Abstract
Polarised microscopy is shown to be a powerful alternative to light scattering for the determination of the viscoelasticity of aligned nematic liquid crystals. We perform experiments in a wide range of temperatures by using an adapted version of the recently introduced differential dynamic microscopy technique, which enables us to extract scattering information directly from the microscope images. A dynamic analysis of the images acquired in different geometries provides the splay, twist and bend viscoelastic ratios. A static analysis allows a successful determination of the bend elastic constant. All our results are in excellent agreement with those obtained with the far more time-consuming depolarised light scattering techniques. Remarkably, a noteworthy extension of the investigated temperature-range is observed, owing to the lower sensitivity of microscopy to multiple scattered light. Moreover, we show that the unique space-resolving capacities of our method enable us to investigate nematics in the presence of spatial disorder, where traditional light scattering fails. Our findings demonstrate that the proposed scattering-with-images approach provides a space-resolved probe of the local sample properties, applicable also to other optically anisotropic soft materials.
Highlights
Devices based on nematic liquid crystals (LC) are very common and include displays for TVs, computers and phones, optical shutters and modulators, and 3D glasses for cinema or television.[1,2,3,4]
The relaxation of LC can be interpreted as a viscoelastic response to a distortion of the director eld and the reorientation time is mostly determined by the so-called viscoelastic ratios, which quantify the importance of the LC viscosity compared to its elasticity.[5]
We show that our imaging-based approach allows extracting the viscoelastic parameters in a heterogeneous planar nematic, by means of a space-resolved experiment that would be practically impossible with Depolarised Dynamic Light Scattering (DDLS)
Summary
Provides the intermediate scattering function for the corresponding wave vector.[9]. This approach has been successfully demonstrated with a variety of samples including colloids and bacteria, both in bright eld,[8,9,10] phase contrast,[11,12,13] and uorescence wide- eld[14] or confocal[15] microscopy. We shall prove here that DDM in combination with properly oriented polarisers – hereina er named polarised differential dynamic microscopy or pDDM – allows performing DDLS experiments with a microscope and permits the full characterisation of LC viscoelastic ratios in nematics. To this aim we rst develop a theoretical description of dynamic microscopy experiments with optically anisotropic samples. The use of pDDM for the extraction of the elastic constants necessitates alignment-dependent theoretical expressions describing the effect of the light propagation in a distorted medium on the image intensity Even though deriving such expressions is beyond the aim of this work, we adapt recent results developed in ref. Where g1 1⁄4 a3 À a2, ha 1⁄4 a4/2, hb 1⁄4 (a2 + a4 + a6)/2, and hc 1⁄4 (Àa2 + a4 + a5)/2
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