Anisotropic liquid dielectrophoresis and interfacial forces

There is a growing technology-driven interest in using external influences to move or shape small quantities of liquids, a process that is referred to as microfluidic actuation. Using electrical, rather than mechanical, forces to achieve this actuation is convenient because this involves relatively simple device architectures that contain no moving parts. Existing non-mechanical microfluidic actuation techniques that are driven by the application of a voltage include electrowetting, which only works with conducting liquids, and dielectrophoresis, which works with both conducting and non-conducting liquids. We have previously shown how dielectrophoresis forces in non-conducting isotropic liquids can be used to create not only forced wetting and liquid spreading, but also liquid film wrinkling in which an engineered electric field distribution imprints a replica of itself as a distortion pattern at the liquid-air interface of the film. In this proposal the possibility that liquid dielectrophoresis can lead to added functionality and greater control within a pure anisotropic liquid, i.e. a nematic liquid crystal rather than a simple isotropic liquid, will be investigated. Liquid dielectrophoresis in pure anisotropic liquids has not been studied before, either experimentally or theoretically. Our proposed integrated collaborative experimental and theoretical research approach aims to understand and exploit the forces that can be created within, and at the surface of, free and confined anisotropic liquids when they are subject to electric fields. The proposed research will investigate an exciting new possibility of using anisotropic liquids along with particular confinement geometries which allow voltage controlled actuated microfluidic pumping to be produced even with simplified electrode architectures. Our industrial supporters include Merck Chemicals Ltd, the world-leading researcher, developer and manufacturer of liquid crystals and reactive mesogens, together with Hewlett-Packard and ADT, who are developing the next generation of information displays based on liquid crystal and microfluidic effects.

New knowledge and methods generated:
(1) We have shown that the deformation of hemispherical droplets of a conducting liquid by the electric field in a capacitor obeys a scaling law in which the change in droplet height is proportional to the square of the electric field and the square of the radius of the droplet at its base. We have developed equations which also explain and describe the experimental dependences of this static deformation on the electric field and on the length-scales in the system when the droplet deviates away from an initial hemispherical shape. We have further explained and quantified the time dependent processes by which the droplets deform immediately after an electric field is applied on short timescales, which is dominated by electric and surface tension effects, before the longer timescale response which is dominated by surface tension and viscosity effect.
(2) We have developed a new relatively simple and compact manometer geometry with an applied electric field which allows a complete investigation of the competition between flow and electric field alignment in a nematic liquid crystal. Normally an electric field applied across a layer of nematic liquid crystal causes the molecules to reorient towards the direction of the electric field when a voltage above a critical value is applied to the plates that confine the liquid crystal - this is the well-known Freederiksz effect. We are able to use electrical forces to draw the nematic liquid crystal into one arm of the manometer and then remove the voltage so the height difference between the two arms decays back towards zero providing access to a range of shear/flow rates in a single experimental run. During this decay we have been able to observe, and explain theoretically, how the flow delays Freedericksz transition to higher voltage.
(3) We have shown how to electrically induce spreading of drops of nematic liquid crystals to create forced wetting on a solid surface. We have quantified and can explain the observed voltage dependence for conventional nematic liquid crystals at different temperatures, and for highly dispersive nematics at different frequencies of the applied voltage. When the droplet has been driven into a film we can create a periodic surface deformation. The research has generated new insight into the role of the alignment of the molecules in the liquid, and associated internal elasticity, in both the spreading and the wrinkling phenomena. We are finishing work to further develop these techniques to provide a new method to observe the effects of, and quantify, certain physical parameters that are usually difficult to measure in nematic liquid crystals.