Differential dynamic microscopy of fluctuating liquid crystals

Fluctuations of the director orientation in liquid crystalline samples can reveal precious information about their viscoelasticity. Laser light scattering (LLS) is a well established tool for extracting such information and has been extensively used for a long time [1]. Although these fluctuations can be easily seen in depolarized microscopy with the naked eye, only in a few isolated cases a quantitative study was attempted [2-4]. We present here experimental results obtained with the recently introduced Differential Dynamic Microscopy (DDM) [5,6] on thin layers of nematic liquid crystals (LC). DDM allows to obtain scattering information from the study of microscopy images. We show that depolarized DDM is perfectly suitable to determine the viscoelastic properties of thin layers of nematic LC, providing direct access to the intermediate scattering function at small scattering wavevectors, which are precluded to ordinary LLS. The differential nature of the technique allows also relaxing the strict cleanliness requirements typically needed in LLS experiments. With a single experiment less than 4 s long, all the three viscoelastic ratios can be measured in a LC sample with suitable alignment, thereby demonstrating a very powerful tool for the rapid characterization of LC. Our results, in agreement with literature values, suggest a routine use of microscopes for the determination of the viscoelastic properties of thermotropic and lyotropic LCs in harsh conditions and for the characterization of various optically anisotropic fluids. References
[1] H. F. Gleeson in Handbook of Liquid Crystals, edited by D. Demus, J. Goodby, G. W. Gray, H.-W. Spiess (Wiley-VCH, Halle, 2008), pp. 699-718.
[2] Y. Galerne, I. Poinsot, and D. Schaegis, Appl. Phys. Lett. 71, 222 (1997)
[3] H. Orihara, A. Sakai, and T. Nagaya, Mol. Cryst. Liq. Cryst. 366, 143 (2001)
[4] A. Yethiraj, R. Mukhopadhyay, and J. Bechhoefer Phys. Rev. E 65, 021702 (2002)
[5] R. Cerbino, and V. Trappe, Phys. Rev. Lett. 100, 188102 (2008)
[6] F. Giavazzi, D. Brogioli, V. Trappe, T. Bellini, and R. Cerbino, Phys. Rev. E 80, 031403 (2009)