Department of Physics, Panjab University, Chandigarh, India

Department seminar

Date and time: 25 July 2014 (Friday), 12:00 noon
Venue: Seminar Hall

Title: Twist bend nematic phase in liquid crystals: A recent discovery

Speaker: Prof. J.K. Vij, Fellow Trinity College Dublin, MRIA; Honorary Professor of Electronic Materials, School of Engineering, Trinity College University of Dublin, Dublin 2, Ireland

Abstract: A fourth state of matter in which the material shows a combination of the properties of liquids and crystals is called a liquid crystal. This state of matter is distinct from the other three commonly known states: gases, liquids and solids (crystals). This fourth state was discovered by an Austrian Botanist Friedrich Reinitzer in 1888. He observed that as the temperature of the solid sample (cholesteryl benzoate) was increased, the crystal changed into a hazy milky liquid, and on further heating, it changed into a clear, transparent liquid. The field of liquid crystals remained of academic interest only to a few interested scientists internationally. The phase in which the molecules show a long-range orientational order and no positional order is called nematic liquid crystal. The best known and most widely used (for example, in modern displays) is the uniaxial nematic, with the rod-like molecules aligned along a single axis, called the director. When the molecules are chiral, the director twists in space, drawing a right-angle helicoid and remaining perpendicular to the helix axis; the structure is called a chiral nematic or cholesteric nematic phase.
A small academic interest continued until the year 1970 by which time some smectic phases with a limited positional and orientational order had also been discovered but by and large the field remained dormant for some 80 years until a crude display using a liquid crystal was demonstrated in the RCA labs in the USA in 1970. This demonstration gave rise to the birth of a large number of research groups in Europe, USA, Japan and India. Pierra de Gennes of the College de France was awarded the 1991 Physics Nobel prize for his seminal contributions to the theories of liquid crystals and of soft matter. Significant technological interest grew to the extent that by the year 2010, liquid crystal displays had almost exclusively replaced the CRT. The outcome has benefitted almost every human being on this planet in the display of information; to name a few - calculators, mobile phones, lap tops, desk top screens of computers and the most recent large TV screens and displays at airports. It will not be out of place to state here that no other technology other than the LCD has saved so much energy and has contributed to the IT revolution.
After some 125 years, a new nematic phase called the twist bend nematic phase has been discovered; its discovery was reported in 2013 by three groups including my research group almost simultaneously [1-3]. The ground work of this phase was laid down almost exclusively in my research group in Dublin [4-7] in collaboration with a group in the U.K. when in the year 2009, one of my graduate students Mamatha Nagaraj and a post doctoral Fellow - V. Panov observed unusual properties of a phase in bimesogenic liquid crystals. We initially called this as the Nx phase but our research papers and presentations interested many others to pursue this work. Here using transmission electron and optical microscopy, we experimentally demonstrate a new nematic order, formed by achiral molecules, in which the director follows an oblique helicoid, maintaining a constant oblique angle with the helix axis and experiencing twist and bend. The oblique helicoids have a nanoscale pitch. The new twist-bend nematic represents a structural link between the uniaxial nematic (no tilt) and a chiral nematic (helicoids with right-angle tilt).
At the 25th international liquid crystal conference held in Dublin June 29 to July 4 2014, this discovery has been hailed as one of the best in the field of liquid crystals during the last 20 years.
1 D. Chen, J. H. Porada, J. B. Hooper, A. Klittnick, Y. Shen, M. R.Tuchband, E. Korblova, D Bedrov, D. M. Walba, M. A. Glaser,J. E. Maclennan, and N. A. Clark, PNAS 110, 15931 (2013).