Margarida M. Telo da Gama

Properties of patchy colloidal particles close to a surface: A Monte Carlo and density functional study

We investigate the behavior of a patchy particle model close to a hard-wall via Monte Carlo simulation and density functional theory (DFT). Two DFT approaches, based on the homogeneous and inhomogeneous versions of Wertheims first order perturbation theory for the association free energy are used. We evaluate, by simulation and theory, the equilibrium bulk phase diagram of the fluid and analyze the surface properties for two isochores, one of which is close to the liquid side of the gas-liquid coexistence curve. We find that the density profile near the wall crosses over from a typical high-temperature adsorption profile to a low-temperature desorption one, for the isochore close to coexistence. We relate this behavior to the properties of the bulk network liquid and find that the theoretical descriptions are reasonably accurate in this regime. At very low temperatures, however, an almost fully bonded network is formed, and the simulations reveal a second adsorption regime which is not captured by DFT. We trace this failure to the neglect of orientational correlations of the particles, which are found to exhibit surface induced orientational order in this regime.

Bicontinuous and mixed gels in binary mixtures of patchy colloidal particles

We investigate the thermodynamics and percolation regimes of model binary mixtures of patchy colloidal particles. The particles of each species have three sites of two types, one of which promotes bonding of particles of the same species while the other promotes bonding of different species. We find up to four percolated structures at low temperatures and densities: two gels where only one species percolates, a mixed gel where particles of both species percolate but neither species percolates separately, and a bicontinuous gel where particles of both species percolate separately forming two interconnected networks. The competition between the entropy and the energy of bonding drives the stability of the different percolating structures. Appropriate mixtures exhibit one or more connectivity transitions between the mixed and bicontinuous gels, as the temperature and/or the composition changes.

Phase diagrams of binary mixtures of patchy colloids with distinct numbers of patches: the network fluid regime

We calculate the network fluid regime and phase diagrams of binary mixtures of patchy colloids, using Wertheims first order perturbation theory and a generalization of Flory–Stockmayers theory of polymerization. The colloids are modelled as hard spheres with the same diameter and surface patches of the same type, A. The only difference between species is the number of their patches – or functionality – f(1)A and f(2)A (with f(2)A > f(1)A). We have found that the difference in functionality is the key factor controlling the behaviour of the mixture in the network (percolated) fluid regime. In particular, when f(2)A ≥ 2f(1)A the entropy of bonding drives the phase separation of two network fluids, which is absent in other mixtures. This drastically changes the critical properties of the system and drives a change in the topology of the phase diagram (from type I to type V) when f(1)A > 2. The difference in functionality also determines the miscibility at high (osmotic) pressures. If f(2)A − f(1)A = 1, the mixture is completely miscible at high pressures, while closed miscibility gaps at pressures above the highest critical pressure of the pure fluids are present if f(2)A − f(1)A > 1. We argue that this phase behaviour is driven by a competition between the entropy of mixing and the entropy of bonding, as the latter dominates in the network fluid regime.

Computing the phase diagram of binary mixtures: A patchy particle case study

We investigate the phase behaviour of 2D mixtures of bi-functional and three-functional patchy particles and 3D mixtures of bi-functional and tetra-functional patchy particles by means of Monte Carlo simulations and Wertheim theory. We start by computing the critical points of the pure systems and then we investigate how the critical parameters change upon lowering the temperature. We extend the successive umbrella sampling method to mixtures to make it possible to extract information about the phase behaviour of the system at a fixed temperature for the whole range of densities and compositions of interest.

PHASE EQUILIBRIA OF MODEL TERNARY MIXTURES : THEORY AND COMPUTER SIMULATION

We report the study of the phase diagram of a three-dimensional continuum model of symmetrical ternary amphiphilic mixtures, representing water, oil, and surfactant, using mean-field approximations as well as Monte Carlo simulations. In line with the results of various lattice models, the continuum model exhibits a region of three-(isotropic) liquid-phase coexistence consisting of water-rich, oil-rich, and surfactant-rich phases. The dependence of the phase diagram on the strength of the anisotropic water–(oil–)surfactant interactions is investigated using a modified mean-field approximation that takes into account, at the lowest level of approximation, the contribution of the water–(oil–)surfactant correlations. The phase behavior of the model ternary mixture is further examined using Monte Carlo simulation techniques in the semigrand canonical ensemble. The results of the simulations for symmetrical mixtures are consistent with the existence of a region of three-(isotropic) liquid-phase coexistence below a tricritical point. This region is analyzed in more detail using the Gibbs Monte Carlo simulation technique. It is shown that the simulation results are in qualitative agreement with the theoretical predictions.

Pathways to folding, nucleation events, and native geometry

We perform extensive Monte Carlo simulations of a lattice model and the Gō potential [N. Gō and H. Taketomi, Proc. Natl. Acad. Sci. U.S.A. 75, 559563 (1978)] to investigate the existence of folding pathways at the level of contact cluster formation for two native structures with markedly different geometries. Our analysis of folding pathways revealed a common underlying folding mechanism, based on nucleation phenomena, for both protein models. However, folding to the more complex geometry (i.e., that with more nonlocal contacts) is driven by a folding nucleus whose geometric traits more closely resemble those of the native fold. For this geometry folding is clearly a more cooperative process.

The Gō model revisited: Native structure and the geometric coupling between local and long-range contacts

Monte Carlo simulations show that long‐range interactions play a major role in determining the folding rates of 48‐mer three‐dimensional lattice polymers modeled by the Gō potential. For three target structures with different native geometries we found a sharp increase in the folding time when the relative contribution of the long‐range interactions to the native states energy is decreased from ∼50% towards zero. However, the dispersion of the simulated folding times is strongly dependent on native geometry and Gō polymers folding to one of the target structures exhibits folding times spanning three orders of magnitude. We have also found that, depending on the target geometry, a strong geometric coupling may exist between local and long‐range contacts, which means that, when this coupling exists, the formation of long‐range contacts is forced by the previous formation of local contacts. The absence of a strong geometric coupling results in a kinetics that is more sensitive to the interaction energy parameters; in this case, the formation of local contacts is not capable of promoting the establishment of long‐range ones when the latter are strongly penalized energetically and this results in longer folding times. Proteins 2005.

The effect of anchoring on the nematic flow in channels

Understanding the flow of liquid crystals in microfluidic environments plays an important role in many fields, including device design and microbiology. We perform hybrid lattice-Boltzmann simulations of a nematic liquid crystal flowing under an applied pressure gradient in two-dimensional channels with various anchoring boundary conditions at the substrate walls. We investigate the relationship between the flow rate and the pressure gradient and the corresponding profile of the nematic director, and find significant departures from the linear Poiseuille relationship. We also identify a morphological transition in the director profile and explain this in terms of an instability in the dynamical equations. We examine the qualitative and quantitative effects of changing the type and strength of the anchoring. Understanding such effects may provide a useful means of quantifying the anchoring of a substrate by measuring its flow properties.

Non-equilibrium adsorption of 2AnB patchy colloids on substrates

We study the irreversible adsorption of spherical 2AnB patchy colloids (with two A-patches on the poles and n B-patches along the equator) on a substrate. In particular, we consider dissimilar AA, AB, and BB binding probabilities. We characterize the patch–colloid network and its dependence on n and on the binding probabilities. Two growth regimes are identified with different density profiles and we calculate a growth mode diagram as a function of the colloid parameters. We also find that, close to the substrate, the density of the network, which depends on the colloid parameters, is characterized by a depletion zone.

Nonequilibrium self-organization of colloidal particles on substrates: adsorption, relaxation, and annealing

Colloidal particles are considered ideal building blocks to produce materials with enhanced physical properties. The state-of-the-art techniques for synthesizing these particles provide control over shape, size, and directionality of the interactions. In spite of these advances, there is still a huge gap between the synthesis of individual components and the management of their spontaneous organization towards the desired structures. The main challenge is the control over the dynamics of self-organization. In their kinetic route towards thermodynamically stable structures, colloidal particles self-organize into intermediate (mesoscopic) structures that are much larger than the individual particles and become the relevant units for the dynamics. To follow the dynamics and identify kinetically trapped structures, one needs to develop new theoretical and numerical tools. Here we discuss the self-organization of functionalized colloids (also known as patchy colloids) on attractive substrates. We review our recent results on the adsorption and relaxation and explore the use of annealing cycles to overcome kinetic barriers and drive the relaxation towards the targeted structures.