Self-Running Liquid Metal Drops that Delaminate Metal Films at Record Velocities

This paper describes a new method
to spontaneously accelerate droplets of liquid metal (eutectic gallium
indium, EGaIn) to extremely fast velocities through a liquid medium
and along predefined metallic paths. The droplet wets a thin metal
trace (a film ∼100 nm thick, ∼ 1 mm wide) and generates
a force that simultaneously delaminates the trace from the substrate
(enhanced by spontaneous electrochemical reactions) while accelerating
the droplet along the trace. The formation of a surface oxide on EGaIn
prevents it from moving, but the use of an acidic medium or application
of a reducing bias to the trace continuously removes the oxide skin
to enable motion. The trace ultimately provides a sacrificial pathway
for the metal and provides a mm-scale mimic to the templates used
to guide molecular motors found in biology (e.g., actin filaments).
The liquid metal can accelerate along linear, curved and U-shaped
traces as well as uphill on surfaces inclined by 30 degrees. The droplets
can accelerate through a viscous medium up to 180 mm/sec which is
almost double the highest reported speed for self-running liquid metal
droplets. The actuation of microscale objects found in nature (e.g.,
cells, microorganisms) inspires new mechanisms, such as these, to
manipulate small objects. Droplets that are metallic may find additional
applications in reconfigurable circuits, optics, heat transfer elements,
and transient electronic circuits; the paper demonstrates the latter.