Nanorings in planar confinement: the role of repulsive surfaces on the formation of lacuna smectics

We study the structure and liquid-crystalline phase behaviour of a model of nonconvex circular soft-repulsive nanorings con ned in a planar slit geometry using molecular-dynamics simulation. The separation distance between the structureless parallel soft-repulsive walls is large enough to allow for the formation of a distinct bulk phase in the central region of the box which is in coexistence with the adsorbed uid thus allowing the analysis of single wall e ects. As the concentration of the particles is increased, the uid adsorbs (wets) onto the planar surfaces leading to the formation of well-de ned smectic-A layers with a spacing proportional to the diameter of the rings. An analysis of the nematic order parameter at distances perpendicular to the surface reveals that the particles in each layer exhibit antinematic behaviour and planar (edge-on) anchoring relative to the short symmetry axis of the rings. This behaviour is in stark contrast to the behaviour observed in convex disc-like particles that have the tendency to form nematic (discotic) structures with hometropic (face-on) anchoring. The smectic phases formed by nanorings in the bulk and under con nement are characterized by the formation of low-density layered liquid-crystalline states with large voids, referred to here as lacuna smectic phases. In contrast to what is typically found for con ned liquid-crystalline systems involving convex particles, no apparent biaxiality is found for the nanorings in planar con nement. We argue that formation of the low-density lacuna smectic layers with planar anchoring is a consequence of the non-convex shape of the circular rings that allow for interpenetration between the particles as observed for nanorings under bulk conditions [Avenda~no et al., Proc. Natl. Acad. Sci. U. S. A. 113, 9699 (2016); H. H. Wensink and C. Avenda~no, Phys. Rev. E 94 062704 (2016)]

Structure and thermodynamics of platelet dispersions

Various properties of fluids consisting of platelike particles differ from the corresponding ones of fluids consisting of spherical particles because interactions between platelets depend on their mutual orientations. One of the main issues in this topic is to understand how structural properties of such fluids depend on factors such as the shape of the platelets, the size polydispersity, the orientational order, and the platelet number density. A statistical mechanics approach to the problem is natural and in the last few years there has been a lot of work on the study of properties of platelet fluids. In this contribution some recent theoretical developments in the field are discussed and experimental investigations are described.Comment: 23 pages, 18 figure

Structure and equation of state of interaction site models for disc-shaped lamellar colloids

We apply RISM (Reference Interaction Site Model) and PRISM (polymer-RISM) theories to calculate the site-site pair structure and the osmotic equation of state of suspensions of circular or hexagonal platelets (lamellar colloids) over a range of ratios of the particle diameter over thickness. Despite the neglect of edge effects, the simpler PRISM theory yields results in good agreement with the more elaborate RISM calculations, provided the correct form factor, characterizing the intramolecular structure of the platelets, is used. The RISM equation of state is sensitive to the number of sites used to model the platelets, but saturates when the hard spheres, associated with the interaction sites, nearly touch; the limiting equation of state agrees reasonably well with available simulation data for all densities up to the isotropic-nematic transition. When properly scaled with the second virial coefficient, the equations of state of platelets with different aspect ratios nearly collapse on a single master curve.Comment: 10 Pages, 11 Figures, Typesetted using RevTeX

On the duality between interaction responses and mutual positions in flocking and schooling.

Recent research in animal behaviour has contributed to determine how alignment, turning responses, and changes of speed mediate flocking and schooling interactions in different animal species. Here, we propose a complementary approach to the analysis of flocking phenomena, based on the idea that animals occupy preferential, anysotropic positions with respect to their neighbours, and devote a large amount of their interaction responses to maintaining their mutual positions. We test our approach by deriving the apparent alignment and attraction responses from simulated trajectories of animals moving side by side, or one in front of the other. We show that the anisotropic positioning of individuals, in combination with noise, is sufficient to reproduce several aspects of the movement responses observed in real animal groups. This anisotropy at the level of interactions should be considered explicitly in future models of flocking and schooling. By making a distinction between interaction responses involved in maintaining a preferred flock configuration, and interaction responses directed at changing it, our work provides a frame to discriminate movement interactions that signal directional conflict from interactions underlying consensual group motion

Scalar <i>φ</i>⁴ field theory for active-particle phase separation

Recent theories predict phase separation among orientationally disordered active particles whose propulsion speed decreases rapidly enough with density. Coarse-grained models of this process show time-reversal symmetry (detailed balance) to be restored for uniform states, but broken by gradient terms; hence detailed-balance violation is strongly coupled to interfacial phenomena. To explore the subtle generic physics resulting from such coupling we here introduce `Active Model B'. This is a scalar $\phi^4$ field theory (or phase-field model) that minimally violates detailed balance via a leading-order square-gradient term. We find that this additional term has modest effects on coarsening dynamics, but alters the static phase diagram by creating a jump in (thermodynamic) pressure across flat interfaces. Both results are surprising, since interfacial phenomena are always strongly implicated in coarsening dynamics but are, in detailed-balance systems, irrelevant for phase equilibria.Comment: 15 pages, 7 figure

A growing bacterial colony in two dimensions as an active nematic

Rod-shaped bacteria are an example of active matter. Here the authors find that a growing bacterial colony harbours internal cellular flows affecting orientational ordering in its interior and at the boundary. Results suggest this system may belong to a new active matter universality class

Probing the critical behavior of colloidal interfaces by gravity

We present a study of the interface between fluid-fluid phase separated colloid-polymer mixtures of identical composition but with varying suspension height. The significance of the sedimentation gradient present in the suspension is controlled by the ratio between the suspension height and the gravitational length of the colloids. We demonstrate that increasing the suspension height, and thus the importance of gravity leads to a systematic roughening of the gas-liquid interface as if one approaches the critical point. By carefully tuning the system height, the suspension can be brought arbitrarily close to criticality, irrespective of the overall composition of colloid and polymer. Our findings are based on measurements of the interfacial tension and capillary wave properties and supported by predictions from a simple density functional theory. © 2011 The Royal Society of Chemistry

Sedimentation-diffusion dynamics in colloid-polymer mixtures

We show experimentally how a phase separated colloid-polymer mixture finds its sedimentation-diffusion equilibrium after initial fluid-fluid demixing. During this equilibration process we measure key parameters of the colloidal interface by assuming local mechanical equilibrium and we inspect the behaviour of the meniscus close to a vertical wall. It turns out that the kinetic pathway associated with the sedimentation process not only strongly depends on the overall colloid and polymer concentrations but also on the height of the suspension. Beyond a certain height the system locally passes through the gas-liquid critical point, which opens new ways to study critical phenomena. © 2010 IOP Publishing Ltd and SISSA

Pair interaction and phase separation in mixtures of colloids and excluded volume polymers

The interactions in polymer-colloid mixtures and their phase stability are calculated using the adsorption method. Starting from appropriate expressions for the correlation length and the osmotic pressure of a polymer solution in the excluded volume interaction limit, this method allows computation of the potential between two plates, two spheres and between a sphere and a plate. The results are in close agreement with computer simulation results. Extended to a many body system of colloids and polymers, the same approach allows one to calculate the thermodynamic properties and phase behavior. Results are presented for the phase behavior of polymer-sphere and polymer-platelet mixtures. The agreement with experimental data is satisfactory and helps to better explain the polymer-colloid phase behavior

Structure and interfacial tension of a hard-rod fluid in planar confinement

The structural properties and interfacial tension of a fluid of hard-spherocylinder rod-like particles in contact with hard structureless flat walls are studied by means of Monte Carlo simulation. The calculated surface tension between the rod fluid and the substrate is characterized by a non-monotonic trend as a function of bulk concentration (density) over the range of isotropic bulk concentrations. As suggested by earlier theoretical studies, a surface-ordering scenario can be confirmed from our simulations: the local orientational order close to the wall changes from uniaxial to biaxial nematic when the bulk concentration reaches about 85% of the value at the onset of the isotropic-nematic phase transition. The surface ordering coincides with a wetting transition whereby the hard wall is wetted by a nematic film. Accurate values of the fluid-solid surface tension, the adsorption, and the average particle-wall contact distance are reported (over a broad range of densities into the dense nematic region for the first time), which may serve as a useful benchmark for future theoretical and experimental studies on confined rod fluids. The simulation data are supplemented with predictions from a second-virial density functional theory, which are in good qualitative agreement with the simulation results