Juho S. Lintuvuori

A self-quenched defect glass in a colloid-nematic liquid crystal composite.

A high concentration of colloidal particles stabilizes a defect network in a liquid crystal and creates a gel-like material. Colloidal particles immersed in liquid crystals frustrate orientational order. This generates defect lines known as disclinations. At the core of these defects, the orientational order drops sharply. We have discovered a class of soft solids, with shear moduli up to 104 pascals, containing high concentrations of colloidal particles (volume fraction ϕ>∼20%) directly dispersed into a nematic liquid crystal. Confocal microscopy and computer simulations show that the mechanical strength derives from a percolated network of defect lines entangled with the particles in three dimensions. Such a “self-quenched glass” of defect lines and particles can be considered a self-organized analog of the “vortex glass” state in type II superconductors.

A new anisotropic soft-core model for the simulation of liquid crystal mesophases

A new anisotropic soft-core model is presented, which is suitable for the rapid simulation of liquid crystal mesophases. The potential is based on a soft spherocylinder, which can be easily tuned to favor different liquid crystal mesophases. The soft-core nature of the potential makes it suitable for long-time step molecular dynamics or dissipative particle dynamics simulations, particularly as a reference model for mesogens or as an anisotropic solvent for use in combination with atomistic models. Results are presented for two variants of the new potential, which show different mesophase behaviors. Variants of the potential can also be linked together to produce more complicated molecular structures. Here, as an example, results are provided for a model multipedal liquid crystal, which has eight liquid crystalline groups linked to a central core via semiflexible chains. Here, despite the complexity of molecular structure, the model succeeds in showing the spontaneous formation of a liquid crystal phase. The results also demonstrate that there is a very strong coupling between the internal structure of the multipedal mesogen and the molecular order of the phase, with the mesogen spontaneously undergoing major structural rearrangement at the transition to the liquid crystal phase.

An investigation of soft-core potentials for the simulation of mesogenic molecules and molecules composed of rigid and flexible segments

Abstract The phase behaviour of three soft core spherocylinder models is investigated with a view to producing an effective potential for use in coarse-grained simulations of liquid crystal phases and polymers composed of rigid and flexible segments. Provided potentials are not made too soft, two of the soft core models are found to work well in terms of successfully reproducing mesophases and in providing considerable improvements in computational speed over other commonly used coarse-grained models. In Monte Carlo simulations a soft-core spherocylinder model in which a cut and shifted Lennard–Jones potential is truncated with a linear tangential potential is found to be particularly effective; while for molecular dynamics a better model is provided by a DPD-like quadratic potential. Here, computational speed-ups of 20 – 30 × are seen in equilibration times in comparison to the well-known soft repulsive spherocylinder (SRS) model. The quadratic potential is used in an additional set of coarse-grained simulations of a liquid crystal with a flexible chain, which exhibits spontaneous formation of a nematic phase. The use of different types of interaction sites is also illustrated by the simulation of a spherocylinder with two “tails” formed from spheres. Here, varying the hardness of the sphere-spherocylinder interaction potential allows the formation of a smectic-A phase which exhibits microphase separation.

A coarse-grained simulation study of mesophase formation in a series of rod-coil multiblock copolymers.

The mesophase behaviour of a rod-coil multiblock copolymer is assessed by means of a new soft-core simulation model, which is suitable for the simulation of combinations of isotropic and anisotropic particles. The simulations demonstrate the presence of isotropic melt, micelles, lamellar, nematic and gyroid phases, with ordered phases able to grow spontaneously from the isotropic melt. The influence of increasing the length of the rigid rod-component of the polymer is studied, with mesophase stability enhanced by increases in rod length. Increased nematic phase stability is demonstrated also as the length of rod component is increased. For longer rods (smaller fraction of coils) the models show evidence of metastable chevron-like structures that initially form on cooling from the polymer melt. These are eventually lost in favour of true lamellar ordering over long annealing runs. The structure of molecules within the phases formed is also assessed. Chains in the lamellar phase are shown to produce both bridging and loop behaviour, with the latter preferred slightly. Simulations at a low occupied volume fraction, corresponding to self-assembly in solution, demonstrate the formation of a structurally ordered nanowire.

Colloidal templating at a cholesteric-oil interface: assembly guided by an array of disclination lines.

We simulate colloids (radius R ~ 1 μm) trapped at the interface between a cholesteric liquid crystal and an immiscible oil at which the helical order (pitch p) in the bulk conflicts with the orientation induced at the interface, stabilizing an ordered array of disclinations. For a weak anchoring strength W of the director field at the colloidal surface, this creates a template, favoring particle positions either on top of or midway between defect lines, depending on α=R/p. For small α, optical microscopy experiments confirm this picture, but for larger α no templating is seen. This may stem from the emergence at moderate W of a rugged energy landscape associated with defect reconnections.

A soft-core Gay-Berne model for the simulation of liquid crystals by Hamiltonian replica exchange

The Gay-Berne (GB) potential has proved highly successful in the simulation of liquid crystal phases, although it is fairly demanding in terms of resources for simulations of large (e.g., N>10(5)) systems, as increasingly required in applications. Here, we introduce a soft-core GB model, which exhibits both liquid crystal phase behavior and rapid equilibration. We show that the Hamiltonian replica exchange method, coupled with the newly introduced soft-core GB model, can effectively speed up the equilibration of a GB liquid crystal phase by frequent exchange of configurations between replicas, while still recovering the mesogenic properties of the standard GB potential.

Colloidal particles at the interface between an isotropic liquid and a chiral liquid crystal

We create an interface between a cholesteric liquid crystal (CLC) and an isotropic liquid (silicone oil) at which homeotropic anchoring leads to a well aligned cholesteric layer and the formation of the fingerprint texture. Fluorescent colloidal particles with planar surface anchoring are dispersed in the CLC and subsequently imaged using confocal microscopy. A majority of these particles decorate the interface between the CLC and the silicone oil. We present a detailed study of the position of the particles along the direction perpendicular to the interface: the final distribution of particles perpendicular to the interface has a clear dependence on the ratio between the particle size and the pitch of the CLC. This suggests, supported by simulations, that there is a particle size and pitch length dependent drive to expel particles, due to the elastic energy cost of remaining in the CLC. We use polarizing optical microscopy to observe changes to the fingerprint texture as the particles perturb the interface. This is combined with a qualitative study of the in-plane ordering of the particles. Chains of particles form perpendicular to the helical axis (parallel to the cholesteric layers), whereas disordered aggregates are seen where the direction of the helical axis is not uniform.

Phase diagram of the uniaxial and biaxial soft-core Gay-Berne model.

Classical molecular dynamics simulations have been used to explore the phase diagrams for a family of attractive-repulsive soft-core Gay-Berne models [R. Berardi, C. Zannoni, J. S. Lintuvuori, and M. R. Wilson, J. Chem. Phys. 131, 174107 (2009)] and determine the effect of particle softness, i.e., of a moderately repulsive short-range interaction, on the order parameters and phase behaviour of model systems of uniaxial and biaxial ellipsoidal particles. We have found that isotropic, uniaxial, and biaxial nematic and smectic phases are obtained for the model. Extensive calculations of the nematic region of the phase diagram show that endowing mesogenic particles with such soft repulsive interactions affect the stability range of the nematic phases, and in the case of phase biaxiality it also shifts it to lower temperatures. For colloidal particles, stabilised by surface functionalisation, (e.g., with polymer chains), we suggest that it should be possible to tune liquid crystal behaviour to increase the range of stability of uniaxial and biaxial phases (by varying solvent quality). We calculate second virial coefficients and show that they are a useful means of characterising the change in effective softness for such systems. For thermotropic liquid crystals, the introduction of softness in the interactions between mesogens with overall biaxial shape (e.g., through appropriate conformational flexibility) could provide a pathway for the actual chemical synthesis of stable room-temperature biaxial nematics.

Swimming in a crystal

We study catalytic Janus particles and Escherichia coli bacteria swimming in a two-dimensional colloidal crystal. The Janus particles orbit individual colloids and hop between colloids stochastically, with a hopping rate that varies inversely with fuel (hydrogen peroxide) concentration. At high fuel concentration, these orbits are stable for 100s of revolutions, and the orbital speed oscillates periodically as a result of hydrodynamic, and possibly also phoretic, interactions between the swimmer and the six neighbouring colloids. Motile E. coli bacteria behave very differently in the same colloidal crystal: their circular orbits on plain glass are rectified into long, straight runs, because the bacteria are unable to turn corners inside the crystal.

Self-Assembly and Nonlinear Dynamics of Dimeric Colloidal Rotors in Cholesterics

We study by simulation the physics of two colloidal particles in a cholesteric liquid crystal with tangential order parameter alignment at the particle surface. The effective force between the pair is attractive at short range and favors assembly of colloid dimers at specific orientations relative to the local director field. When pulled through the fluid by a constant force along the helical axis, we find that such a dimer rotates, either continuously or stepwise with phase-slip events. These cases are separated by a sharp dynamical transition and lead, respectively, to a constant or an ever-increasing phase lag between the dimer orientation and the local nematic director.