Band Formation in Mixtures of Oppositely Charged Colloids Driven by an ac Electric Field

Abstract

We present experiments on pattern formation in a Brownian system of oppositely charged colloids driven by an ac electric field. Using confocal laser scanning microscopy we observe complete segregation of the two particle species into bands perpendicular to a field of sufficient strength when the frequency is in a well-defined range. Because of its Brownian nature the system spontaneously returns to the equilibrium mixture after the field is turned off. We show that band formation is linked to the time scale associated with collisions between particles moving in opposite directions. DOI: 10.1103/PhysRevLett.106.228303 PACS numbers: 82.70.Dd, 47.54.-r, 89.75.Kd Pattern formation is found on many different length scales and as a consequence is studied in molecular systems [1,2], geology [3], the animal kingdom [4,5], granular systems [6], and colloids [7]. Granular and colloidal systems are especially interesting since they can be studied in real space on the single-particle level due to their slow dynamics and relatively large size compared to atoms and molecules [8,9]. A visually compelling case of pattern formation is found in binary granular systems containing two different particles species that segregate when the mixture is subjected to mechanical agitation [10][11][12]. Recently, it was shown that periodically sheared granular suspensions [13] can undergo an out-of-equilibrium phasetransition from a dynamic fluctuating state into a quiescent ''absorbing state'', which can also be found in epidemics and on the surface of certain heterogeneous catalysts [14].Computer simulations have recently identified binary mixtures of colloidal particles driven in opposite directions by an external field as an interesting test bed for pattern formation in complex systems [15,16]. Such colloidal systems are all the more interesting because they are thermal systems: Brownian motion allows any induced nonequilibrium state to spontaneously relax back to the equilibrium mixture, and presents pattern formation as a competition between field-induced ordering and thermal disordering. Such a binary colloidal system is hard to realize in experiment, however, because of strict requirements placed on a sufficiently strong interaction with the driving field and the thermodynamic stability of the mixture. Nevertheless, we recently succeeded in developing a system of oppositely charged colloids that can be studied in 3D real space when exposed to an external electric field. We performed a combined experimental and Brownian dynamics simulation study, which showed that as a result of collisions between particles moving in opposite directions, the different particle species segregate into lanes parallel to the field [17]. We also demonstrated that the fraction of particles in a lanelike environment increased monotonically with field strength. Once particles were in a lane, fluctuations perpendicular to the field axis decreased and the electrophoretic mobility increased with respect to particles not in a lane.Here, we employ this binary ...

Co-Assembly of Oppositely Charged Particles into Linear Clusters and Chains of Controllable Length

“…In both cases, however, the large opposite charge of the two types of particles leads to the formation of permanent chains. In this respect our work differs from earlier studies of particle pattern formation in AC electric fields, such as the work on sterically stabilized binary suspensions of oppositely charged microparticles28 where the lanes formed upon application of the external field were temporary and disassembled on removing the electric field.…”

Electrophoresis of concentrated colloidal dispersions in low-polar solvents

“…Once the particles have arrived on the surface, they form a real image on the electronic template that can be made permanent by evaporation of the solvent, leaving the particles behind. Driven by the wish to realize soft interactions for micron-sized colloids [4,5], and the design of advanced materials like electronic ink [6], interest in interactions between charged particles in nonaqueous solvents has again increased in the last decade [7][8][9][10][11].…”

Exploiting non-equilibrium phase separation for self-assembly

“…In particular, mixtures of oppositely charged colloids on a substrate demix into striped patterns when an external electric AC field is applied. 3,4 Different patterns can be obtained by changing the frequency of the electric field. [5][6][7] Similar demixing behavior has been observed in granular systems under vibration and related model systems.…”