Capillary and electrostatic limitations to the contact angle in electrowetting-on-dielectric

Abstract

The shape of a conducting liquid droplet placed on a hydrophobic dielectric surface is simulated numerically by solving the Laplace-Young capillary equation. The electric force, acting on the conducting surface, distorts the droplet shape leading to a change in the apparent contact angle; its variation is compared with a theoretical Young-Lippman prediction. At sufficiently large values of voltage, applied to the droplet, the numerical algorithm fails to converge, which is interpreted as the break-up of the droplet surface with small droplets being ejected from the surface. These highly charged droplets, as well as any other electric charges near the triple contact line, generated for example by the electric corona discharge, cause a change of the distribution of the electric forces. This effect can be helpful in explaining saturation of the apparent contact angle: an appropriately selected surface charge near the contact line can completely stop droplet distortion, and the contact angle variation, despite the increased droplet voltage. Furthermore, the simulation results show the effect of the permittivity of the medium surrounding the droplet, on the contact angle variation.

Effects of dynamic contact angle on numerical modeling of electrowetting in parallel plate microchannels

“…Adamiak (2006) nicely summarized concerns regarding contact line and necessity of considering macroscopic contact angle far enough from it. Since treatment of the interface is so that it captures approximate location of the contact line as described in Sect.…”

Electrowetting on dielectric driven droplet resonance and mixing enhancement in parallel-plate configuration

“…2,7,26,27 It infers that at the same driving frequency, a greater droplet deformation can be excited by actuating the droplet with a larger alternating driving electric signal. For the electric driving potential used in this study (see Fig.…”

“…1) has become a major platform for driving droplets in microfluidic systems. Fundamentals of EWOD, including basic operations on a droplet, 2-4 dynamics of contact angle variation, 5,6 and contact angle saturation 7,8 have been extensively studied. Integrated functional chips 9-11 and in-droplet particle manipulation 12,13 based on EWOD have also been demonstrated.…”

Dynamics Behaviors of Droplet on Hydrophobic Surfaces Driven by Electric Field

“…Another method is to apply an external force to drive the dynamic behavior of sessile droplets. The existing methods include applied electric fields [13,14,15,16,17], magnetic fields [18], pressure [19], air flow [20,21,22], laser [23] and so on. In these methods, the electric field driving the dynamic behavior of sessile droplets has been developed as a relatively technology exhibiting promising perspective for the droplet control [24,25].…”

“…Wei et al [16] experimentally conducted the rolling behavior of the water droplet on superhydrophobic surfaces under electrical fields, and built a finite element modeling (FEM) simulation model to indicate that an electrostatic force produced by electrical fields drove a water droplet to roll. Adamiak [17] numerically simulated the deformation of an ideally conducting liquid droplet deposited on the flat dielectric surfaces by solving the capillary Laplace–Young equation. Liu [26] conducted numerical simulations and experiments on the dynamic mechanism of water droplet formation with different applied voltages and droplet distribution, and drove surface discharges on the insulator surface under an AC electric field.…”