Home Page of Jim Lutsko

Last updated: July 18, 2018 Aider ŕ mieux vivre dans la rue

Research Interests
  • Protein nucleation and crystal growth
  • Nonlinear diffusion
  • Theory of Nucleation
  • Zeolites
  • Crystallization
  • Bubble Cavitation and Sonoluminescence
  • Reactive flows far from equilibrium
  • Structure of nonequilibrium fluids
  • Granular Fluids
  • Nonextensive statistical mechanics
  • Fuzzy rule induction
Publications PHYSF475 Nanophysique 2018-2019 Useful Links
American Physical Society
American Institute of Physics
Materials Research Society

Info
Cirriculum Vitae
Cirriculum Vitae (en française)
Email: jlutsko AT ulb.ac.be
Address:
Center for Nonlinear Phenomena and Complex Systems
University Libre de Bruxelles
Campus Plaine, CP 231, 1050 Bruxelles, Belgium
Classical density functional theory, unconstrained crystallization, and polymorphic behavior
While in principle, classical density functional theory (cDFT) should be a powerful tool for the study of crystallization, in practice this has not so far been the case. Progress has been hampered by technical problems which have plagued the study of the crystalline systems using the most sophisticated fundamental measure theory models. In this paper, the reasons for the difficulties are examined and it is proposed that the tensor functionals currently favored are in fact numerically unstable. By reverting to an older, more heuristic model it is shown that all of the technical difficulties are eliminated. Application to a Lennard-Jones fluid results in a demonstration of power of cDFT to describe crystallization in a highly inhomogeneous system. First, we show that droplets attached to a slightly hydrophobic wall crystallize spontaneously upon being quenched. The resulting crystallites are clearly faceted structures and are predominantly HCP structures. In contrast, droplets in a fully periodic calculational cell remain stable to lower temperatures and eventually show the same spontaneous localization of the density into “atoms” but in an amorphous structure having many of the structural characteristics of a glass. A small change of the protocol leads, at the same temperature, to the formation of crystals, this time with the fcc structure typical of bulk Lennard-Jones solids. The fcc crystals have lower free energy than the amorphous structures which in turn are more stable than the liquid droplets. It is demonstrated that as the temperature is raised, the free energy differences between the structures decrease until the solid clusters become less stable than the liquid droplets and spontaneously melt. The presence of energy barriers separating the various structures is therefore clearly demonstrated.
This work is a product of the AMECRYS project: "Revolutionizing Downstream Processing of Monoclonal Antibodies by Continuuous Template-Assisted Membrane Crystallization"
The local density of a droplet hanging from a surface as a function of temperature. As the temperature is lowered, more and more structure develops until at sufficiently low temperature, a crystalline structure spontaneously forms.
Lattice induced crystallization of nanodroplets: the role of finite-size effects and substrate properties in controlling polymorphism
Targeting specific technological applications requires the control of nanoparticle properties, especially the crystalline polymorph. Freezing a nanodroplet deposited on a solid substrate leads to the formation of crystalline structures. We study the inherent mechanisms underlying this general phenomenon by means of molecular dynamics simulations. Our work shows that different crystal structures can be selected by finely tuning the solid substrate lattice parameter. Indeed, while for our system, face-centered cubic is usually the most preponderant structure, the growth of two distinct polymorphs, hexagonal centered packing and body-centered cubic, was also observed even when the solid substrate was face-centered cubic. Finally, we also demonstrated that the growth of hexagonal centered packing is conditioned by the appearance of large enough body-centered cubic clusters thus suggesting the presence of a cross-nucleation pathway. Our results provide insights into the impact of nanoscale effects and solid substrate properties towards the growth of polymorphic nanomaterials.
This work is a product of the AMECRYS project: "Revolutionizing Downstream Processing of Monoclonal Antibodies by Continuuous Template-Assisted Membrane Crystallization"
Polymoprph selection via substrate strain.
Solute particle near a nanopore: influence of size and surface properties on the solvent-mediated forces
Nanoscopic pores are used in various systems to attract nanoparticles. In general the behaviour is a result of two types of interactions: the material specific affinity and the solvent-mediated influence also called the depletion force. The latter is more universal but also much more complex to understand since it requires modeling both the nanoparticle and the solvent. Here, we employed classical density functional theory to determine the forces acting on a nanoparticle near a nanoscopic pore as a function of its hydrophobicity and its size. A simple capillary model is constructed to predict those depletion forces for various surface properties. For a nanoscopic pore, complexity arises from both the specific geometry and the fact that hydrophobic pores are not necessarily filled with liquid. Taking all of these effects into account and including electrostatic effects, we establish a phase diagram describing the entrance and the rejection of the nanoparticle from the pore.
This work is a product of the AMECRYS project: "Revolutionizing Downstream Processing of Monoclonal Antibodies by Continuuous Template-Assisted Membrane Crystallization"
Solvent-mediated forces create effective interactions between nanoparticles and surfaces.
Step Crowding Effects Dampen the Stochasticity of Crystal Growth Kinetics
Crystals grow by laying down new layers of material which can either correspond in size to the height of one unit cell (elementary steps) or multiple unit cells (macrosteps). Surprisingly, experiments have shown that macrosteps can grow under conditions of low supersaturation and high impurity density such that elementary step growth is completely arrested. We use atomistic simulations to show that this is due to two effects: the fact that the additional layers bias fluctuations in the position of the bottom layer towards growth and by a transition, as step height increases, from a 2D to a 3D nucleation mechanism.
This article was featured in Physics News and Commentary: see the Synopsis: Growing Crystals in Macrosteps
A macrostep bridging impurities on the crystal surface
Mesoscopic Impurities Expose a Nucleation-Limited Regime of Crystal Growth
Nanoscale self-assembly is naturally subject to impediments at the nanoscale. The recently developed ability to follow processes at the molecular level forces us to resolve older, coarse-grained concepts in terms of their molecular mechanisms. In this Letter, we highlight one such example. We present evidence based on experimental and simulation data that one of the cornerstones of crystal growth theory, the Cabrera-Vermilyea model of step advancement in the presence of impurities, is based on incomplete physics. We demonstrate that the piercing of an impurity fence by elementary steps is not solely determined by the Gibbs-Thomson effect, as assumed by Cabrera-Vermilyea. Our data show that for conditions leading up to growth cessation, step retardation is dominated by the formation of critically sized fluctuations. The growth recovery of steps is counter to what is typically assumed, not instantaneous. Our observations on mesoscopic impurities for lysozyme expose a nucleation-dominated regime of growth that has not been hitherto considered, where the system alternates between zero and near-pure velocity. The time spent by the system in arrest is the nucleation induction time required for the step to amass a supercritical fluctuation that pierces the impurity fence.
AFM images showing steps breaking through array of mesoscopic impurities.
Observing classical nucleation theory at work by monitoring phase transitions with molecular precision.
It is widely accepted that many phase transitions do not follow nucleation pathways as envisaged by the classical nucleation theory. Many substances can traverse intermediate states before arriving at the stable phase. The apparent ubiquity of multi-step nucleation has made the inverse question relevant: does multistep nucleation always dominate single-step pathways? Here we provide an explicit example of the classical nucleation mechanism for a system known to exhibit the characteristics of multi-step nucleation. Molecular resolution atomic force microscopy imaging of the two-dimensional nucleation of the protein glucose isomerase demonstrates that the interior of subcritical clusters is in the same state as the crystalline bulk phase. Our data show that despite having all the characteristics typically associated with rich phase behaviour, glucose isomerase 2D crystals are formed classically. These observations illustrate the resurfacing importance of the classical nucleation theory by re-validating some of the key assumptions that have been recently questioned... Read more ... The article is open source
AFM images showing crystallinity of sub-critical and super-critical clusters.
2-variable 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. 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 Classical Nucleation Theory 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. Read more...
Free energy surfaces as functions of radius and density of a cluster. Left: sub-critical, Right: super-critical - the line shows the most likely nucleation pathway.
The physical basis of step pinning.
The growth of crystals from solution is a fundamental process of relevance to such diverse areas as X-ray-diffraction structural determination and the role of mineralization in living organisms. A key factor determining the dynamics of crystallization is the effect of impurities on step growth. For over fifty years, all discussions of impurity-step interaction have been framed in the context of the Cabrera--Vermilyea (CV) model for step blocking, which has nevertheless proven difficult to validate experimentally. Here we report on extensive computer simulations which clearly falsify the CV model, suggesting a more complex picture. While reducing to the CV result in certain limits, our approach is more widely applicable, encompassing non-trivial impurity-crystal interactions, mobile impurities and negative growth, among others. Read the paper... view movies
Left: Snapshot of kineic Monte Carlo simulation. Right: Step velocity shown as size of circles as a function of impurity size, L, and impurity spaceing, n_{c} shown for three different supersaturations. Blue indicates positive velocity, red is negative velocity and white is statistically zero. The CV prediction for zero velocity is everything below the broken line marked CV.

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