M2 ENS Paris-Cachan 2019

In November 2019 three students from Ecole Normale Supérieure Paris-Cachan - Juliette Plo, Dihya Sadi and Elio Thellier - came to our lab for a two months internship untitled "From liquid crystals to displays". During the internship they worked simultaneously on two projects: 1°- detection of the isotropic - nematic phase transition and the measure of the quadropolar order parameter by means of RMN. 2° - detection of the magnetic and dielectric anisotropies of nematics by means of a new setup, built during the internship, inspired by the method invented by Mieczyslaw Jezewski and Wilhelm Kast about 100 years ago. Results of the internship were reported in an article untitled "Fréedericksz transition on air" published in American Journal Physics - well known bulletin of the American Association of Physics Teachers.

Abstract: The operational principle of twisted nematic displays involves the dielectric anisotropy of nematics. This crucial property was discovered about a hundred years ago by Jezewski and Kast who used a so-called resonance method in which the frequency of an LC tank circuit was set by the capacitance of a capacitor filled with a nematic liquid crystal. Jezewski and Kast observed that the resonance frequency changed upon application of a magnetic field to the capacitor. They interpreted the corresponding change in the dielectric permittivity as being due to reorientation of molecules by the magnetic field. Here, we describe a modern, simple, and low-cost version of this experiment. Instead of the LC oscillator working with vacuum lamps, we use an op-amp RC oscillator in which a twisted nematic display plays the role of the capacitor. For the purpose of classroom demonstrations, the oscillator frequency f is detected by a software-defined radio operating in the double-side band mode (DSB). Upon an appropriate tuning of the reception frequency f, even small changes of f become audible. This setup is very convenient for demonstration and measurements of all characteristics of the Freedericksz transition driven by magnetic or electric fields.