746 research outputs found
Identification of structure in condensed matter with the topological cluster classification
We describe the topological cluster classification (TCC) algorithm. The TCC
detects local structures with bond topologies similar to isolated clusters
which minimise the potential energy for a number of monatomic and binary simple
liquids with particles. We detail a modified Voronoi bond detection
method that optimizes the cluster detection. The method to identify each
cluster is outlined, and a test example of Lennard-Jones liquid and crystal
phases is considered and critically examined.Comment: 28 pages, 28 figure
Long-Lived Non-Equilibrium Interstitial-Solid-Solutions in Binary Mixtures
We perform particle resolved experimental studies on the heterogeneous
crystallisation process of two compo- nent mixtures of hard spheres. The
components have a size ratio of 0.39. We compared these with molecular dynamics
simulations of homogenous nucleation. We find for both experiments and
simulations that the final assemblies are interstitial solid solutions, where
the large particles form crystalline close-packed lattices, whereas the small
particles occupy random interstitial sites. This interstitial solution
resembles that found at equilibrium when the size ratios are 0.3 [Filion et
al., Phys. Rev. Lett. 107, 168302 (2011)] and 0.4 [Filion, PhD Thesis, Utrecht
University (2011)]. However, unlike these previous studies, for our system sim-
ulations showed that the small particles are trapped in the octahedral holes of
the ordered structure formed by the large particles, leading to long-lived
non-equilibrium structures in the time scales studied and not the equilibrium
interstitial solutions found earlier. Interestingly, the percentage of small
particles in the crystal formed by the large ones rapidly reaches a maximum of
around 14% for most of the packing fractions tested, unlike previous
predictions where the occupancy of the interstitial sites increases with the
system concentration. Finally, no further hopping of the small particles was
observed
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
Recasting a model atomistic glassformer as a system of icosahedra
We consider a binary Lennard-Jones glassformer whose super-Arrhenius dynamics
are correlated with the formation of icosahedral structures. Upon cooling these
icosahedra organize into mesoclusters. We recast this glassformer as an
effective system of icosahedra which we describe with a population dynamics
model. This model we parameterize with data from the temperature regime
accessible to molecular dynamics simulations. We then use the model to
determine the population of icosahedra in mesoclusters at arbitrary
temperature. Using simulation data to incorporate dynamics into the model we
predict relaxation behavior at temperatures inaccessible to conventional
approaches. Our model predicts super-Arrhenius dynamics whose relaxation time
remains finite for non-zero temperature.Comment: 10 pages, 9 figure
Crystallization of dense binary hard-sphere mixtures with marginal size ratio
Molecular dynamics simulations are performed for binary hard-sphere mixtures with a size ratio of =0.9 and a volume fraction of =0.58 over a range of compositions. We show how, at this high volume fraction, crystallization depends sensitively on the composition. Evidence is presented that crystallization in these mixtures does not proceed by the standard nucleation and growth paradigm. Rather, some crystallite forms almost immediately and then an interplay between compositional fluctuations and crystal growth is able to dramatically extend the time scale on which further crystallization occurs. This can be seen as a form of geometric frustration
Using mutual information to measure order in model glass-formers
Whether or not there is growing static order accompanying the dynamical
heterogeneity and increasing relaxation times seen in glassy systems is a
matter of dispute. An obstacle to resolving this issue is that the order is
expected to be amorphous and so not amenable to simple order parameters. We use
mutual information to provide a general measurement of order that is sensitive
to multi-particle correlations. We apply this to two glass-forming systems (2D
binary mixtures of hard disks with different size ratios to give varying
amounts of hexatic order) and show that there is little growth of amorphous
order in the system without crystalline order. In both cases we measure the
dynamical length with a four-point correlation function and find that it
increases significantly faster than the static lengths in the system as density
is increased. We further show that we can recover the known scaling of the
dynamic correlation length in a kinetically constrained model, the 2-TLG.Comment: 10 pages, 12 Figure
The sediment of mixtures of charged colloids: segregation and inhomogeneous electric fields
We theoretically study sedimentation-diffusion equilibrium of dilute binary,
ternary, and polydisperse mixtures of colloidal particles with different
buoyant masses and/or charges. We focus on the low-salt regime, where the
entropy of the screening ions drives spontaneous charge separation and the
formation of an inhomogeneous macroscopic electric field. The resulting
electric force lifts the colloids against gravity, yielding highly
nonbarometric and even nonmonotonic colloidal density profiles. The most
profound effect is the phenomenon of segregation into layers of colloids with
equal mass-per-charge, including the possibility that heavy colloidal species
float onto lighter ones
Controlling kinetic pathways in demixing microgel-micelle mixtures
[Image: see text] We investigate the temperature-dependent phase behavior of mixtures of poly(N-isopropylacrylamide) (pNIPAM) microgel colloids and a triblock copolymer (PEOâPPOâPEO) surfactant. Usually, gelation in these systems results from an increase in temperature. Here we investigate the role of the heating rate, and surprisingly, we find that this causes the mechanism of aggregation to change from one which is driven by depletion of the microgels by the micelles at low temperatures to the association of the two species at high temperatures. We thus reveal two competing mechanisms for attractions between the microgel particles which can be controlled by changing the heating rate. We use this heating-rate-dependent response of the system to access multiple structures for the same system composition. Samples were found to demix into phases rich and poor in microgel particles at temperatures below 33 °C, under conditions where the microgels particles are partially swollen. Under rapid heating full demixing is bypassed, and gel networks are formed instead. The temperature history of the sample, therefore, allows for kinetic selection between different final structures, which may be metastable
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