Electrowetting is the manipulation by
electric field of a liquid's wetting properties. Typically, electrowetting is studied by applying an electric field
between a metal surface, coated with an insulating layer, and a liquid (Fig. 1) [1]. The insulating layer
is used to prevent electrolysis,
resulting in what is commonly referred to as electrowetting
on dielectric (EWOD).
Electrowetting has become a widely used tool for manipulating small amounts of liquids on surfaces. Applications range
from ‘lab-on-a-chip’ devices to adjustable lenses to novel electronic displays [2].
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| Fig. 1 - Electrowetting experimental setup (not to scale). |
Watch electrowetting at work, click here to watch a video.
Qualitatively, we are able to manipulate the wetting properties of a liquid by controlling the surface
concentration of charged species. We know, for example, from making bubbles with
soapy water that when a solute is enriched at an interface, the surface tension decreases when the solution concentration
is increased. Thus, if can somehow “push” the solute particles away
from the interface, or alternatively crowd them onto the interface, we can control that interface’s surface tension. This is essentially what we are doing by applying an electric field to the liquid.
Quantitatively, we can explain this using Gibbsian interfacial thermodynamics. Some relationships which we must consider in a quantitative explanation are: the contact angle dependence
on interfacial tensions (Young’s Equation, Eqn. (1)), and the interfacial tension dependence on applied voltage (Eqn.
(2)).
Where
σsl
= solid liquid interfacial tension
σsv
= solid vapor interfacial tension
σlv
= liquid vapor interfacial tension
ρsl
= solid liquid surface charge density
θ = contact
angle
U = applied voltage
In an EWOD situation
(Fig. 1), we can assume that the voltage drop across the dielectric layer is much much larger than the drop across the electric
double layer, which gives c ≈ cd = εoεd
/d (c = total capacitance, cd = metal/insulator/liquid capacitance, d = dielectric layer thickness, εd = dielectric constant
of the insulator). With this, and assuming the liquid is a perfect conductor,
we can integrate (2) and get
Where
σ0,sl
= initial (unbiased) solid liquid interfacial tension
σsl (U)
= voltage dependant solid liquid interfacial tension
Substituting Young’s
equation (1) into (3) yields the contact angle dependence on applied voltage
In this equation,
we assume that the surface of the insulating layer does not give rise to spontaneous adsorption of charge in the absence of
an applied voltage, and that σlv is independent of voltage. Equation
(4) has been shown to be a good model for electrowetting so long as the applied voltage is not too high, at which the effect
begins to saturate out (i.e. contact angle becomes independent of voltage).
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