Equilibrium drop surface profiles in electric fields

Abstract

Electrowetting is becoming a more and more frequently used tool to manipulate liquids in various microfluidic applications. On the scale of the entire drop, the effect of electrowetting is to reduce the apparent contact angle of partially wetting conductive liquids upon application of an external voltage. Microscopically, however, strong electric fields in the vicinity of the three phase contact line give rise to local deformations of the drop surface. We determined the equilibrium surface profile using a combined numerical, analytical, and experimental approach. We find that the local contact angle in electrowetting is equal to Young’s angle independent of the applied voltage. Only on the scale of the thickness of the insulator and beyond does the surface slope assume a value consistent with the voltage-dependent apparent contact angle. This behaviour is verified experimentally by determining equilibrium surface profiles for insulators of various thicknesses between 10 and 250 µm. Numerically and analytically, we find that the local surface curvature diverges algebraically upon approaching the contact line with an exponent −1<μ<0. We discuss the relevance of the local surface properties for dynamic aspects of the contact line motion.

A numerical technique to simulate display pixels based on electrowetting

“…Close to the three-phase contact line, however, local electric fringe fields give rise to a strong deformation and high curvature of the liquid-liquid interface resulting from the balance of the local Maxwell stress and Laplace pressure at any point on the interface. As a result, Young's angle is still maintained on a local scale ≪ d (Buehrle et al 2003;Mugele and Buehrle 2007).…”

“…Buehrle et al (2003), Mugele and Buehrle (2007), the display pixel case requires a combination of electrohydrodynamic and electrowetting modelling. As long as the electrolytic aqueous phase does not have a contact point with the dielectric layer, the problem is purely electrohydrodynamic since there is no threephase contact line.…”

On-Chip Double Emulsion Droplet Assembly Using Electrowetting-on-Dielectric and Dielectrophoresis

“…Because of their reconfigurability, flexibility, and scalability, droplet-based microfluidic devices [11][12][13][14][15][16][17][18][19][68][69][70][71][72][73][74][75] promise to overcome some of the drawbacks of continuous flow systems -such as complexity, non-reconfigurability (application-specific). Several droplet actuation methods have been developed for droplet translocation, including electrostatic [13,14], thermocapillary [15,16], electrowetting [12,18,19,[28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][68][69][70]…”

Electrowetting on a lotus leaf

“…In this sense, EW is one of the most promising techniques to carry out elementary operations on droplets, such as generating, transporting, splitting, and merging, [24][25][26][27][28] but these operations can often be limited by an irreversible behavior. When applied voltage is larger than a certain voltage, the hydrophobic property of the sample surface could not recover completely.…”