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Fabrication, folding, and propulsion of complex colloidal molecules under applied magnetic fields
Yang, Tao
Yang, Tao
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2018
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Directed colloidal assembly under external fields can be used to assemble building blocks into complicated structures that mimic complex molecules or structures found in nature with similar or even advanced functionality. As many biological molecules or structures including protein or flagella have semi-flexible or flexible backbones, permanent yet flexible bonds must be used in assembling structures to achieve comparable functionality such as protein folding or flagella propulsion. While other external fields such as electrical and optical fields can be used for assembly, magnetic fields provide non-invasive and deep penetration into the human body with minimal side effects. In this thesis, therefore we focus on the use of magnetic fields with one-dimensional (1D) magnetic chains and two-dimensional (2D) microwheels used as model assembly systems. We first explore the use of thiol-click reactions to build up flexible bonds between particles and demonstrate the tunability of both binding strength and chain flexibility over a wide range by varying linker length and reaction temperature. Furthermore, an excellent match between experimental results and predictions from classical polymer theory provides confidence that magnetic colloidal chains are excellent macroscopic analogues of synthetic polymers. With increased chain flexibility using large molecular weight hydrophilic polymers, we show that assembled chains can form closed rings or “lassos” in the presence of a planar rotating magnetic field. By adding an additional AC magnetic field along the direction perpendicular to the substrate, lassos can propel in a controllable fashion. We further demonstrate its use for reversible cargo transport and release without need for chemistry for attachment or disengagement. Extending to 2D assembly models, the translation of microwheels on both flat and topographic surfaces was also investigated. In this, we demonstrate that coupling between rotating wheel-shaped bots and nearby walls can be enhanced through surface topography. We show potential utility using gravitational potential energy barriers to separate isomers of different symmetry.
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