Hydrophobic Dielectrics of Fluoropolymer / BaTiO3 Nanocomposites for Low-Voltage and Charge Storing Electrowetting Devic
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0949-C05-06
Hydrophobic Dielectrics of Fluoropolymer / BaTiO3 Nanocomposites for Low-Voltage and Charge Storing Electrowetting Devices Murali K. Kilaru1, Gui Lin2, James E. Mark3, and Jason C. Heikenfeld4 1 ECE, University of Cincinnati, Rhodes 933, Cincinnati, OH, 45221 2 Chemistry, University of Cincinnati, Crosley 1500, Cincinnati, OH, 45221 3 Chemistry, University of Cincinnati, Crosley 1501B, Cincinnati, OH, 45221 4 ECE, University of Cincinnati, Rhodes 836A, Cincinnati, OH, 45221
ABSTRACT A hydrophobic fluoropolymer:BaTiO3 nanocomposite is presented for achieving lower-voltage electrowetting operation. The composite fluoropolymer film exhibits as much as a 10X increase in measured dielectric constant. Increased surface roughness with BaTiO3 content increased the sessile drop contact angle from θ~104° to 121° with compositions ranging from 0 to 97 vol. % of BaTiO3 in the fluoropolymer. A larger change in electrowetted contact angle is observed for the 50 vol. % of BaTiO3 nanocomposite (∆θ~60°) compared to a standard fluoropolymer film of 1 um thickness (∆θ~30°). Furthermore, required operating voltage for electrowetting decreases by ~30V for films with high BaTiO3 content. Strong charge storage in the nanocomposite was observed via a new bistable electrowetting effect. For samples with strong charge storage, the droplet remains wetted (>2 minutes) even after removing the applied voltage. Rapid (< 1s) droplet de-wetting could be achieved by briefly applying a reverse polarity voltage to remove the stored charge in the nanocomposite. INTRODUCTION Electrowetting continues to see rapid growth in applications including optics [1], electronic paper[2], and lab-on chip devices[3]. These many applications create a broad-reaching research base for electrowetting. However, if electrowetting is to realize its fullest potential, new hydrophobic dielectric systems need to be developed. Particularly needed are new dielectrics which reduce electrowetting operating voltage. In order to appreciate routes for achieving lower operating voltage, the fundamental electrowetting effect must first be understood. Electrowetting of an electrically conductive liquid occurs when a voltage bias (V) is applied between the liquid and an underlying dielectric coated electrode. The dielectric is usually a fluoropolymer, which is hydrophobic. Hydrophobicity provides the water droplet with a large initial contact angle. Electrowetting is governed by the Young-Lippmann Eq.[4].
1 ε ⋅V 2 cos(θV ) = cos(θ 0 ) + ⋅ 2 γ ⋅z
(1)
where θ0 is Young’s contact angle, determined by interfacial surface tensions at zero applied voltage, θV is the electrowetted contact angle at an electrical potential of V, ε is the electric permittivity of the dielectric layer beneath the droplet, γ is the liquid/air interfacial surface
tension (~73 mN/m for water), and z is the thickness of dielectric layer. These parameters are depicted in Fig. 1.
FIG 1, Basic electrowetting operation. According to Eq. 1, in order to obtain the same contact angle change at lower volta
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