title: Dynamic self-assembly of charged colloidal strings and walls in simple fluid flows.
abstract: Colloidal particles can self-assemble into various ordered structures in fluid flows that have potential applications in biomedicine, materials synthesis and encryption. These dynamic processes are also of fundamental interest for probing the general principles of self-assembly under non-equilibrium conditions. Here, we report a simple microfluidic experiment, where charged colloidal particles self-assemble into flow-aligned 1D strings with regular particle spacing near a solid boundary. Using high-speed confocal microscopy, we systematically investigate the influence of flow rates, electrostatics and particle polydispersity on the observed string structures. By studying the detailed dynamics of stable flow-driven particle pairs, we quantitatively characterize interparticle interactions. Based on the results, we construct a simple model that explains the intriguing non-equilibrium self-assembly process. Our study shows that the colloidal strings arise from a delicate balance between attractive hydrodynamic coupling and repulsive electrostatic interaction between particles. Finally, we demonstrate that, with the assistance of transverse electric fields, a similar mechanism also leads to the formation of 2D colloidal walls.
authors.affiliation: Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA. xcheng@umn.edu and Films & Film Products Research Laboratories, Toray Industries, Inc, 1-1, Sonoyama 1-chome, Otsu, Shiga 520-8558, Japan.
authors.forename: Yu
authors.initials: Y
authors.lastname: Abe
authors.affiliation: Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA. xcheng@umn.edu.
authors.forename: Bo
authors.initials: B
authors.lastname: Zhang
authors.affiliation: Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA. xcheng@umn.edu.
authors.forename: Leonardo
authors.identifier: http://orcid.org/0000-0002-9516-1346
authors.initials: L
authors.lastname: Gordillo
authors.affiliation: Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA. xcheng@umn.edu.
authors.forename: Alireza Mohammad
authors.identifier: http://orcid.org/0000-0002-2031-9057
authors.initials: AM
authors.lastname: Karim
authors.affiliation: Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA. xcheng@umn.edu.
authors.forename: Lorraine F
authors.initials: LF
authors.lastname: Francis
authors.affiliation: Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA. xcheng@umn.edu.
authors.forename: Xiang
authors.identifier: http://orcid.org/0000-0002-2759-764X
authors.initials: X
authors.lastname: Cheng