  Depending on the contact line motion, colloid-rich drolets evaporation can
leave a ring-like or a spot-like residue. Herein, we determine this outcome by
controlling the contact line motion using coplanar direct current
electrowetting-on-dielectrics (DC-EWOD). Combined with theoretical calculations
of the droplet shape and its evaporation rate, the time-dependent actuation
voltage is first derived from experiments and simulations. Thanks to the
additional control over the contact angle, the contact line can be maintained
in pinned state even on surfaces that exhibit little contact angle hysteresis
such as homogenous flat Teflon coatings. In the absence of EWOD control,
polystyrene particles and Escherichia coli suspended in the droplet formed a
dot-like pattern at the center of the initial contact, whereas application of
the mechanism resulted in ring-like patterns of a controllable radius. Unlike
chemically or structurally patterned substrates, the contact line could recover
its mobility at any preset time before reaching the control limit, which is
useful to accurately and consistently fabricate self-assembled nanostructures
of desired patterns on different surfaces.
