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Axisymmetric and Nonaxisymmetric Oscillations of Sessile Compound Droplets in an Open Digital Microfluidic Platform
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posted on 2017-09-18, 00:00 authored by Shubhi Bansal, Prosenjit SenManipulating droplets of biological fluids in an electrowetting on
dielectric (EWOD)-based digital microfluidic platform is a significant
challenge because of biofouling and surface contamination. This problem
is often addressed by operating in an oil environment. We study an
alternate configuration of sessile compound droplets having an aqueous
core surrounded by a smaller oil shell. In contrast to the conventional
EWOD platform, an open digital microfluidic platform enabled by the
core–shell configuration will allow electrical, mechanical,
or optical probes to get unrestricted access to the droplet, thus
enabling highly flexible and dynamically reconfigurable lab-on-chip
systems. Understanding droplet oscillations is essential as they are
known to enhance mixing. To our knowledge, this is the first study
of axisymmetric and nonaxisymmetric oscillations of compound droplets
actuated using EWOD platforms. Mode shapes for both axisymmetric and
nonaxisymmetric oscillations were studied and explained. Enhancement
in the axisymmetric oscillation of the core by decreasing the shell
volume was obtained experimentally and modeled theoretically. Smaller
shell volumes reduce the damping losses, allowing the appearance of
nonaxisymmetric modes over a larger range of operating parameters.
The oscillation frequency regime for obtaining prominent nonaxisymmetric
oscillations for different shell volumes was identified. Compound
droplets provide a mechanism to reduce biofouling, sample contamination,
and evaporation. We demonstrate axisymmetric and nonaxisymmetric oscillations
of compound droplets with the biological core of red blood cells,
providing crucial first steps for promoting applications such as rapid
efficient assays, mixing of biological fluids, and fluidic photonics
on hysteresis-free surfaces.