5 research outputs found
The role of fivefold symmetry in suppressing crystallization
Although long assumed to have an important role in the suppression of crystallization and the development of glassformers, the effect of local fivefold symmetry has never been directly tested. Here we consider whether such suppression of crystallization has a kinetic or thermodynamic nature and investigate its mechanism. We introduce a model in which the degree of fivefold symmetry can be tuned by favouring arrangements of particles in pentagonal bipyramids. We thus show that fivefold symmetry has both kinetic and thermodynamic effects on the mechanism of crystallization to a face-centred cubic crystal. Our results suggest that the mechanism of crystallization suppression is related to the surface tension between fluid and crystal. Interestingly, the degree of fivefold symmetry has little effect on crystal growth rate, suggesting that growth may be only weakly coupled to fluid structure in hard sphere like systems. Upon increasing the fivefold symmetry, we find a first-order transition to an alternative icosahedra-rich phase. At intermediate bias strengths we find a one-component glassformer
A structural comparison of models of colloid-polymer mixtures
We study the structure of colloidal fluids with reference to colloid-polymer
mixtures. We compare the one component description of the Asakura-Oosawa (AO)
idealisation of colloid-polymer mixtures with the full two-component model. We
also consider the Morse potential, a variable range interaction, for which the
ground state clusters are known. Mapping the state points between these
systems, we find that the pair structure of the full AO model is equally well
described by the Morse potential or the one component AO approach. We employ a
recently developed method to identify in the bulk fluid the ground state
clusters relevant to the Morse potential. Surprisingly, when we measure the
cluster populations, we find that the Morse fluid is significantly closer the
full AO fluid than the one component AO description.Comment: 13 pages, accepted by J. Phys. Condens: Matter special issue for
CECAM meeting 'New Trends in Simulating Colloids: from Models to
Applications
The effect of attractions on the local structure of liquids and colloidal fluids
We revisit the role of attractions in liquids and apply these concepts to
colloidal suspensions. Two means are used to investigate the structure; the
pair correlation function and a recently developed topological method. The
latter identifies structures topologically equivalent to ground state clusters
formed by isolated groups of 5 < m < 13 particles, which are specific to the
system under consideration. Our topological methodology shows that, in the case
of Lennard-Jones, the addition of attractions increases the system's ability to
form larger (m>8) clusters, although pair-correlation functions are almost
identical. Conversely, in the case of short-ranged attractions, pair
correlation functions show a significant response to adding attraction, while
the liquid structure exhibits a strong decrease in clustering upon adding
attractions. Finally, a compressed, weakly interacting system shows a similar
pair structure and topology.Comment: 22 page
Structure and kinetics in the freezing of nearly hard spheres
We consider homogeneous crystallisation rates in confocal microscopy
experiments on colloidal nearly hard spheres at the single particle level.
These we compare with Brownian dynamics simuations by carefully modelling the
softness in the interactions with a Yukawa potential, which takes account of
the electrostatic charges present in the experimental system. Both structure
and dynamics of the colloidal fluid are very well matched between experiment
and simulation, so we have confidence that the system simulated is close to
that in the experiment. In the regimes we can access, we find reasonable
agreement in crystallisation rates between experiment and simulations, noting
that the larger system size in experiments enables the formation of critical
nuclei and hence crystallisation at lower supersaturations than the
simulations. We further examine the structure of the metastable fluid with a
novel structural analysis, the topological cluster classification. We find that
at densities where the hard sphere fluid becomes metastable, the dominant
structure is a cluster of m=10 particles with five-fold symmetry. At a particle
level, we find three regimes for the crystallisation process: metastable fluid
(dominated by m=10 clusters), crystal and a transition region of frequent
hopping between crystal-like environments and other (m\neq10) structuresComment: 10 page