44 research outputs found
Simulation of a Hard-Spherocylinder Liquid Crystal with the pe
The pe physics engine is validated through the simulation of a liquid crystal
model system consisting of hard spherocylinders. For this purpose we evaluate
several characteristic parameters of this system, namely the nematic order
parameter, the pressure, and the Frank elastic constants. We compare these to
the values reported in literature and find a very good agreement, which
demonstrates that the pe physics engine can accurately treat such densely
packed particle systems. Simultaneously we are able to examine the influence of
finite size effects, especially on the evaluation of the Frank elastic
constants, as we are far less restricted in system size than earlier
simulations
Frustration of the isotropic-columnar phase transition of colloidal hard platelets by a transient cubatic phase
Using simulations and theory, we show that the cubatic phase is metastable
for three model hard platelets. The locally favored structures of perpendicular
particle stacks in the fluid prevent the formation of the columnar phase
through geometric frustration resulting in vitrification. Also, we find a
direct link between structure and dynamic heterogeneities in the cooperative
rotation of particle stacks, which is crucial for the devitrification process.
Finally, we show that the life time of the glassy cubatic phase can be tuned by
surprisingly small differences in particle shape.Comment: Submitted to Phys. Rev. Let
Fundamental measure theory for non-spherical hard particles: predicting liquid crystal properties from the particle shape
Density functional theory (DFT) for hard bodies provides a theoretical description of the effect of particle shape on inhomogeneous fluids. We present improvements of the DFT framework fundamental measure theory (FMT) for hard bodies and validate these improvements for hard spherocylinders. To keep the paper self-contained, we first discuss the recent advances in FMT for hard bodies that lead to the introduction of fundamental mixed measure theory (FMMT) in our previous paper (2015 Europhys. Lett. 109 26003). Subsequently, we provide an efficient semi-empirical alternative to FMMT and show that the phase diagram for spherocylinders is described with similar accuracy in both versions of the theory. Finally, we present a semi-empirical modification of FMMT whose predictions for the phase diagram for spherocylinders are in excellent quantitative agreement with computer simulation results
Freezing of parallel hard cubes with rounded edges
The freezing transition in a classical three-dimensional system of parallel
hard cubes with rounded edges is studied by computer simulation and
fundamental-measure density functional theory. By switching the rounding
parameter s from zero to one, one can smoothly interpolate between cubes with
sharp edges and hard spheres. The equilibrium phase diagram of rounded parallel
hard cubes is computed as a function of their volume fraction and the rounding
parameter s. The second order freezing transition known for oriented cubes at s
= 0 is found to be persistent up to s = 0.65. The fluid freezes into a
simple-cubic crystal which exhibits a large vacancy concentration. Upon a
further increase of s, the continuous freezing is replaced by a first-order
transition into either a sheared simple cubic lattice or a deformed
face-centered cubic lattice with two possible unit cells: body-centered
orthorhombic or base-centered monoclinic. In principle, a system of parallel
cubes could be realized in experiments on colloids using advanced synthesis
techniques and a combination of external fields.Comment: Submitted to JC
Vacancy-stabilized crystalline order in hard cubes
We examine the effect of vacancies on the phase behavior and structure of
systems consisting of hard cubes using event-driven molecular dynamics and
Monte Carlo simulations. We find a first-order phase transition between a fluid
and a simple cubic crystal phase that is stabilized by a surprisingly large
number of vacancies, reaching a net vacancy concentration of ~6.4% near bulk
coexistence. Remarkably, we find that vacancies increase the positional order
in the system. Finally, we show that the vacancies are delocalized and
therefore hard to detect.Comment: Published online in PNAS early edition September 10, 201
Differently Shaped Hard Body Colloids in Confinement: From passive to active particles
We review recent progress in the theoretical description of anisotropic hard
colloidal particles. The shapes considered range from rods and dumbbells to
rounded cubes, polyhedra and to biaxial particles with arbitrary shape. Our
focus is on both static and dynamical density functional theory and on computer
simulations. We describe recent results for the structure, dynamics and phase
behaviour in the bulk and in various confining geometries, e.g. established by
two parallel walls which reduce the dimensionality of the system to two
dimensions. We also include recent theoretical modelling for active particles,
which are autonomously driven by some intrinsic motor, and highlight their
fascinating nonequilibrium dynamics and collective behaviour.Comment: 15 pages, 6 figures, EPJ ST (accepted
Self-assembly and entropic effects in pear-shaped colloid systems. I. Shape sensitivity of bilayer phases in colloidal pear-shaped particle systems
The role of particle shape in self-assembly processes is a double-edged sword. On the one hand, particle shape and particle elongation are often considered the most fundamental determinants of soft matter structure formation. On the other hand, structure formation is often highly sensitive to details of shape. Here, we address the question of particle shape sensitivity for the self-assembly of hard pear-shaped particles by studying two models for this system: (a) the pear hard Gaussian overlap (PHGO) and (b) the hard pears of revolution (HPR) model. Hard pear-shaped particles, given by the PHGO model, are known to form a bicontinuous gyroid phase spontaneously. However, this model does not replicate an additive object perfectly and, hence, varies slightly in shape from a “true” pear-shape. Therefore, we investigate in the first part of this series the stability of the gyroid phase in pear-shaped particle systems. We show, based on the HPR phase diagram, that the gyroid phase does not form in pears with such a “true” hard pear-shaped potential. Moreover, we acquire first indications from the HPR and PHGO pair-correlation functions that the formation of the gyroid is probably attributed to the small non-additive properties of the PHGO potential
Self-assembly and entropic effects in pear-shaped colloid systems. II. Depletion attraction of pear-shaped particles in a hard-sphere solvent
We consider depletion effects of a pear-shaped colloidal particle in a hard-sphere solvent for two different model realizations of the pear-shaped colloidal particle. The two models are the pear hard Gaussian overlap (PHGO) particles and the hard pears of revolution (HPR). The motivation for this study is to provide a microscopic understanding for the substantially different mesoscopic self-assembly properties of these pear-shaped colloids, in dense suspensions, that have been reported in the previous studies. This is done by determining their differing depletion attractions via Monte Carlo simulations of PHGO and HPR particles in a pool of hard spheres and comparing them with excluded volume calculations of numerically obtained ideal configurations on the microscopic level. While the HPR model behaves as predicted by the analysis of excluded volumes, the PHGO model showcases a preference for splay between neighboring particles, which can be attributed to the special non-additive characteristics of the PHGO contact function. Lastly, we propose a potentially experimentally realizable pear-shaped particle model, the non-additive hard pear of revolution model, which is based on the HPR model but also features non-additive traits similar to those of PHGO particles to mimic their depletion behavior
Purely entropic self-assembly of the bicontinuous Ia3Ě…d gyroid phase in equilibrium hard-pear systems
We investigate a model of hard pear-shaped particles which forms the bicontinuous Ia3d structure by entropic self-assembly, extending the previous observations of Barmes et al. (2003 Phys. Rev. E 68, 021708. (doi:10.1103/PhysRevE.68.021708)) and Ellison et al. (2006 Phys. Rev. Lett. 97, 237801. (doi:10.1103/PhysRevLett.97.237801)). We specifically provide the complete phase diagram of this system, with global density and particle shape as the two variable parameters, incorporating the gyroid phase as well as disordered isotropic, smectic and nematic phases. The phase diagram is obtained by two methods, one being a compression–decompression study and the other being a continuous change of the particle shape parameter at constant density. Additionally, we probe the mechanism by which interdigitating sheets of pears in these systems create surfaces with negative Gauss curvature, which is needed to form the gyroid minimal surface. This is achieved by the use of Voronoi tessellation, whereby both the shape and volume of Voronoi cells can be assessed in regard to the local Gauss curvature of the gyroid minimal surface. Through this, we show that the mechanisms prevalent in this entropy-driven system differ from those found in systems which form gyroid structures in nature (lipid bilayers) and from synthesized materials (di-block copolymers) and where the formation of the gyroid is enthalpically driven. We further argue that the gyroid phase formed in these systems is a realization of a modulated splay-bend phase in which the conventional nematic has been predicted to be destabilized at the mesoscale due to molecular-scale coupling of polar and orientational degrees of freedo
Self-assembly and entropic effects in pear-shaped colloid systems. II. Depletion attraction of pear-shaped particles in a hard-sphere solvent
We consider depletion effects of a pear-shaped colloidal particle in a hard-sphere solvent for two different model realizations of the pear-shaped colloidal particle. The two models are the pear hard Gaussian overlap (PHGO) particles and the hard pears of revolution (HPR). The motivation for this study is to provide a microscopic understanding for the substantially different mesoscopic self-assembly properties of these pear-shaped colloids, in dense suspensions, that have been reported in the previous studies. This is done by determining their differing depletion attractions via Monte Carlo simulations of PHGO and HPR particles in a pool of hard spheres and comparing them with excluded volume calculations of numerically obtained ideal configurations on the microscopic level. While the HPR model behaves as predicted by the analysis of excluded volumes, the PHGO model showcases a preference for splay between neighboring particles, which can be attributed to the special non-additive characteristics of the PHGO contact function. Lastly, we propose a potentially experimentally realizable pear-shaped particle model, the non-additive hard pear of revolution model, which is based on the HPR model but also features non-additive traits similar to those of PHGO particles to mimic their depletion behavior.The authors thank Universities Australia and the German Academic Exchange Service (DAAD) for funds through a collaboration funding scheme, through the grant “Absorption and
confinement of complex fluids.” They also thank the DFG through
Grant No. ME1361/11-2 and through the research group “Geometry and Physics of Spatial Random Systems” (GPSRS) for funding.
They gratefully acknowledge Klaus Mecke’s support and advice in
useful discussions. P.W.A.S. acknowledges a Murdoch University
Postgraduate Research Scholarship. G.E.S.-T. is grateful to the Food
Science Department at the University of Copenhagen and the Physical Chemistry group at Lund University for their hospitality and
to Copenhagen University, the Camurus Lipid Research Foundation, and the Danish National Bank for enabling a sabbatical stay
in Denmark and Sweden