We study the interplay of activity, order and flow through a set of coarse-grained equations governing the hydrodynamic velocity, concentration and stress fields in a suspension of active, energydissipating particles. We make several predictions for the rheology of such systems, which can be tested on bacterial suspensions, cell extracts with motors and filaments, or artificial machines in a fluid. The phenomena of cytoplasmic streaming, elastotaxis and active mechanosensing find natural explanations within our model. PACS numbers: 87.16.Ac, 87.15.Ya, 87.10.+e An active particle [1, 2] absorbs energy from its surroundings or from an internal fuel tank and dissipates it in the process of carrying out internal movements usually resulting in translatory or rotary motion. This broad definition includes macroscopic machines and organisms, living cells, and their components such as actin-myosin and ion pumps [3]. In this paper, we consider the interplay of activity, order and flow via coarse-grained equations governing the hydrodynamic velocity, concentration and stress fields in a suspension containing active particles of linear size ℓ, at concentration φ, each particle exerting a typical force f on the ambient fluid, with the activity of an individual particle correlated over a time τ 0 (say the 'run' time of a bacterium), and collective fluctuations in the activity correlated over length scales ξ and timescales τ . Rather than focussing on ordered phases [4], instabilities [4,5], or patterns (asters, vortices, spirals) formed in such assemblies [6] which our equations are of course capable of predicting, we apply them in the isotropic phase, with a view to understanding how a system such as a biological cell, composed of active elements, responds to deformation or mechanical stress. In addition to throwing light on full-cell rheometry [7,8], our equations form the framework for an analysis of any experiment probing the mechanical consequences of biological activity.Our simple model makes rather interesting predictions: An orientationally ordered state of active particles has a nonzero, macroscopic, anisotropic stress in contrast with thermal equilibrium nematics. Activity contributes an amount δη ∼ f ℓc 0 τ to the viscosity, with a sign determined by the type of active particle, and always enhances the apparent (noise) temperature. The latter greatly enhances the amplitude of the t −d/2 long-time tails [9] in the velocity autocorrelation. On approaching an orientationally ordered state, active suspensions with δη > 0 behave like passive systems near translational freezing, showing strong shear thickening and Maxwelllike viscoelasticity. Nonlinear fluctuation corrections give a dynamic modulusobservable over a large dynamic range, since τ is large. Cytoplasmic streaming [10], in which material flows from the depolymerising trailing edge to the polymerising leading edge of a crawling amoeboid cell, finds a natural explanation in our model, as do elastotaxis [11] and active mechanosensing [12], where cells orient t...

Pattern formation in 3D random media has been a topic of interest in soft matter and biological systems. However, the onset of long-range microscopic ordering has not been explored in randomly moving self-propelled particles due to a lack of model systems as well as local probe techniques. In this article, we report on a novel experiment, using motile Escherichia coli bacteria as a model system, to study the onset of dynamic correlation and collective movement in three-dimension. We use fluctuation of an optically trapped micron-size bead as a detector of correlated bacterial motion, and further study this behavior by analyzing the motility of fluorescent bacteria in a confocal volume. We find evidence of dynamic correlation at very low volume fractions (0.01). We show that the magnitude of this correlation strongly depends on the interbacterial distances and their coupling modes. This opens up possibilities to probe long-range pattern formation in actively propelled cells or organisms coupled through hydrodynamics and/or chemical signaling.

We present the electron density map of the asymmetric ripple phase of dilauroylphosphatidylcholine. We find that the primary feature characterizing the "asymmetry" of the rippled bilayers is the difference in the bilayer thickness in the two arms, and not the asymmetry of the bilayer height profile as is generally assumed. This difference in the bilayer thickness can be attributed to a mean tilt of the hydrocarbon chains of the lipid molecules along the direction of the ripple wave vector. We propose a Landau theory for this phase which takes into account the anisotropic elastic properties of a bilayer with tilt order.

Many achiral polymers crystallize into spherulites with gigantic chirality, as is evident from spectacular images of periodic banding observed in a polarized optical microscope, arising from the twisting of the lamellae making up the spherulites. We present a new mechanism of the spontaneous chiral symmetry breaking, by accounting for topological defects in finite crystalline ribbons made of achiral molecules in equilibrium. We show that disclinations stabilize a twisted helicoidal ribbon, with spontaneous selection of its width and chiral period, which are proportional to each other, as a universal law.

We use fluorescence confocal polarised microscopy (FCPM) to study tubular growth upon hydration of dry DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) in water and water-glycerol mixtures. We have developed a model to relate the FCPM intensity profiles to the multilamellar structures of the tubules. Insertion of an additional patch inside a tubule produces a beaded structure, while a straight configuration is retained if the growth is on the outside. We use a simple model to suggest that reduction in overall curvature energy drives bead formation.