  The fluid-fluid interface is a complex environment for a floating object
where the statics and dynamics may be governed by capillarity, gravity,
inertia, and other external body forces. Yet, the alignment of these forces in
intricate ways might result in beautiful pattern formation and self-assembly of
these objects, as in the case of bubble rafts or colloidal particles. While
interfacial self-assembly has been explored widely, controlled manipulation of
floating objects, e.g. drops, at the fluid-fluid interface still remains a
challenge largely unexplored. In this work, we reveal the self-assembly and
manipulation of water drops floating at an oil-air interface. We show that the
assembly occurs due to electrostatic interactions between the drops and their
environment. We highlight the role of the boundary surrounding the system by
showing that even drops with a net zero electric charge can self-assemble under
certain conditions. Using experiments and theory, we show that the depth of the
oil bath plays an important role in setting the distance between the
self-assembled drops. Furthermore, we demonstrate ways to manipulate the drops
actively and passively at the interface.
