Active patchy colloids and shape-tunable dynamics under AC electric fields

Abstract

The colloidal science has witnessed two major advancements in recent years. One is the realization of active colloids with enhanced motions, controlled trajectories and collective self-organization, not only mimicking those of living organisms but also promising in exploiting them as micro-machinery and robotics. The other is the introduction of patchy particles with directional interactions which endows low coordinate, complex open structures. In this thesis, a merger of active colloids and patchy particles is presented. This strategy enables a plethora of intriguing dynamics for individual particles as well as their assemblies. Starting from the patchy colloids of various low-symmetry shapes synthesized in bulk via a cluster-encapsulation-dewetting method, it is shown that the shape parameters, including number of patches, size, symmetry, and site-specific functionalization, can be precisely regulated. This method is versatile and can be extended to seeds of different materials and synthesis of high-order patchy particles and clusters. More importantly, low-symmetry patchy colloids also carry essential information encoded in their shapes and material heterogeneity that programs their locomotion and self-assembly under AC electric fields. It is demonstrated that the propulsion velocity and the ability to brake and steer can be regulated by two-patch particles possessing low-symmetry patches with a variety of bending angles. The assembly of mono-patch particles and big dielectric spheres results in the nonequilibrium structures such as colloidal spinners and “colloidal molecules” according to the patchy particles’ aspect ratios. Looking into the colloidal molecules, various dynamic bonds are revealed, which are highly selective and directional stemming from multiple interactions achieved by controlling the particle size, shape, and material heterogeneity. The assembled colloidal molecules with the desired colloidal bonds display controllable propulsion, steering, reconfigurations as well as other dynamics thanks to their distinct bond properties. The working principle is general, further extended to the co-assembly of synthetic particles with bacteria and living cells, giving rise to hybrid colloidal molecules of various forms. Finally, a series of collective, dissipative assemblies that form colloidal chains, walls, and stacking configurations are realized using metallodielectric particles with two patches. Given the certain ionic strength, the configuration of assembly is tunable by a single parameter, the electric field frequency. This work demonstrates a desirable level of control towards the targeted dynamic assembly, which on one hand broadens the scope of colloidal assembly and dynamics one can possibly realize, and on the other hand enables active matter to perform sophisticated tasks specifically in the field of biomedical engineering and organ-on-a-chip devices.