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At least 19 recordsLinked to original sources
arXiv2013

Understanding coil-to-globule transition of polymers with the aid of a novel cluster analysis technique

In this article, a novel cluster analysis algorithm was employed in the study of polymer coil to globule transition via single chain Monte Carlo simulations. The algorithm, which has been recently applied in Molecular Dynamics simulations of atomistic systems that tend to phase separate [arXiv: 1307.7366 [cond-mat.soft]], provides us with a convenient means to map out the dynamics of "pearls" formation along the backbone chain together with extracting meaningful quantitative information about their shape and size distribution. Preliminary findings tend to favour a two-stage model of collapse kinetics, although a more complicated picture emerges when looking at the details of cluster formation along the chain.

cond-mat.soft↗
arXiv2019

Reexamination of Tolman's law and the Gibbs adsorption equation for curved interfaces

In manuscript arXiv:1703.08719 [cond-mat.soft], it was claimed that the well-known deduction of Tolman's law is not rigorous, since Tolman's argument implies that two different definitions of the surface tension, called $γ$ and $\barγ$ in the manuscript, coincide. This claim is retracted as it can be shown by free-energy minimization that $γ= \barγ$ indeed holds for the Laplace radius. Joachim Groß, Philipp Rehner, Carlos Vega, Øivind Wilhelmsen, and the anonymous reviewers of The Journal of Chemical Physics contributed to finding the mistake in the manuscript.

cond-mat.soft↗
arXiv2014

Periodic ordering of clusters and stripes in a two-dimensional lattice model. II. Results of Monte Carlo simulation

The triangular lattice model with nearest-neighbor attraction and third-neighbor repulsion, introduced in [J. Pekalski, A. Ciach and N. G. Almarza, arXiv:1401.0801 [cond-mat.soft]] is studied by Monte Carlo simulation. Introduction of appropriate order parameters allowed us to construct a phase diagram, where different phases with patterns made of clusters, bubbles or stripes are thermodynamically stable. We observe, in particular, two distinct lamellar phases - the less ordered one with global orientational order and the more ordered one with both orientational and translational order. Our results concern spontaneous pattern formation on solid surfaces, fluid interfaces or membranes that is driven by competing interactions between adsorbing particles or molecules.

cond-mat.soft↗
arXiv2026

Spectral insights into active matter: Exceptional Points and the Mathieu equation

We show that recent numerical findings of universal scaling relations in systems of noisy, aligning self-propelled particles by Kürsten [Kürsten, arXiv:2402.18711v2 [cond-mat.soft] (2025)] can robustly be explained by perturbation theory and known results for the Mathieu equation with purely imaginary parameter. In particular, we highlight the significance of a cascade of exceptional points that leads to non-trivial fractional scaling exponents in the singular-perturbation limit of high activity. Crucially, these features are rooted in the Fokker-Planck operator corresponding to free self-propulsion. This can be viewed as a dynamical phase transition in the dynamics of noisy active matter. We also predict that these scaling relations depend on the symmetry of the alignment interactions and discuss the relevance of this structure in the free propagation for self-alignment and cohesion-type interactions.

cond-mat.stat-mech↗
arXiv2019

Morphology and flow patterns in highly asymmetric active emulsions

We investigate numerically, by a hybrid lattice Boltzmann method, the morphology and the dynamics of an emulsion made of a polar active gel, contractile or extensile, and an isotropic passive fluid. We focus on the case of a highly off-symmetric ratio between the active and passive components. In absence of any activity we observe an hexatic-ordered droplets phase, with some defects in the layout. We study how the morphology of the system is affected by activity both in the contractile and extensile case. In the extensile case a small amount of activity favors the elimination of defects in the array of droplets, while at higher activities, first aster-like rotating droplets appear, and then a disordered pattern occurs. In the contractile case, at sufficiently high values of activity, elongated structures are formed. Energy and enstrophy behavior mark the transitions between the different regimes.

cond-mat.soft↗
arXiv2009

Stochastic Eulerian-Lagrangian Methods for Fluid-Structure Interactions with Thermal Fluctuations and Shear Boundary Conditions

A computational approach is introduced for the study of the rheological properties of complex fluids and soft materials. The approach allows for a consistent treatment of microstructure elastic mechanics, hydrodynamic coupling, thermal fluctuations, and externally driven shear flows. A mixed description in terms of Eulerian and Lagrangian reference frames is used for the physical system. Microstructure configurations are represented in a Lagrangian reference frame. Conserved quantities, such as momentum of the fluid and microstructures, are represented in an Eulerian reference frame. The mathematical formalism couples these different descriptions using general operators subject to consistency conditions. Thermal fluctuations are taken into account in the formalism by stochastic driving fields introduced in accordance with the principles of statistical mechanics. To study the rheological responses of materials subject to shear, generalized periodic boundary conditions are developed where periodic images are shifted relative to the unit cell to induce shear. Stochastic numerical methods are developed for the formalism. As a demonstration of the methods, results are presented for the shear responses of a polymeric fluid, lipid vesicle fluid, and a gel-like material.

cond-mat.soft↗
arXiv2022

An Introduction to Motility-Induced Phase Separation

Active particles may undergo phase separation when interactions oppose self-propulsion, in the absence of any cohesive forces. The corresponding Motility-Induced Phase Separation (MIPS) is arguably the simplest non-trivial collective feature that distinguishes active from passive particles. It is observed in a large variety of systems which we review in this chapter. We describe in depth the case of motile particles interacting via quorum-sensing interactions, whose theoretical framework is by now well-established. We close the chapter by discussing the features observed in systems undergoing MIPS that still challenge our understanding.

cond-mat.soft↗
arXiv2010

No free lunch for effective potentials: general comment for Faraday FD144

I briefly review the problem of representability -- a single coarse-grained effective pair potential cannot simultaneously represent all the properties of an underlying more complex system -- as well as a few other subtleties that can arise in interpreting coarse-grained potentials.

cond-mat.soft↗
arXiv2014

A Supersolid Skin Covering both Water and Ice

The mysterious nature and functionality of water and ice skins remain baffling to the community since 1859 when Farady firstly proposed liquid skin lubricating ice. Here we show the presence of supersolid phase that covers both water and ice using Raman spectroscopy measurements and quantum calculations. In the skin of two molecular layers thick, molecular undercoordination shortens the H-O bond by ~16% and lengthens the OH nonbond by ~25% through repulsion between electron pairs on adjacent O atoms, which depresses the density from 0.92 for bulk ice to 0.75 gcm-3. The O:H-O cooperative relaxation stiffens the H-O stretching phonon from 3200/3150 cm-1 to the same value of 3450 cm-1 and raises the melting temperature of both skins by up to ~310 K. Numerical derivatives on the viscosity and charge accumulation suggests that the elastic, polarized, and thermally stable supersolid phase makes the ice frictionless and water skin hydrophobic and ice like at room temperature.

cond-mat.soft↗
arXiv2007

Approximate square-root-time relaxation in glass-forming liquids

We present data for the dielectric relaxation of 43 glass-forming organic liquids, showing that the primary (alpha) relaxation is often close to square-root-time relaxation. The better an inverse power-law description of the high-frequency loss applies, the more accurately is square-root-time relaxation obeyed. These findings suggest that square-root-time relaxation is generic to the alpha process, once a common view, but since long believed to be incorrect. Only liquids with very large dielectric losses deviate from this picture by having consistently narrower loss peaks. As a further challenge to the prevailing opinion, we find that liquids with accurate square-root-time relaxation cover a wide range of fragilities.

cond-mat.soft↗
arXiv2008

Crystallization of the Kob-Andersen binary Lennard-Jones liquid

At density 1.2 the Kob-Andersen binary Lennard-Jones liquid partly crystallizes in the temperature interval [0.39, 0.44] after typically 10-100 microseconds (Argon units). The crystallization is initiated by a phase separation where the large (A) particles cluster in a volume void of B particles. We investigate a modification of the Kob-Andersen system where the attraction between particles of the same type is removed, thus disfavoring phase separation. We have not been able to crystallize this new system.

cond-mat.soft↗
arXiv2009

Strong pressure-energy correlations in liquids as a configuration space property: Simulations of temperature down jumps and crystallization

Computer simulations recently revealed that several liquids exhibit strong correlations between virial and potential energy equilibrium fluctuations in the NVT ensemble [U. R. Pedersen {\it et al.}, Phys. Rev. Lett. {\bf 100}, 015701 (2008)]. In order to investigate whether these correlations are present also far from equilibrium constant-volume aging following a temperature down jump from equilibrium was simulated for two strongly correlating liquids, an asymmetric dumbbell model and Lewis-Wahnstr{ö}m OTP, as well as for SPC water that is not strongly correlating. For the two strongly correlating liquids virial and potential energy follow each other closely during the aging towards equilibrium. For SPC water, on the other hand, virial and potential energy vary with little correlation as the system ages towards equilibrium. Further proof that strong pressure-energy correlations express a configuration space property comes from monitoring pressure and energy during the crystallization (reported here for the first time) of supercooled Lewis-Wahnstr{ö}m OTP at constant temperature.

cond-mat.soft↗
arXiv2009

"Isomorphs" in liquid state diagrams

A liquid is termed strongly correlating if its virial and potential energy thermal equilibrium fluctuations in the NVT ensemble are more than 90% correlated [Phys. Rev. Lett. 100, 015701 (2008)]. The fluctuations of a strongly correlating liquid are well approximated by those of an inverse power-law intermolecular potential. Building on this fact we here define "isomorphic lines" in the state diagram of a strongly correlating liquid. It is shown from computer simulations of the Kob-Andersen binary Lennard-Jones liquid that no aging is associated with jumps between two isomorphic points. Isomorphic state points have the same excess entropy, the same reduced average relaxation time, the same (reduced) dynamics, and the same scaled radial distribution functions. Finally we calculate the equation for isomorphs in the virial / potential energy diagram for Lennard-Jones type liquids and show that all such 12-6 liquids have the same isomorphs; these may be scaled into a master isomorph.

cond-mat.soft↗
arXiv2008

Physical ageing studied by a device allowing for rapid thermal equilibration

Ageing of organic glasses to the equilibrium liquid state is studied by measuring the dielectric loss utilizing a microregulator where temperature is controlled by means of a Peltier element. Compared to conventional equipment the new device adds almost two orders of magnitude to the span of observable ageing times. Data for five organic glass-forming liquids are presented. The existence of an "inner clock" is confirmed by a model-free test showing that the ageing of structure is controlled by the same material time that controls the dielectric properties. At long times relaxation is not stretched, but simple exponential, and there is no "expansion gap" between the limits of the relaxation rates following up and down jumps to the same temperature.

cond-mat.soft↗
arXiv2011

Particle Monte Carlo simulation of string-like colloidal assembly in 2 dimensions

We simulate structural phase behavior of polymer-grafted colloidal particles by molecular Monte Carlo technique. Interparticle potential, which has a finite repulsive square-step outside a rigid core of the colloid, was previously confirmed via numerical self-consistent field calculation. This model potential is purely repulsive. We simulate these model colloids in the canonical ensemble in 2 dimensions and find that these particles containing no interparticle attraction self-assemble and align in a string-like assembly, at low temperature and high density. This string-like colloidal assembly is related to percolation phenomena. Analyzing the cluster size distribution and the average string length, we build phase diagrams and discover that the average string length diverges around the region where the melting transition line and the percolation transition line cross. This result is similar to Ising spin systems, in which the percolation transition line and the order-disorder line meet at a critical point.

cond-mat.soft↗
arXiv2015

Differential capacitance of the electric double layer: The interplay between ion finite size and dielectric decrement

We study the electric double layer by combining the effects of ion finite size and dielectric decrement. At high surface potential, both mechanisms can cause saturation of the counter-ion concentration near a charged surface. The modified Grahame equation and differential capacitance are derived analytically for a general expression of a permittivity epsilon(n) that depends on the local ion concentration, n, and under the assumption that the co-ions are fully depleted from the surface. The concentration at counter-ion saturation is found for any epsilon(n), and a criterion predicting which of the two mechanisms (steric vs. dielectric decrement) is the dominant one is obtained. At low salinity, the differential capacitance as function of surface potential has two peaks (so-called camel-shape). Each of these two peaks is connected to a saturation of counter-ion concentration caused either by dielectric decrement or by their finite size. Because these effects depend mainly on the counter-ion concentration at the surface proximity, for opposite surface-potential polarity either the cations or anions play the role of counter-ions, resulting in an asymmetric camel-shape. At high salinity, we obtain and analyze the crossover in the differential capacitance from a double-peak shape to a uni-modal one. Finally, several nonlinear models of the permittivity decrement are considered, and we predict that the concentration at dielectrophoretic saturation shifts to higher concentration than those obtained by the linear decrement model.

cond-mat.soft↗
arXiv2020

Static and Dynamic Properties of Block-Copolymer Based Grafted Nanoparticles Across the Non-Ergodicity Transition

We present a systematic investigation of static and dynamic properties of block copolymer micelles with crosslinked cores, representing model polymer-grafted nanoparticles, over a wide concentration range from dilute regime to an arrested (crystalline) state, by means of light and neutron scattering, complemented by linear viscoelasticity. We have followed the evolution of their scattering intensity and diffusion dynamics throughout the non-ergodicity transition and the observed results have been contrasted against appropriately coarse-grained Langevin Dynamics simulations. These stable model soft particles of the core-shell type are situated between ultrasoft stars and hard spheres, and the well-known star pair interaction potential is not appropriate to describe them. Instead, we have found that an effective brush interaction potential provides very satisfactory agreement between experiments and simulations, offering insights into the interplay of softness and dynamics in spherical colloidal suspensions.

cond-mat.soft↗