Abstract
Colloidal particles suspended in liquid crystals can exhibit various effective anisotropic interactions that can be tuned and utilized in self-assembly processes. We simulate a two-dimensional system of hard disks suspended in a solution of dense hard needles as a model system for colloids suspended in a nematic lyotropic liquid crystal. The novel event-chain Monte Carlo technique enables us to directly measure colloidal interactions in a microscopic simulation with explicit liquid crystal particles in the dense nematic phase. We find a directional short-range attraction for disks along the director, which triggers chaining parallel to the director and seemingly contradicts the standard liquid crystal field theory result of a quadrupolar attraction with a preferred {45^{circ }} angle. Our results can be explained by a short-range density-dependent depletion interaction, which has been neglected so far. Directionality and strength of the depletion interaction are caused by the weak planar anchoring of hard rods. The depletion attraction robustly dominates over the quadrupolar elastic attraction if disks come close. Self-assembly of many disks proceeds via intermediate chaining, which demonstrates that in lyotropic liquid crystal colloids depletion interactions play an important role in structure formation processes.
Highlights
Colloidal particles suspended in liquid crystals can exhibit various effective anisotropic interactions that can be tuned and utilized in self-assembly processes
Effective interactions become even more interesting for colloids suspended in anisotropic fluids such as liquid crystals (LCs), which can be seen as colloidal mixtures of larger colloidal particles suspended in a liquid of small rod-like particles, in particular for lyotropic L Cs6
Hard needles can be viewed as two endpoints connected by an infinitely thin, hard line, see Fig. 1a, and we sample in the Monte Carlo (MC) simulation by moving the two endpoints
Summary
Colloidal particles suspended in liquid crystals can exhibit various effective anisotropic interactions that can be tuned and utilized in self-assembly processes. These have been first explored systematically by Ramaswamy et al.[13], Ruhwandl and Terentjev[14] and by Poulin et al who observed a chaining of water droplets inside a L C15 The nature of these elastic LC-mediated interactions strongly depends on the details of the interaction between colloidal particle and LC host, i.e., how the LC molecules or rods are anchored on the colloid (normal, conic or planar, weak or strong anchoring), and can be of dipolar, quadrupolar, or even more complicated n ature[16,17,18]. LC colloidal assemblies can be engineered, for example, by tuning the surface anchoring or engineering nematic defect structures[24,28, 29]
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