Two distinct, stable alignment states have been observed for a nematic liquid crystal confined in a layer with thickness of 12 m and in square wells with sides of length between 20 and 80 m. 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.

The wetting of solid surfaces can be modified by altering the surface free energy balance between the solid, liquid and vapour phases. Here we show that liquid dielectrophoresis (L-DEP) induced by non-uniform electric fields can be used to enhance and control the wetting of dielectric liquids. In the limit of thick droplets, we show theoretically that the cosine of the contact angle follows a simple voltage squared relationship analogous to that found for electrowetting-on-dielectric (EWOD). Experimental observations confirm this predicted dielectrowetting behavior and show that the induced wetting is reversible. Our findings provide a non-contact electrical actuation process for meniscus and droplet control.

There has been intense recent interest in photonic devices based on microfluidics that include displays [1,2] and refractive tunable microlenses and optical beamsteerers [3][4][5] that work using the principle of electrowetting [6,7]. Here we report a novel approach to optical devices in which static wrinkles are produced at the surface of a thin film of oil as a result of dielectrophoretic forces [8][9][10]. We have demonstrated this voltage programmable surface wrinkling effect with periodic devices with pitch lengths of between 20 µm and 240 µm and with response times of less than 40 µs. By careful choice of oils, it is possible to optimise either for high amplitude sinusoidal wrinkles at micrometer-scale pitches or for more complex non-sinusoidal profiles with higher Fourier components at the longer pitches. This provides the possibility for rapidly responsive voltage programmable polarisation insensitive transmission and reflection diffraction devices and for arbitrary surface profile optical devices. * E-mail: carl.brown@ntu.ac.uk, Tel: 44 115 8483184 2 The structure of the device is shown in figure 1. The side view, in figure 1(a), shows the glass substrate coated with patterned gold/titanium conducting electrodes, on which there is a thin solid dielectric layer (either photoresist or a dielectric stack), upon which is coated a thin layer of oil. The electrodes were arranged as an array of stripes parallel to the y direction in the xy plane. This geometry allowed every other electrode to be electrically connected as shown in the plan view in figure 1(b).The electrically induced wrinkling at the oil surface will be considered first for a device with an electrode pitch of p = 80 µm. When a small volume (0.1 µL) of 1-decanol was initially dispensed onto the device it formed a spherical cap with a contact angle of 5°. Every other stripe in the electrode array was biased with an A.C. voltage with r.m.s. magnitude V O and the inter-digitated stripes between them were earthed, as shown in figure 1. This creates a periodic electric field profile in the plane of the oil layer which is highly non-uniform. A polarisable dielectric material in a region containing non-uniform electric fields experiences a force, known as a dielectrophoretic force, in the direction of the increase in magnitude of the electric field [8][9][10]. When the r.m.s. electrode voltage was greater than V O = 20 Volts the dielectrophoretic forces spread the oil into a thin film with a uniform thickness, h = 12 µm, across the area covered by the electrodes.Increasing the voltage between neighbouring electrodes gave rise to a periodic undulation at the surface of the oil. The period of the wrinkle was equal to the electrode pitch, 80 µm, and the peaks and troughs of the wrinkle lay parallel to the electrode fingers along the y-direction. This undulation arises because the highest electric field gradients occur in the gaps between the electrodes and so the dielectrophoretic forces in these regions cause the oil to collect there preferentially. The...

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 nonwetting 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 Tanner's 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.

Dewetting films are not the time reversal of spreading droplets.