Random lasing in a solution of reflective colloidal particles: the effect of interfaces and inter-particle correlations

The propagation of light across 2D and 3D slabs of reflective colloidal particles in a fluid-like state has been investigated by simulation. The colloids are represented as hard spheres with and without an attractive square-well tail. Representative configurations of particles have been generated by Monte Carlo. The path of rays entering the slab normal to its planar surface has been determined by exact geometric scattering conditions, assuming that particles are macroscopic spheres fully reflective at the surface of their hard-core potential. The analysis of light paths provides the transmission and reflection coefficients, the mean-free path, the average length of transmitted and reflected paths, the distribution of scattering events across the slab, and the angular spread of the outcoming rays as a function of dimensionality and thermodynamic state. The results highlight the presence of a sizeable population of very long paths, which play an important role in random lasing from solutions of metal particles in an optically active fluid. The output power spectrum resulting from the stimulated emission amplification decays asymptotically as an inverse power law. The present study goes beyond the standard approach based on a random walk confined between two planar interfaces and parametrised in terms of the mean-free path and scattering matrix. Here, instead, the mean free path, the correlation among scattering events, and memory effects are not assumed a priori but emerge from the underlying statistical mechanics model of interacting particles. Moreover, the approach joins smoothly the ballistic regime of light propagation at low density with the diffusive regime at high density of scattering centres. These properties are exploited to investigate the effect of weak polydispersivity and of large density fluctuations at the critical point of the model with the attractive potential tail

Precursors of fluidisation in the creep response of a soft glass

Using extensive numerical simulations, we study the fluidisation process of dense amorphous materials subjected to an external shear stress, using a three-dimensional colloidal glass model. In order to disentangle possible boundary effects from finite size effects in the process of fluidisation, we implement a novel geometry-constrained protocol with periodic boundary conditions. We show that this protocol is well controlled and that the longtime fluidisation dynamics is, to a great extent, independent of the details of the protocol parameters. Our protocol, therefore, provides an ideal tool to investigate the bulk dynamics prior to yielding and to study finite size effects regarding the fluidisation process. Our study reveals the existence of precursors to fluidisation observed as a peak in the strain-rate fluctuations, that allows for a robust definition of a fluidisation time. Although the exponents in the power-law creep dynamics seem not to depend significantly on the system size, we reveal strong finite size effects for the onset of fluidisation

Modelling the kinetics of amyloid fibril nucleation

A kinetic theory has been developed within the framework of preexisting nucleation theory and applied, for the first time, to investigate the one-step formation of amyloid fibrils. Atomistic Nucleation Theory (ANT) for fibrils, in particular, has been successfully applied to model real peptides and proteins, in order to investigate at the molecular level the nucleation of amyloid fibrils from a homogeneous solution. Kinetic parameters predicted by the theory, such as the nucleation rates, have been compared successfully to the results of experiments. The present theoretical study has shown that variations in solubility are the primary origin of the changes in the nucleation rates between a protein and its point-mutations. The same ANT approach allows the analysis of the fibril size distribution, whose results, once again, are consistent with experimental observations. In the last stage of the investigation, computer simulations have been carried out to test selected assumptions underlying the theory. For the first time, the nucleation of strongly anisotropic systems has been investigated using kinetic Monte Carlo (KMC) simulations. Novel and unexpected features, never discussed before in either experiments or simulations studies, have been revealed by the simulations. Although obtained within the study of amyloid fibrils nucleation, these last results are of general validity, providing useful insight on the nucleation of all systems whose molecules interact via strongly anisotropic forces

On the effects of the degrees of freedom on calculating diffusion properties in nanoporous materials

If one carries out a molecular simulation of N particles using periodic boundary conditions, linear momentum is conserved and hence the number of degrees of freedom is set to 3N-3. In most programs, this number of degrees of freedom is the default setting. However, if one carries out a molecular simulation in an external field, one needs to ensure that degrees of freedom are changed from this default setting to 3N, as in an external field the velocity of the center of mass can change. Using the correct degrees of freedom is important in calculating the temperature and in some algorithms to simulate at constant temperature. For sufficiently large systems, the difference between 3N and 3N-3 is negligible in the way. However, there are systems in which the comparison with experimental data requires molecular dynamics simulations of a small number of particles. In this work, we illustrate the effect of an incorrect setting of degrees of freedom in molecular dynamic simulations studying the diffusion properties of guest molecules in nanoporous materials. We show that previously published results have reported a surprising diffusion dependence on the loading, which could be traced back to an incorrect setting of the degrees of freedom. As the correct settings are convoluted and counter-intuitive in some of the most commonly used molecular dynamics programs, we carried out a systematic study on the consequences of the various commonly used (incorrect) settings. Our conclusion, is that for systems smaller than 50 particles the results are most likely unreliable as these are either performed at an incorrect temperature or the temperature is incorrectly used in some of the results. Furthermore, a novel and efficient method to calculate diffusion coefficients of guest molecules into nanoporous materials at zero loading conditions is introduced

Precursors of fluidisation in the creep response of a soft glass

International audienc

The effect of organic ions on the formation and collapse of nanometric bubbles in ionic liquid/water solutions: A molecular dynamics study

Molecular dynamics simulation is applied to investigate the effect of two ionic liquids (IL) on the nucleation and growth of (nano-)cavities in water under tension and on the cavities’ collapse following the release of tension. Simulations of the same phenomena in two pure water samples of different sizes are carried out for comparison. The first IL, i.e., tetra-ethyl ammonium mesylate ([Tea][Ms]), is relatively hydrophilic and its addition to water at 25 wt% concentration decreases its tendency to nucleate cavities. Apart from quantitative details, cavity formation and collapse are similar to those taking place in water, and qualitatively follow the Rayleigh-Plesset (RP) equation. The second IL, i.e., tetrabutyl phosphonium 2,4-dimethylbenzene sulfonate ([P4444 ][DMBS]), is amphiphilic, and forms nanostructured solutions with water. At 25 wt% concentrations, [P4444 ][DMBS] favours the nucleation of bubbles, that tend to form at the interface between water-rich and IL-rich domains. Cavity collapse in [P4444 ][DMBS]/water solutions are greatly hindered by a shell of ions decorating the interface between the solution and the vapour phase. A similar effect is observed for the equilibration of a population of bubbles of different sizes. The drastic slowing down of bubbles’ relaxation processes suggests ways to produce long-lived nanometric cavities in the liquid phase that could be useful for nanotechnology and drug delivery