Because of their unique characteristics, colloids have been used to investigate the fundamental physics of soft materials including both equilibrium phase behavior and kinetic processes. Unlike atoms, colloidal sizes can be conveniently tailored and are typically large enough to be probed individually with interaction strengths and effective ranges that can be modulated over several orders of magnitude. Despite these significant advantages, only relatively simple colloidal models such as spheres have been created; such systems in turn assemble into crystals of fairly limited symmetry, precluding the study of problems associated with complex structure. To push towards the synthesis of more complicated colloidal molecules, we use combined applied magnetic and anisotropic optical fields to fabricate colloidal chains. By integrating these induced forces within microfluidic channels and in flow, we grow colloidal chains one particle at one time, mimicking step-growth polymerization. The key advantage of this method is the ability to precisely control chain length and sequence, both essential for studies of self-assembly. In this, chain length is determined by a balance between the hydrodynamic shear stress, applied magnetic field, and the optical forces applied. Once a desired chain length is achieved, we fix the assembly in situ via the use of thiol-functionalized magnetic beads and a functionalized polyethylene glycol crosslinker. With the ability to perform directed assembly and irreversible fixation in flow, a route to the high-throughput synthesis of colloidal molecules can be achieved.
Tao Yang, Ning Wu, and David W. M. Marr, "Combined magneto- and opto-fluidic assembly of colloidal chains of controllable length (Presentation Recording)," Proc. SPIE 9548, Optical Trapping and Optical Micromanipulation XII, 954819 (Presented at SPIE Nanoscience + Engineering: August 11, 2015; Published: 5 October 2015); https://doi.org/10.1117/12.2187146.4519370471001.
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