Although myosin II filaments are known to exist in non-muscle cells, their dynamics and organization are incompletely understood. Here, we combined structured illumination microscopy with pharmacological and genetic perturbations, to study the process of actomyosin cytoskeleton self-organization into arcs and stress fibres. A striking feature of the myosin II filament organization was their 'registered' alignment into stacks, spanning up to several micrometres in the direction orthogonal to the parallel actin bundles. While turnover of individual myosin II filaments was fast (characteristic half-life time 60 s) and independent of actin filament turnover, the process of stack formation lasted a longer time (in the range of several minutes) and required myosin II contractility, as well as actin filament assembly/disassembly and crosslinking (dependent on formin Fmnl3, cofilin1 and α-actinin-4). Furthermore, myosin filament stack formation involved long-range movements of individual myosin filaments towards each other suggesting the existence of attractive forces between myosin II filaments. These forces, possibly transmitted via mechanical deformations of the intervening actin filament network, may in turn remodel the actomyosin cytoskeleton and drive its self-organization.

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.In ordinary fluids, acoustic waves with sufficiently large wavelength have arbitrarily low frequency due to Galilean invariance [1]. When either a global rotation or an external magnetic field is present, Galilean invariance is explicitly broken by either Coriolis or Lorentz forces within the fluid, respectively. Hence, the spectrum of acoustic waves becomes gapped in the bulk. Yet, a peculiar phenomenon can occur at edges or interfaces: chiral edge modes propagate robustly irrespective of interface geometry. This phenomenon analogous to edge states in the quantum Hall effect [2][3][4] was unveiled in the context of equatorial waves [5] and explored in out-of-equilibrium and active fluids [6,7]. Similar phenomena occur in lattices of circulators [8,9], polar active fluids under confinement [10] and coupled mechanical oscillators [11][12][13], including gyroscopes [14,15] and oscillators subject to Coriolis forces [16,17].In addition to Coriolis or Lorentz body forces, fluids in which time-reversal and parity are broken generically exhibit a dissipationless viscosity that is odd under each of these symmetries [18,19]. The viscosity tensor η ijkl relates the strain rate v kl ≡ ∂ k v l to the viscous part of the stress tensor σ ij = η ijkl v kl . Odd viscosity refers to the antisymmetric part of the viscosity tensor η o ijkl = −η o klij [18,19]. In an isotropic two-dimensional fluid, odd viscosity is specified by a single pseudoscalar η o , see Supplementary Information (SI) for details [19]. Odd viscosity changes sign under either time-reversal or parity, and hence must vanish when at least one of these symmetries is present. Conversely, odd viscosity is generically non-vanishing as soon as both time-reversal and parity are broken [20][21][22]. For instance, microscopic Coriolis or Lorentz forces are sufficient to induce a non-zero odd viscosity [23,24], in addition to the corresponding body forces. Odd viscosity has been studied theoretically in various systems (see SI for a partial review) including polyatomic gases [25], magnetized plasmas [24,26], flu-ids of vortices [27][28][29][30], chiral active fluids [31], quantum Hall states and chiral superfluids/superconductors [32][33][34][35][36][37][38][39][40][41][42]. Its presence has been exp...

A colloidal solution is a homogeneous dispersion of particles or droplets of one phase (solute) in a second, typically liquid, phase (solvent). Colloids are ubiquitous in biological, chemical and technological processes, homogenizing highly dissimilar constituents. To stabilize a colloidal system against coalescence and aggregation, the surface of each solute particle is engineered to impose repulsive forces strong enough to overpower van der Waals attraction and keep the particles separated from each other. Electrostatic stabilization of charged solutes works well in solvents with high dielectric constants, such as water (dielectric constant of 80). In contrast, colloidal stabilization in solvents with low polarity, such as hexane (dielectric constant of about 2), can be achieved by decorating the surface of each particle of the solute with molecules (surfactants) containing flexible, brush-like chains. Here we report a class of colloidal systems in which solute particles (including metals, semiconductors and magnetic materials) form stable colloids in various molten inorganic salts. The stability of such colloids cannot be explained by traditional electrostatic and steric mechanisms. Screening of many solute-solvent combinations shows that colloidal stability can be traced to the strength of chemical bonding at the solute-solvent interface. Theoretical analysis and molecular dynamics modelling suggest that a layer of surface-bound solvent ions produces long-ranged charge-density oscillations in the molten salt around solute particles, preventing their aggregation. Colloids composed of inorganic particles in inorganic melts offer opportunities for introducing colloidal techniques to solid-state science and engineering applications.

The actin cytoskeleton is a critical regulator of cytoplasmic architecture and mechanics, essential in a myriad of physiological processes. Here we demonstrate a liquid phase of actin filaments in the presence of the physiological cross-linker, filamin. Filamin condenses short actin filaments into spindle-shaped droplets, or tactoids, with shape dynamics consistent with a continuum model of anisotropic liquids. We find that cross-linker density controls the droplet shape and deformation timescales, consistent with a variable interfacial tension and viscosity. Near the liquid-solid transition, cross-linked actin bundles show behaviors reminiscent of fluid threads, including capillary instabilities and contraction. These data reveal a liquid droplet phase of actin, demixed from the surrounding solution and dominated by interfacial tension. These results suggest a mechanism to control organization, morphology, and dynamics of the actin cytoskeleton.actin | phase separation | liquid crystal | cytoskeleton T he cellular cytoplasm is a hierarchical array of diverse, soft materials assembled from biological molecules that work in concert to support cell physiology (1). The actin cytoskeleton constitutes a spectrum of materials constructed from the semiflexible polymer actin (F-actin) that are crucial in diverse physical processes ranging from cell division and migration to tissue morphogenesis (2, 3). Cross-linking and regulatory proteins assemble actin filaments into bundles and networks with varied composition, mechanics, and physiological function (4). The mechanical properties of actin assemblies regulate force generation and transmission to dynamically control morphogenic processes from the subcellular to tissue length scales (5, 6).A mechanistic understanding of cytoplasmic mechanics is obscured by the rich complexity of in vivo cytoskeletal assemblies (7) and has been investigated via in vitro model systems (8, 9). Vastly different material properties have been accessed through varying filament length, concentration, and cross-linking. For semidilute concentrations of long actin filaments (>1 μm), the mean spacing between actin filaments, or mesh size, is much smaller than the filament length. In this case, cross-linking proteins mechanically constrain actin filaments to result in space-spanning networks that are viscoelastic gels (10). The structure of cross-linked actin networks is kinetically determined, reflecting a metastable state (11, 12) that requires motor-driven stresses for significant shape changes (13). In contrast, highly concentrated solutions of short actin filaments (<1 μm) align due to entropic effects and form equilibrium liquid crystal phases (14). Liquid crystal theory has been introduced as a framework to understand actin cortex mechanics and mitotic spindle shape (5, 15), but the existence of liquid crystal-like phases at physiological conditions is uncertain.Liquid-like phases of proteins and nucleic acids have been found within the cytoplasm and are thought to be important in subcellular...