Papers by Ramón Castañeda-Priego
arXiv (Cornell University), Sep 29, 2022
The interplay of phase separation and dynamical arrest can lead to the formation of gels and glas... more The interplay of phase separation and dynamical arrest can lead to the formation of gels and glasses, which is relevant for such diverse fields as hard and soft condensed matter physics, materials science, food engineering and pharmaceutical industry. Here, the non-equilibrium states as well as the interactions of globular proteins are analyzed. Lysozyme in brine is chosen as a model system with short-range attractions. The metastable gas-liquid binodal and the dynamical arrest line as well as the second virial coefficient B 2 have been determined for various solution conditions by cloud-point measurements, optical microscopy, centrifugation experiments and light scattering. If temperature is expressed in terms of B 2 , the binodals obtained under various conditions fall onto a master curve, as suggested by the extended law of corresponding states. Arrest lines for different salt concentrations overlap within experimental errors, whereas they do not overlap if the temperature axis is replaced by B 2 . This indicates that the binodals are not sensitive to the details of the potential, but can be described by one integral parameter, i.e. B 2 , whereas the arrest line appears governed by its attractive part. The experimental findings are supported by numerical results of the non-equilibrium self-consistent generalized Langevin equation theory.
Journal of Physics: Condensed Matter, Feb 21, 2022
The discrete hard-sphere (HS), square-well (SW), and square-shoulder (SS) potentials have become ... more The discrete hard-sphere (HS), square-well (SW), and square-shoulder (SS) potentials have become the battle horse of molecular and complex fluids because they contain the basic elements to describe the thermodynamic, structural, and transport properties of both types of fluids. The mathematical simplicity of these discrete potentials allows us to obtain some analytical results despite the nature and complexity of the modeled systems. However, the divergent forces arising at the potential discontinuities may lead to severe issues when discrete potentials are used in computer simulations with uniform time steps. One of the few routes to avoid these technical problems is to replace the discrete potentials with continuous and differentiable forms built under strict physical criteria to capture the correct phenomenology. The match of the second virial coefficient between the discrete and the soft potentials has recently been successfully used to construct a continuous representation that mimics some physical properties of HSs (Báez et al 2018 J. Chem. Phys. 149 164907). In this paper, we report an extension of this idea to construct soft representations of the discrete SW and SS potentials. We assess the accuracy of the resulting soft potential by studying structural and thermodynamic properties of the modeled systems by using extensive Brownian and molecular dynamics computer simulations. Besides, Monte Carlo results for the original discrete potentials are used as benchmark. We have also implemented the discrete interaction models and their soft counterparts within the integral equations theory of liquids, finding that the most widely used approximations predict almost identical results for both potentials.
arXiv (Cornell University), May 30, 2023
Brownian motion is a universal characteristic of colloidal particles embedded in a host medium, a... more Brownian motion is a universal characteristic of colloidal particles embedded in a host medium, and it is the fingerprint of molecular transport or diffusion, a generic feature of relevance not only in Physics but also in several branches of Science and Engineering. Since its discovery, Brownian motion or colloid dynamics has been important in elucidating the connection between the molecular details of the diffusing macromolecule and the macroscopic information of the host medium. However, colloid dynamics is far from being completely understood. For example, the diffusion of non-spherical colloids and the effects of geometry on the dynamics of either passive or active colloids are a few representative cases that are part of the current challenges in Soft Matter Physics. In this contribution, we take a step forward to introduce a covariant description of the colloid dynamics in curved spaces. This formalism will allow us to understand several phenomena, for instance, the effects of curvature on the kinetics during spinodal decomposition and the thermodynamic properties of the colloidal dispersion, just to mention a few examples. This theoretical framework will also serve as the starting point to highlight the role of geometry on colloid dynamics, an aspect that is of paramount importance to understanding more complex phenomena, such as the diffusive mechanisms of proteins embedded in cell membranes.
Journal of Chemical Physics, Aug 21, 2022
We introduce a general physical formulation that allows one to obtain uniquely the effective inte... more We introduce a general physical formulation that allows one to obtain uniquely the effective interactions between particles by contracting the bare forces, even in highly concentrated systems. We tested it by studying depletion forces in binary and ternary colloidal mixtures with a total packing fraction up to 55%. Our result opens up the possibility of finding an efficient route to determine effective interactions at finite concentration and even at thermodynamic conditions near to meta-stable or out of equilibrium states.
Journal of Chemical Physics, Dec 1, 2005
We study the effective interactions among large hard spherical colloidal particles induced by sma... more We study the effective interactions among large hard spherical colloidal particles induced by small hard rodlike particles and compare them with those induced by small hard spherical particles to highlight the specific effects due to the anisotropic shape of the former. This is done by determining the effective pair potentials within the framework of the reference interaction site model approach. The rodlike particles are modeled as N nonoverlapping spherical units arranged in a straight line, so that their total length is N times their transversal diameter. These results are compared against those obtained in the Asakura-Oosawa limit.
arXiv (Cornell University), Jun 8, 2022
The term single file (SF) dynamics refers to the motion of an assembly of particles through a cha... more The term single file (SF) dynamics refers to the motion of an assembly of particles through a channel with cross-section comparable to the particles' diameter. Single file diffusion (SFD) is then the diffusion of a tagged particle in a single file, i.e., under the condition that particle passing is not allowed. SFD accounts for a large variety of processes in nature, including diffusion of colloids in synthetic and natural-shaped channels, biological motors along molecular chains, electrons in proteins and liquid helium, ions through membrane, just to mention a few examples. Albeit introduced in '65, over the last decade the classical notion of SF dynamics has been generalized to account through a more realistic modeling of, among others, particles properties, file geometry, particle-particle and channel-particles interactions, thus paving the way to remarkable applications in, for example, the technology of bio-integrated nanodevices. We then provide a comprehensive review of the recent advances in the theory of SF dynamics and the ensuing experimental realisations.
Physica D: Nonlinear Phenomena, Oct 1, 2011
This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright
Physical Review E, Jan 2, 2007
Our interest goes to the different virial contributions to the equation of state of charged collo... more Our interest goes to the different virial contributions to the equation of state of charged colloidal suspensions. Neglect of surface effects in the computation of the colloidal virial term leads to spurious and paradoxical results. This pitfall is one of the several facets of the danger of a naive implementation of the so called One Component Model, where the micro-ionic degrees of freedom are integrated out to only keep in the description the mesoscopic (colloidal) degrees of freedom. On the other hand, due incorporation of wall induced forces dissolves the paradox brought forth in the naive approach, provides a consistent description, and confirms that for salt-free systems, the colloidal contribution to the pressure is dominated by the micro-ionic one. Much emphasis is put on the no salt case but the situation with added electrolyte is also discussed.
Journal of Chemical Physics, Mar 17, 2023
Journal of Physics: Condensed Matter, Sep 11, 2004
In this work we investigate the experimentally observed electrohydrodynamic instabilities in a co... more In this work we investigate the experimentally observed electrohydrodynamic instabilities in a confined suspension of lambda-DNA under strong electric fields. We model the underlying stochastic motion of the DNA coils on a coarse grained level, using continuous functions to describe the charge density of the system in space and time. We find, within our approach, that in contrast to previous
Journal of Chemical Physics, Dec 7, 2010
Recently, it has been demonstrated that electrostatic interactions between charged colloids in co... more Recently, it has been demonstrated that electrostatic interactions between charged colloids in contact with a symmetric electrolyte can accurately be described by using the so-called renormalized jellium (RJ) model proposed by Trizac and Levin.1 This mean-field approximation, based on the Poisson–Boltzmann equation (PBE), requires that the charge of the background around a tagged colloid be renormalized self-consistently to coincide with the colloid effective charge; such a requirement leads to precise thermodynamic properties even in highly charged colloidal suspensions.2–5 In this note, we revisit the renormalized jellium approximation and, in particular, provide a simple explicit recipe that facilitates the renormalization procedure being easy of implementing in different situations. We here apply such recipe in the case of charged spheres and discuss its extension to the case of charged rods. The system under consideration is a charged colloidal suspension of volume fraction η composed of, obviously, charged colloids of radius a, and counter ions in contact with a symmetric (1:1) salt reservoir of concentration 2cs , with cs the density of positive or negative salt ions; the solvent is included through the dielectric constant e. The RJ model assumes that the charge of Nc − 1 colloidal particles around a tagged macroion is smeared out in the whole suspension to form a homogeneous background with charge Zbacke, e being the elementary charge. This background charge is Zback = Zbare, with Zbare the colloidal bare charge, and not known a priori, but it is determined self-consistently to be equal to the effective charge, Zeff, of the tagged macroion.1 An extraordinary advantage of the RJ model is that Zeff is directly associated to the system osmotic pressure, the screening parameter, and the effective pair interaction between colloids (when it is explicitly considered at the level of the Yukawa approximation).1, 2 Nevertheless, since it is an explicit function of the system state, i.e., Zeff = Zeff(η, Zbare, cs), its evaluation depends on the specific conditions of the system and, currently, the entire charge renormalization procedure is carried out through an iterative process.6 This iterative procedure demands the construction of a function of the form Zeff = Zeff(Zback, Zbare). The selfconsistently condition is reached in the intersection of the function with the straight line Zeff = Zback. However, it is still possible to reformulate this scheme and avoid the full iterative procedure to gain clarity in the way in which the RJ can be applied and extended to study the physical properties of charge-stabilized colloidal suspensions. The complete original procedure can then be reformulated as follows. We start with the requirement of selfconsistency from the beginning. This means that the condition Zback = Zeff must be explicitly incorporated into the PBE. Our main assumption is that there exists a unique Zeff for a given Zbare; this avoids completely the inclusion of Zback and facilitates drastically the renormalization scheme. Additionally, one should also rephrase the typical boundary conditions (BCs) to solve the PBE with simple BCs at one point [see Eqs. (3) and (4)]. To achieve this, we use the fact that the farfield solution of the PBE has a Yukawa-like form that depends on Zeff. Therefore, in the case of charged spherical colloids, the PBE now reads d2φ dr2 + 2 r dφ dr = −3η ZeffλB a − ρ+(∞)e−φ + ρ−(∞)e, (1)
Physical Review E, May 25, 2006
Physical review, Dec 10, 2021
During the past decade, there has been a hot debate about the physical mechanisms that determine ... more During the past decade, there has been a hot debate about the physical mechanisms that determine when a colloidal dispersion approaches the gel transition. However, there is still no consensus on a possible unique route that leads to the conditions for the formation of a gel-like state. Based on gel states identified in experiments, Valadez-Pérez et al. [Phys. Rev. E 88, 060302(R) (2013)] proposed rigidity percolation as the precursor of colloidal gelation in adhesive hard-sphere dispersions with coordination number n b equal to 2.4. Although this criterion was originally established to describe mechanical transitions in network-forming molecular materials with highly directional interactions, it worked well to explain gel formation in colloidal suspensions with isotropic short-range attractive forces. Recently, this idea has also been used to account for the dynamical arrest experimentally observed in attractive spherocylinders. Then, by assuming that rigidity percolation also drives gelation in spherical colloids interacting with short-ranged and highly directional potentials, we locate the thermodynamic states where gelation seems to occur in dispersions made up of patchy colloids. To check whether the criterion n b = 2.4 also holds in patchy colloidal systems, we apply the so-called bond-bending analysis to determine the fraction of floppy modes at some percolating clusters. This analysis confirms that the condition n b = 2.4 is a good approximation to determine those percolating clusters that are either mechanically stable or rigid. Furthermore, our results point out that not all combinations of patches and coverages lead to a gel-like state. Additionally, we systematically study the structure and the cluster size distribution along those thermodynamic states identified as gels. We show that for high coverage values, the structure is very similar for systems that have the same coverage regardless the number or the position of the patches on the particle surface. Finally, by using dynamic Monte Carlo computer simulations, we calculate both the mean-square displacement and the intermediate scattering function at and in the neighborhood of the gel-like states.
Journal of Chemical Physics, Feb 11, 2011
Understanding clustering of complex fluids is of interest in material science because the formati... more Understanding clustering of complex fluids is of interest in material science because the formation of aggregates in the suspension leads to changes in the material properties. Recently, using a mixed closure relation and a thermodynamic self-consistency criterion, Bomont et al. have shown the temperature dependence, at a fixed density, of the cluster formation in systems with short-range attractions and longrange repulsions which are modeled with the hard-core double Yukawa potential. 1 In this communication, we provide evidence that the cluster formation is a common behavior in systems with competitive interactions. In particular, we demonstrate that, based on the same thermodynamic self-consistency criterion, equally accurate structural information is obtained irrespective of the chosen mixed closure relation. Additionally, we explore the dependence of the clustering on the density and potential parameters. Our findings are corroborated with Monte Carlo computer simulations.
Springer eBooks, 2003
We present a general method for constructing effective pair interaction potentials in colloidal s... more We present a general method for constructing effective pair interaction potentials in colloidal systems, which results from a contraction of the description of colloidal mixtures based on the integral equations theory of simple liquids. In order to illustrate its applicability, we calculate the entropy driven potentials in mixtures of hard spheres, as well as the screened coulombic interactions between charged particles. The accuracy of our results is pointed out by comparison with computer simulations for the case of entropy driven potentials and experimental data for the case of charged colloids.
Frontiers research topics, 2023
Physical Review-Section E-Statistical Nonlinear and Soft Matter Physics, 2011
Physical Review Letters, Mar 11, 2011
We measure the dynamical arrest transition in a model, thermoreversible, adhesive hard sphere dis... more We measure the dynamical arrest transition in a model, thermoreversible, adhesive hard sphere dispersion. At low volume fractions, φ, below the critical point, gelation occurs within the gas-liquid phase boundary. For φ slightly below and above the critical concentration, the phase boundary follows the predicted percolation transition. At high φ, it melds into the predicted attractive-driven glass transition. Our results demonstrate that for φ above ~ 20% physical gelation is an extension of the attractive-driven glass line and occurs without competition for macroscopic phase separation. Colloidal glasses are characterized by a dynamical arrest of the disperse phase and transition out of equilibrium resulting in a loss of ergodicity [1]. For hard spheres, the repulsive driven glass (RDG) transition is well known to occur at volume fraction φ ~ 0.58 [2]. The addition of a short-range attraction, as in the case of adhesive hard spheres (AHS), can induce an additional glassy state, an attractive driven glass (ADG) [3]. Whilst dynamical arrest at high concentrations can be well described with simulation techniques and mode coupling theory (MCT) the transition at intermediate φ, often termed gelation, is less clear [4]. In the recent literature there has been
Revista Mexicana De Fisica, 2007
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Papers by Ramón Castañeda-Priego