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.
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We thank Pranav Agrawal, Kevin Dorfman, Woo Jin Hyun, Satish Kumar, Truong Pham, Yi Peng and Seunghwan Shin for their help with experiments and fruitful discussions. This research was supported by the University of Minnesota Industrial Partnership for Research in Interfacial and Materials Engineering (IPRIME). Y. A. acknowledges the financial support from the Toray Industries. The research was also partially supported by the David & Lucile Packard Foundation. L. G. was partially supported by Conicyt PAI/Postdoctorado Becas Chile 74150032. Portions of this work were performed in the University of Minnesota NanoFabrication Center, which receives partial support from the NSF through the NNIN.