Electric-field assembly of a binary mixture of spherical colloids

Abstract

While nature self assembles simple building blocks such as atoms and molecules into complex structures, achieving similar levels of complexities with synthetic colloidal building blocks via self- or directed-assembly is much more difficult. Previous research mostly focuses on single component particles and the assembled structures are limited. In this thesis we investigate the behavior of a binary mixture of spherical particles with different sizes within the full frequency spectrum of AC electric fields. In the frequency regime of 10-100 kHz, we observe the formation of three-dimensional small clusters ranging from tetramer to decamer, driven by the dipolar attraction between large and small particles. Both coordination number and bond lengths of these colloidal molecules can be selectively tuned by frequency. These clusters can also be linked by polyvinylpyrrolidone and they remain intact after the removal of electric fields. In the low frequency regime less than 1 kHz, two-dimensional lattices made up of triangles, squares, and pentagons are observed via planar aggregation between large and small particles, driven by a strong electrohydrodynamic flow. Furthermore, we have also investigated the propulsion of linked linear chains of particles due to an unbalanced electrohydrodynamic flow around the chains in the low frequency regime. We find strong correlations between bond angles of adjacent particle pairs during chain translation. Our findings lay the foundation for a more systematic study of active motion of asymmetric colloidal assembly under electric fields. The permanently linked colloidal molecules could also be used as new building blocks for assembly in the future.