Active control of the freezing process of a ferrofluid droplet with magnetic fields

Different freezing behaviors of a ferrofluid droplet under magnetic lift and squeeze conditions. [Display omitted] •Active method of controlling freezing of a ferrofluid droplet using magnetic field.•Elongated or squeezed droplet shape under the magnetic lift or squeeze conditions.•Extended or shortened freezing time under the magnetic lift or squeeze conditions.•Scaling analysis of a relationship between droplet freezing time and frozen height. Most existing studies of droplet freezing on cold solid surfaces focus on the use of different surface wettability and roughness which may be considered as passive methods. Here we report an active method using an applied magnetic field to control the deformation and freezing process of a sessile ferrofluid droplet on a cold surface. By changing the direction and magnitude of the magnetic field, we can elongate or squeeze the shape of ferrofluid droplets. Consequently, the droplet freezing time can be extended or shortened under the magnetic lift or squeezing conditions, respectively. Additionally, we use the modified Young-Laplace equation with consideration of an additional magnetic force effect to analyze the shape variation of a ferrofluid droplet with magnetic field, and also present a scaling analysis for the freezing time.