The Influence of Electric and Magnetic Fields on the Structure of Flat Drops of Magnetic Fluids upon Drying

Abstract

Drop casting is a widely used approach for surface modification and the creation of functional coatings. Drying of sessile droplets is often associated with a so-called coffee-ring effect which originates from a non-uniform evaporation rate in the liquid–air and results in the formation of non-planar deposits. In the last two decades, this phenomenon attracted a lot of attention as a potential approach towards surface modification and structuring, resulting in an increased interest to control the morphology of the formed deposits. Here, we describe the effect of magnetic and electric fields individually and in combination on the processes of deposit formation for drops of water- or kerosene-based magnetic fluids. For the water-based fluid, the relative inner deposit density showed a linear dependence on the applied electric potential in the range from −5 to +5 V and varied from 100% to 31%, while kerosene-based fluid did not respond to the electric field. The combination of electric and magnetic fields allowed more complex control over the morphology of the deposits and to separate the annular deposit from the main one by the rim up to 0.3 mm. The theoretical basis behind the observed effects is discussed.