Phase diagram and aggregation dynamics of a monolayer of paramagnetic colloids

Abstract

We have developed a tunable colloidal system and a corresponding theoretical model for studying the phase behavior of particles assembling under the influence of long-range magnetic interactions. A monolayer of paramagnetic particles is subjected to a spatially uniform magnetic field with a static perpendicular component and a rapidly rotating in-plane component. The sign and strength of the interactions vary with the tilt angle θ of the rotating magnetic field. For a purely in-plane field, θ=90^{∘}, interactions are attractive and the experimental results agree well with both equilibrium and out-of-equilibrium predictions based on a two-body interaction model. For tilt angles 50^{∘}≲θ≲55^{∘}, the two-body interaction gives a short-range attractive and long-range repulsive interaction, which predicts the formation of equilibrium microphases. In experiments, however, a different type of assembly is observed. Inclusion of three-body (and higher-order) terms in the model does not resolve the discrepancy. We further characterize the anomalous regime by measuring the time-dependent cluster size distribution.

Structure and kinetics in colloidal films with competing interactions

“…For two-dimensional (2D) systems, much previous simulation and theoretical work has shown how short-range attraction combined with longer-range repulsion, and even pure repulsion exhibiting multiscale features such as a "shoulder" at short length scale, can generate a complex plethora of structures in and out of equilibrium, including stripes, chains, clusters, and ordered crystals [12][13][14]. Similar patterns are seen in experimental systems such as magnetic particles and biological membranes [6,[15][16][17] and have been discussed in the general context of geometrically frustrated materials [18]. 2D systems are particularly interesting for applications such as structured membranes, filters, films, coatings, and biological materials.…”

Phase diagram of two-dimensional colloids with Yukawa repulsion and dipolar attraction

“…50, where it has been shown that three-body interactions become significant (up to ∼60% comparing to the pair energy) at distances smaller than two particle diameters, while the many-body effects of higher orders can be neglected. 32,35,50 Therefore, the relevant experimental conditions can be satisfied for 2D suspensions of particles having a magnetic core and nonmagnetic shell and exposed to rapidly rotating in-plane magnetic fields. The thickness of the nonmagnetic shell should not be (at least) about a half of the core diameter.…”

“…[1][2][3] Two-dimensional (2D) colloidal crystals self-assembled in external fields can act as seeds for 3D structures used in photonics [4][5][6][7][8] as well as for porous media and membranes used for photocatalysis, electrochemical energy storage and conversion, and chemical applications. [9][10][11][12][13] Although tunable interactions can be achieved in different ways (including optical, chemical, and flow-mediated mechanisms 2 ), the use of electric [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] and magnetic 16,[30][31][32][33][34][35][36][37][38][39][40][41] fields is among the most promising due to their technological flexibility, the long-range character of the obtained interactions, and the ability to change them in situ.…”

“…From a fundamental point of view, colloidal suspensions with tunable interactions allow us to perform particle-resolved studies 1,2,[42][43][44][45] to understand basic generic mechanisms of melting and crystallization, condensation and evaporation, spinodal decomposition, slow dynamics in glasses, nucleation, and coalescence, occurring in different regimes of interactions between particles. 1,26,43,[46][47][48] Dipolar attractions induced by external rotating fields have attracted interest in the framework of particle-resolved studies of 2D systems in magnetic 32,35,48 and electric 18,25,26,49 fields. These studies used 2D colloidal suspensions of particles, which were synchronously polarized by an in-plane rotating field, yielding isotropic dipolar attractions ∝1/r 3 at large distances, whose magnitude is determined by the field magnitude and the material properties of the solvent and colloids.…”

Equilibrium Structures and Stationary Patterns on Magnetic Colloidal Fluids

“…In this chapter, we review the studies of static structural and dynamical properties in colloidal magnetic fluids, which provide a comprehensive description of their bulk phase behavior at thermal equilibrium. Presently, model magnetic colloidal systems can be prepared with tunable interaction among particles of the hard sphere and longrange dipolar types [11][12][13][14], especially the colloidal system constituted of micronsize polystyrene spheres that are electrically charged and dispersed in organic solvents [11]. In such a system, particles acquire an induced electric dipole moment under the external static electric field.…”