  With an eye towards next-generation, smart, micro/nanofluidic devices,
capable of responding to external stimuli or changes in environment, we
demonstrate a means to achieve dynamic control of the spatio-temporal
properties of the electric field in a standardized microfluidic chamber.
Typical top-down patterning, currently used to design the field distribution,
is replaced by freely-suspended colloids which locally disturb the electric
field from the bottom-up. Even under uniform forcing, polarization of the
colloid induces the formation of strong, three-dimensional gradients at its
surface - essentially repurposing it into a portable floating electrode whose
precise location can be manipulated to reconfigure the electric field in real
time. Focusing on active Janus colloids as a sample platform, we measure the
strength of the induced gradients and highlight the advantages of a
colloid-based system by revealing a prototype for an all-in-one cargo carrier,
capable of on-demand, selective, label-free assembly and transport of
micro/nano sized targets.
