Straining soft colloids in aqueous nematic liquid crystals

Liquid crystals (LCs), because of their long-range molecular ordering, are anisotropic, elastic fluids. Herein, we report that elastic stresses imparted by nematic LCs can dynamically shape soft colloids and tune their physical properties. Specifically, we use giant unilamellar vesicles (GUVs) as soft colloids and explore the interplay of mechanical strain when the GUVs are confined within aqueous chromonic LC phases. Accompanying thermal quenching from isotropic to LC phases, we observe the elasticity of the LC phases to transform initially spherical GUVs (diameters of 2–50 µm) into two distinct populations of GUVs with spindle-like shapes and aspect ratios as large as 10. Large GUVs are strained to a small extent (R/r < 1.54, where R and r are the major and minor radii, respectively), consistent with an LC elasticity-induced expansion of lipid membrane surface area of up to 3% and conservation of the internal GUV volume. Small GUVs, in contrast, form highly elongated spindles (1.54 < R/r < 10) that arise from an efflux of LCs from the GUVs during the shape transformation, consistent with LC-induced straining of the membrane leading to transient membrane pore formation. A thermodynamic analysis of both populations of GUVs reveals that the final shapes adopted by these soft colloids are dominated by a competition between the LC elasticity and an energy (∼0.01 mN/m) associated with the GUV–LC interface. Overall, these results provide insight into the coupling of strain in soft materials and suggest previously unidentified designs of LC-based responsive and reconfigurable materials.The majority of living materials are soft. This characteristic emerges from noncovalent interactions that lead to the formation of supramolecular structures that reorganize in response to subtle chemical and mechanical cues (1). The regulation of mechanical strain in particular and the engineering of responses to it across a hierarchy of spatial scales (from the molecular to the supramolecular to the cellular level) are increasingly understood to be one of the central sciences of living systems (2, 3).Inspired in part by the functionality of biological materials, a wide range of soft synthetic materials has been assembled by noncovalent interactions of molecular and macromolecular components (1, 4). In particular, liquid crystals (LCs) (Fig. 1A), which are phases that combine the molecular mobility of liquids with the long-range orientational ordering of crystalline solids, have provided the basis for a spectrum of responsive materials, including systems where electrical fields and mechanical strain compete to control electrooptical properties (5, 6). More recently, micro- and nanometer-sized colloidal particles dispersed in LCs have been used to form tunable self-assembled structures for photonic crystals and metamaterials (7). In the systems studied to date, however, the colloids have been “hard” compared with the LC, leading to mechanical straining of the LC but not the colloids (8).Open in a separate windowFig. 1.(A) Schematic illustration of the long-range orientational order of a nematic LC, which can be characterized by a local director n^. (B) The chemical structure of a DSCG molecule. (C and E) Fluorescence and (D and F) crossed polar micrographs of GUVs in 15% (wt/wt) DSCG at (C and D) 48 °C (isotropic phase) and (E and F) at 25 °C (nematic phase). (Scale bars: 10 µm.)In this paper, we move beyond these past studies and consider the more complex situation in which soft colloids are dispersed in LCs, such that a coupling exists between colloid shape and LC strain. Specifically, we have used micrometer-sized synthetic giant unilamellar vesicles (GUVs) as model soft colloids and dispersed them in LCs. We hypothesized that elastic stresses arising from deformation of the LC would strain the GUVs, potentially giving rise, for example, to anisometric GUV shapes, expansion of the surface area of GUV membranes, and temporary poration and/or permanent rupture of the GUV bilayers. To test this hypothesis, we used the lyotropic chromonic LC phase formed from aqueous solutions of disodium cromoglygate (DSCG) (Fig. 1B). We used DSCG, because it is not amphiphilic, and thus, we predicted that it would not disrupt the lipid bilayers of GUVs (in contrast, many surfactants that form lyotropic phases solubilize lipid bilayers). DSCG molecules stack into anisometric assemblies when dissolved in water (9–11) and form mesophases in a manner that depends on temperature and the concentration. We note here that the ordering of nematic DSCG and other chromonic LCs has been explored in confined spherical (12) and cylindrical geometries (13) as well as surrounding rigid spherical inclusions (14).The results described in this paper yield fundamental insights into the ways in which elastic stresses are coupled to particle shape in soft matter systems, hinting at previously unidentified designs of LC-based responsive and/or active materials. In addition, we note that recent experiments suggest that curvature strain within bacterial and mitochondrial membranes may locally concentrate certain families of lipids to regions of highest membrane curvature. The elastically strained GUVs described in this paper may provide the basis of an experimental platform to further investigate biophysical questions relating to membrane curvature strain (2). Our results also have the potential to provide insights into the recent observation that elastic stresses imparted by LCs can alter bacterial cell shape (15).