  Manipulation of small-scale matter is a fundamental topic in micro- and
nanorobotics. Numerous magnetic robotic systems have been developed for the
manipulation of microparticles in an ambient environment, liquid as well as on
the air-liquid interface. These systems move intrinsically magnetic or
magnetically tagged objects by inducing a magnetic torque or force. However,
most of the materials found in nature are non-magnetic. Here, we report a
ferrofluidic manipulator for automatic two-dimensional manipulation of
non-magnetic objects floating on top of a ferrofluid. The manipulation system
employs eight centimeter-scale solenoids, which can move non-magnetic particles
by deforming the air-ferrofluid interface. Using linear programming, we can
control the motion of the non-magnetic particles with a predefined trajectory
of a line, square, and circle with a precision of 25.1+/-19.5 um, 34.4+/-28.4
um and 33.4+/-26.6 um, respectively. The ferrofluidic manipulator is versatile
with the materials and the shapes of the objects under manipulation. We have
successfully manipulated particles made of polyethylene, polystyrene, a silicon
chip, and poppy and sesame seeds. This work shows a promising venue for the
manipulation of living and non-living matter at the air-liquid interface.
