  The deposition of particles on a surface by an evaporating sessile droplet is
important for phenomena as diverse as printing, thin-film deposition and
self-assembly. The shape of the final deposit depends on the flows within the
droplet during evaporation. These flows are typically determined at the onset
of the process by the intrinsic physical, chemical and geometrical properties
of the droplet and its environment. Here, we demonstrate deterministic
emergence and real-time control of Marangoni flows within the evaporating
droplet by an external point-source of vapor. By varying the source location,
we can modulate these flows in space and time to pattern colloids on surfaces
in a controllable manner.
