We present the first experimental results demonstrating a novel approach to con-trolling the size as well as the spatial patterning of defect domains in a smectic liquid crystal by geometric confinement in surface modified microchannels. By confining the liquid crystal 8CB (4'-octyl-4-cyanobiphenyl) in micron sized rectangular channels with controlled surface polarity, we were able to generate defect domains that are not only nearly uniform in size but also arranged in quasi-two-dimensionally ordered patterns. Atomic force microscopy measurements revealed that the defects have a toroidal topology, which we argue is dictated by the boundary conditions imposed by the walls of the microchannel. We show that the defects can be considered as colloidal objects, which interact with each other to form ordered patterns. This method opens the possibility to exploit the unique optical and rheological properties associated with LC defects to making new materials. For example, the control of the shape, size and spatial arrangement of the defects at the mesoscale suggests applications in patterning, templating, and when extended to lyotropic liquid crystals, a process leading to uniform size spherical particles for chemical encapsulation and delivery.
On the other hand, we present the direct imaging of the internal structure of the toroidal defects in SmA liquid crystal droplet, by laser scanning confocal fluorescence microscopy (LSCFM). The consideration on the fluorescence anisotropy of rod molecule, based on the boundary conditions, gives us the understanding in the molecular orientation of liquid crystals in the defects, and therefore, we successfully distinguish the toroidal topology from the onion like spherulite.
Additionally, we studied on the confinement effect of the undulation instability of thermotropic liquid crystal 8CB near the transition temperature of nematicsmectic phase and it's transformation into focal conic toroidal defects. We present the strong correlation between the confining dimension (the width and depth of Si-fabricated channel) and the undulation period, and interestingly we found that this correlation occurred from as early as nematic phase.
We present, in the second chapter, the biaxial confinement of a semi-flexible filament in a channel by in situ video fluorescence microscopy. As the channel width decreases, F-actin undergoes a transition from a 2D random phase to a 1D biaxially confined phase, leading to an increased persistence length. A theoretical calculation shows that the tangent-tangent correlation function in the confined phase decreases to a minimum, then saturates to a constant at long distances, indicating that confinement induces long-range order in the actin filament. The location of the minimum of the experimental correlation function is consistent with our theoretical calculation.
이 논문은 surface modified microchannel의 geometric confinement에 의해 smectic liquid crystal 4'-octyl-4-cyanobiphenyl (8CB)의 defect size 및 spatial patterning of defects를 mesoscale에서 control하는 새로운 시도를 보여준다. Microchannel에서 생성된 defect domains은 마치 colloidal objects처럼 서로 간의 interaction에 의해 uniform in size 뿐만 아니라 quasi-two dimensionally ordered patterns을 형성한다. AFM에 의해 측정된 surface topographic depressions은 toroidal topological model에 대한 실험적 증거가 된다. Mesoscale에서 LC defect size 및 spatial arrangement의 control에 대한 이 논문은 patterning, templating으로의 응용 뿐 아니라, chemical encapsulation 또는 drug delivery를 위한 lyotropic liquid crystal의 uniform sized spherical particle process에 적용 가능성을 제안하는 연구이다.
그리고, laser scanning confocal fluorescence microscopy (LSCFM)를 이용하여 SmA liquid crystal toroidal defect의 internal structure를 연구한다. Boundary condition에 기초하여, 즉, channel walls에서 8CB moleculs의 tangential alignment, air/LC interface에서 homeotropic orientation에 기초하여 rod molecules의 fluorescence anisotropy 효과를 고려할 경우, defect를 구성하는 liquid crystal의 molecular orientation에 대한 이해가 가능하기 때문에, onion like spherulite로부터 toroidal topology를 실험적으로 구별할 수 있음을 이 논문은 보여준다.
한편으로, confined space에서 thermotropic liquid crystal의 phase transformations에 대한 실험 결과를 보여준다. 특히, nematic-smectic transition temperature 근처에서 undulation instability의 confinement effect, 그리고, undulating phase에서 focal conic toroidal defect로의 transforms을 관찰한다. 그 결과, confining dimension (microchannel의 width and depth)과 liquid crystal의 undulation period 사이에 strong correlation이 존재하며, 이 correlation은 이미 nematic phase의 초기단계에서부터 관찰된다는 사실을 알 수 있다.
이 논문의 두번째는 video fluorescence microscopy를 이용한 microchannel에서 semi-flexible filament의 biaxial confinement effect 연구 결과를 다룬다. Microchannel의 width를 감소시킴에 따라, F-actin의 persistence length가 증가하고, 2D random phase에서 1D biaxially confined phase로 phase transition 현상을 발견한다. Tangent-tangent correlation function in confined phase의 이론적 계산 결과에 의하면, correlation function에 minimum이 존재하고, 그 minimum의 위치는 실험에 의해 측정된 correlation function의 minimun 위치와 일치함을 이 논문은 보여준다.
