Confinement

The interest in confining colloidal liquid crystals arises from the interplay between "simple" packing problems satisfying local boundary conditions, competing elasticities and defects. Advances in lithography techniques mean that confining length-scales for molecular Liquid Crystals (LCs) become of the order of the extrapolation length, which is set by the ratio between the elasticity and the anchoring strength for colloidal LCs the confinement may even reach the particle length L. Further advances in colloidal model systems, imaging techniques, and image analysis explain why experiments are now catching up with predictions made by theory and simulations, which have benefited from a steady increase in computational power.

Instances of extremely confined colloidal liquid crystals are found in biological systems, such as actin filament in cells or DNA in viruses, but also in industrial applications like inklet printing and fiber spinning of giant graphene oxyde, or printing of semiconductor nanomaterials.

Louis' research focused on the confinement of liquid crystals in the smectic phase in rectangular [1] and circular [2] cavities. To do so, he first developed a model experimental set-up where he may study isotropic, nematic and smectic phases at coexistence in a single confining chamber and down to the single particle level [1]. He is presently collaborating with theoreticians (René Wittmann and Hartmut Löwen) to further characterize the behavior of smectic liquid crystals in annular confinement [2].

In figure 1, we show how confinement can be use to control the organization of the particles. On the left (sharp angles) the smectic layers are organized in a single domain. However, on the right (wide angles) the layers are organized into three domains. This transition can be characterized to compare the energy of bending deformation and line defects [3].

threestatelozenges.pdf

Figure 1: confinement of silica rods in lozenges. Top row: Bright field images (10x) of silica rods confined into lozenges. Bottom row: Smectic layers are detected and colored as a function of their orientation.

References

[1] Louis B G Cortes et al 2017 J. Phys.: Condens. Matter 29 064003

[2] In preparation.

[3] Colloidal liquid crystals: phase behavior, dynamics and confinement, PhD thesis, Louis Cortes. (Soon available for download.)