Mass transfer enhancement and surface functionalization in digital microfluidics using AC electrowetting: the smaller, the better

This paper addresses modelling and computation of stirring and chemical transport at drop scale due to shape oscillations of small amplitude induced by coplanar AC electrowetting on dielectrics (EWOD). An axisymmetric flow inside a drop deformed by standing capillary waves is promoted by coplanar EWOD in the oscillating regime ( 10-500 Hz). This can be seen as a new stirring/mixing mechanism at microscale provided that a deformable fluid interface is available. The latter requirement is fully satisfied in biomedical applications of digital microfluidics where droplets of volume similar to 1 mu L are commonly handled. A steady streaming model at large Strouhal number is taken into account in order to investigate time-dependent chemical transport in a sessile drop under oscillating EWOD. Numerical calculations based on finite element method are performed to predict the transport of a biochemical solute with possible affinity for drop surface and supporting solid surface as well. A dedicated weak form is also derived to take into account diffusional transport along the liquid surface. Mass transfer enhancement is investigated by considering the variance of the chemical concentration in the transient regime. Special attention is given to investigate mass transfers and surface ageing by considering the respective roles of the Thiele number, the Damkohler number, the Peclet and Surface Peclet numbers.