Self and collective correlation functions in a gel of tetrahedral patchy particles (original) (raw)
Understanding tetrahedral liquids through patchy colloids
We investigate the structural properties of a simple model for tetrahedral patchy colloids in which the patch width and the patch range can be tuned independently. For wide bond angles, a fully bonded network can be generated by standard Monte Carlo or molecular dynamics simulations of the model, providing a neat method for generating defect-free random tetrahedral networks. This offers the possibility of focusing on the role of the patch angular width on the structure of the fully bonded network. The analysis of the fully bonded configurations as a function of the bonding angle shows how the bonding angle controls the system compressibility, the strength of the pre-peak in the structure factor and ring size distribution. Comparison with models of liquid water and silica allows us to find the best mapping between these continuous potentials and the colloidal one. Building on previous studies focused on the connection between angular range and crystallization, the mapping makes it possible to shed new light on the glass-forming ability of network-forming tetrahedral liquids. arXiv:1309.2198v1 [cond-mat.soft]
Phase behaviour of pure and mixed patchy colloids — Theory and simulation
Current Opinion in Colloid & Interface Science, 2017
We review the phase behaviour of pure and mixed patchy colloids, as revealed (mostly) by theory and computer simulation. These experimentally-realisable systems are excellent models for investigating the general problem of the interplay between (equilibrium) phase transitions and self-assembly in soft condensed matter. We focus on how liquid-vapour condensation can be preempted by the formation of different types of aggregates, in particular rings, which we argue is relevant to the criticality of empty fluids and network fluids, and possibly also of dipolar fluids. In this connection we also discuss percolation and gelation in pure and mixed patchy colloids. Finally, we describe the rich phase behaviour of (mostly binary) patchy colloid mixtures.
Molecular Physics, 2009
We investigate the influence of strong directional, or bonding, interactions on the phase diagram of complex fluids, and in particular on the liquid-vapour critical point. To this end we revisit a simple model and theory for associating fluids which consists of spherical particles having a hardcore repulsion, complemented by three short-ranged attractive sites on the surface (sticky spots). Two of the spots are of type A and one is of type B; the interactions between each pair of spots have strengths ǫ AA , ǫ BB and ǫ AB. The theory is applied over the whole range of bonding strengths and results are interpreted in terms of the equilibrium cluster structures of the coexisting phases. In systems where unlike sites do not interact (i.e., where ǫ AB = 0), the critical point exists all the way to ǫ BB /ǫ AA = 0. By contrast, when ǫ BB = 0, there is no critical point below a certain finite value of ǫ AB /ǫ AA. These somewhat surprising results are rationalised in terms of the different network structures of the two systems: two long AA chains are linked by one BB bond (X-junction) in the former case, and by one AB bond (Y-junction) in the latter. The vapour-liquid transition may then be viewed as the condensation of these junctions and, we find that X-junctions condense for any attractive ǫ BB (i.e., for any fraction of BB bonds), whereas condensation of the Y-junctions requires that ǫ AB be above a finite threshold (i.e., there must be a finite fraction of AB bonds).
Structure and phase transitions in confined binary colloid mixtures
The Journal of Chemical Physics, 2003
We report the results of a study of crystallization in quasi-two-dimensional binary mixtures of large and small colloids. The experiments sample the parameter spaces of colloid particle diameter ratio, large particle density, and small particle packing fraction. The depletion potential between the large particles, induced by the presence of the small particles in the system, affects the density at which the large particles undergo a liquid-to-solid freezing transition. For systems with a large to small particle diameter ratio of 4.6, the addition of small particles increases the large particle liquidus transition density, a seemingly counterintuitive result given that the depletion potential is purely attractive when the small particle packing fraction is low. When the large to small particle diameter ratio is 8.8, the same trend in the large particle liquidus transition density is seen, but to a lesser extent. The other system properties for the system with diameter ratio 8.8 show the same trends as for the system with diameter ratio 4.6. Liquid-liquid phase separation is observed for binary mixtures with diameter ratios of 20 and 40. Although the particles used in our experiment can be well modeled as hard spheres, our results cannot be readily explained by extant descriptions of the depletion interaction developed for three-dimensional binary hard sphere mixtures. Inversion of the pair correlation functions obtained from our measurements yields a depletion interaction that is much stronger than predicted for the same densities and diameter ratio in a three-dimensional hard sphere mixture. Our results imply that the depletion interaction is strongly dependent on the degree of confinement of the system.
Crystallization and gelation in colloidal systems with short-ranged attractive interactions
Physical Review E, 2008
We systematically study the relationship between equilibrium and nonequilibrium phase diagrams of a system of short-ranged attractive colloids. Using Monte Carlo and Brownian dynamics simulations we find a window of enhanced crystallization that is limited at high interaction strength by a slowing down of the dynamics and at low interaction strength by the high nucleation barrier. We find that the crystallization is enhanced by the metastable gas-liquid binodal by means of a two-stage crystallization process. First, the formation of a dense liquid is observed and second the crystal nucleates within the dense fluid. In addition, we find at low colloid packing fractions a fluid of clusters, and at higher colloid packing fractions a percolating network due to an arrested gas-liquid phase separation that we identify with gelation. We find that this arrest is due to crystallization at low interaction energy and it is caused by a slowing down of the dynamics at high interaction strength. Likewise, we observe that the clusters which are formed at low colloid packing fractions are crystalline at low interaction energy, but glassy at high interaction energy. The clusters coalesce upon encounter.
Phase transitions in nanoconfined binary mixtures of highly oriented colloidal rods
Physical Chemistry Chemical Physics, 2010
We analyse a binary mixture of colloidal parallel hard cylindrical particles with identical diameters but dissimilar lengths L 1 and L 2 , with s = L 2 /L 1 = 3, confined by two parallel hard walls in a planar slit-pore geometry, using a fundamental-measure density functional theory. This model presents (arXiv:1002.0612v, accepted in Phys. Rev. E) nematic (N) and two types of smectic (S) phases, with first-and second-order N-S bulk transitions and S-S demixing, and surface behaviour at a single hard wall which includes complete wetting by the S phase mediated (or not) by an infinite number of surface-induced layering (SIL) transitions. In the present paper the effects of confinement on this model colloidal fluid mixture are studied. Confinement brings about profound changes in the phase diagram, resulting from competition between the three relevant length scales: pore width h, smectic period d and length ratio s. Four main effects are identified: (i) Second-order bulk N-S transitions are suppressed. (ii) Demixing transitions are weakly affected, with small shifts in the µ 1 − µ 2 (chemical potentials) plane. (iii) Confinement-induced layering (CIL) transitions occurring in the two confined one-component fluids in some cases merge with the demixing transition. (iv) Surface-induced layering (SIL) transitions occurring at a single surface as coexistence conditions are approached are also shifted in the confined fluid. Trends with pore size are analysed by means of complete µ 1 − µ 2 and p −x (pressure-mean pore composition) phase diagrams for particular values of pore size. This work, which is the first one to address the behaviour of liquid-crystalline mixtures under confinement, could be relevant as a first step to understand self-assembling properties of mixtures of metallic nanoparticles under external fields in restricted geometry.
Predicting the Phase Diagram of Two-Dimensional Colloidal Systems with Long-Range Interactions †
The Journal of Physical Chemistry B, 2006
The phase diagram of a two-dimensional model system for colloidal particles at the air-water interface was determined using Monte Carlo computer simulations in the isothermic-isobaric ensemble. The micrometerrange binary colloidal interaction has been modeled by hard disklike particles interacting via a secondary minimum followed by a weaker longer-range repulsive maximum, both of the order of k B T. The repulsive part of the potential drives the clustering of particles at low densities and low temperatures. Pinned voids are formed at higher densities and intermediate values of the surface pressure. The analysis of isotherms, translational and orientational correlation functions as well as structure factor gives clear evidence of the presence of a melting first-order transition. However, the melting process can be also followed by a metastable route through a hexatic phase at low surface pressures and low temperatures, before crystalization occurs at higher surface pressure.
We numerically investigate colloidal dimers with asymmetric interaction strengths to study how the interplay between molecular geometry, excluded volume effects and attractive forces determines the overall phase behavior of such systems. Specifically, our model is constituted by two rigidly-connected tangent hard spheres interacting with other particles in the first instance via identical square-well attractions. Then, one of the square-well interactions is progressively weakened, until only the corresponding bare hard-core repulsion survives, giving rise to a "Janus dumbbell" model. We investigate structure, thermodynamics and phase behavior of the model by means of successive umbrella sampling and Monte Carlo simulations. In most of the cases, the system behaves as a standard simple fluid, characterized by a gas-liquid phase separation, for sufficiently low temperatures. In these conditions we observe a remarkable linear scaling of the critical temperature as a function of the interaction strength.