Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.
Experiments are carried to investigate the interfacial
morphologies of a thin film of micro magnetic drops
under a constant perpendicular field. Strong dependences
of the drop breakup patterns on their initial sizes and
wetting conditions are observed. Three modes of
well-ordered breakup instability patterns are identified for
drop sizes ranged from D≈800μm to 2,200μm for a field
strength of H=346Oe on a dry plate. The pattern of
breaking sub-scale droplets can be categorized into three
main parts: (1) a central droplet, (2) the outer fluids in
forms of either an outer residual annulus or an outer array
of primary subdroplets, and (3) the middle region that
might evolve into a single circular array of middle
subdroplets. A more complex and disorder mode IV
instability is recorded for large drop sizes D≥2,300μm.
Nevertheless, the central droplet is pulled apart for an
even large droplet D≥2,600μm, and is referred as a new
mode V instability although the topology remains features
of the mode IV instability. Because of height variation
along the ferrofluid surface, domination of the central
droplet is significant. On the other hand, a prewetted plate
leads to a nearly flat fluid surface. The breakups of
sub-scale droplets are nearly evenly distributed. The sizes
sub-scale droplets are weakly dependent on their initial
diameters. The number of breaking sub-scale droplets N
and the diameter of initial droplet D can be approximated
by a correlation of N~D2 .
