Fig. 3: System design for encapsulation and ejection.

a Larger droplet \(\left({R}_{d}=50 \, \upmu {{\rm{m}}}\right)\) can encapsulate the bead at a much lesser supply voltage (\({V}_{s}\approx 85V\)). b Smaller droplet \(\left({R}_{d}=20 \, \upmu {{\rm{m}}}\right)\) requires a larger supply voltage (\({V}_{s}\approx 135V\)) to overcome the capillary force and encapsulate the bead. At the lower voltage (\({V}_{s} \, \approx \, 117 \, {{\rm{V}}}\)) the dielectrophoretic force cannot overcome the capillary force to encapsulate the bead into the smaller droplet (It was enough to encapsulate the bead in the \({R}_{d}=25 \, \upmu {{\rm{m}}}\) droplet). c The low viscosity of silicone oil 1 cSt enables the ejection of the bead from the droplet at \({V}_{s} \, \approx \, 0.1\, {{\rm{V}}}\). With increase in oil viscosity the increased dissipation of the kinetic energy of the droplet prevents complete separation from the bead. d Silicone oil 1 cSt (dashed line) renders the hydrophilic (in air, dotted line) streptavidin surface slightly hydrophobic. The surfactant Span80 further reduces the interfacial tension between the silicone oil and the reagent and increases the contact angle (solid line) that the reagent droplet forms on a streptavidin surface making it hydrophobic. It is critical for the ejection of the bead from the droplet. Interfacial tension and contact angle measurements are recorded with time variations to account for the surface adsorption. (Refer to Experimental Procedure for Bead-Droplet Interaction subsection of Methods for details.) The streptavidin and glass thicknesses are not drawn to scale. The scale bars on experimental frames in (a, b) represent \(40 \, \upmu {{\rm{m}}}\). The symbol ϕ in the color plots is the phase variable in the phase field simulations. \(\phi=-1\) in the oil medium (represented in blue) and \(\phi=1\) in the aqueous medium (represented in red). \(-1 < \phi < 1\) represents the boundary region between the droplet and the oil medium.