Dielectric Effects in the Self-Assembly of Binary Colloidal Aggregates

Abstract

Electrostatic interactions play an important role in numerous self-assembly phenomena, including colloidal aggregation. Although colloids typically have a dielectric constant that differs from the surrounding solvent, the effective interactions that arise from inhomogeneous polarization charge distributions are generally neglected in theoretical and computational studies. We introduce an efficient technique to resolve polarization charges in dynamical dielectric geometries, and demonstrate that dielectric effects qualitatively alter the predicted self-assembled structures, with surprising colloidal strings arising from many-body effects.PACS numbers: 77.84. Nh, 82.70.Dd, 61.20.Ja, 77.22.Ej Colloids are ubiquitous in systems of physical, chemical, and biological interest. In suspension, dissociation of surface groups frequently causes these particles to carry an electrical charge, resulting in electrostatic interactions that play an important role in colloidal stability, aggregation, and self-assembly [1][2][3]. Far less is known about the effect of induced polarization charges. Although molecular dynamics (MD) and Monte Carlo simulations of charged colloids are now commonplace, they rarely take into account dielectric effects and instead treat the dielectric constant as spatially uniform. This is particularly striking in view of the large dielectric contrast between typical colloids and an aqueous solution (e.g., κ ≈ 2.5 for polystyrene vs. κ ≈ 80 for water at 293 K), which induces significant polarization charges at the colloidal surface. Densely packed and anisotropic arrangements of dielectric objects make this approximation even less justified. Thus, there is a pressing need to assess the role of dielectric effects in self-assembly phenomena.Proper treatment of dielectric effects has been limited by computational complexity. Only the simplest dielectric geometries permit analytical solution. For an interacting system of dielectric spheres, a series expansion has been derived [4], but this still requires expensive numerical evaluation. A more general approach is to numerically solve the induced bound charge self-consistently over discretized dielectric interfaces [5][6][7][8][9][10]. This approach does not constrain the geometry, but has not yet been efficient enough to allow simulation of dynamical dielectric objects, such as mobile colloids. Indeed, existing work has largely treated the dielectric geometry as static, focusing on ion distributions in planar [11][12][13] or spherical [14,15] geometries.In this Letter, we address this situation by presenting the first study of a dielectric system with a fully dynamic geometry, exploring the effect of polarization charges that respond to and influence the motion of charged colloids. Using an optimized simulation method [16] we explicitly demonstrate that dielectric interactions can qualitatively alter self-assembly in a prototypical size-asymmetric binary mixture of charged colloids in solution. In particular, polarization charge that binds a colloid pair ca...

Generic, phenomenological, on-the-fly renormalized repulsion model for self-limited organization of terminal supraparticle assemblies

“…In previous work (18,19,23), the cluster-cluster interaction was derived from the particle-particle interaction after the clusters had been formed. Only recently have Barros and Luijten proposed a numerical technique to resolve the polarization charge distributions in dynamic dielectric geometries arising during assembly (34). They demonstrated that dynamic polarization indeed alters--both qualitatively and quantitatively--predicted self-assembled structures.…”

Direct observation of particle interactions and clustering in charged granular streams

“…This enables the initiation of clustering at relative velocities much larger than the upper limit for sticking after a head-on collision, a long-standing issue known from pre-planetary dust aggregation 1,12 . Moreover, Coulomb interactions together with dielectric polarization are found to stabilize characteristic molecule-like configurations, providing new insights for the modelling of clustering dynamics in a wide range of microscopic dielectric systems, such as charged polarizable ions, biomolecules and colloids [13][14][15][16] . One of the key difficulties in studying the interplay between repulsive contact forces, short-range cohesion and long-range electrostatic forces during cluster formation has been to obtain sufficiently detailed experimental data.…”

“…The sum E 0 of the translational kinetic energy (in the centre-of-mass reference frame) and electrostatic potential energy determines whether r(t) forms an elliptical (E 0 < 0), parabolic (E 0 = 0), or hyperbolic (E 0 > 0) trajectory. For dielectric particles, important corrections arise from induced polarization 15,16,23 . These polarization forces are always attractive, becoming increasingly important at close approach.…”

Computer simulations of single particles in external electric fields

“…For example, Barros et al [40] proposed to use a Generalized Minimum Residual method [41] to calculate the distribution of surface bound charges on nanoparticles and applied this approach to study the self-assembly of binary colloids. They observed an unexpected string structure formation due to the dielectric effect [42]. Boundary methods such as that sketched above also open up the possibility to study phenomena associated with inducedcharge electrokinetics [43].…”