  In a non-uniform magnetic field, the droplets of colloids of nickel nanorods
and nanoparticles aggregate to form a cusp at the droplet surface not deforming
the entire droplet shape. When the field is removed, nanorods diffuse away and
cusp disappears. Spherical particles can form cusps in a similar way, but they
stay aggregated after release of the field; finally, the aggregates settle down
to the bottom of the drop. X-ray phase contrast imaging reveals that nanorods
in the cusps stay parallel to each other without visible spatial order of their
centers of mass. Formation of cusps can be explained with a model that includes
magnetostatic and surface tension forces. The discovered possibility of
controlled assembly and quenching of nanorod orientation under the cusped
liquid surface offers vast opportunities for alignment of carbon nanotubes,
nanowires and nanoscrolls, prior to spinning them into superstrong and
multifunctional fibers. Magneto and electrostatic analogy suggests that similar
ideal alignment can be achieved with the rod-like dipoles subject to a strong
electric filed.
