arXiv subjects

Tanniemola B. Liverpool

Publications and source records attributed to Tanniemola B. Liverpool.

At least 19 recordsLinked to original sources

arXiv2026

Quantifying cell shape and density fluctuations in epithelial tissue in vivo

Controlling changes in cell shape are crucial for many biological processes, such as tissue development and wound healing. Tissues typically are heterogeneous with a variety of cell shapes and sizes. Of particular interest are local deviations from an average cell shape and size. These fluctuations may extend and transmit across tissues, potentially offering valuable insights into tissue characteristics such as variations in effective "stiffness" or rigidity. In this study, we present a theoretical framework that captures the dynamics of epithelial cell shapes within tissue, incorporating both their average behaviour and fluctuation patterns. We model cells as interacting soft ellipsoids of varying size and aspect ratio. Coarse-graining our model, we obtain a set of continuum stochastic differential equations from which we derive spatial-temporal correlation functions. These correlation functions fit with our experimental data from the developmental process of the \textit{Drosophila} pupal wing. From the correlation functions, critical parameters representing active cell shape changes and effective tissue "stiffness" can be determined.

cond-mat.stat-mech↗

arXiv2026

Exotic rheology of materials with active rearrangements

The flow of biological tissues during development is controlled through the active stresses generated by cells interacting with their mechanical environment in the tissue. Many developmental processes are driven by convergence-extension flows where the tissue has an emergent negative shear modulus and viscosity. This exotic rheology is generated through active T1 transitions where rearrangements are opposite the applied stress direction. Here, we introduce a mean-field elasto-plastic model which shows convergence-extension, based on the Hebraud-Lequeux model, that includes both passive and active elastic elements with opposite stress responses. We find that the introduction of active elements profoundly changes the rheology. Beyond a threshold fraction of active elements, it gives rise to non-monotonic flow curves and negative stresses at positive strain rates. Controlled by the active fraction and the stress diffusion rate, we find both yield stress materials and fluids, with either a positive or negative yield stress or viscosity. Furthermore, allowing for residual active stresses generates rheological curves with an asymmetry in strain rate dependence, resembling hysteresis. These features are characteristic of metamaterials, and highlight how biology uses disordered, active materials with exotic rheology.

cond-mat.soft↗

arXiv2026

Breakup of an active chiral fluid

The nonlinear breakup dynamics of a strip of active chiral fluid is considered, and it is shown that the strip thickness goes to zero as a power law in finite time. Applying slender body theory to the hydrodynamic equations of active chiral fluids, we predict the exponents analytically, and our predictions are shown to be in excellent agreement with numerical simulations. Qualitative agreement between experiment and simulation is also found.

cond-mat.soft↗

arXiv2026

Dynamics of Wound Closure in Living Nematic Epithelia

We study theoretically the closure of a wound in a layer of epithelial cells in a living tissue after damage. Our analysis is informed by our recent experiments observing re-epithelialisation in vivo of Drosophila pupae. On time and length-scales such that the evolution of the epithelial tissue near the wound is well captured by that of a 2D active fluid with local nematic order, we consider the free-surface problem of a hole in a bounded region of tissue, and study the role that active stresses far from the hole play in the closure of the hole. For parallel anchored nematic order at the wound boundary (as we observe in our experiments), we find that closure is accelerated when the active stresses are contractile and slowed down when the stresses are extensile. Parallel anchoring also leads to the appearance of topological defects which annihilate upon wound closure.

cond-mat.soft↗

arXiv2025

Correlated cell movements drive epithelial finger formation

Epithelia form protective barriers in multicellular organisms. To maintain homeostasis, they must be able to regenerate and heal damaged areas. This occurs through collective cell migration, during which finger-like protrusions commonly appear. Whether these protrusions are driven by specialised leader cells, biochemical cues, or generic physical interactions remains unclear. Integrating in vitro imaging, agent-based simulations, and continuum modelling, we show that correlated active cell motion alone suffices to produce fingers. Leader cells, signalling, and proliferation modulate, but do not trigger, this pattern. Our results show that the key mechanism underlying a complex biological process can be understood using a general framework of the physics of dense active matter.

cond-mat.soft↗

arXiv2025

The physics and mathematics of living and dying matter

We introduce and study a class of active matter models in which we keep track of fuel (stored energy) consumption. They are by construction, thermodynamically consistent. Using these models it is possible for us to observe and follow how active behaviour develops and also how it dissipates as the energy runs out. It is also straightforward to define, calculate and keep track of macroscopic thermodynamic quantities.

cond-mat.soft↗

arXiv2025

Cyclically sheared colloidal gels: structural change and delayed failure time

We present experiments and simulations on cyclically sheared colloidal gels, and probe their behaviour on several different length scales. The shearing induces structural changes in the experimental gel, changing particles' neighborhoods and reorganizing the mesoscopic pores. These results are mirrored in computer simulations of a model gel-former, which show how the material evolves down the energy landscape under shearing, for small strains. By systematic variation of simulation parameters, we characterise the structural and mechanical changes that take place under shear, including both yielding and strain-hardening. We simulate creeping flow under constant shear stress, for gels that were previously subject to cyclic shear, showing that strain-hardening also increases gel stability. This response depends on the orientation of the applied shear stress, revealing that the cyclic shear imprints anisotropic structural features into the gel.

cond-mat.soft↗

arXiv2025

Non-reciprocal interactions drive emergent chiral crystallites

We study a new type of 2D active material that exhibits macroscopic phases with two emergent broken symmetries: self-propelled achiral particles that form dense hexatic clusters, which spontaneously rotate. We experimentally realise active colloids that self-organise into both polar and hexatic crystallites, exhibiting exotic emergent phenomena. This is accompanied by a field theory of coupled order parameters formulated on symmetry principles, including non-reciprocity, to capture the non-equilibrium dynamics. We find that the presence of two interacting broken symmetry fields leads to the emergence of novel chiral phases built from (2D) achiral active colloids (here Quincke rollers). These phases are characterised by the presence of both clockwise and counterclockwise rotating clusters. We thus show that spontaneous rotation can emerge in non-equilibrium systems, even when the building blocks are achiral, due to non-reciprocally coupled broken symmetries. This interplay leads to self-organized stirring through counter-rotating vortices in confined colloidal systems, with cluster size controlled by external electric fields.

cond-mat.soft↗

arXiv2024

Yielding in colloidal gels: from local structure to meso-scale strand breakage and macroscopic failure

We study creep flow and yielding of particulate depletion gels under constant shear stress, combining data on different length and time scales. We characterise the breakage of meso-scale strands in the gel. Breakage events are distributed homogeneously in space, corresponding to macroscopically ductile flow. At the microscale, a spatio-temporal analysis of structural and mechanical metrics connects properties of strands before and after they fail, indicating that strand breakage is statistically predictable. Using results from different scales, we discuss the interplay between creeping and aging dynamics, and we demonstrate a viscosity bifurcation.

cond-mat.soft↗

arXiv2024

Traveling strings of active dipolar colloids

We study an intriguing new type of self-assembled active colloidal polymer system in 3D. It is obtained from a suspension of Janus particles in an electric field that induces parallel dipoles in the particles as well as self-propulsion in the plane perpendicular to the field. At low packing fractions, in experiment, the particles self-assemble into 3D columns that are self-propelled in 2D. Explicit numerical simulations combining dipolar interactions and active self-propulsion find an activity dependent transition to a string phase by increasing dipole strength. We classify the collective dynamics of strings as a function of rotational and translational diffusion. Using an anisotropic version of the Rouse model of polymers with active driving, we analytically compute the strings' collective dynamics and centre of mass motion, which matches simulations and is consistent with experimental data. We also discover long range correlations of the fluctuations along the string contour that grow with the active persistence time, a purely active effect that disappears in the thermal limit.

cond-mat.soft↗

arXiv2023

Complex flow profiles in microscopic active crystals

Active solids emerge from self-actuating components interacting with each other to form crystalline patterns. In equilibrium, commensurability underpins our understanding of nanoscale friction and particle-level dynamics of crystals. However, these concepts have yet to be imported into the realm of active matter. Here, we develop an experimental platform and a theoretical description for microscopic clusters composed of active particles confined and self-assembled into small crystals. In our experiments, these crystallites form upon circular confinement of active rollers, with a magic number of 61 rollers per well. Competition between solidity and self propulsion leads to self shearing and complex flow inversion behaviour, along with self sliding states and activity induced melting. We discover active stick slip dynamics, which periodically switch between a commensurate static state and an incommensurate self-sliding state characterised by a train of localised defects. We describe the steady state behaviour using a discretised model of active hydrodynamics. We then quantify the intermittent stick slip dynamics using a self-propelled extension of the Frenkel Kontorova (FK) model, a fundamental workhorse of slipping and flow in crystals. Our findings in a colloidal model system point to a wealth of phenomena in incommensurate active solids as design principles for both assembly and robotics down to the nanoscale.

cond-mat.soft↗

arXiv2023

Rheology of suspensions of flat elastic particles

We consider a suspension of non-interacting flat elastic particles in a Newtonian fluid. We model a flat shape as three beads, carried along by the flow according to Stokes' law, and connected by nonlinear springs, chosen such that the energy is quadratic in the area. In analogy with common dumbbell models involving two beads connected by linear springs, we solve the stochastic equations of motion exactly to compute the constitutive law for the stress tensor of a flat elastic particle suspension. A lower convected time derivative naturally arises as part of the constitutive law, but surprisingly the rheological response in strong extensional and strong contracting flows is similar to that of the classical Oldroyd-B model associated with dumbbell suspensions.

physics.flu-dyn↗

arXiv2023

Mechano-chemical active feedback generates convergence extension in epithelial tissue

Convergence extension, the simultaneous elongation of tissue along one axis while narrowing along a perpendicular axis, occurs during embryonic development. A fundamental process that contributes to shaping the organism, it happens in many different species and tissue types. Here we present a minimal continuum model, that can be directly linked to the controlling microscopic biochemistry, which shows spontaneous convergence extension. It is comprised of a 2D viscoelastic active material with a mechano-chemical active feedback mechanism coupled to a substrate via friction. Robust convergent extension behaviour emerges beyond a critical value of the activity parameter and is controlled by the boundary conditions and the coupling to the substrate. Oscillations and spatial patterns emerge in this model when internal dissipation dominates over friction, as well as in the active elastic limit.

cond-mat.soft↗

arXiv2022

Exact results for sheared polar active suspensions with variable liquid crystalline order

We consider a confined sheared active polar liquid crystal with a uniform orientation and study the effect of variations in the magnitude of polarization. Restricting our analysis to one-dimensional geometries, we demonstrate that with asymmetric boundary conditions, this system is characterized, macroscopically, by a linear shear stress vs. shear strain relationship that does not pass through the origin: At a zero strain rate, the fluid sustains a non-zero stress. Analytic solutions for the polarization, density, and velocity fields are derived for asymptotically large or small systems and are shown by comparison with precise numerical solutions to be good approximations for finite-size systems.

cond-mat.soft↗

arXiv2022

The nonlinear motion of cells subject to external forces

To develop a minimal model for a cell moving in a crowded environment such as in tissue, we investigate the response of a liquid drop of active matter moving on a flat rigid substrate to forces applied at its boundaries. We consider two different self-propulsion mechanisms, active stresses and treadmilling polymerisation, and we investigate how the active drop motion is altered by these surface forces. We find a highly non-linear response to forces that we characterise using drop velocity, drop shape, and the traction between the drop and the substrate. Each self-propulsion mechanism gives rise to two main modes of motion: a long thin drop with zero traction in the bulk, mostly occurring under strong stretching forces, and a parabolic drop with finite traction in the bulk, mostly occurring under strong squeezing forces. In each case there is a sharp transition between parabolic, and long thin drops as a function of the applied forces and indications of drop break-up where large forces stretch the drop.

cond-mat.soft↗

arXiv2022

An equation of state for active matter

We characterise the steady states of a suspension of two-dimensional active brownian particles (ABPs). We calculate the steady-state probability distribution to lowest order in Peclet number. We show that macroscopic quantities can be calculated in analogous way to equilibrium systems using this probability distribution. We then derive expressions for the macroscopic pressure and position-orientation correlation functions. We check our results by direct comparison with extensive numerical simulations. A key finding is the importance of many-body effective interactions even at very low densities.

cond-mat.stat-mech↗

arXiv2021

Crystallisation and Polymorph Selection in Active Brownian Particles

We explore crystallisation and polymorph selection in active Brownian particles with numerical simulation. In agreement with previous work [Wysocki $\textit{et al.}$ $\textit{Europhys. Lett.}$, $\textbf{105}$ 48004 (2014)], we find that crystallisation is suppressed by activity and occurs at higher densities with increasing $Péclet$ number ($Pe$). While the nucleation rate decreases with increasing activity, the crystal growth rate increases due to the accelerated dynamics in the melt. As a result of this competition we observe the transition from a nucleation and growth regime at high $Pe$ to "spinodal nucleation" at low $Pe$. Unlike the case of passive hard spheres, where preference for FCC over HCP polymorphs is weak, activity causes the annealing of HCP stacking faults, thus strongly favouring the FCC symmetry at high $Pe$. When freezing occurs more slowly, in the nucleation and growth regime, this tendency is much reduced and we see a trend towards the passive case of little preference for either polymorph.

cond-mat.soft↗

arXiv2020

How many ways a cell can move: the modes of self-propulsion of an active drop

Numerous physical models have been proposed to explain how cell motility emerges from internal activity, mostly focused on how crawling motion arises from internal processes. Here we offer a classification of self-propulsion mechanisms based on general physical principles, showing that crawling is not the only way for cells to move on a substrate. We consider a thin drop of active matter on a planar substrate and fully characterize its autonomous motion for all three possible sources of driving: (i) the stresses induced in the bulk by active components, which allow in particular tractionless motion, (ii) the self-propulsion of active components at the substrate, which gives rise to crawling motion, and (iii) a net capillary force, possibly self-generated, and coupled to internal activity. We determine travelling-wave solutions to the lubrication equations as a function of a dimensionless activity parameter for each mode of motion. Numerical simulations are used to characterize the drop motion over a wide range of activity magnitudes, and explicit analytical solutions in excellent agreement with the simulations are derived in the weak-activity regime.

cond-mat.soft↗