Voltage forced wetting, spreading and wrinkling of a nematic liquid crystal layer

Liquid dielectrophoresis (L-DEP) describes the phenomenon by which the force on the dipoles of dielectric liquids in non-uniform electric fields can be used to drive the liquid to occupy regions of high electric field intensity [1]. Applications that exploit liquid dielectrophoresis effects include switchable microlenses, optical shutters, beamsteerers, diffraction gratings, and electronic paper displays [2, 3]. In recent work we have shown how L-DEP forces created using voltages applied between co-planar interdigital electrodes can reduce the equilibrium contact angle of an isotropic sessile droplet on a solid surface. This produces voltage forced wetting and can also drive liquid spreading more strongly than Tanner’s law to create a thin film of liquid [4, 5]. Further increase in the applied voltage causes a periodic “wrinkle” deformation to appear on the liquid-air interface of the spread film [3]. Nematic liquid crystals are particularly interesting liquids for L-DEP studies since they are often designed to exhibit large anisotropic polarizabilities, which provides a high driving force for L-DEP, as well as possessing internal elasticity and exhibiting anisotropic flow properties.

Using a nematic liquid crystal, we have observed the influence of elasticity on both the profile and the amplitude of a static surface wrinkle deformation formed at the free surface of a thin nematic film by L-DEP forces. These elastic forces are associated with the periodic distortions of the nematic n-director within the film, which also couple to the surface deformation. We have investigated these effects experimentally using nematic mixtures with large positive and negative dielectric anisotropy values, and with planar or homeotropic anchoring of the nematic n-director at the solid surface. The nematic n-director orientation was inferred from polarised light measurements of the spatial variation of the phase retardation in spread films obtained using a Mach-Zehnder interferometer, and quantitative surface profile measurements were obtained using optical coherence tomography. These quantitative measurements compare favourably with predictions from nematic continuum theory combined with surface evolution models driven by calculated periodic variations in the Maxwell stress at the liquid crystal-air boundary.

We gratefully acknowledge funding from Merck Chemicals Ltd. and the UK EPSRC.

References
[1] T.B. Jones, M. Gunji, M. Washizu and M.J. Feldman, J. Appl. Phys. 89, 1441 (2001)
[2] S. Xu, H. Ren, and S-T. Wu, J. Phys. D: Appl. Phys. 46, 483001 (2013)
[3] C.V. Brown, G.G. Wells, M.I. Newton, and G. McHale, Nature Photon. 3, 403 (2009)
[4] G. McHale, C.V. Brown, M.I. Newton, G.G. Wells, N. Sampara, Phys. Rev. Lett. 107, 186101 (2011)
[5] G. McHale, C.V. Brown, and N. Sampara, Nature Comms. 4, 1605 (2013)