Ferrofluid Drop To Spike Reversible Transition Due To An Approaching Magnet*

Ferrofluid sessile droplets when exposed to magnetic field may go through surface normal instability, spike-like structures. This phenomenon occurs when the magnetization (M) of the ferrofluid droplet exceeds a critical magnetization (M_c) depending upon the interfacial tension, gravitational potential and the magnetic field strength (B) and its gradient (∇B) experienced by the ferrofluid-air interface. We discovered and showed that depending upon the concentration of magnetic nanoparticles (MNPs) in the ferrofluid, there may exist another reversible transition from spikes back to droplet shape of ferrofluid. The existing theory based on total energy minimization was extended to include the non-uniformity of the magnetic field on the plane perpendicular to the direction of magnetization. The theoretically predicted gap (δ) between the magnet and droplets for the first (δ_c1) and second (δ_c2) transitions were in a good agreement with experimental findings. The experimental characteristic wavelength characterized by the area averaged separation distance (s_expt) between the spikes also closely matched with theoretical estimations (λ_model). The instability induced spike formation has many applications such as sub-microlitre sized droplet formation, as object transport vehicle, etc.