Viscoelasticity of nematic liquid crystals at a glance

Many modern devices, such as for instance displays for computers and phones, are made of liquid crystals (LC) and their working principle depends strongly on the characteristic time of molecular reorientation after the application of external fields. Such reorientation time is mostly determined by the so-called viscoelastic ratios that quantify the importance of the LC viscosity compared to its elasticity. A standard way of measuring such viscoelastic ratios is small-angle Depolarized Dynamic Light Scattering (DDLS), which exploits the temporal intensity fluctuations in the depolarized scattered light to infer information about the lifetime of the orientational fluctuations of the LC director. Small-angle DDLS measurements are quite long and demanding and do not allow a space resolved characterization of an inhomogeneous sample. Here we show that the recently introduced Differential Dynamic Microscopy [1] can be used to perform a high throughput, multi-angle scattering experiments on liquid crystals in the nematic phase by exploiting a simple Fourier analysis of short movies acquired with a polarizing microscope. We succeed in measuring the splay, twist and bend viscoelastic ratios for nematic samples of a commercial liquid crystal (6CB) with controlled alignment of the director at the cell surface. In a wide range of temperatures, the results are in excellent agreement with literature data obtained with depolarized dynamic light scattering experiments. We show that our scattering-with-images approach enables a local, space-resolved characterization of optically anisotropic soft materials. This method is demonstrated here by extracting the viscoelastic ratios of spatially disordered nematics in regions of 20 mum size.