Coplanar electrowetting-induced stirring as a tool to manipulate biological samples in lubricated digital microfluidics. Impact of ambient phase on drop internal flow pattern

Abstract

Oscillating electrowetting on dielectrics (EWOD) with coplanar electrodes is investigated in this paper as a way to provide efficient stirring within a drop with biological content. A supporting model inspired from Ko et al. [Appl. Phys. Lett. 94, 194102 (2009)] is proposed allowing to interpret oscillating EWOD-induced drop internal flow as the result of a current streaming along the drop surface deformed by capillary waves. Current streaming behaves essentially as a surface flow generator and the momentum it sustains within the (viscous) drop is even more significant as the surface to volume ratio is small. With the circular electrode pair considered in this paper, oscillating EWOD sustains toroidal vortical flows when the experiments are conducted with aqueous drops in air as ambient phase. But when oil is used as ambient phase, it is demonstrated that the presence of an electrode gap is responsible for a change in drop shape: a pinch-off at the electrode gap yields a peanut-shaped drop and a symmetry break-up of the EWOD-induced flow pattern. Viscosity of oil is also responsible for promoting an efficient damping of the capillary waves which populate the surface of the actuated drop. As a result, the capillary network switches from one standing wave to two superimposed traveling waves of different mechanical energy, provided that actuation frequency is large enough, for instance, as large as the one commonly used in electrowetting applications (f $ 500 Hz and beyond). Special emphasis is put on stirring of biological samples. As a typical application, it is demonstrated how beads or cell clusters can be focused under flow either at mid-height of the drop or near the wetting plane, depending on how the nature of the capillary waves is (standing or traveling), and therefore, depending on the actuation frequency (150 Hz-1 KHz).

Biosensing at Drop Scale Assisted by Electrowetting at Moderate Frequency

“…2, pure water leads to a solid-liquid surface pressure apparently larger than the one of PBS buffer. This is in agreement with the fact that the PBS buffer is characterized by an averaged concentration of about The variables R 0 and A n denote the drop radius and the amplitude of standing drop oscillations [7] for the eigenmode n. Contact angle measurements presented here are performed for n = 2 with T 2 =0.15 or T 2 =0.30 (Fig. 1).…”

“…The additional capacity contribution d oil /ε 0 ε oil is due to the presence of an ultrathin film of silicone oil (thickness: d oil = 260 nm, permittivity: ε oil = 2) which warrants lubrication of the solid surface [6,7]. Finally, the surface tensions γ S G and γ LG being constant, the variation of γ S L , expressed in term of the physicochemical surface pressure, Π S L , writes, according to the latter equations:…”

“…2). And the numerical calculations of drop flow patterns [7] show that, in contrast to mode n=2 for which the vortices are observed throughout the drop, the vortices observed at high mode, n=6, concentrate within the Stuart layer standing along the drop surface with a steady corner eddy located near the contact line [9]. This could explain why the efficiency of stirring-enhanced surface ageing might be higher for modes 4 and 6; a point which is in line with the presented measurements.…”

“…The present paper shows how coplanar EWOD in the oscillating regime (see electrode pair design in Fig. 1 of [7]) could reveal helpful for label-free biosensing based on such DCA measurements. Here, extensive use is made of BSA molecule since it is currently involved in heterogeneous reactions, for instance in the case of avidin adsorption on (BSA-coated) oxide surfaces of biochips [5].…”

Improving the dielectric properties of an electrowetting-on-dielectric microfluidic device with a low-pressure chemical vapor deposited Si3N4 dielectric layer

“…In this work, we used an EWOD chip of another kind with coplanar electrodes. The electrodes are designed on only the bottom plate of a EWOD DMF chip 20,21 to actuate droplets. The top plate has a hydrophobic layer coated on a glass substrate, which is not involved in droplet actuation; this feature provides much potential for integrating additional functions on the chip.…”