C. V. Brown

Voltage-dependent anchoring of a nematic liquid crystal on a grating surface

The switching properties of most liquid-crystal electro-optic devices rely mainly on the reorientation of the average molecular direction (director) within the bulk of the liquid-crystal layer. Reorientation of the director at or near the surfaces of the layer usually has an insignificant effect on device performance. Here we describe a different configuration in which a nematic liquid crystal is placed between a flat surface treated to induce a parallel anchoring of the director and a grating surface treated to give a perpendicular anchoring. We show that this configuration leads to an effective azimuthal anchoring at the grating surface that depends on the applied voltage when the nematic phase has negative dielectric anisotropy (that is, the director has a tendency to align perpendicular to the applied field). This leads to a voltage-controlled twist effect in the liquid-crystal cell that is highly sensitive to the grating profile. Furthermore, this twist effect possesses an electro-optic response which is far less dependent on viewing angle compared to many other liquid-crystal display configurations. We therefore suggest that this technology might find application in the next generation of liquid-crystal displays.

Multistable alignment states in nematic liquid crystal filled wells

Two distinct, stable alignment states have been observed for a nematic liquid crystal confined in a layer with thickness of 12μm and in square wells with sides of length between 20 and 80μm. The director lies in the plane of the layer and line defects occur in two corners of the squares. The positions of the defects determine whether the director orientation is across the diagonal or is parallel to two opposite edges of the square. The device is multistable because both the diagonal and parallel states are stable when rotated by multiples of 90° in plane.

Double minimum in the surface stabilized ferroelectric liquid crystal switching response

A double minimum has recently been observed in the time–voltage switching response for a smectic C* liquid crystal layer in the surface stabilized geometry (“Ferroelectric Liquid Crystal Device,” K. P. Lymer and J. C. Jones, U.K. Patent No. GB2338797, 17th June 1999). Liquid crystal continuum theory is used to demonstrate that this unusual switching behavior arises if the equilibrium orientation of the molecular director rotates around the smectic cone as a function of distance through one half of the layer only. The double minimum is shown to evolve for large differences between the e2 and e1 components of the smectic C biaxial permittivity tensor.

Voltage-induced spreading and superspreading of liquids

The ability to quickly spread a liquid across a surface and form a film is fundamental for a diverse range of technological processes, including printing, painting and spraying. Here we show that liquid dielectrophoresis or electrowetting can produce wetting on normally non-wetting surfaces, without needing modification of the surface topography or chemistry. Additionally, superspreading can be achieved without needing surfactants in the liquid. Here we use a modified Hoffman-de Gennes law to predict three distinct spreading regimes: exponential approach to an equilibrium shape, spreading to complete wetting obeying a Tanners law-type relationship and superspreading towards a complete wetting film. We demonstrate quantitative experimental agreement with these predictions using dielectrophoresis-induced spreading of stripes of 1,2 propylene glycol. Our findings show how the rate of spreading of a partial wetting system can be controlled using uniform and non-uniform electric fields and how to induce more rapid superspreading using voltage control.

Method for the measurement of the K22 nematic elastic constant

A technique has been developed for the measurement of the K22 twist elastic constant in nematic liquid crystal materials. This involves the measurement of the Freedericksz transition voltages in untwisted linear and π-twist regions in a wedge cell geometry. The method avoids the need for the accurate determination of the cell thickness and cholesteric pitch and is far more straightforward to implement than other methods in the literature. The validity of this method is demonstrated for the well-characterized material E7.

Optical diffraction from a liquid crystal phase grating

The finite-difference time-domain method has been used in the numerical analysis of the optical diffraction properties of a liquid crystal phase grating. The grating is formed using a nematic material that is switched using striped electrodes with a unity mark-space ratio. Three different surface pretilts have been investigated: 0°, 30° parallel, and 30° antiparallel. The tilt geometry determines the degree of suppression of the zero order and the asymmetry of the diffraction. Highly efficient beam-steering devices are shown to be possible using the antiparallel tilt alignment.

Study of elastic constant ratios in nematic liquid crystals

A technique has recently been proposed for determining the elastic constant ratio K22∕K11 in nematic liquid crystals [E. P. Raynes, C. V. Brown, and J. F. Stromer, Appl. Phys. Lett. 82, 13 (2003)]. This technique has been applied to five nematic materials that cover a range of K33∕K11 elastic constant ratio values. An unusually high value of K22∕K11 is found in one material. High values of the ratio K22∕K11 tended to occur in materials that also have high values of the ratio K33∕K11. The results are validated with independent measurements of K22 from cholesteric helix unwinding.

Amplitude scaling of a static wrinkle at an oil-air interface created by dielectrophoresis forces

Dielectrophoresis forces have been used to create a static periodic wrinkle with a sinusoidal morphology on the surface of a thin layer of 1-decanol oil. The surface deformation occurs when a voltage V is applied between adjacent coplanar strip electrodes in an interdigitated array onto which the oil film is coated. It has been shown experimentally that the peak-to-peak amplitude A of the wrinkle scales according to the functional form A ∝ V2 exp(-αh/p) for a range of oil film thicknesses h (between 15 and 50 μm) and wrinkle pitches p (160, 240, and 320 μm).

Numerical analysis of nematic liquid crystal alignment on asymmetric surface grating structures

The influence of an asymmetric periodic grooved cell surface on the 2D static director configuration of a nematic liquid crystal has been investigated. The minimum in the Frank-Oseen free energy was solved numerically with the Rapini-Papoular form of the surface anchoring energy at the nematic-grating interface. Results are presented for the variation of pretilt angle in the tilted bulk director field as a function of the surface groove depth, pitch and asymmetry and the bulk parameters. The simulations demonstrate the existence of two energetically degenerate high and low pretilted bulk alignment configurations. The pretilt values in these two regimes and also for the low tilt regime with finite surface anchoring are consistent with experimental results. An effective increase in the resolution of the model is obtained by using an irregular grid to describe the surface profile.

Not spreading in reverse: The dewetting of a liquid film into a single drop

Dewetting films are not the time reversal of spreading droplets. Wetting and dewetting are both fundamental modes of motion of liquids on solid surfaces. They are critically important for processes in biology, chemistry, and engineering, such as drying, coating, and lubrication. However, recent progress in wetting, which has led to new fields such as superhydrophobicity and liquid marbles, has not been matched by dewetting. A significant problem has been the inability to study the model system of a uniform film dewetting from a nonwetting surface to a single macroscopic droplet—a barrier that does not exist for the reverse wetting process of a droplet spreading into a film. We report the dewetting of a dielectrophoresis-induced film into a single equilibrium droplet. The emergent picture of the full dewetting dynamics is of an initial regime, where a liquid rim recedes at constant speed and constant dynamic contact angle, followed by a relatively short exponential relaxation of a spherical cap shape. This sharply contrasts with the reverse wetting process, where a spreading droplet follows a smooth sequence of spherical cap shapes. Complementary numerical simulations and a hydrodynamic model reveal a local dewetting mechanism driven by the equilibrium contact angle, where contact line slip dominates the dewetting dynamics. Our conclusions can be used to understand a wide variety of processes involving liquid dewetting, such as drop rebound, condensation, and evaporation. In overcoming the barrier to studying single film-to-droplet dewetting, our results provide new approaches to fluid manipulation and uses of dewetting, such as inducing films of prescribed initial shapes and slip-controlled liquid retraction.