Polar Planar vs. Polar Vertical Reverse Tilt Domain Walls Charles Rosenblatt, Case Western Reserve University, DMR Left: Polarized photomicrograph of textures, showing superposition of pinned filaments and Schlieren brushes. a) T=25°C, b) T=40°, c) T=55°, d) T=70°. Below: Calculated energy per unit length of domain wall vs. pretilt angle θ 0 for polar horizontal and polar vertical walls. The three sets of curves represent three different angles ( ϕ = 0, π/4, and π/2) of the director with respect to the wall, where Δ ϕ = π for all three cases. For any given pretilt angle  just below T NI, the stable configuration corresponds to that with the lowest energy. Special treatment of a glass substrate allows us to control the polar orientation (called the “pretilt angle”) of liquid crystal molecules. The two extreme orientations are vertical (pretilt angle  = 0) and planar (  = 90 o ), with intermediate angles both possible and scientifically interesting. If the surface treatment does not specify an azimuthal orientation , then domain structures can appear, where the azimuthal angle of the liquid crystal can vary smoothly over the surface. But if two domains where  differs by approximately 180 o meet, the filament-like wall that separates the domains can have two different structures (shown schematically in the figure at the right), depending upon the value of  just below the transition temperature T NI from the liquid crystalline nematic phase to the isotropic phase. The image above shows “Polar Vertical” domain walls as a function of increasing temperature. As the temperature is increased, the pretilt angle  becomes smaller (closer to the vertical orientation), with a decrease in optical contrast across the wall. At sufficiently high temperature the walls vanish, but a “memory” of the walls is retained, so that on cooling the walls return exactly as they had been before heating.

Polar Planar vs. Polar Vertical Reverse Tilt Domain Walls Charles Rosenblatt, Case Western Reserve University, DMR Broader Impact:  A deeper understanding of domain structure and interfacial energies in liquid crystals  Connection linking the soft condensed matter community with the more traditional solid state community.  Presentation of results at several international venues, including France, Italy, China, Hong Kong, and Korea.  Formalized university-to-university international exchange programs with the Liquid Crystal National Laboratory, Universita della Calabria (Italy) and with Université Pierre et Marie Curie (Paris, France). A similar agreement with Nagaoka University of Technology (Japan) is nearly finalized. Education: Two graduate students (Ruiting Wang and Minhua Zhu), two postdocs (Timothy Atherton and Ji-Hoon Lee), and one visiting professor (Daeseung Kang) were involved in this project. Zhu now works at Kent Optronics in New York, Wang is a student in New York, Atherton and Lee are currently postdocs in my group (Lee is a research fellow funded by the Korean Research Foundation), and Kang is a professor at Soongsil University in Seoul.