Electrowetting charges the solid-liquid interface to change the contact area of a droplet of a conducting liquid. It is a powerful technique used to create variable focus liquid lenses, electronic paper and other devices, but it depends upon ions within the liquid. Liquid dielectrophoresis (L-DEP) is a bulk force acting on the dipoles throughout a dielectric liquid and is not normally considered to be a localized effect acting at the interface between the liquid and a solid or other fluid. In this work, we show theoretically how non-uniform electric fields generated by interdigitated electrodes can effectively convert L-DEP into an interface localized form. We show that for droplets of sufficient thickness, the change in the cosine of the contact angle is proportional to the square of the applied voltage and so obeys a similar equation to that for electrowetting – this we call dielectrowetting. However, a major difference to electrowetting is that the strength of the effect is controlled by the electrode spacing and the liquid permittivity rather than the properties of an insulator in a sandwich structure. Experimentally, we show that that this dielectrowetting equation accurately describes the contact angle of a droplet of oil viewed across parallel interdigitated electrodes. Importantly, the induced spreading can be complete, such that contact angle saturation does not occur. We then show that for thin films, L-DEP can shape the liquid-air interface creating a spatially periodic wrinkle and that such a wrinkle can be used to create a voltage programmable phase diffraction grating.
We have recently reported a novel approach to producing voltage programmable optical devices in which static wrinkles
are created at the surface of a thin film of oil [<i>Nature Photonics</i> 3(7), 403 (2009)]. The oil coats a 2d electrode pattern
and dielectrophoretic forces created from the non-uniform fringing electric field profiles near to the electrodes determine
how this pattern is "imprinted" at the remote oil/air interface. Sinusoidal wrinkles have been created on the surface of
decanol oil with pitch lengths of between 20 and 240 micrometers and have been rapidly switched on in less than 40
microseconds. Non-sinusoidal surface wrinkles with higher harmonics appear when electrodes with the larger pitches are
used in conjunction with an oil that has a lower dielectric constant, for example hexadecane. It is found that the higher
Fourier components of the shape of the periodic wrinkle decay faster than the fundamental component as the thickness of
the oil film is increased.