useful in various situations. Our main guideline was to expose the beauty of the theory and to present many different aspects of it while keeping the text short, simple and accessible, sometimes at the expense of diving deep or providing thorough expositions. Hopefully this book could serve as a smooth entry to Analytic Group Theory.

math.GR↗

arXiv2023

Mechanical Forces Quench Frontal Polymerization: Experiments and Theory

Frontal polymerization is a promising energy-saving method for rapid fabrication of polymer components with good mechanical properties. In these systems, a small energy input is sufficient to convert monomers, from a liquid or soft solid state, into a stiff polymer component. Once the reaction is initiated, it propagates as a self-sustaining front that is driven by the heat released from the reaction itself. While several studies have been proposed to capture the coupling between thermodynamics and extreme chemical kinetics in these systems, and can explain experimentally observed thermo-chemical instabilities, only few have considered the potential influence of mechanical forces that develop in these systems during fabrication. Nonetheless, some experiments do indicate that local volume changes induced by the competing effects of thermal expansion and chemical shrinkage, can lead to significant deformation or even failure in the resulting component. In this work, we present a unique experimental approach to elucidate the effect of mechanics on the propagation. Our experiments reveal that residual stresses that arise in frontal polymerization are not only a potential cause of undesired deformations in polymer products, but can also quench the reaction front. This thermo-chemo-mechanically coupled effect is captured by our theoretical model, which explains the mechanical limitations on frontal polymerization and can guide future fabrication. Overall, the findings of this work suggest that mechanical coupling needs to be taken into consideration to enable industrial applications of frontal polymerization at large scales.

cond-mat.soft↗

arXiv2023

A large deformation theory for coupled swelling and growth with application to growing tumors and bacterial biofilms

There is significant interest in modelling the mechanics and physics of growth of soft biological systems such as tumors and bacterial biofilms. Solid tumors account for more than 85% of cancer mortality and bacterial biofilms account for a significant part of all human microbial infections.These growing biological systems are a mixture of fluid and solid components and increase their mass by intake of diffusing species such as fluids and nutrients (swelling) and subsequent conversion of some of the diffusing species into solid material (growth). Experiments indicate that these systems swell by large amounts and that the swelling and growth are intrinsically coupled. However, many existing theories for swelling coupled growth employ linear poroelasticity, which is limited to small swelling deformations, and employ phenomenological prescriptions for the dependence of growth rate on concentration of diffusing species and the stress-state in the system. In particular, the termination of growth is enforced through the prescription of a critical concentration of diffusing species and a homeostatic stress. In contrast, by developing a fully coupled swelling-growth theory that accounts for large swelling through nonlinear poroelasticity, we show that the emergent driving stress for growth automatically captures all the above phenomena. Further, we show that for the soft growing systems considered here, the effects of the homeostatic stress and critical concentration can be encapsulated under a single notion of a critical swelling ratio. The applicability of the theory is shown by its ability to capture experimental observations of growing tumors and biofilms under various mechanical and diffusion-consumption constraints. Additionally, compared to generalized mixture theories, our theory is amenable to relatively easy numerical implementation with a minimal physically motivated parameter space.

cond-mat.soft↗

arXiv2022

Torsional Instabilities in the Delamination of Soft Adhesives

Soft adhesive contacts are ubiquitous in nature and are increasingly used in synthetic systems, such as flexible electronics and soft robots, due to their advantages over traditional joining techniques. While methods to study the failure of adhesives typically apply tensile loads to the adhesive joint, less is known about the performance of soft adhesives under shear and torsion, which may become important in engineering applications. A major challenge that has hindered the characterization of shear/torsion-induced delamination is imposed by the fact that, even after delamination, contact with the substrate is maintained, thus allowing for frictional sliding and re-adhesion. In this work, we address this gap by studying the controlled delamination of soft cylinders under combined compression and torsion. Our experimental observations expose the nucleation of delamination at an imperfection and its propagation along the circumference of the cylinder. The observed sequence of `stick-slip' events and the sensitivity of the delamination process to material parameters are explained by a theoretical model that captures axisymmetric delamination patterns, along with the subsequent frictional sliding and re-adhesion. By opening up an avenue for improved characterization of adhesive failure, our experimental approach and theoretical framework can guide the design of adhesives in future applications.

cond-mat.soft↗

arXiv2022

Periodic necking of misfit hyperelastic filaments embedded in a soft matrix

The necking instability is a precursor to tensile failure and rupture of materials. A quasistatically loaded free-standing uniaxial specimen typically exhibits necking at a single location, corresponding to a long wavelength bifurcation mode. If confined to a substrate or embedded in a matrix, the same filament can exhibit periodic necking thus creating segments of finite length. While periodic instabilities have been extensively studied in ductile metal filaments and thin sheets, less is known about necking in hyperelastic materials. There is a renewed interest in the role of necking in novel materials to advance fabrication processes and to explain fragmentation phenomena observed in 3D printed active biological matter. In both cases materials are not well described by existing frameworks that employ J2 plasticity, and existing studies ignore the role of misfit stretches that may emerge in these systems through chemical or biological contraction. To address these limitations, we first experimentally demonstrate the role of the matrix on the necking and fragmentation of a compliant embedded filament. Using a strain softening generalized hyperelastic model, our analytical bifurcation analysis explains the experimental observations and agrees with numerical predictions. The analysis reveals 3 distinct bifurcation modes: a long wavelength necking, recovering the Considere criterion; a periodic necking observed in our experiments; and a short wavelength mode characterized by localization along the center cord of the filament and independent of the film-to-matrix stiffness ratio. We find that the softening coefficient and the filament misfit stretch can significantly influence the stability threshold and observed wavelength, respectively. Our results can guide the design and fabrication of composite materials and explain the fragmentation processes observed in active biological materials.

cond-mat.soft↗

arXiv2022

Biofilms as self-shaping growing nematics

Active nematics are the nonequilibrium analog of passive liquid crystals in which anisotropic units consume free energy to drive emergent behavior. Similar to liquid crystal (LC) molecules in displays, ordering and dynamics in active nematics are sensitive to boundary conditions; however, unlike passive liquid crystals, active nematics, such as those composed of living matter, have the potential to regulate their boundaries through self-generated stresses. Here, using bacterial biofilms confined by a hydrogel as a model system, we show how a three-dimensional, living nematic can actively shape itself and its boundary in order to regulate its internal architecture through growth-induced stresses. We show that biofilms exhibit a sharp transition in shape from domes to lenses upon changing environmental stiffness or cell-substrate friction, which is explained by a theoretical model considering the competition between confinement and interfacial forces. The growth mode defines the progression of the boundary, which in turn determines the trajectories and spatial distribution of cell lineages. We further demonstrate that the evolving boundary defines the orientational ordering of cells and the emergence of topological defects in the interior of the biofilm. Our findings reveal novel self-organization phenomena in confined active matter and provide strategies for guiding the development of programmed microbial consortia with emergent material properties.

q-bio.QM↗

arXiv2022

Interfacial Cavitation

Cavitation has long been recognized as a crucial predictor, or precursor, to the ultimate failure of various materials, ranging from ductile metals to soft and biological materials. Traditionally, cavitation in solids is defined as an unstable expansion of a void or a defect within a material. The critical applied load needed to trigger this instability - the critical pressure - is a lengthscale independent material property and has been predicted by numerous theoretical studies for a breadth of constitutive models. While these studies usually assume that cavitation initiates from defects in the bulk of an otherwise homogeneous medium, an alternative and potentially more ubiquitous scenario can occur if the defects are found at interfaces between two distinct media within the body. Such interfaces are becoming increasingly common in modern materials with the use of multi-material composites and layer-by-layer additive manufacturing methods. However, a criterion to determine the threshold for interfacial failure, in analogy to the bulk cavitation limit, has yet to be reported. In this work we fill this gap. Our theoretical model captures a lengthscale independent limit for interfacial cavitation, and is shown to agree with our observations at two distinct lengthscales, via two different experimental systems. To further understand the competition between the two cavitation modes (bulk versus interface) we expand our investigation beyond the elastic response to understand the ensuing unstable propagation of delamination at the interface. A phase diagram summarizes these results, showing regimes in which interfacial failure becomes the dominant mechanism.

cond-mat.soft↗

arXiv2022

A Reciprocal Theorem for Finite Deformations in Incompressible Bodies

The reciprocal theorems of Maxwell and Betti are foundational in mechanics but have so far been restricted to infinitesimal deformations in elastic bodies. In this manuscript, we present a reciprocal theorem that relates solutions of a specific class of large deformation boundary value problems for incompressible bodies; these solutions are shown to identically satisfy the Maxwell-Betti theorem. The theorem has several potential applications such as development of alternative convenient experimental setups for the study of material failure through bulk and interfacial cavitation, and leveraging easier numerical implementation of equivalent auxiliary boundary value problems. The following salient features of the theorem are noted: (i) it applies to dynamics in addition to statics, (ii) it allows for large deformations, (iii) generic body shapes with several potential holes, and (iv) any general type of boundary conditions.

cond-mat.soft↗

arXiv2022

The crucial role of elasticity in regulating liquid-liquid phase separation in cells

Liquid-liquid phase separation has emerged as a fundamental mechanism underlying intracellular organization, with evidence for it being reported in numerous different systems. However, there is a growing concern regarding the lack of quantitative rigor in the techniques employed to study phase separation, and their ability to account for the complex nature of the cellular milieu, which affects key experimentally observable measures, such as the shape, size and transport dynamics of liquid droplets. Here we bridge this gap by combining recent experimental data with theoretical predictions that capture the subtleties of nonlinear elasticity and fluid transport. We show that within a biologically accessible range of material parameters, phase separation is highly sensitive to elastic properties and can thus be used as a mechanical switch to rapidly transition between different states in cellular systems. Furthermore, we show that this active mechanically mediated mechanism can drive transport across cells at biologically relevant timescales and could play a crucial role in promoting spatial localization of condensates; whether cells exploit such mechanisms for transport of their constituents, remains an open question.

cond-mat.soft↗