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Title: Voltage induced spreading and liquid optical devices
Author: Sampara, N.
Awarding Body: Nottingham Trent University
Current Institution: Nottingham Trent University
Date of Award: 2013
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An experimental investigation has been carried out into Liquid dielectrophoresis (LDEP), in which dielectric forces are used to actuate bulk liquid spreading and to imprint a deformation pattern at a liquid air surface. Our basic conguration uses a set of co-planar stripe interdigital electrodes to create a non-uniform A.C. electric field that exponentially decays in amplitude and penetrates above the structure into a dielectric liquid droplet. Dielectric forces drive a voltage-induced spreading of the droplet into a low contact angle droplet/film, an effect which will be referred to here as "dielectrowetting". Further application of the A.C. voltage to the electrodes results in a static sinusoidal wrinkle forming at the oil-air interface on the spread film. Three main areas of investigations are reported, the first of which is a study of the statics and dynamics of the voltage induced spreading of a stripe of the material 1,2 propylene glycol. In the limit of thick droplets, a theoretical prediction has been derived that the cosine of the static equilibrium contact angle is proportional to the square of the amplitude of the voltage applied to the inter-digital electrodes. This relationship is analogous to that found for electrowetting-on-dielectric (EWOD). Experimental observations confirm this predicted dielectrowetting behavior for the advancing static contact angle of the voltage induced spreading of a stripe of the material 1,2 propylene glycol. The predicted scaling relationship with the electrode dimensions has been confirmed for electrodes of linewidth 20 um, 40 um and 80 um. It is also shown that with an appropriate surface treatment the induced wetting is made reversible. The dynamic contact angle ( o) of the liquid droplet was also measured as a function of voltage during spreading and the results are found to agree with the predictions of the Hoffman-de-Gennes macroscopic viscous dissipation approach extended to account for the electric field. Three distinct regimes have been investigated experimentally and theoretically: partial wetting and exponential approach to equilibrium shape, spreading to complete wetting obeying a Tanner's law relationship, and super-spreading towards a complete wetting film. The second area is the fabrication of the solid phase transmission grating with a minimized zeroth order using LDEP and a UV curable liquid resin. A new electric-field assisted approach to fabricate solid diffractive optical devices is demonstrated in which the optical properties of the device can be tuned by adjusting the applied voltage whilst the optical medium is in its liquid phase. Continuous adjustment of the voltage, and hence the optical diffraction pattern, during UV curing can produce a solid grating with suppressed intensity of the transmitted zero order. The third area is a study of the dynamics of the formation of a periodic wrinkle deformation at the oil-air interface on a spread dielectric liquid film. An experimental study of how the switch on time ton and the switch off/relaxation time toff of the wrinkle scales as a function of the applied voltage V is presented. The amplitude A(V, t) of the wrinkle as a function of time was obtained from fitting the time dependent intensities of the positive zeroth, first and second diffracted orders of laser light transmitted through the oil film assuming that it acts as a thin sinusoidal phase grating. The results for relaxation have been compared with an analytical expression for toff that has been derived by considering the LaPlace pressure restoring force at the oil-air interface under a creeping flow approximation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available