  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. Those 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 novel
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
floating on the air-liquid interface by deforming the air-ferrofluid interface.
Using linear programming, we can control the motion of non-magnetic particles
with a predefined trajectory of a line, square, and circle with a precision of
57.4+/-33.6 um, 74+/-44.4 um, and 67.2+/-38.6 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.
