Assembly and Actuation in Concentrated Suspensions of Janus Ellipsoids

The emergence of anisotropic colloidal particles as building blocks for self-assembly has opened up a route to achieve bottom-top assembly of diverse functional structures with complex symmetries. For instance, Janus ellipsoidal particles that posses both shape and interaction anisotropy self- assemble into fiber-like structures in which the degree of positional order is controlled by the electrolyte concentration (Nature Materials 14 117-124 (2015). These structures may also be reversibly actuated by application of an external electric field. Here, we have studied the equilibrium self-assembly of dense suspensions of Janus ellipsoidal particles via confocal laser-scanning microscopy (CLSM). The work examines the role of the particle concentration on the kinetics of the assembly process and the phases that can be obtained. At particular electrolyte concentrations (~1mM), where the screened electrostatic repulsion between the particles facilitates the assembly process, we have observed a domain structure of the assembled fibers, with local nematic order within each domain. To exploit the reconfigurability of these structures in applications where actuation by external stimuli is desirable, we investigate the actuation capability in a highly populated network of the assembled fibers. Depending on the frequency of the applied field, the fibers align with the external field, the domain boundaries disappear, and field-induced interactions between neighboring fibers are observed. Our findings are relevant to generating shape-memory colloidal fibers suitable for applications where rapid, on demand reconfiguration is needed.