Wetting phenomenon of sessile ferrofluid droplets on a heterogeneous solid substrate

Abstract

The present study investigates the wetting phenomenon of sessile ferrofluid droplets on heterogeneous substrates, exhibiting hydrophilic and hydrophobic regions. Dripping the ferrofluid on the interface that delimits the regions with different hydrophilicity, the relaxation dynamics of the contact angle on each side is analyzed, considering the effects associated with a non-uniform magnetic flux density. Our results show that the wetting dynamic is governed by the contact line evolution on the hydrophilic side, being strongly affected by the magnetix flux density. Due to the surface heterogeneity, it is noticed that sessile ferrofluid drops present a different shape from the typical spherical cap observed on homogeneous substrates. In addition, it is verified that the interplay of substrate heterogeneity and drop surface deformation leads to a slow dewetting on the hydrophobic surface, which depends on the field strength. The present findings may be significant for microfluidic devices exploiting magnetically controlled ferrofluid movement on heterogeneous soft materials.