31 research outputs found
A two-parameter extension of classical nucleation theory.
A two-variable stochastic model for diffusion-limited nucleation is developed using a formalism derived from fluctuating hydrodynamics. The model is a direct generalization of the standard classical nucleation theory (CNT). The nucleation rate and pathway are calculated in the weak-noise approximation and are shown to be in good agreement with direct numerical simulations for the weak-solution/strong-solution transition in globular proteins. We find that CNT underestimates the time needed for the formation of a critical cluster by two orders of magnitude and that this discrepancy is due to the more complex dynamics of the two variable model and not, as often is assumed, a result of errors in the estimation of the free energy barrier
Dynamical density functional theory for orientable colloids including inertia and hydrodynamic interactions
Over the last few decades, classical density-functional theory (DFT) and its
dynamic extensions (DDFTs) have become powerful tools in the study of colloidal
fluids. Recently, previous DDFTs for spherically-symmetric particles have been
generalised to take into account both inertia and hydrodynamic interactions,
two effects which strongly influence non-equilibrium properties. The present
work further generalises this framework to systems of anisotropic particles.
Starting from the Liouville equation and utilising Zwanzig's
projection-operator techniques, we derive the kinetic equation for the Brownian
particle distribution function, and by averaging over all but one particle, a
DDFT equation is obtained. Whilst this equation has some similarities with
DDFTs for spherically-symmetric colloids, it involves a
translational-rotational coupling which affects the diffusivity of the
(asymmetric) particles. We further show that, in the overdamped (high friction)
limit, the DDFT is considerably simplified and is in agreement with a previous
DDFT for colloids with arbitrary shape particles.Comment: dynamical density functional theory ; colloidal fluids ;
arbitrary-shape particles ; orientable colloid
How ice grows from premelting films and water droplets
Close to the triple point, the surface of ice is covered by a thin liquid
layer (so-called quasi-liquid layer) which crucially impacts growth and melting
rates. Experimental probes cannot observe the growth processes below this
layer, and classical models of growth by vapor deposition do not account for
the formation of premelting films. Here, we develop a mesoscopic model of
liquid-film mediated ice growth, and identify the various resulting growth
regimes. At low saturation, freezing proceeds by terrace spreading, but the
motion of the buried solid is conveyed through the liquid to the outer
liquid-vapor interface. At higher saturations water droplets condense, a large
crater forms below, and freezing proceeds undetectably beneath the droplet. Our
approach is a general framework that naturally models freezing close to three
phase coexistence and provides a first principle theory of ice growth and
melting which may prove useful in the geosciences.Comment: 32 pages, 10 figure
3D visualization of additive occlusion and tunable full-spectrum fluorescence in calcite
From biomineralization to synthesis, organic additives provide an effective means of controlling crystallization processes. There is growing evidence that these additives are often occluded within the crystal lattice. This promises an elegant means of creating nanocomposites and tuning physical properties. Here we use the incorporation of sulfonated fluorescent dyes to gain new understanding of additive occlusion in calcite (CaCO3), and to link morphological changes to occlusion mechanisms. We demonstrate that these additives are incorporated within specific zones, as defined by the growth conditions, and show how occlusion can govern changes in crystal shape. Fluorescence spectroscopy and lifetime imaging microscopy also show that the dyes experience unique local environments within different zones. Our strategy is then extended to simultaneously incorporate mixtures of dyes, whose fluorescence cascade creates calcite nanoparticles that fluoresce white. This offers a simple strategy for generating biocompatible and stable fluorescent nanoparticles whose output can be tuned as required