  This research introduces a new drop fluidics, which uses a deformable and
stretchable elastomeric film as the platform, instead of the commonly used
rigid supports. Such a soft film impregnated with magnetic particles can be
modulated with an external electromagnetic field that produces a vast array of
topographical landscapes with varying surface curvature, which, in conjunction
with capillarity, can direct and control motion of water droplets efficiently
and accurately. When a thin layer of oil is present on this film that is
deformed locally, a centrosymmetric wedge is formed. A water droplet placed on
this oil laden film becomes asymmetrically deformed thus producing a gradient
of Laplace pressure within the droplet setting it to motion. A simple theory is
presented that accounts for the droplet speed in terms of such geometric
variables as the volume of the droplet and the thickness of the oil film
covering the soft elastomeric film, as well as such material variables as the
viscosity of the oil and interfacial tension of the oil-water interfaces.
Following the verification of the theoretical result using well-controlled
model systems, we demonstrate how the electromagnetically controlled
elasto-capillary force can be used to manipulate the motion of single and/or
multiple droplets on the surface of the elastomeric film and how such
elementary operations as drop fusion and thermally addressed chemical
transformation can be carried out in aqueous droplets. It is expected that the
resulting drop fluidics would be suitable for digital control of drop motion by
simply switching on and off the electromagnetic fields applied at different
positions underneath the elastomeric film. We anticipate that this method of
directing and manipulating water droplets is poised for its applications in
various biochemical reaction engineering, an example of which is Polymerase
Chain Reaction (PCR).
