441 research outputs found
Phase Separation and Self-Assembly in a Fluid of Mickey Mouse Particles
Recent developments in the synthesis of colloidal particles allow for control
over shape and inter-particle interaction. One example, among others, is the
so-called "Mickey Mouse" (MM) particle for which the self-assembly properties
have been previously studied yielding a stable cluster phase together with
elongated, tube-like structures. Here, we investigate under which conditions a
fluid of Mickey Mouse particles can yield phase separation and how the
self-assembly behaviour affects the gas-liquid coexistence. We vary the
distance between the repulsive and the attractive lobes (bond length), and the
interaction range, and follow the evolution of the gas-liquid (GL) coexistence
curve. We find that upon increasing the bond length distance the binodal line
shifts to lower temperatures, and that the interaction range controls the
transition between phase separation and self-assembly of clusters. Upon further
reduction of the interaction range and temperature, the clusters assume an
increasingly ordered tube-like shape, ultimately matching the one previously
reported in literature. These results are of interest when designing particle
shape and particle-particle interaction for self-assembly processes
Crystal nucleation of colloidal hard dumbbells
Using computer simulations we investigate the homogeneous crystal nucleation
in suspensions of colloidal hard dumbbells. The free energy barriers are
determined by Monte Carlo simulations using the umbrella sampling technique. We
calculate the nucleation rates for the plastic crystal and the aperiodic
crystal phase using the kinetic prefactor as determined from event driven
molecular dynamics simulations. We find good agreement with the nucleation
rates determined from spontaneous nucleation events observed in event driven
molecular dynamics simulations within error bars of one order of magnitude. We
study the effect of aspect ratio of the dumbbells on the nucleation of plastic
and aperiodic crystal phases and we also determine the structure of the
critical nuclei. Moreover, we find that the nucleation of the aligned CP1
crystal phase is strongly suppressed by a high free energy barrier at low
supersaturations and slow dynamics at high supersaturations.Comment: Accepted by J. Chem. Phy
Fabrication of colloidal Laves phases via hard tetramers and hard spheres: bulk phase diagram and sedimentation behaviour
Colloidal photonic crystals display peculiar optical properties which make
them particularly suitable for application in different fields. However, the
low packing fraction of the targeted structures usually poses a real challenge
in the fabrication stage. Here, we propose a novel route to colloidal photonic
crystals via a binary mixture of hard tetramers and hard spheres. By combining
theory and computer simulations, we calculate the phase diagram as well as the
stacking diagram of the mixture, and show that a colloidal analogue of the
MgCu2 Laves phase -- which can serve as a precursor of a photonic bandgap
structure -- is a thermodynamically stable phase in a large region of the phase
diagram. Our findings show a relatively large coexistence region between the
fluid and the Laves phase, which is potentially accessible by experiments.
Furthermore, we determine the sedimentation behaviour of the suggested mixture,
by identifying several stacking sequences. Our work uncovers a new
self-assembly path towards a photonic structure with a band gap in the visible
region
Gas-liquid phase separation in oppositely charged colloids: stability and interfacial tension
We study the phase behavior and the interfacial tension of the screened
Coulomb (Yukawa) restricted primitive model (YRPM) of oppositely charged hard
spheres with diameter s using Monte Carlo simulations. We determine the
gas-liquid and gas-solid phase transition using free energy calculations and
grand-canonical Monte Carlo simulations for varying inverse Debye screening
length k. We find that the gas-liquid phase separation is stable for k s <= 4,
and that the critical temperature decreases upon increasing the screening of
the interaction (decreasing the range of the interaction). In addition, we
determine the gas-liquid interfacial tension using grand-canonical Monte Carlo
simulations. The interfacial tension decreases upon increasing the range of the
interaction. In particular, we find that simple scaling can be used to relate
the interfacial tension of the YRPM to that of the restricted primitive model,
where particles interact with bare Coulomb interactions.Comment: 17 pages, 6 Figures, accepted for publication in J. Chem. Phy
Isotropic-Nematic transition of long thin hard spherocylinders confined in a quasi-two-dimensional planar geometry
We present computer simulations of long thin hard spherocylinders in a narrow
planar slit. We observe a transition from the isotropic to a nematic phase with
quasi-long-range orientational order upon increasing the density. This phase
transition is intrinsically two dimensional and of the Kosterlitz-Thouless
type. The effective two-dimensional density at which this transition occurs
increases with plate separation. We qualitatively compare some of our results
with experiments where microtubules are confined in a thin slit, which gave the
original inspiration for this work.Comment: 8 pages, 10 figure
A Novel Chiral Phase of Achiral Hard Triangles and an Entropy-Driven Demixing of Enantiomers
We investigate the phase behavior of a system of hard equilateral and
right-angled triangles in two dimensions using Monte Carlo simulations. Hard
equilateral triangles undergo a continuous isotropic-triatic liquid crystal
phase transition at packing fraction . Similarly, hard right-angled
isosceles triangles exhibit a first-order phase transition from an isotropic
fluid phase to a rhombic liquid crystal phase with a coexistence region . Both these liquid crystal phases undergo a
continuous phase transition to their respective close-packed crystal structures
at high pressures. Although the particles and their close-packed crystals are
both achiral, the solid phases of equilateral and right-angled triangles
exhibit spontaneous chiral symmetry breaking at sufficiently high packing
fractions. The colloidal triangles rotate either in clockwise or anti-clockwise
direction with respect to one of the lattice vectors for packing fractions
higher than . As a consequence, these triangles spontaneously form a
regular lattice of left- or right-handed chiral holes which are surrounded by
six triangles in the case of equilateral triangles and four or eight triangles
for right-angled triangles. Moreover, our simulations show a spontaneous
entropy-driven demixing transition of the right- and left-handed "enantiomers".Comment: 9 pages, 10 figure
Phase Diagrams of Binary Mixtures of Oppositely Charged Colloids
Phase diagrams of binary mixtures of oppositely charged colloids are
calculated theoretically. The proposed mean-field-like formalism interpolates
between the limits of a hard-sphere system at high temperatures and the
colloidal crystals which minimize Madelung-like energy sums at low
temperatures. Comparison with computer simulations of an equimolar mixture of
oppositely charged, equally sized spheres indicate semi-quantitative accuracy
of the proposed formalism. We calculate global phase diagrams of binary
mixtures of equally sized spheres with opposite charges and equal charge
magnitude in terms of temperature, pressure, and composition. The influence of
the screening of the Coulomb interaction upon the topology of the phase diagram
is discussed. Insight into the topology of the global phase diagram as a
function of the system parameters leads to predictions on the preparation
conditions for specific binary colloidal crystals.Comment: Submitte
Vapour-Liquid Coexistence of an Active Lennard-Jones fluid
We study a three-dimensional system of self-propelled Lennard-Jones particles
using Brownian Dynamics simulations. Using recent theoretical results for
active matter, we calculate the pressure and report equations of state for the
system. Additionally, we chart the vapour-liquid coexistence and show that the
coexistence densities can be well described using simple power laws. Lastly, we
demonstrate that our out-of-equilibrium system shows deviations from both the
law of rectilinear diameters and the law of corresponding states.Comment: 8 pages, 8 figure
- …