Chemoresponsive assemblies of microparticles at liquid crystalline interfaces

Abstract

Assemblies formed by solid particles at interfaces have been widely studied because they serve as models of molecular phenomena, including molecular self-assembly. Solid particles adsorbed at interfaces also provide a means of stabilizing liquid-liquid emulsions and synthesizing materials with tunable mechanical, optical, or electronic properties. Whereas many past studies have investigated colloids at interfaces of isotropic liquids, recently, new types of intercolloidal interactions have been unmasked at interfaces of liquid crystals (LCs): The long-range ordering of the LCs, as well as defects within the LCs, mediates intercolloidal interactions with symmetries that differ from those observed with isotropic liquids. Herein, we report the decoration of interfaces formed between aqueous phases and nematic LCs with prescribed densities of solid, micrometer-sized particles. The microparticles assemble into chains with controlled interparticle spacing, consistent with the dipolar symmetry of the defects observed to form about each microparticle. Addition of a molecular surfactant to the aqueous phase results in a continuous ordering transition in the LC, which triggers reorganization of the microparticles, first by increasing the spacing between microparticles within chains and ultimately by forming two-dimensional arrays with local hexagonal symmetry. The ordering transition of the microparticles is reversible and is driven by surfactant-induced changes in the symmetry of the topological defects induced by the microparticles. These results demonstrate that the orderings of solid microparticles and molecular adsorbates are strongly coupled at the interfaces of LCs and that LCs offer the basis of methods for reversible, chemosensitive control of the interfacial organization of solid microparticles.

Fig. 1.

(A) Cartoon depicting the use of sedimentation to deposit microparticles at interfaces between nematic 5CB and aqueous phases. Polarized (B, D, and F) and bright-field (C, E, and G) optical micrographs of interfacial assemblies of microparticles with areal densities of 4,500 (B and C), 9,900 (D and E), and 14,800 microparticles/mm² (F and G). Polarized (H) and bright-field (I) optical micrographs of the same microparticles sedimented onto a 5CB–aqueous interface heated to 40 °C (above the nematic-to-isotropic transition temperature of 5CB). A false white border has been added to indicate in the edge of the sample (H). The microparticles in (I) appear larger than the microparticles in the other images because of a slight tilting of the sample in (I). (Scale bar: 50 μm.)

Fig. 2.

(A and F) Schematic illustrations of nematic 5CB confined within a specimen grid supported on OTS-treated glass surfaces. The orientation of the nematic 5CB at the interface to the aqueous phase is shown to be parallel to the interface in (A, planar anchoring) and perpendicular to the interface in (F). Polarized (B) and bright-field (C) micrographs of a DMOAP-treated microparticle at the 5CB–aqueous interface when the ordering of 5CB at the aqueous interface was planar. Illustrations of the side view (D) and top view (E) of the LC orientation surrounding a single DMOAP-treated microparticle when the orientation of 5CB at the 5CB–aqueous interface was planar. (G) Polarized and (H) bright-field micrographs of a DMOAP-treated microparticle at the 5CB–aqueous interface when the ordering of 5CB at the aqueous interface was induced to be perpendicular by the presence of 1,300 μM SDS in the aqueous phase. Illustrations of the side view (I) and top view (J) of the orientational profile of the LC around an isolated DMOAP-treated microparticle when the orientation of 5CB at the 5CB–aqueous interface is perpendicular. (Scale bars: 2 μm.)

Fig. 3.

Optical micrographs of SDS-induced reordering of microparticle assemblies formed at the nematic 5CB–aqueous interface. The concentrations of SDS in the aqueous phase were 150 (A and B), 550 (C and D), 700 (E and F), 950 (G and H), and 1,300 μM (I and J). Images in the left column were obtained with crossed polars, and images in the right column were obtained in bright-field imaging conditions. (Scale bar: 50 μm.)

Fig. 4.

Bright-field optical micrographs demonstrating the reversible ordering transition of microparticles assembled at a nematic 5CB–aqueous interface upon the adsorption and desorption of the surfactant SDS. The concentration of SDS in the aqueous phase was < 5 (A and C) and 1,300 μM (B). Additional images can be found in Fig. S1. (Scale bar: 50 μm.)

Fig. 5.

(A) Center-to-center, nearest-neighbor spacing of microparticles within chains of microparticles formed at the LC–aqueous interface, plotted as a function of the concentration of SDS in the aqueous phase. Images are bright-field micrographs of microparticles at the nematic 5CB–aqueous interface with aqueous phase SDS concentrations of 150, 700, and 1,300 μM. (Scale bars: 5 μm.) (B) Center-to-center, nearest-neighbor spacing of microparticles, measured as a function of time, for microparticles assembled at the 5CB–aqueous interface in the presence of 700 μM SDS in the aqueous phase. (C) Probability of finding a given number of nearest neighbors for microparticles in regions free of compact aggregates when the SDS concentration in the aqueous phase was 1,300 μM. (D) Increase in the area fraction of the 5CB–aqueous interface (SDS concentration of 1,300 μM) occupied by regions of 2D microparticle assemblies with hexagonal symmetry, plotted as a function of the overall concentration of microparticles at the interface.

Fig. 6.

(A) Polarized optical micrograph of microparticle chains at the nematic 5CB–aqueous interface when the azimuthal ordering of the 5CB at the interface was controlled by patterning the underlying substrate. (B) Cartoon depicting the azimuthal alignment of the LC. (Scale bar: 100 μm.)