Control of colloidal self-assembly by liquid-liquid crystalline phase separation

Abstract

Nucleation and growth (N&G) -the mechanism of forming a new thermodynamic phase- is one of the most important physical phenomena underlying gas-liquid (GLPS), liquid-liquid (LLPS), and solid-liquid (SLPS) phase separation. In principle, the phase transition rate and properties of the emerging new phase, such as the shape, structure, and size distribution, are determined by the interplay between thermodynamics and kinetics of N&G. In heterogeneous colloidal systems based on filamentous biological colloids, N&G occurs via a distinct phase separation mechanism and bears both fundamental and technological significance. In this system, the phase separation is purely entropic - a phenomenon recognized as liquid-liquid crystalline phase separation (LLCPS)- and two separate vertical lines define the two-phase region's binodal curve. Within binodal concentrations, where isotropic and nematic phases co-exist, orientational fluctuations of constituting filaments drive N&G that proceeds via the spontaneous formation of microdroplets of the nematic phase in the isotropic phase. These microdroplets, called tactoids, offer a rich liquid crystalline phase behavior and are now emerging as interesting components for structural investigations and developing new materials. Upon growth of tactoids with time, with induction time in the order of hours to days, their self-selected shape, composition, and structure change following volume-compositions trajectories, that approach two vertical binodal asymptotes. This implies that our understanding of heterogeneous colloidal systems is restricted to the N&G paradigm relying on the interplay between thermodynamics and kinetics, and it remains a hurdle to intervene in tactoids growth progress and thus control the induction time, shape, size, composition, orientation, and structure of tactoids. In this thesis, on the fundamental side, we study effects of kinetics on phase separation by separating them from thermodynamics in heterogeneous colloidal systems. This, besides insights from non-equilibrium features of tactoids, enables us, on the practical side, to address long-lasting challenges in liquid crystals, including the need for more control over tactoids formation in terms of the shape and internal structures, formation time, stability, orientation, internal components, and density of tactoids. Specifically, we first examine non-equilibrium features of liquid crystalline tactoids. By carefully designing a microfluidic system, we expose tactoids to an extensional flow field, showing that the shape of tactoids can be manipulated by hydrodynamic forces, allowing us to induce structural transformation and tune the self-assembly structure in tactoids. We combine free energy functional theory and experimental measurements to rationalize how liquid crystalline structures change when the confinement shape changes. Additionally, we study the shape and structural deformation of tactoids under shear flow, enabling us to deform tactoids in a different direction with respect to the collective orientation of colloids within tactoids. Next, we study the shape and structural relaxation of tactoids when they are released from an out-of-equilibrium state, allowing us to disentangle kinetics of the self-assembly of tactoids from shape dynamics. Based on insights obtained from tactoids' out-of-equilibrium properties, we show, by carefully selecting colloidal systems and controlling phase separation in microfluidic devices, that it becomes possible to disentangle kinetics effects from thermodynamics to form tactoids. Using rod-like colloids, we extrude a solution set at one thermodynamic nematic branch inside the other isotropic branch, realizing nematic or cholesteric droplets where the composition is set by thermodynamics, while dynamic flow parameters define the structure and morphology. Our results unveil new physical phenomena, such as orders of magnitude shorter timescales of tactoids formation, a wider phase diagram for tactoids, and internal cholesteric structures that are not observable via the N&G pathway. Our approach enables the on-demand fabrication of multicomponent heterogeneous liquid crystal jets/tactoids, enhancing their potential and introducing new fundamental and technological directions in hybrid structured fluids. Finally, we show that evaporation-induced progressive up-concentration inside of drying droplets makes it possible to cross, at different speeds, various thermodynamic stability states in solutions of rigid colloids. This allows us to access both metastable states, where phase separation occurs via N&G, and unstable states, where phase separation occurs via the more elusive spinodal decomposition, leading to the formation of tactoids of different shapes. Our findings not only deepen our understanding of the fundamental interplay between kinetics and thermodynamics in the formation of heterogeneous colloidal systems and out-of-equilibrium features of liquid crystalline systems, but they may also spark previously unanticipated opportunities in physics, biophysics, materials science, and the numerous technological applications that rely on N&G.