Dynamic Self-Assembly and Patterns in Electrostatically Driven Granular Media

Abstract

We show that granular media consisting of metallic microparticles immersed in a poorly conducting liquid in strong DC electric field self-assemble a rich variety of novel phases. These phases include static precipitate: honeycombs and Wigner crystals; and novel dynamic condensate: toroidal vortices and pulsating rings. The observed structures are explained by the interplay between charged granular gas and electrohydrodynamic convective flows in the liquid.PACS numbers: 45.70.Qj,05.65.+b,47.15.Cb,47.55.Kf Understanding the unifying principles of self-assembly of complex systems such as macro-molecules [1], diblock copolymers [2], micro-magnetic systems [3], ensembles of charged particles [4] is the key to future advances in nanoscience. Large ensembles of small particles display fascinating collective behavior when they acquire an electric charge and respond to competing long-range electromagnetic and short-range contact forces. Many industrial technologies face the challenge of assembling and separating such single-or multi-component micro and nano-size ensembles. The dynamics of conducting microparticles in electric field in the air was studied in [5,6]. Phase transitions and clustering instability of the electrostatically driven granular gas were found. The studies of self-assembly of colloidal particles in aqueous solutions revealed the importance of self-induced electrohydrodynamic (EHD) convective flows on the formation of various precipitate states [4,7,8,9,10]. Ordered clusters of particles vibrated in liquid were studied in Ref. [11].In this Letter we report new dynamic phenomena occurring in granular gas in poorly conducting liquid subject to strong electric field (up to 20 kV/cm). We show that metallic particles (120 µm diameter Bronze spheres) immersed in a toluene-ethanol mixture in DC electric field self-assemble into a rich variety of novel phases. These phases include static precipitates: honeycombs and Wigner crystals; and novel dynamic condensates: toroidal vortices and pulsating rings (Figs. 1 and 2). The observed phenomena are attributed to interaction between particles and EHD flows produced by the action of the electric field on ionic charges in the bulk of liquid. This provides a new mechanism for self-assembly of microparticles in non-aqueous solutions.To form the electro-cell, granular media consisting of 3 g (about 0.5 × 10 6 ) mono-dispersed 120 µm bronze spheres was placed into a 1.5 mm gap between two horizontal 12.5×12.5 cm glass plates covered by a transparent conducting layer of indium tin-dioxide (the particles constitute less than a monolayer coverage on the bottom plate). An electric field perpendicular to the plates was created by a DC high voltage source (0-3 kV) connected to the inner surface of each plate [5,6]. The liquid was introduced into the cell through two teflon microcapillaries connected to opposing side walls of the cell. Real time images were acquired using a high speed, up to 1000 frames per second, digital camera suspended over the transparent glass plates of...

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