arXiv subjects

Kinjal Dasbiswas

Publications and source records attributed to Kinjal Dasbiswas.

15 recordsLinked to original sources

arXiv2026

Cooperative effect of local active stresses on the macroscopic contractility of elastic fiber networks

The collective action of actively contractile units embedded in elastic biopolymer networks plays a crucial role in regulating the network's macroscopic mechanical response. Here, we investigate how the macroscopic boundary stress in model elastic fiber networks depends on the number and nature of embedded contractile units, each exerting an isotropic force dipole, as well as on the bending stiffness of fibers. We find that the macroscopic stress increases nonlinearly with the number of dipoles due to mutual stiffening of initially soft, bending-dominated networks. Using effective medium theory, we relate this enhanced contractility to an increase in the effective average network coordination number due to constraints imposed by the force dipoles. By comparing three distinct force dipole models that differ in their local structures, we demonstrate that the specific manner in which an active unit constrains the network strongly influences the onset and nature of the stiffening transition. Our results highlight that not only the quantity but also the local geometry of force-generating units critically determines the macroscopic mechanical behavior. This framework provides a physical basis for understanding how biological systems-such as molecular motors in the cytoskeleton, or adherent cells in the extracellular matrix-can modulate network-scale nonlinear elastic properties through local tuning of active force-generating units.

cond-mat.soft↗

arXiv2025

Gliding microtubules exhibit tunable collective rotation driven by chiral active forces

How chirality propagates across scales remains an open question in many biological and synthetic systems. An especially clear manifestation of this propagation is found in in vitro gliding assays of cytoskeletal filaments on surfaces, driven by molecular motors. These assays have become model systems of active matter dynamics, as they spontaneously organize into diverse dynamical states, including collective motions with chiral rotation. However, the microscopic mechanisms underlying these chiral collective dynamics have remained unclear. Here, we investigate rotating active nematic order in microtubule gliding assay experiments under two stabilization conditions, each on two types of substrates. We propose that chirality in active forces exerted by motors on microtubules represents a viable mechanism for this large-scale chirality. Using Brownian dynamics simulations of self-propelled, semiflexible filaments with chiral activity, we demonstrate that coherently rotating active nematic order emerges by this mechanism even in the absence of curvature, i.e. shape chirality, of the constituent filaments. Moreover, we predict that the angular speed and handedness of the collective rotation can be tuned by modulating filament stiffness. Our findings identify a new set of sufficient microscopic ingredients for predictable propagation of chiral handedness from the molecular to the material scale in living and active matter.

cond-mat.soft↗

arXiv2024

Elastic interactions compete with persistent cell motility to drive durotaxis

The directed migration of cells toward stiffer substrate regions or durotaxis is relevant to tissue development and tumor progression. Here, we introduce a phenomenological model for single cell durotaxis that incorporates both elastic deformation-mediated cell-substrate interactions and the stochasticity of cell migration. Our model is motivated by a key observation in an early demonstration of durotaxis: a single, contractile cell at a sharp interface between a softer and a stiffer region of an elastic substrate reorients and migrates towards the stiffer region. We model migrating cells as self-propelling, persistently motile agents that exert contractile traction forces on their elastic substrate. The resulting substrate deformations induce elastic interactions with mechanical boundaries, captured by an elastic potential. Cell dynamics are governed by two critical parameters: the strength of the traction-induced boundary interaction (A) and the persistence of cell motility (Pe). The resulting elastic forces and torques align cells perpendicular (parallel) to the boundary and accumulate (deplete) them at clamped (free) boundaries. A clamped boundary induces an attractive potential, promoting durotaxis, while a free boundary generates a repulsive potential, preventing anti-durotaxis. By analyzing steady-state position and orientation probabilities, we show how accumulation and depletion depend on elastic potential strength and motility. We compare our findings with biological microswimmers and other active particles that accumulate at confining boundaries. The model defines metrics for boundary accumulation and durotaxis, presenting a phase diagram with three regimes: durotaxis, adurotaxis, and motility-induced accumulation. Our model predicts how durotaxis depends on cell contractility and motility, offering insights and testable predictions for future experiments.

cond-mat.soft↗

arXiv2023

Shape Transitions in Network Model of Active Elastic Shells

Morphogenesis involves the transformation of initially simple shapes, such as multicellular spheroids, into more complex $3D$ shapes. These shape changes are governed by mechanical forces including molecular motor-generated forces as well as hydrostatic fluid pressure, both of which are actively regulated in living matter through mechano-chemical feedback. Inspired by autonomous, biophysical shape change, such as occurring in the model organism hydra, we introduce a minimal, active, elastic model featuring a network of springs in a globe-like spherical shell geometry. In this model there is coupling between activity and the shape of the shell: if the local curvature of a filament represented by a spring falls below a critical value, its elastic constant is actively changed. This results in deformation of the springs that changes the shape of the shell. By combining excitation of springs and pressure regulation, we show that the shell undergoes a transition from spheroidal to either elongated ellipsoidal or a different spheroidal shape, depending on pressure. There exists a critical pressure at which there is an abrupt change from ellipsoids to spheroids, showing that pressure is potentially a sensitive switch for material shape. More complex shapes, involving loss of cylindrical symmetry, can arise when springs are excited both above (spring constants increase) and below (spring constants decrease) the curvature threshold. We thus offer biologically inspired design principles for autonomous shape transitions in active elastic shells.

cond-mat.soft↗

arXiv2023

Range and strength of mechanical interactions of force dipoles in elastic fiber networks

Mechanical forces generated by myosin II molecular motors drive diverse cellular processes, most notably shape change, division and locomotion. These forces may be transmitted over long range through the cytoskeletal medium - a disordered, viscoelastic network of biopolymers. The resulting cell size scale force chains can in principle mediate mechanical interactions between distant actomyosin units, leading to self-organized structural order in the cell cytoskeleton. To investigate this process, we model the actin cytoskeleton on a percolated fiber lattice network, where fibers are modeled as linear elastic elements that can both bend and stretch, whereas myosin motors exert contractile force dipoles. We quantify the range and heterogeneity of force transmission in these networks in response to a force dipole, showing how these depend on varying bond dilution and fiber bending-to-stretching stiffness ratio. By analyzing clusters of nodes connected to highly strained bonds, as well as the decay rate of strain energy with distance from the force dipole, we show that long-range force transmission is screened out by fiber bending in diluted networks. We further characterize the difference in the propagation of tensile and compressive forces. This leads to a dependence of the mechanical interaction between a pair of force dipoles on their mutual separation and orientation. In more homogeneous networks, the interaction between force dipoles recapitulates the power law dependence on separation distance predicted by continuum elasticity theory, while in diluted networks, the interactions are short-ranged and fluctuate strongly with local network configurations. Altogether, our work suggests that elastic interactions between force dipoles in disordered, fibrous media can act as an organizing principle in biological materials.

cond-mat.soft↗

arXiv2023

Optimal mechanical interactions direct multicellular network formation on elastic substrates

Cells self-organize into functional, ordered structures during tissue morphogenesis, a process that is evocative of colloidal self-assembly into engineered soft materials. Understanding how inter-cellular mechanical interactions may drive the formation of ordered and functional multicellular structures is important in developmental biology and tissue engineering. Here, by combining an agent-based model for contractile cells on elastic substrates with endothelial cell culture experiments, we show that substrate deformation-mediated mechanical interactions between cells can cluster and align them into branched networks. Motivated by the structure and function of vasculogenic networks, we predict how measures of network connectivity like percolation and fractal dimension, as well as local morphological features including junctions, branches, and rings depend on cell contractility and density, and on substrate elastic properties including stiffness and compressibility. We predict and confirm with experiments that cell network formation is substrate stiffness-dependent, being optimal at intermediate stiffness. Overall, we show that long-range, mechanical interactions provide an optimal and general strategy for multi-cellular self-organization, leading to more robust and efficient realization of space-spanning networks than through just local inter-cellular interactions.

cond-mat.soft↗

arXiv2022

Collective states of active particles with elastic dipolar interactions

Many types of mammalian cells exert active contractile forces and mechanically deform their elastic substrate, to accomplish biological functions such as cell migration. These substrate deformations provide a mechanism by which cells can sense other cells, leading to long range mechanical intercell interactions and possible self organization. Here, we treat cells as noisy motile particles that exert contractile dipolar stresses on elastic substrates as they move. By combining this minimal model for the motility of individual cells with a linear elastic model that accounts for substrate mediated cell cell interactions, we examine emergent collective states that result from the interplay of cell motility and long range elastic dipolar interactions. In particular, we show that particles self assemble into flexible, motile chains which can cluster to form diverse larger scale compact structures with polar order. By computing key structural and dynamical metrics, we distinguish between the collective states at weak and strong elastic interactions, as well as at low and high motility. We also show how these states are affected by confinement, an important characteristic of the complex mechanical microenvironment inhabited by cells. Our model predictions are generally applicable to active matter with dipolar interactions ranging from biological cells to synthetic colloids endowed with electric or magnetic dipole moments.

cond-mat.soft↗

arXiv2021

Modeling mechanochemical pattern formation in elastic sheets of biological matter

Inspired by active shape morphing in developing tissues and biomaterials, we investigate two generic mechanochemical models where the deformations of a thin elastic sheet are driven by, and in turn affect, the concentration gradients of a chemical signal. We develop numerical methods to study the coupled elastic deformations and chemical concentration kinetics, and illustrate with computations the formation of different patterns depending on shell thickness, strength of mechanochemical coupling and diffusivity. In the first model, the sheet curvature governs the production of a contractility inhibitor and depending on the threshold in the coupling, qualitatively different patterns occur. The second model is based on the stress--dependent activity of myosin motors, and demonstrates how the concentration distribution patterns of molecular motors are affected by the long-range deformations generated by them. Since the propagation of mechanical deformations is typically faster than chemical kinetics (of molecular motors or signaling agents that affect motors), we describe in detail and implement a numerical method based on separation of timescales to effectively simulate such systems. We show that mechanochemical coupling leads to long-range propagation of patterns in disparate systems through elastic instabilities even without the diffusive or advective transport of the chemicals.

cond-mat.soft↗

arXiv2021

Self-organization and shape change by active polarization in nematic droplets

Active forces occurring within cells can drive crucial biological processes that involve spontaneous organization and shape change, such as cell division. Motivated by recent in vitro experiments of nematic droplets of cytoskeletal filaments and motors that self-organize and divide, we present a minimal hydrodynamic model that combines the nonequilibrium kinetics of motor-filament interactions with equilibrium nematic phase separation. The motors organize within droplets and structure filaments into polarized aster defects. At large motor activity, they can even deform or divide the droplet, or form multi-aster chains of droplets. Our predicted phase diagram recapitulates these experimentally observed shapes.

cond-mat.soft↗

arXiv2020

Mechanochemical induction of wrinkling morphogenesis on elastic shells

Morphogenetic dynamics of tissue sheets require coordinated cell shape changes regulated by global patterning of mechanical forces. Inspired by such biological phenomena, we propose a minimal mechanochemical model based on the notion that cell shape changes are induced by diffusible biomolecules that influence tissue contractility in a concentration-dependent manner -- and whose concentration is in turn affected by the macroscopic tissue shape. We perform computational simulations of thin shell elastic dynamics to reveal propagating chemical and three-dimensional deformation patterns arising due to a sequence of buckling instabilities. Depending on the concentration threshold that actuates cell shape change, we find qualitatively different patterns. The mechanochemically coupled patterning dynamics are distinct from those driven by purely mechanical or purely chemical factors. Using numerical simulations and theoretical arguments, we analyze the elastic instabilities that result from our model and provide simple scaling laws to identify wrinkling morphologies.

cond-mat.soft↗

arXiv2019

Nucleation and shape dynamics of model nematic tactoids around adhesive colloids

Recent experiments have shown how nematically-ordered tactoid shaped actin droplets can be reorganized and divided by the action of myosin molecular motors. In this paper, we consider how similar morphological changes can potentially be achieved under equilibrium conditions. Using simulations, both atomistic and continuum, and a phenomenological model, we explore how the nucleation dynamics, shape changes, and the final steady state of a nematic tactoid droplet can be modified by interactions with model adhesive colloids that mimic a myosin motor cluster. Our results provide a prescription for the minimal conditions required to stabilize tactoid reorganization and division in an equilibrium colloidal-nematic setting.

cond-mat.stat-mech↗

arXiv2019

Topological waves in fluids with odd viscosity

Fluids in which both time-reversal and parity are broken can display a dissipationless viscosity that is odd under each of these symmetries. Here, we show how this odd viscosity has a dramatic effect on topological sound waves in fluids, including the number and spatial profile of topological edge modes. Odd viscosity provides a short-distance cutoff that allows us to define a bulk topological invariant on a compact momentum space. As the sign of odd viscosity changes, a topological phase transition occurs without closing the bulk gap. Instead, at the transition point, the topological invariant becomes ill-defined because momentum space cannot be compactified. This mechanism is unique to continuum models and can describe fluids ranging from electronic to chiral active systems.

cond-mat.soft↗

arXiv2018

Describing screening in dense ionic fluids with a charge-frustrated Ising model

Charge correlations in dense ionic fluids give rise to novel effects such as long-range screening and colloidal stabilization which are not predicted by the classic Debye-Huckel theory. We show that a Coulomb or charge-frustrated Ising model, which accounts for both long-range Coulomb and short-range molecular interactions, simply describes some of these ionic correlations. In particular, we obtain at mean field level and in simulations, a non-monotonic dependence of the screening length on the temperature. Using a combination of simulations and mean field theories, we study how the correlations in the various regimes are affected by the strength of the short ranged interactions.

cond-mat.soft↗

arXiv2017

Theory of epithelial cell shape transitions induced by mechanoactive chemical gradients

Cell shape is determined by a balance of intrinsic properties of the cell as well as its mechanochemical environment. Inhomogeneous shape changes underly many morphogenetic events and involve spatial gradients in active cellular forces induced by complex chemical signaling. Here, we introduce a mechanochemical model based on the notion that cell shape changes may be induced by external diffusible biomolecules that influence cellular contractility (or equivalently, adhesions) in a concentration-dependent manner -- and whose spatial profile in turn is affected by cell shape. We map out theoretically the possible interplay between chemical concentration and cellular structure. Besides providing a direct route to spatial gradients in cell shape profiles in tissues, our results indicate that the dependence on cell shape helps create robust mechanochemical gradients.

physics.bio-ph↗

arXiv2017

Topological localization in out-of-equilibrium dissipative systems

In this paper we report that notions of topological protection can be applied to stationary configurations that are driven far from equilibrium by active, dissipative processes. We show this for physically two disparate cases : stochastic networks governed by microscopic single particle dynamics as well as collections of driven, interacting particles described by coarse-grained hydrodynamic theory. In both cases, the presence of dissipative couplings to the environment that break time reversal symmetry are crucial to ensuring topologically protection. These examples constitute proof of principle that notions of topological protection, established in the context of electronic and mechanical systems, do indeed extend generically to processes that operate out of equilibrium. Such topologically robust boundary modes have implications for both biological and synthetic systems.

cond-mat.stat-mech↗