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Séminaire Charles Rosenblatt

Surface chirality and liquid crystals

Charles Rosenblatt

Liquid Crystal Group, Department of Physics
Case Western Reserve University, Ohio, USA

Chirality and its consequences, such as optical rotatory power, most often occur when inversion symmetry is absent in the constituent components, such as molecules or self-assembled structures. But this is not an absolute requirement : Achiral molecules have been shown to self-organize into macroscopically chiral phases, and chiral molecules such as DNA have been used as templates to induce chirality in inherently achiral materials.

In this talk I will discuss how we mechanically generate chirality in two dimensions, with consequences at the molecular level, by nanoscribing a polymer-coated substrate. Here 2D chirality is defined as the inability to superpose an object onto its mirror image by rotation and translation within a plane ; the letter “F” is chiral in 2D, whereas the letter “E” is achiral.

We then coat the scribed surface with a liquid crystal and, by measuring the molecular rotation as a function of applied field — this is the so-called "electroclinic effect" — we demonstrate not only the existence of mechanically generated surface chirality, but also determine its relative strength. In a related project, we demonstrate and quantify how carbon nanotubes can induce bulk chiral effects in a neighboring achiral liquid crystal, including an electroclinic effect and a bulk twist distortion in the nematic phase.