Experiments of Interfacial Instability on a Ferrofluid Droplet*

The interfacial morphologies of an extremely thin layer of ferrofluid droplet under a constant perpendicular magnetic field are investigated. Striking patterns consisting of numerous sub-scale droplets that develop from Rosensweig instability are observed. For a dry plate the breaking pattern of sub-scale droplets can be characterized by a dimensionless magnetic Bond number, $Bo_m$. In general, a more pronounced instability, which is evident by a greater number of breaking sub-scale droplets $N$, arises with a higher $Bo_m$. For a magnetic Bond number that is larger than a critical value, the central droplet is torn apart. For a prewetted plate, a nearly flat fluid surface is achieved due to a smaller contact angle, which then leads to virtually evenly distributed sub-scale droplets. A global size for all breaking sub-scale droplets is observed regardless of their initial diameters. On the other hand, when a ferrofluid droplet is immersed in a thin layer of a nonmagnetic fluid, a formation of intriguing interfacial structures is observed, and the development of a hybrid-type ferrohydrodynamic instability is verified, where peak and labyrinthine ferrofluid patterns coexist and share a coupled dynamic evolution.