arXiv subjects

Christos N. Likos

Publications and source records attributed to Christos N. Likos.

At least 37 records · Page 2Linked to original sources

arXiv2015

Soft self-assembled nanoparticles with temperature-dependent properties

The fabrication of versatile building blocks that are reliably self-assemble into desired ordered and disordered phases is amongst the hottest topics in contemporary material science. To this end, microscopic units of varying complexity, aimed at assembling the target phases, have been thought, designed, investigated and built. Such a path usually requires laborious fabrication techniques, especially when a specific funcionalisation of the building blocks is required. Telechelic star polymers, i.e., star polymers made of a number $f$ of di-block copolymers consisting of solvophobic and solvophilic monomers grafted on a central anchoring point, spontaneously self-assemble into soft patchy particles featuring attractive spots (patches) on the surface. Here we show that the tunability of such a system can be widely extended by controlling the physical and chemical parameters of the solution. Indeed, at fixed external conditions the self-assembly behaviour depends only on the number of arms and/or on the ratio of solvophobic to solvophilic monomers. However, changes in temperature and/or solvent quality makes it possible to reliably change the number and size of the attractive patches. This allows to steer the mesoscopic self-assembly behaviour without modifying the microscopic constituents. Interestingly, we also demonstrate that diverse combinations of the parameters can generate stars with the same number of patches but different radial and angular stiffness. This mechanism could provide a neat way of further fine-tuning the elastic properties of the supramolecular network without changing its topology.

cond-mat.soft↗

arXiv2015

Validity of Stokes-Einstein Relation in Soft Colloids up to the Glass Transition

We investigate the dynamics of kinetically frozen block copolymer micelles of different softness across a wide range of particle concentrations, from the fluid to the onset of glassy behavior, through a combination of rheology, dynamic light scattering and pulsed field gradient NMR spectroscopy. We additionally perform Brownian dynamics simulations based on an ultrasoft coarse-grained potential, which are found to be in quantitative agreement with experiments, capturing even the very details of dynamic structure factors S(Q, t) on approaching the glass transition. We provide evidence that for these systems the Stokes-Einstein relation holds up to the glass transition; given that it is violated for dense suspensions of hard colloids, our findings suggest that its validity is an intriguing signature of ultrasoft interactions.

cond-mat.soft↗

arXiv2015

An Anisotropic Effective Model for the Simulation of Semiflexible Ring Polymers

We derive and introduce anisotropic effective pair potentials to coarse-grain solutions of semiflexible rings polymers of various lengths. The system has been recently investigated by means of full monomer-resolved computer simulations, revealing a host of unusual features and structure formation, which, however, cannot be captured by a rotationally-averaged effective pair potential between the rings' centers of mass [M. Bernabei et al., Soft Matter 9, 1287 (2013)]. Our new coarse-graining strategy is to picture each ring as a soft, penetrable disc. We demonstrate that for the short- and intermediate-length rings the new model is quite capable of capturing the Physics in a quantitative fashion, whereas for the largest rings, which resemble flexible ones, it fails at high densities. Our work opens the way for the physical justification of general, anisotropic penetrable interaction potentials.

cond-mat.soft↗

arXiv2014

Structures and pathways for clathrin self-assembly in the bulk and on membranes

We present a coarse-grained model of clathrin that is simple enough to be computationally tractable yet includes key observed qualitative features: a triskelion structure with excluded volume between legs; assembly of polymorphic cages in the bulk; formation of buds on a membrane. We investigate the assembly of our model using both Monte Carlo simulations and molecular dynamics with hydrodynamic interactions, in the latter employing a new membrane boundary condition. In the bulk, a range of known clathrin structures are assembled. A membrane budding pathway involving the coalescence of multiple small clusters is identified.

cond-mat.soft↗

arXiv2014

Equilibrium properties of charged microgels: a Poisson-Boltzmann-Flory approach

The equilibrium properties of ionic microgels are investigated using a combination of the Poisson-Boltzmann and Flory theories. Swelling behavior, density profiles, and effective charges are all calculated in a self-consistent way. Special attention is given to the effects of salinity on these quantities. It is found that the equilibrium microgel size is strongly influenced by the amount of added salt. Increasing the salt concentration leads to a considerable reduction of the microgel volume, which therefore releases its internal material -- solvent molecules and dissociated ions -- into the solution. Finally, the question of charge renormalization of ionic microgels in the context of the cell model is briefly addressed.

cond-mat.soft↗

arXiv2014

Influence of Rigidity and Knot Complexity on the Knotting of Confined Polymers

We employ computer simulations and thermodynamic integration to analyse the effects of bending rigidity and slit confinement on the free energy cost of tying knots, $ΔF_{\rm knotting}$, on polymer chains under tension. A tension-dependent, non-zero optimal stiffness $κ_{\rm min}$ exists, for which $ΔF_{\rm knotting}$ is minimal. For a polymer chain with several stiffness domains, each containing a large amount of monomers, the domain with stiffness $κ_{\rm min}$ will be preferred by the knot. A {\it local} analysis of the bending in the interior of the knot reveals that local stretching of chains at the braid region is responsible for the fact that the tension-dependent optimal stiffness has a non-zero value. The reduction in $ΔF_{\rm knotting}$ for a chain with optimal stiffness relative to the flexible chain can be enhanced by tuning the slit width of the 2D confinement and increasing the knot complexity. The optimal stiffness itself is independent of the knot types we considered, while confinement shifts it towards lower values.

cond-mat.soft↗

arXiv2013

Cluster Glasses of Semiflexible Ring Polymers

We present computer simulations of concentrated solutions of unknotted nonconcatenated semiflexible ring polymers. Unlike in their flexible counterparts, shrinking involves a strong energetic penalty, favoring interpenetration and clustering of the rings. We investigate the slow dynamics of the centers-of-mass of the rings in the amorphous cluster phase, consisting of disordered columns of oblate rings penetrated by bundles of prolate ones. Scattering functions reveal a striking decoupling of self- and collective motions. Correlations between centers-of-mass exhibit slow relaxation, as expected for an incipient glass transition, indicating the dynamic arrest of the cluster positions. However, self-correlations decay at much shorter time scales. This feature is a manifestation of the fast, continuous exchange and diffusion of the individual rings over the matrix of clusters. Our results reveal a novel scenario of glass-formation in a simple monodisperse system, characterized by self-collective decoupling, soft caging and mild dynamic heterogeneity.

cond-mat.soft↗

arXiv2013

Dynamics of Self-Assembly of Model Viral Capsids in the Presence of a Fluctuating Membrane

A coarse-grained computational model is used to investigate the effect of a fluctuating fluid membrane on the dynamics of patchy-particle assembly into virus capsid-like cores. Results from simulations for a broad range of parameters are presented, showing the effect of varying interaction strength, membrane stiffness and membrane viscosity. Furthermore, the effect of hydrodynamic interactions is investigated. Attraction to a membrane may promote assembly, including for sub-unit interaction strengths for which it does not occur in the bulk, and may also decrease single-core assembly time. The membrane budding rate is strongly increased by hydrodynamic interactions. The membrane deformation rate is important in determining the finite-time yield. Higher rates may decrease the entropic penalty for assembly and help guide sub-units towards each other but may also block partial cores from being completed. For increasing sub-unit interaction strength, three regimes with different effects of the membrane are identified.

cond-mat.soft↗

arXiv2012

Effect of bending rigidity on the knotting of a polymer under tension

A coarse-grained computational model is used to investigate how the bending rigidity of a polymer under tension affects the formation of a trefoil knot. Thermodynamic integration techniques are applied to demonstrate that the free-energy cost of forming a knot has a minimum at non-zero bending rigidity. The position of the minimum exhibits a power-law dependence on the applied tension. For knotted polymers with non-uniform bending rigidity, the knots preferentially localize in the region with a bending rigidity that minimizes the free-energy.

cond-mat.soft↗

arXiv2012

Microscopically-resolved simulations prove the existence of soft cluster crystals

We perform extensive monomer-resolved computer simulations of suitably-designed amphiphilic dendritic macromolecules over a broad range of densities, proving the existence and stability of cluster crystals formed in these systems, as predicted previously on the basis of effective pair potentials [B. M. Mladek et al., Phys. Rev. Lett. 96, 045701 (2006)]. Key properties of these crystals, such as the adjustment of their site occupancy with density and the possibility to heal defects by dendrimer migration, are confirmed on the monomer-resolved picture. At the same time, important differences from the predictions of the pair potential picture, stemming from steric crowding, arise as well and they place an upper limit in the density for which such crystals can exist.

cond-mat.soft↗

arXiv2012

Telechelic star polymers as self-assembling units from the molecular to the macroscopic scaleBarbara Capone

By means of multiscale molecular simulations, we show that telechelic-star polymers are a simple, robust and tunable system, which hierarchically self-assembles first into soft-patchy particles and then into targeted crystalline structures. The self-aggregating patchy behavior can be fully controlled by the number of arms per star and by the fraction of attractive monomeric units at the free ends of the arms. Such self-assembled soft-patchy particles while forming, upon augmenting density, gel-like percolating networks and stable ordered structures, preserve properties as particle size, number and arrangement of patches per particle. In particular, we demonstrate that the flexibility inherent in the soft-patchy particles brings forward a novel mechanism that leads to the stabilisation of diamond and simple cubic crystals over a wide range of densities, and for molecular sizes ranging from about 10 nm up to the micrometer scale.

cond-mat.soft↗

arXiv2012

Influence of Fluctuating Membranes on Self-Assembly of Patchy Colloids

A coarse-grained computational model is used to investigate the effect of a fluid membrane on patchy-particle assembly into biologically-relevant structures motivated by viral cores and clathrin. For cores, we demonstrate a non-monotonic dependence of the promotion of assembly on membrane stiffness. If the membrane is significantly deformable, cores are enveloped in buds, although this effect is suppressed for very flexible membranes. In the less deformable regime, we observe no marked enhancement for cores, even for strong adhesion to the surface. For clarthrin-like particles, we again observe the formation of buds, whose morphology depends on membrane-flexibility.

cond-mat.soft↗

arXiv2010

Monomer-resolved simulations of cluster-forming dendrimers

We present results of monomer-resolved Monte Carlo simulations for a system of amphiphilic dendrimers of second generation. Our investigations validate a coarse-grained level description based on the zero-density limit effective pair-interactions for low and intermediate densities, which predicted the formation of stable, finite aggregates in the fluid phase. Indeed, we find that these systems form a homogeneous fluid for low densities, which on increasing the density spontaneously transforms into a fluid of clusters of dendrimers. Although these clusters are roughly spherical in nature for intermediate densities, also more complex structures are detected for the highest densities considered.

cond-mat.soft↗

arXiv2010

Dynamics in binary cluster crystals

As a result of the application of coarse-graining procedures to describe complex fluids, the study of systems consisting of particles interacting through bounded, repulsive pair potentials has become of increasing interest in the last years. A well known example is the so-called Generalized Exponential Model (GEM-$m$), for which the interaction between particles is described by the potential $v(r)=ε\exp[-(r/σ)^m]$. Interactions with $m > 2$ lead to the formation of a novel phase of soft matter consisting of cluster crystals. Recent studies on the phase behavior of binary mixtures of GEM-$m$ particles have provided evidence for the formation of novel kinds of alloys, depending on the cross interactions between the two species. This work aims to study the dynamic behavior of such binary mixtures by means of extensive molecular dynamics simulations, and in particular to investigate the effect of the addition of non-clustering particles on the dynamic scenario of one-component cluster crystals. Analogies and differences with the one-component case are revealed and discussed by analyzing self- and collective dynamic correlators.

cond-mat.soft↗

arXiv2010

Phonon dispersions of cluster crystals

We analyze the ground states and the elementary collective excitations (phonons) of a class of systems, which form cluster crystals in the absence of attractions. Whereas the regime of moderate-to-high-temperatures in the phase diagram has been analyzed in detail by means of density functional considerations (Likos C N, Mladek B M, Gottwald D and Kahl G 2007 {\it J.~Chem.~Phys.}\ {\bf 126} 224502), the present approach focuses on the complementary regime of low temperatures. We establish the existence of an infinite cascade of isostructural transitions between crystals with different lattice site occupancy at $T=0$ and we quantitatively demonstrate that the thermodynamic instabilities are bracketed by mechanical instabilities arising from long-wavelength acoustical phonons. We further show that all optical modes are degenerate and flat, giving rise to perfect realizations of Einstein crystals. We calculate analytically the complete phonon spectrum for the whole class of models as well as the Helmholtz free energy of the systems. On the basis of the latter, we demonstrate that the aforementioned isostructural phase transitions must terminate at an infinity of critical points at low temperatures, brought about by the anharmonic contributions in the Hamiltonian and the hopping events in the crystals.

cond-mat.soft↗

arXiv2008

Cluster Crystals in Confinement

This paper has been withdrawn by the author(s), due to technical copyright problems with the Royal Society of Chemistry.

cond-mat.soft↗

arXiv2008

Multiple glass transitions in star polymer mixtures: Insights from theory and simulations

The glass transition in binary mixtures of star polymers is studied by mode coupling theory and extensive molecular dynamics computer simulations. In particular, we have explored vitrification in the parameter space of size asymmetry $δ$ and concentration $ρ_2$ of the small star polymers at fixed concentration of the large ones. Depending on the choice of parameters, three different glassy states are identified: a single glass of big polymers at low $δ$ and low $ρ_2$, a double glass at high $δ$ and low $ρ_2$, and a novel double glass at high $ρ_2$ and high $δ$ which is characterized by a strong localization of the small particles. At low $δ$ and high $ρ_2$ there is a competition between vitrification and phase separation. Centered in the $(δ, ρ_2)$-plane, a liquid lake shows up revealing reentrant glass formation. We compare the behavior of the dynamical density correlators with the predictions of the theory and find remarkable agreement between the two.

cond-mat.soft↗

arXiv2008

Crystallization of magnetic dipolar monolayers: a density functional approach

We employ density functional theory to study in detail the crystallization of super-paramagnetic particles in two dimensions under the influence of an external magnetic field that lies perpendicular to the confining plane. The field induces non-fluctuating magnetic dipoles on the particles, resulting into an interparticle interaction that scales as the inverse cube of the distance separating them. In line with previous findings for long-range interactions in three spatial dimensions, we find that explicit inclusion of liquid-state structural information on the {\it triplet} correlations is crucial to yield theoretical predictions that agree quantitatively with experiment. A non-perturbative treatment is superior to the oft-employed functional Taylor expansions, truncated at second or third order. We go beyond the usual Gaussian parametrization of the density site-orbitals by performing free minimizations with respect to both the shape and the normalization of the profiles, allowing for finite defect concentrations.

cond-mat.soft↗