Collaborative Research: Self-regulated non-equilibrium assembly of chiral colloidal clusters via electrokinetic interactions

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Non-technical abstract Autonomous motion and transport of microscopic objects are essential for maintaining the bioactivities of all living species. Although natural systems have evolved to possess extremely delicate biochemical motors, the development of synthetic motors lags way behind in complexity and efficiency. This research aims to provide the fundamental knowledge necessary for developing new types of synthetic microrobots driven by non-invasive alternating-current electric fields. In particular, the research team investigates the interplay between two types of electric-field-induced solvent flow surrounding microscopic particles. Precise control of such kind of particle interactions provides a new mechanism for cargo capture, transport, and delivery by the colloidal micromotors in a lab-on-a-chip device. Moreover, the organized structures formed by those micromotors are excellent building blocks for making functional materials that exhibit exotic optical properties for applications in superlenses, cloaking devices, and molecular sensing. In addition, this award also plans to develop hands-on learning modules to engage underrepresented groups in science and engineering. Technical abstract This project aims to answer a fundamental question in colloidal physics: what is the nature of the electrokinetic flow around a charged dielectric particle near an electrode when subjected to a perpendicularly applied alternating-current electric field? A series of recent experiments strongly suggest that the classical theories on electrohydrodynamic flow and induced-charge electroosmosis flow are insufficient to capture the propulsion and non-equilibrium assembly of charged dielectric particles because it only considers the electroosmotic flow originating from the electrode. Instead, this project investigates the impact of the concentration polarization of the electric double layer around the charged particle on a new type of electrokinetic flow (the concentration-polarization-induced electroosmosis) via complementary experimental and theoretical studies. In addition, the hydrodynamic interactions originating from multiple types of electrokinetic flow are exploited to achieve the self-regulated out-of-equilibrium assembly of multiple colloids into uniform clusters with complex symmetries. Finally, perturbation theory and Brownian dynamics simulations are used to investigate the role of hydrodynamic interactions in the assembly.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.