Impact of nonionic stabilizers and ionic solutes on liquid crystal shell stability and defect configuration

Liquid crystals combine fluidity with long-range molecular order and respond sensitively to interfaces, curvature, and chemical or biological species. Spherical liquid-crystal shells, where a thin nematic layer is confined between two aqueous phases, provide a platform for studying topological defects and for reconfigurable photonics and sensing. This thesis explores how nonionic polymer stabilizers and ionic solutes affect the stability, director-field configuration, and light-guiding behavior of nematic liquid-crystal shells.

The intrinsic birefringence of nematic shells enables polarization-controlled total internal reflection. Under suitable incident polarization, light is guided within the shell and emerges as bright spots for tangential anchoring and as rotating arcs for radial anchoring. These reflection signatures depend on refractive-index contrast and shell geometry and provide a simple, reflection-only route for distinguishing anchoring conditions using low-cost microscopy, without requiring transmission imaging.

We track the motion of topological defects in tangentially aligned shells as the director field anneals under buoyancy-induced wall-thickness gradients. Both unit-strength +1 and half-strength +1/2 surface defects migrate toward the thinner hemisphere during this relaxation. By varying the concentration and degree of hydrolysis of poly(vinyl alcohol), we show that moderately hydrolyzed polymer permits gravity-driven annealing, whereas highly hydrolyzed polymer suppresses this relaxation and leaves shells partitioned into defect-rich microdomains, consistent with stronger interfacial anchoring and a more rigid adsorbed layer.

A kinetic description of shell stability as a function of polymer concentration accounts for both shell collapse and fusion events. We develop a coupled kinetic scheme that describes frequent shell rupture (“popping”) and the much rarer fusion events that can precede it, focusing on the dominant rupture pathway while incorporating representative fusion rates. This analysis shows how encounter frequencies depend on interfacial polymer coverage and how stabilizer content governs shell lifetimes.

Electrolyte composition and bacterial lipopolysaccharide can further restructure director fields and optical textures in polymer-stabilized shells. Adding a saline buffer to the aqueous phases, thereby increasing the ionic strength, is associated with a shift in defect populations from configurations dominated by positive half-strength defects toward textures with a higher fraction of +1 defects. This evolution is interpreted using our proposed interfacial capacitor picture: a curvature-induced asymmetry between inner and outer electrical double layers generates internal fields that may promote additional defect coalescence in thin regions. Preliminary observations indicate that amphiphilic lipopolysaccharide molecules can nucleate lipid-rich domains across the shells, including cases where domains appear to emerge from defect cores; these domains can then grow and migrate along shell-thickness gradients, lowering elastic and interfacial free energy.

Taken together, the results show that nonionic polymers and ionic solutes reshape director fields in nematic liquid-crystal shells and thereby tune light guiding, defect dynamics, shell lifetimes, and biochemical response. Collectively, these optical, mechanical, and interfacial controls support the use of nematic shells as soft, reconfigurable photonic and biosensing materials.