Feedback Controlled Self-Assembly of Elliptical Colloidal Particles: Navigating between Liquid, Nematic and Crystal States

Controlled self-assembly of ellipse-shaped colloidal particles holds promise for a range of applications, including stimuli-responsive coatings, advanced liquid-crystal displays, and new-generation electromagnetic devices (i.e. invisibility cloaks). While research into efficient control strategies for hard ellipse systems has accelerated in recent years, achieving an optimal controller remains challenging due to system heterogeneities, susceptibility to defects, and slow diffusion-limited relaxation. Here a precise control over the assembly and disassembly kinetics of ellipses is demonstrated across fluid, nematic, and crystal phase equilibria using closed-loop feedback-controlled actuation of an AC-electric field. By calculating order parameters from real-time particle arrangements and altering the applied field via feedback control, our controller enables the active reconfiguration of colloidal ellipses from any starting ensemble to a target microstructure in second-time scales. Further, an osmotic force balance model utilizing the local density profile is employed to accommodate varying numbers of particles, electrode gap widths, and particle dimensions, thereby expanding the controller application to a broad range of experimental conditions and potential technological implementations. Drawing from a systematic analysis of phase transitions and free energy landscapes, our method can serve as a basis for manipulating particle ensembles for use in various metamaterial applications.