Electrowetting — From statics to dynamics |
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Authors: | Longquan Chen Elmar Bonaccurso |
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Affiliation: | Experimental Interface Physics, Center of Smart Interfaces, Technische Universität Darmstadt, Alarich-Weiss-Str. 10, 64287 Darmstadt, Germany |
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Abstract: | More than one century ago, Lippmann found that capillary forces can be effectively controlled by external electrostatic forces. As a simple example, by applying a voltage between a conducting liquid droplet and the surface it is sitting on we are able to adjust the wetting angle of the drop. Since Lippmann's findings, electrocapillary phenomena – or electrowetting – have developed into a series of tools for manipulating microdroplets on solid surfaces, or small amounts of liquids in capillaries for microfluidic applications. In this article, we briefly review some recent progress of fundamental understanding of electrowetting and address some still unsolved issues. Specifically, we focus on static and dynamic electrowetting. In static electrowetting, we discuss some basic phenomena found in DC and AC electrowetting, and some theories about the origin of contact angle saturation. In dynamic electrowetting, we introduce some studies about this rather recent area. At last, we address some other capillary phenomena governed by electrostatics and we give an outlook that might stimulate further investigations on electrowetting. |
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Keywords: | AC, alternating current DC, direct current EWOD, electrowetting on dielectric HD, hydrodynamic (model) MK, molecular kinetic (theory) A, area (m2) C, capacitance per unit area (F/m2) E, electric field strength (V/m) F, force (N) H, drop height (m) H?, characteristic drop height (m) L, characteristic length (m) LC, capillary length (m) R, drop wetting radius (m) R?, characteristic drop radius (m) R0, initial drop radius T, absolute temperature (K) U, contact line velocity (m/s) U?, characteristic contact line velocity (m/s) V, voltage, applied potential (V) VS, saturation voltage (V) Vth, threshold voltage (V) Veff, effective voltage (V) c, coefficient d, thickness (m) f, frequency (Hz) f0, molecular jump frequency (Hz) fC, critical frequency (Hz) g, gravitational acceleration (m/s2) kB, Boltzmann constant l, slip length (m) t, time (s) α, wetting exponent γ, surface tension (N/m) γLS, liquid&ndash solid interfacial tension (N/m) γSV, solid&ndash vapor interfacial tension (N/m) ε, relative permittivity ε0, free space permittivity λ, molecular displacement (m) μ, viscosity (Pa s) θeq, equilibrium contact angle θ, contact angle θA, advancing contact angle θR, receding contact angle Δθ, contact angle hysteresis ρ, density (kg/m3) σ, conductivity (S) |
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