Abstract
The electrostatically assisted wettability enhancement of dielectric solid surfaces, commonly termed as electrowetting-on-dielectric (EWOD), facilitates many microfluidic applications due to simplicity and energy efficiency. The application of a voltage difference between a conductive droplet and an insulated electrode substrate, where the droplet sits, is enough for realizing a considerable contact angle change. The contact angle modification is fast and almost reversible; however it is limited by the well-known saturation phenomenon which sets in at sufficiently high voltages. In this work, we experimentally show and computationally support the effect of elasticity and thickness of the dielectric on the onset of contact angle saturation. We found that the effect of elasticity is important especially for dielectric thickness smaller than 10 μm and becomes negligible for thickness above 20 μm. We attribute our findings on the effect of the dielectric thickness on the electric field, as well as on the induced electric stresses distribution, in the vicinity of the three phase contact line. Electric field and electric stresses distribution are numerically computed and support our findings which are of significant importance for the design of soft materials based microfluidic devices.
Highlights
In a typical EWOD configuration, the application of an electric voltage difference between an insulated metal substrate and a conductive liquid mass, e.g. droplets, introduces an excess of electrostatic energy
Contrary to Lippmann’s prediction indicating complete wetting at sufficiently high voltage, the reduction of the apparent angle reaches a limit; the limiting phenomenon is widely known as contact angle saturation.[14,15,16]
Our static contact angle and hysteresis measurements accord with the latter observations showing that the advancing apparent contact angle and the hysteresis increase as the substrates’ elastic modulus decreases
Summary
Electrowetting-on-dielectric (EWOD) is already established as a key methodology for active control of the apparent wettability[1] and mobility of liquid droplets.[2,3] Electrowetting, i.e. the electrostatic enhancement of solid wettability, has been utilized in technological applications such as lab-on-a-chip devices,[4,5,6] optofluidic displays[7,8] as well as liquid lenses[9,10] and energy harvesting systems.[11,12] In a typical EWOD configuration, the application of an electric voltage difference between an insulated metal substrate (a flat metal electrode coated by a thin dielectric layer) and a conductive liquid mass, e.g. droplets, introduces an excess of electrostatic energy. Other groups have reported lower saturation angles of about 15–301 using ionic liquids[25] or glycols,[17] these results have no impact on the fact that the limitation of the electrostatic wettability enhancement is universal for any EWOD configuration Several applications, such as microfluidics, involve soft substrates. For thinner dielectrics (d o 10 mm), the field distribution is sharper and the electrical stress is focused at the vicinity of the TPL, explaining the deviation of the microscopic contact angle from Youngs angle that is observed in electrowetting experiments on very thin insulators.[36] These studies have shown that the liquid profile at the TPL highly sharpens due to high intensification of the electrostatic stress at that region, as the dielectric thickness decreases to less than a few tens of microns. We use two liquids, water and propylene glycol, to extend the validity of our observations
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