Vesicle shape transformations driven by confined active filaments

In active matter systems, deformable boundaries provide a mechanism to organize internal active stresses. To study a minimal model of such a system, we perform particle-based simulations of an elastic vesicle containing a collection of polar active filaments. The interplay between the active stress organization due to interparticle interactions and that due to the deformability of the confinement leads to a variety of filament spatiotemporal organizations that have not been observed in bulk systems or under rigid confinement, including highly-aligned rings and caps. In turn, these filament assemblies drive dramatic and tunable transformations of the vesicle shape and its dynamics. We present simple scaling models that reveal the mechanisms underlying these emergent behaviors and yield design principles for engineering active materials with targeted shape dynamics.

Shape Dynamics of the TT¯ Deformation

I will show how the flow triggered by deforming two-dimensional conformal field theories on a torus by the TT¯ operator is identical to the evolution generated by the (radial) quantum Shape Hamiltonian in 2 + 1 dimensions. I will discuss how the gauge invariances of the Shape Dynamics, i.e., volume-preserving conformal invariance and diffeomorphism invariance along slices of constant radius are realized as Ward identities of the deformed quantum field theory. I will also comment about the relationship between the reduction to shape space on the gravity side and the solvability of the irrelevant operator deformation of the conformal field theory

Origami: Single-cell oriented 3D shape dynamics of folding epithelia from fluorescence microscopy images

A common feature of morphogenesis is the formation of three-dimensional structures from the folding of two-dimensional epithelial sheets aided by spatio-temporal cell shape changes at the cellular-level. Studying cell shape dynamics and polarised processes that underpin them, requires orienting cells within the epithelial sheet. In epithelia with highly curved surfaces, assigning cell orientation can be difficult to automate in silico . We present ‘Origami’, a MATLAB-based image analysis pipeline to compute oriented cell shape-features. Our automated method accurately computed cell orientation in regions with opposing curvature in synthetic epithelia and fluorescence images of zebrafish embryos. As proof of concept, we identified different cell shape signatures in the developing zebrafish inner ear, where the epithelium deforms in opposite orientations to form different structures. Origami is designed to be user-friendly and is generally applicable to fluorescence images of curved epithelia.

Shape Recovery of Deformed Biomolecular Droplets: Dependence on Condensate Viscoelasticity

ABSTRACTPhase-separated biomolecular condensates often appear as micron-sized droplets. Due to interfacial tension, the droplets usually have a spherical shape and, upon deformation, tend to recover their original shape. Likewise, interfacial tension drives the fusion of two droplets into a single spherical droplet. In all previous studies on shape dynamics, biomolecular condensates have been modeled as purely viscous. However, recent work has shown that biomolecular condensates are viscoelastic, with shear relaxation occurring not instantaneously as would in purely viscous fluids. Here we present an exact analytical solution for the shape recovery dynamics of biomolecular droplets, which exhibits rich time dependence due to viscoelasticity. For condensates modeled as purely viscous, shape recovery is an exponential function of time, with the time constant given by the “viscocapillary” ratio, i.e., viscosity over interfacial tension. For viscoelastic droplets, shape recovery becomes multi-exponential, with shear relaxation yielding additional time constants. The longest of these time constants can be dictated by shear relaxation and independent of interfacial tension, thereby challenging the currently prevailing viscocapillarity-centric view derived from purely viscous fluids. These results highlight the importance of viscoelasticity in condensate shape dynamics and expand our understanding of how material properties affect condensate dynamics in general, including aging. The analytical solution presented here can also be used for validating numerical solutions of fluid-dynamics problems and for fitting experimental and molecular simulation data.

Development of the voiceless sibilant fricative contrast in three-year-olds: an ultrasound and acoustic study

The study analysed spectral and tongue shape dynamics of voiceless alveolar and postalveolar fricatives produced by ten children learning Scottish English. Synchronised ultrasound tongue imaging data and acoustic data were used to characterise children's productions of the phonemic contrast. Six children had consistently accurate productions of both fricative targets, with some cross-consonant phonetic differences in the direction previously demonstrated for older children and adults, as well as some immature acoustic and articulatory dynamic patterns. Instrumental analyses made it possible to describe tongue shape for phonemic errors and phonetically distorted realisations. There was some evidence of articulatory contrast in production preceding contrast in perception. The observed patterns can be explained by the complex articulatory demands on the fricative production, in combination with the developing control of articulators. The paper concludes by discussing the implications of the results for phonological theory and for speech therapy practice.