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Achille Giacometti

Publications and source records attributed to Achille Giacometti.

At least 73 records · Page 4Linked to original sources
arXiv2013

Structure and phase behavior of colloidal dumbbells with tunable attractive interactions

We investigate thermodynamic and structural properties of colloidal dumbbells in the framework provided by the Reference Interaction Site Model (RISM) theory of molecular fluids and Monte Carlo simulations. We consider two different models: in the first one we set identical square-well attractions on the two tangent spheres composing the molecule (SW-SW model); in the second scheme, one of square-well interactions is switched off (HS-SW model). Appreciable differences emerge between the physical properties of the two models. Specifically, the $k \to 0$ behavior of SW-SW structure factors $S(k)$ points to the presence of a gas-liquid coexistence, as confirmed by subsequent fluid phase equilibria calculations. Conversely, the HS-SW $S(k)$ develops a low-$k$ peak, signaling the presence of aggregates; such a process destabilizes the gas-liquid phase separation, promoting at low temperatures the formation of a cluster phase, whose structure depends on the system density. We further investigate such differences by studying the phase behavior of a series of intermediate models, obtained from the original SW-SW by progressively reducing the depth of one square-well interaction. RISM structural predictions positively reproduce the simulation data, including the rise of $S(k \to 0$) in the SW-SW model and the low-$k$ peak in the HS-SW structure factor. As for the phase behavior, RISM agrees with Monte Carlo simulations in predicting a gas-liquid coexistence for the SW-SW model (though the critical parameters appears overestimated by the theory) and its progressive disappearance moving toward the HS-SW model.

cond-mat.soft↗
arXiv2013

The isotropic-to-nematic phase transition in hard helices: theory and simulation

We investigate the isotropic-to-nematic phase transition in systems of hard helical particles, using Onsager theory and Monte Carlo computer simulations. Motivation of this work resides in the ubiquity of the helical shape motif in many natural and synthetic polymers, as well as in the well known importance that the details of size and shape have in determining the phase behaviour and properties of (soft) condensed matter systems. We discuss the differences with the corresponding spherocylinder phase diagram and find that the helix parameters affect the phase behaviour and the existence of the nematic phase. We find that for high helicity Onsager theory significantly departs from numerical simulations even when a modifed form of the Parsons-Lee rescaling is included to account for the non-convexity of particles.

cond-mat.soft↗
arXiv2013

Janus fluid with fixed patch orientations: theory and simulations

We study thermophysical properties of a Janus fluid with constrained orientations, using analytical techniques and numerical simulations. The Janus character is modeled by means of a Kern-Frenkel potential where each sphere has one hemisphere of square-well and the other of hard-sphere character. The orientational constraint is enforced by assuming that each hemisphere can only point either North or South with equal probability. The analytical approach hinges on a mapping of the above Janus fluid onto a binary mixture interacting via a "quasi" isotropic potential. The anisotropic nature of the original Kern-Frenkel potential is reflected by the asymmetry in the interactions occurring between the unlike components of the mixture. A rational-function approximation extending the corresponding symmetric case is obtained in the sticky limit, where the square-well becomes infinitely narrow and deep, and allows a fully analytical approach. Notwithstanding the rather drastic approximations in the analytical theory, this is shown to provide a rather precise estimate of the structural and thermodynamical properties of the original Janus fluid.

cond-mat.soft↗
arXiv2012

One dimensional Potts model with many-body interactions and the Generalized Model of Polypeptide Chain for the helix-coil transition

Helix-coil transition in polypeptides is an example of a spin model with a preferred spin direction, in the sense that a theoretical formulation of this problem requires to assign a preferred value of spin to the helical conformation in order to account for different symmetries of the helical {\sl vs.} the coil states. This leads to the spin Hamiltonian of the {\sl Generalized Model of Polypeptide Chain} (GMPC) variety as opposed to the Potts model variety, both with many-body interactions. We compare the explicit solution of the Potts model and the solution of the GMPC within the transfer-matrix formalism. Comparison of both secular equations reveals that the largest eigenvalue of the Potts model with $Δ$ many-body interactions is identical to the largest eigenvalue of the GMPC model with $Δ-1$ many-body interactions, indicating the equivalence of both free energies. In distinction, the second largest eigenvalues do not coincide, leading to different thermal behavior of the spatial correlation length, related to the helix-coil transition interval. Spin models with built-in spin anisotropy thus engender different physical properties in the thermodynamic limit that we explore in detail.

cond-mat.stat-mech↗
arXiv2012

Self-assembly mechanism in colloids: perspectives from Statistical Physics

Motivated by recent experimental findings in chemical synthesis of colloidal particles, we draw an analogy between self-assembly processes occurring in biological systems (e.g. protein folding) and a new exciting possibility in the field of material science. We consider a self-assembly process whose elementary building blocks are decorated patchy colloids of various types, that spontaneously drive the system toward a unique and predetermined targeted macroscopic structure. To this aim, we discuss a simple theoretical model -- the Kern-Frenkel model -- describing a fluid of colloidal spherical particles with a pre-defined number and distribution of solvophobic and solvophilic regions on their surface. The solvophobic and solvophilic regions are described via a short-range square-well and a hard-sphere potentials, respectively. Integral equation and perturbation theories are presented to discuss structural and thermodynamical properties, with particular emphasis on the computation of the fluid-fluid (or gas-liquid) transition in the temperature-density plane. The model allows the description of both one and two attractive caps, as a function of the fraction of covered attractive surface, thus interpolating between a square-well and a hard-sphere fluid, upon changing the coverage. By comparison with Monte Carlo simulations, we assess the pros and the cons of both integral equation and perturbation theories in the present context of patchy colloids, where the computational effort for numerical simulations is rather demanding.

cond-mat.soft↗
arXiv2012

Osmotic pressure induced coupling between cooperativity and stability of a helix-coil transition

Most helix-coil transition theories can be characterized by a set of three parameters: energetic, describing the (free) energy cost of forming a helical state in one repeating unit; entropic, accounting for the decrease of entropy due to the helical state formation; and geometric, indicating how many repeating units are affected by the formation of one helical state. Depending on their effect on the helix-coil transition, solvents or co-solutes can be classified with respect to their action on these parameters. Solvent interactions that alter the entropic cost of helix formation by their osmotic action can affect both the stability (transition temperature) and the cooperativity (transition interval) of the helix-coil transition. A consistent inclusion of osmotic pressure effects in a description of helix-coil transition for poly(L-glutamic acid) in solution with polyethylene glycol can offer an explanation of the experimentally observed linear dependence of transition temperature on osmotic pressure as well as the concurrent changes in the cooperativity of the transition.

q-bio.BM↗
arXiv2012

Coil-globule transition of a homopolymer chain in a square-well potential: Comparison between Monte Carlo canonical replica exchange and Wang-Landau sampling

We study the equilibrium properties of a flexible homopolymer where consecutive monomers are represented by impenetrable hard spheres that are tangent to each other, and non-consecutive monomers interact via a square-well potential. To this aim, we use both replica exchange canonical simulations and micro-canonical Wang-Landau techniques for relatively short chains, and perform a close comparative analysis of the corresponding results. These investigations are then further exploited to reproduce, at a much shorter scale and, hence, computational effort, the phase diagram previously studied with much longer chains. This opens up the possibility of improving the model and introduce specificities typical, among other examples, of protein folding.

cond-mat.soft↗
arXiv2012

Fluid-Fluid and Fluid-Solid transitions in the Kern-Frenkel model from Barker-Henderson thermodynamic perturbation theory

We study the Kern-Frenkel model for patchy colloids using Barker-Henderson second-order thermodynamic perturbation theory. The model describes a fluid where hard sphere particles are decorated with one patch, so that they interact via a square-well (SW) potential if they are sufficiently close one another, and if patches on each particle are properly aligned. Both the gas-liquid and fluid-solid phase coexistences are computed and contrasted against corresponding Monte-Carlo simulations results. We find that the perturbation theory describes rather accurately numerical simulations all the way from a fully covered square-well potential down to the Janus limit (half coverage). In the region where numerical data are not available (from Janus to hard-spheres), the method provides estimates of the location of the critical lines that could serve as a guideline for further efficient numerical work at these low coverages. A comparison with other techniques, such as integral equation theory, highlights the important aspect of this methodology in the present context.

cond-mat.soft↗
arXiv2011

The penetrable square-well model: extensive versus non-extensive phase

The phase diagram of the penetrable square-well fluid is investigated through Monte Carlo simulations of various nature. This model was proposed as the simplest possibility of combining bounded repulsions at short scale and short-range attractions. We prove that the model is thermodynamically stable for sufficiently low values of the penetrability parameter, and in this case the system behaves similarly to the square-well model. For larger penetration, there exists an intermediate region where the system is metastable, with well defined fluid-fluid and fluid-solid transitions, at finite size, but eventually becomes unstable in the thermodynamic limit. We characterize the unstable non-extensive phase appearing at high penetrability, where the system collapses into an isolated blob of a few clusters of many ovelapping particles each.

cond-mat.soft↗
arXiv2011

Effect of Polydispersity and Anisotropy in Colloidal and Protein Solutions: an Integral Equation Approach

Application of integral equation theory to complex fluids is reviewed, with particular emphasis to the effects of polydispersity and anisotropy on their structural and thermodynamic properties. Both analytical and numerical solutions of integral equations are discussed within the context of a set of minimal potential models that have been widely used in the literature. While other popular theoretical tools, such as numerical simulations and density functional theory, are superior for quantitative and accurate predictions, we argue that integral equation theory still provides, as in simple fluids, an invaluable technique that is able to capture the main essential features of a complex system, at a much lower computational cost. In addition, it can provide a detailed description of the angular dependence in arbitrary frame, unlike numerical simulations where this information is frequently hampered by insufficient statistics. Applications to colloidal mixtures, globular proteins and patchy colloids are discussed, within a unified framework.

cond-mat.soft↗
arXiv2011

Competition for hydrogen bond formation in the helix-coil transition and protein folding

The problem of the helix-coil transition of biopolymers in explicit solvents, like water, with the ability for hydrogen bonding with solvent is addressed analytically using a suitably modified version of the Generalized Model of Polypeptide Chains. Besides the regular helix-coil transition, an additional coil-helix or reentrant transition is also found at lower temperatures. The reentrant transition arises due to competition between polymer-polymer and polymer-water hydrogen bonds. The balance between the two types of hydrogen bonding can be shifted to either direction through changes not only in temperature, but also by pressure, mechanical force, osmotic stress or other external influences. Both polypeptides and polynucleotides are considered within a unified formalism. Our approach provides an explanation of the experimental difficulty of observing the reentrant transition with pressure; and underscores the advantage of pulling experiments for studies of DNA. Results are discussed and compared with those reported in a number of recent publications with which a significant level of agreement is obtained.

cond-mat.soft↗
arXiv2011

Phase diagram of the penetrable square well-model

We study a system formed by soft colloidal spheres attracting each other via a square-well potential, using extensive Monte Carlo simulations of various nature. The softness is implemented through a reduction of the infinite part of the repulsive potential to a finite one. For sufficiently low values of the penetrability parameter we find the system to be Ruelle stable with square-well like behavior. For high values of the penetrability the system is thermodynamically unstable and collapses into an isolated blob formed by a few clusters each containing many overlapping particles. For intermediate values of the penetrability the system has a rich phase diagram with a partial lack of thermodynamic consistency.

cond-mat.soft↗
arXiv2010

Cluster theory of Janus particles

We apply a simple statistical mechanics cluster approximation for studying clustering in the Kern and Frenkel model of Janus fluids. The approach is motivated by recent Monte Carlo simulations work on the same model revealing that the vapor coexisting with the liquid phase contains clusters of different sizes and shapes whose equilibrium concentrations in general depend on the interaction range as well as on thermodynamic parameters. The approximation hinges on a separation between the intra- and inter-cluster contribution to thermodynamics, where only the former is explicitly computed by Monte Carlo simulations. Two levels of a simple liquid theory approximation are exploited for the description of the latter. In the first we use the ideal-gas expressions and obtain a qualitative agreement with extensive Monte Carlo bulk simulations. This can be improved to a semi-quantitative agreement, by using a hard-sphere description for the cluster-cluster correlations.

cond-mat.soft↗
arXiv2010

A numerical study of one-patch colloidal particles: from square-well to Janus

We perform numerical simulations of a simple model of one-patch colloidal particles to investigate: (i) the behavior of the gas-liquid phase diagram on moving from a spherical attractive potential to a Janus potential and (ii) the collective structure of a system of Janus particles. We show that, for the case where one of the two hemispheres is attractive and one is repulsive, the system organizes into a dispersion of orientational ordered micelles and vesicles and, at low $T$, the system can be approximated as a fluid of such clusters, interacting essentially via excluded volume. The stability of this cluster phase generates a very peculiar shape of the gas and liquid coexisting densities, with a gas coexistence density which increases on cooling, approaching the liquid coexistence density at very low $T$.

cond-mat.soft↗
arXiv2010

Effects of patch size and number within a simple model of patchy colloids

We report on a computer simulation and integral equation study of a simple model of patchy spheres, each of whose surfaces is decorated with two opposite attractive caps, as a function of the fraction $χ$ of covered attractive surface. The simple model explored --- the two-patch Kern-Frenkel model --- interpolates between a square-well and a hard-sphere potential on changing the coverage $χ$. We show that integral equation theory provides quantitative predictions in the entire explored region of temperatures and densities from the square-well limit $χ= 1.0$ down to $χ\approx 0.6$. For smaller $χ$, good numerical convergence of the equations is achieved only at temperatures larger than the gas-liquid critical point, where however integral equation theory provides a complete description of the angular dependence. These results are contrasted with those for the one-patch case. We investigate the remaining region of coverage via numerical simulation and show how the gas-liquid critical point moves to smaller densities and temperatures on decreasing $χ$. Below $χ\approx 0.3$, crystallization prevents the possibility of observing the evolution of the line of critical points, providing the angular analog of the disappearance of the liquid as an equilibrium phase on decreasing the range for spherical potentials. Finally, we show that the stable ordered phase evolves on decreasing $χ$ from a three-dimensional crystal of interconnected planes to a two-dimensional independent-planes structure to a one-dimensional fluid of chains when the one-bond-per-patch limit is eventually reached.

cond-mat.soft↗
arXiv2009

Local orientational ordering in fluids of spherical molecules with dipolar-like anisotropic adhesion

We discuss some interesting physical features stemming from our previous analytical study of a simple model of a fluid with dipolar-like interactions of very short range in addition to the usual isotropic Baxter potential for adhesive spheres. While the isotropic part is found to rule the global structural and thermodynamical equilibrium properties of the fluid, the weaker anisotropic part gives rise to an interesting short-range local ordering of nearly spherical condensation clusters, containing short portions of chains having nose-to-tail parallel alignment which runs antiparallel to adjacent similar chains.

cond-mat.soft↗
arXiv2009

Phase diagram of Janus Particles

We deeply investigate a simple model representative of the recently synthesized Janus particles, i.e. colloidal spherical particles whose surface is divided into two areas of different chemical composition. When the two surfaces are solvophilic and solvophobic, these particles constitute the simplest example of surfactants. The phase diagram includes a colloidal-poor (gas) colloidal-rich (liquid) de-mixing region, which is progressively suppressed by the insurgence of micelles, providing the first model where micellization and phase-separation are simultaneously observed. The coexistence curve is found to be negatively sloped in the temperature-pressure plane, suggesting that Janus particles can provide a colloidal system with anomalous thermodynamic behavior.

cond-mat.soft↗