On the use of AC electrowetting for biosensing based on dynamic contact angle
Résumé
A model system based on bovine serum albumin (BSA)-laden drop is selected. AC ElectroWetting-On-Dielectrics (EWOD), increasingly used in digital microfluidics for medical assays, is implemented in coplanar configuration. The moderate frequency required by EWOD actuation enables shape oscillations of the BSA-laden sessile drop and promotes internal micro-streaming. The non-reversible impact of EWOD-induced micro-streaming upon surface pressures is investigated. In contrast to the liquidgas drop surface where thermodynamical equilibrium is instantaneously achieved, the adsorption of BSA molecules at the lubricated solid substrate can be characterized by a time-dependent solidliquid surface pressure. As demonstrated from dynamic contact angle (DCA) imaging, adsorption of BSA is promoted under AC EWOD. A significant dependence of the solidliquid surface pressure on the resonant capillary frequencies is observed: for a large enough actuation frequency corresponding to an oscillating mode, n = 6, numerical simulations are performed which show that corner eddies located in the Stuart boundary layer, near the oscillating contact line, yield an efficient molecular renewal. In this way, oscillating EWOD could be used purposely as a tool to speed up label-free detection from DCA measurements (C) 2016 Elsevier B.V. All rights reserved.