Liquid-crystal mediated nanoparticle interactions and gel formation

Abstract

Colloidal particles embedded within nematic liquid crystals exhibit strong anisotropic interactions arising from preferential orientation of nematogens near the particle surface. Such interactions are conducive to forming branched, gel-like aggregates. Anchoring effects also induce interactions between colloids dispersed in the isotropic liquid phase, through the interactions of the pre-nematic wetting layers. Here we utilize computer simulation using coarse-grained mesogens to perform a molecular-level calculation of the potential of mean force between two embedded nanoparticles as a function of anchoring for a set of solvent conditions straddling the isotropic-nematic transition. We observe that strong, nontrivial interactions can be induced between particles dispersed in mesogenic solvent, and explore how such interactions might be utilized to induce a gel state in the isotropic and nematic phases.

Recent advances in molecular simulation: A chemical engineering perspective

“…In the case of weak planar anchoring, there was a single minimum in the free energy corresponding to configurations where the nanoparticles were in close proximity, sharing layers of liquid‐crystal molecules that wet their surface. For strongly homeotropic anchoring interactions, however, the free energy exhibited two minima caused by interactions between the halolike structures of bound liquid‐crystal molecules that formed around each nanoparticle and the surrounding interstitial fluid . The metastable minimum at approximately 10σ, where σ is the minor axis of a liquid‐crystal molecule, corresponded to the halos coming into contact, as illustrated by the image on the right in the bottom panel of Figure .…”

“…Free‐energy methods have also been recently applied to examine the interactions between colloidal nanoparticles embedded in nematic liquid crystals . Such particles can exhibit unusual effective interactions that are mediated by the structure of liquid‐crystal molecules near their surface . This structure is, in turn, controlled by anchoring effects between the nanoparticles and liquid‐crystal molecules that arise because of their mutual interactions.…”

“…The ability to judiciously tune such interactions to develop new colloidal liquid‐crystal composites could result in improved materials for designing technologies ranging from semiconductors to sensors . To this end, de Pablo and coworkers performed MD simulations to study the behavior of nanoparticles in nematic liquid crystals. In their CGM, the nanoparticles and liquid‐crystal molecules were described as simple spheres and oblate ellipsoidal particles, respectively.…”

“…de Pablo and coworkers observed a rich variety of physical behaviors in their simulations that were found to be sensitive to the density of the liquid‐crystal phase and the relative strength of the planar and homeotropic anchoring interactions. Figure , for example, shows the free‐energy profiles as a function of the particle separation distances that were computed for systems with weakly planar and strongly homeotropic anchoring interactions.…”

“…Less than 3 decades ago, the state of the art was embodied by the first direct simulations of phase equilibria for simple fluids such as argon . Today, chemical engineers routinely apply molecular simulation to study challenging problems related to drug design and formulation; biomolecular crowding; protein folding and aggregation; wetting phenomena and hydration thermodynamics; nucleation and growth processes; the thermophysical properties of complex fluids such as ionic liquids and liquid crystals; the phase behavior of polymeric, colloidal, and self‐assembled systems; and the synthesis, design and characterization of advanced materials for applications ranging from adsorption‐separation processes to energy conversion and storage …”