Probing Into the Drying Pattern Dynamics of a Ferrofluid Droplet Under the Actuation of Magnetic Field

Evaporation of liquids, especially colloids, is a subject of research interest since decades. Evaporation patterns of colloidal particles can determine key factors about the liquid properties and the underlying physics. Particle deposition, self-assembly, and inkjet printing are various applications, where the control of droplet evaporation and its patterns can be useful. Here, kerosene-based ferrofluid droplets were allowed to evaporate in the presence of static magnetic field. Earlier, the evaporation pattern of the same fluid in ambient conditions was reported, where ring pattern with central accumulation was observed. This was attributed to thermal Marangoni flow. In this case, magnetic field helps in circumventing this pattern, and directional dependence on magnetic field in the final patterns was also observed. Magnetic field parallel to the substrate created chain structures in the direction of the field, whereas magnetic field perpendicular to the substrate resulted in completely uniform pattern. For higher volume fraction of the chosen samples, the influence of external magnetic field was seen earlier than the samples of lower volume fraction. Effect of substrate wettability was observed under similar conditions for glass substrate and Polydimethylsiloxane (PDMS) substrate. These experimental results are compared/supported by the available existing theoretical results/theories. The inferences obtained from this analysis are expected to be useful in the optimization of particle deposition of magnetic colloids, typically of demanding significance in fields, such as nanochromatography, lithography, surface engineering, self-assembly, inkjet printing, and so on.