Contact angle of the ferronanofluid and influence of the magnetic field on the drying droplet

The unique magnetic properties of ferronanofluids distinguish them from the rest of the nanofluids group. To discover their full potential, their thermophysical properties have to be investigated. The aim of this study is to provide more insight into ferronanofluid wettability, including the contact angle on copper and aluminium surfaces. The presence of a ferronanofluid droplet in a magnetic field causes an interaction. The effect of this interplay is depicted by droplet side profile photographs and microscope imaging of a dried deposit. Two regimes of a droplet deformation due to this interplay are identified: pinning-fixed and de-pinning regime. In the first one, the droplet contact line is maintained, while the contact angle is increased. In the de-pinning regime, the droplet starts to flow, elongating its contact line and decreasing the contact angle. The regime transition occurs at a magnetic Bond number of 2.5. Cracking of the ferronanofluid deposit is also observed. Drying the droplet under the magnetic field affects the crack pattern by changing the alignment of cracks to match the direction of the magnetic field and reducing the number of cracks. This shows the influence of the ferronanoparticles ordering, and therefore of the magnetic stress, on the material fracture.