Low-Density Ordered Phase in Brownian Dipolar Colloidal Suspensions

Abstract

We study the low volume fraction and electric field phase behavior of a Brownian colloidal suspension. On the application of a uniform ac field, we find a novel phase where chains of particles aggregate to form a well defined cellular network, consisting of particle-free ''voids'' surrounded by a percolating network of particle-rich walls. This cellular structure is stable to very long times, indicative of an equilibrium thermodynamic phase. The cell-cell spacing is not sensitive to the concentration of the sample but scales with sample thickness. Any self-consistent mechanism for the existence of this void phase must consist of long-ranged repulsions and shorter-ranged attractions. DOI: 10.1103/PhysRevLett.102.198301 PACS numbers: 47.57.JÀ, 64.70.pv Brownian colloidal suspensions with tunable interactions are important model systems for understanding phase transitions in atomic systems [1][2][3][4]. Real-space microscopy and light scattering experiments have been carried out on crystal nucleation dynamics [5,6] as well as structure and dynamics at the colloidal glass transition [7][8][9]. In the presence of an external electric field, Brownian colloidal spheres experience an anisotropic dipolar interaction and form chains (the ''string fluid'') and multichain aggregates [10,11], as well as body centered tetragonal (BCT) crystals [12][13][14]. Multiple competing interactions lead to richer phase behavior. In colloid-polymer mixtures both network-forming gel phases and glassy behavior can be observed by control of competing attractive and repulsive interactions [15,16]. Anisotropic interactions between colloids [17] in a liquid crystal medium also lead to a network of particle aggregates [18]. In dipolar colloids with added electrostatic repulsions, numerous crystalline phases are observed [19,20]. The dipolar interaction is of great interest because it consists of both competing on-axis (along the electric field) attractions and in-plane repulsions, and because it is an important driving force for nanoparticle selforganization [21].Simulations of spheres with both dipolar and van der Waals interactions have shown many variants (linear aggregates, droplets, columns) of phase separation into regions of higher and lower densities [22,23]. However, structure formation in the low-density regime of Brownian colloidal suspensions in an electric field has not attracted much experimental attention. Recently, interesting field induced cellular structures were reported in a granular medium [24], but the mechanism (and whether they represented equilibrium structures) was not clear.In this Letter, we report real-space confocal microscopy studies of the effect of an ac external electric field on the low-density structure of Brownian colloidal suspensions. We report the existence of a novel ''gel-like'' phase at low volume fractions ( < 4:0%) where chains of particles aggregate in time to form a cellular structure composed of ''voids''-particle-free domains enclosed completely by particle-rich walls. We propose that the co...

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