Influence and regulation of magnetic field on wettability of ferrofluid droplet on hydrophobic surface

The controllable dynamic behavior of ferrofluid droplets under the magnetic field can be used to realize directional transport of small droplets or bubbles in microfluidic devices, anti-icing, droplet condensation, mineral flotation and other fields. At present, the mechanism, influencing factors and regulation methods of the field-assisted wetting behavior of magnetic fluid on the superhydrophobic surface are not clear. The wetting behavior and droplet shape evolutions of water-based ferrofluid on a hydrophobic surface under an external magnetic field are studied experimentally. Under the vertical magnetic field, the effects of the magnetic induction intensity and ferrofluid droplet size on the droplet wetting behaviors are investigated, and the contact line diameter and contact angle of the droplet are measured experimentally. The experimental results show that the apparent contact angle of the ferrofluid droplets decreases from above 90° to below 90° under the action of the weak magnetic field. Under the magnetic field, the nanomagnetic particles in the magnetic fluid form a chain structure along the direction of the magnetic field line and the droplet contact angle changes. Through a scaling analysis, the theoretical relationship of the magnetic field and the contact angle is established and it successfully predicts our experimental results. The work is valuable for controlling the wetting properties of the ferrofluid droplets on the solid surfaces under the magnetic field.