Liquid crystals: a playground for exploring physics

The teaching of experimental physics at the Master’s level has recently undergone a notable evolution thanks to the setting up of platforms equipped with state-of-the-art equipment, similar to that used in the research laboratories of the Université Paris-Saclay.

Researchers and professors of Laboratoire de Physique des Solides (LPS) have proposed a set of original experiments dedicated to the study of liquid crystals. Among these, the experimental project of 6 weeks (at a rate of 1 day per week), carried out at the LPS by three students of the M1 of fundamental physics of the ENS-Paris-Saclay, demonstrated the complementarity that can exist between the activities of teaching and research. The students worked on liquid crystals during their immersion course entitled “Liquid crystals: from physics to displays”. Their involvement, the quality of their work and the originality of the project resulted in an article “Fréedericksz transition on air” published in the American Journal of Physics (AJP), a reference teaching journal of the American Association of Physics Teachers.

The operating principle of displays based on twisted nematic liquid crystal involves the dielectric anisotropy of the nematic phases. Mieczyslaw Jeżewski and Wilhelm Kast discovered this fundamental property about 100 years ago. They had proposed a so-called resonance method in which the resonant frequency of an LC circuit is a function of the capacitance of a capacitor filled with nematic liquid crystal. Jeżewski and Kast then observed that the resonant frequency was changed when a magnetic field was applied to the capacitor and related the change in dielectric permittivity to the reorientation of the molecules. The paper published in AJP describes a modern, simple, and inexpensive version for performing this experiment to detect the magnetic and dielectric anisotropies of nematic phases (Figure 1).

Figure 1. (left)Modern circuit developed in this work based on the resonance method used by Jeżewski et Kast. (right) Close-up view of the nematic cell subjected to the magnetic field.

Instead of using an LC oscillator operating with vacuum tubes, we use an operational amplifier RC oscillator in which a twisted nematic display cell acts as the capacitor. In order to make this experiment demonstrative in a public presentation, the resonant frequency of the oscillator can also be detected by a software radio operating in dual sideband mode. After appropriate adjustment of the receiving frequency, small variations of the resonance frequency become audible, highlighting changes in the configuration of the liquid crystal. This device is very convenient to determine the characteristics of the Freedericksz transition generated by magnetic (Figure 1) or electric fields. The observation of a display cell based on these twisted nematics by means of a polarizing microscope also allows discovering walls and disinclinations generated by symmetry breaks (Figure 2).

These experiments illustrate the multidisciplinary nature of liquid crystal physics, at the intersection of electromagnetism, statistical physics, optics … and prove the interest of presenting these fascinating systems to students in the framework of courses on soft matter physics. Eventually, associated with other techniques used for the study of liquid crystals (X-ray diffraction, NMR, polarized light microscopy…), this new device will reinforce the experimental platforms of the Physics Department.

Figure 2. Generation of disinclinations and walls by symmetry breaks in the twisted nematic cell designed to display the letter “M”.

Video abstract of the article

Fréedericksz transition on air
J. Plo, D. Sadi, E. Thellier, P. Pieranski, M. Zeghal, P. Judeinstein
American J. of Physics, 2021, 89, 603-611
doi: 10.1119/10.0003350

Pawel Pieranski
Mehdi Zeghal
Patrick Judeinstein