Sylvain Grollau

Slow dynamics of thin nematic films in the presence of adsorbed nanoparticles

Recent experiments indicate that liquid crystals can be used to optically report the presence of biomolecules adsorbed at solid surfaces. In this work, numerical simulations are used to investigate the effects of biological molecules, modeled as spherical particles, on the structure and dynamics of nematic ordering. In the absence of adsorbed particles, a nematic in contact with a substrate adopts a uniform orientational order, imposed by the boundary conditions at this surface. It is found that the relaxation to this uniform state is slowed down by the presence of a small number of adsorbed particles. However, beyond a critical concentration of adsorbed particles, the liquid crystal ceases to exhibit uniform orientational order at long times. At this concentration, the domain growth is characterized by a first regime where the average nematic domain size LD obeys the scaling law LDt approximately t1/2; at long times, a slow dynamics regime is attained for which LD tends to a finite value corresponding to a metastable state with a disordered texture. The results of simulations are consistent with experimental observations.

Monte Carlo simulations and dynamic field theory for suspended particles in liquid crystalline systems

Monte Carlo simulations and dynamic field theory are used to study spherical particles suspended in a nematic liquid crystal. Within these two approaches, we investigate the binding of the defects to the particles, the adsorption of a particle at a solid surface, and two particles interacting with each other. Quantitative comparisons indicate good agreement between the two approaches. A Monte Carlo method based on the combination of canonical expanded ensemble simulations with a density-of-state formalism is used to determine the potential of mean force between one particle and a hard wall. On the other hand, the potential of mean force is evaluated using a dynamic field theory, where the time-dependent evolution of the second rank tensor includes two major aspects of liquid crystalline materials, namely the excluded volume and the long-range order elasticity. The results indicate an effective repulsive force that acts between the particle and the wall. Layer formation at the surface of the hard wall gives rise to local minima in the potential of mean force. The director profile for a particle at contact with a solid surface is characterized by a disclination line distorted and attracted towards the wall. The structure of the nematic for two particles at short distances is also investigated. Our results indicate a structure where the two particles are separated by a circular disclination line. The potential of mean force associated with this configuration indicates an effective attractive interaction between the two particles.

Multiscale Simulation of Liquid Crystals

Nematic liquid crystals are characterized by the occurrence of disclination lines, topological defects where the average molecular orientation changes abruptly. Recent experiments have shown that, in addition to their application in displays, liquid crystals permit the detection of ligand-receptor binding by optical amplification. The optimal design of LC-based biosensors requires an understanding of the effects of the presence of biomolecules on the structure and dynamics of nematic liquid crystals. We present a multiscale approach that combines molecular simulations and mesoscale modeling: Monte Carlo simulations are used to study the interactions of diluite colloidal particles, as well as the structure of topological defects; these results compare satisfactorily with the corresponding theoretical calculations at the mesoscale level. The mesoscale modeling of a multi-particle sensor shows that adsorbed biomo- lecules modify the relaxation dynamics in the device: at low surface-coverage densities, the equilibrium structure is characterized by a slightly perturbed uniform nematic order; at a critical density, the dynamics exhibits a slowdown at late stages, characteristic of the inability of the nematic to achieve a uniform order. These results are compared with experimental observations of the nematic response in biosensors.