  Vertical drop impacts of ferrofluids onto glass slides in a non-uniform
magnetic field have been studied using high-speed photography. Outcomes have
been classified based on the motion of the fluid-surface contact lines, and
formation of peaks (Rosensweig instabilities) which affect the height of the
spreading drop. The largest peaks are nucleated at the edge of a spreading
drop, similarly to crown-rim instabilities in drop impacts with conventional
fluids, and remain there for an extended time. Impact Weber numbers ranged from
18.0 to 489, and the vertical component of the B-field was varied between 0 and
0.37 T at the surface by changing the vertical position of a simple disc magnet
placed below the surface. The falling drop was aligned with the vertical
cylindrical axis of the 25 mm diameter magnet, and the impacts produced
Rosensweig instabilities without splashing. At high magnetic field strengths a
stationary ring of ferrofluid forms approximately above the outer edge of the
magnet.
