Contact Angle Saturation in Electrowetting

Abstract

Electrowetting is the phenomenon of contact angle decrease under the influence of an external voltage applied across the solid/liquid interface. Electrowetting offers an interesting possibility to enhance the wettability of hydrophobic materials without altering the chemical composition of the system and thus could be incorporated in various microfluidic devices. Electrowetting is fundamentally an electrocapillary effect occurring on an insulated solid electrode (hence the change of the solid/liquid interfacial tension with voltage follows Lippmann's equation). A limiting contact angle value larger than zero is achieved even at very large external voltages. Saturation precludes full wetting of the substrate and restricts the magnitude of the capillary force variation. Contact angle saturation has been given various interpretations (e.g., charge trapping, air ionization) but appears to reflect a natural thermodynamic limit rather than being simply a defective property. The limiting value of the contact angle is given by the Young equation when the value of the solid/liquid interfacial tension reaches zero. The model is in excellent agreement with our own results and often gives an adequate description of published data. It also suggests that the saturation limit is determined by the material properties of the system and electrowetting at voltages exceeding this threshold is essentially a nonequilibrium process.

Wettability Increase by “Corona” Ionization

“…' EFFECTS OF HUMIDITY ON THE CONTACT ANGLE SATURATION As shown in the section above, in our experiments the saturation regime for the contact angle has been reached and the contact angle saturation value is smaller than the limit predicted by Quinn et al 8 This result is consistent with the comparison made in Table 1 12 attributing saturation to air ionization was more concurrent that a limiting effect. Recently, additional light has been shed on the saturation effect in liquidÀvaporÀsolid 33 and in liquidÀ liquidÀsolid, 30,34 attributing the saturation to trapped charge.…”

“…has been experimentally demonstrated 8 for a wide range of voltage values prior to a saturation regime. In eq 1, θ 0 is the contact angle before the voltage is applied, θ V is the contact angle after a voltage V is applied, ε 0 is the vacuum permittivity, ε r and d are the relative permittivity and the thickness of the dielectric layer, respectively, and γ LV is the surface tension of the liquidÀgas interface.…”

“…Vallet et al 12 attributed to air ionization the saturation phenomena of the contact angle, Blake 13 used a corona charging procedure to assist the study of contact angle change for a constant speed moving substrate, and Arifin et al 14 investigated the effect of the electrohydrodynamic thrust on the movement of particles suspended on a fluid. Moreover, Quinn et al 8,15 attributed contact angle saturation to the fact that the surface tension between liquid and solid Received: January 31,2011 ABSTRACT: Experiments showing an increase in the wettability of a hydrophobic surface when using corona air ionization are shown. Photoluminiscence observations support the predictions of charge accumulation at the triple line and confirm previous experiments.…”

The Nonlinear Phenomena of Thin Polydimethylsiloxane (PDMS) Films in Electrowetting

“…The contact angle change is small or the contact angle does not decrease, when the voltage is over a critical voltage. This phenomenon is called the contact angle saturation [31,32]. The "modified" Lippmann's equation [30] (6) where s θ is the contact angle at saturation, L is…”

An Electrowetting Model for Rough Surfaces Under Low Voltage

“…The classical effect of electrowetting on dielectric (EWOD) [5,6] is to make a droplet flatten and spread by applying voltage between the electrodes in the droplet and under the dielectric coating. As the electrical control of the microfluidic motion is significantly more promising for microdevices and easy to manipulate, electrowetting has attracted much research interest [7][8][9][10][11][12][13].…”