Influences of wetting condition to Rosensweig instability of a ferrofluid droplet

Abstract

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 .