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Drying behavior of magnetic nanofluid in ambient conditions

The study of evaporation patterns of liquid drops is a growing field of research with numerous applications in inkjet printing, controlled particle deposition, self-assembly, etc. After the liquid in a drop completely evaporates, it leaves behind the constituent particles in various patterns on the substrate. This depends on factors such as ambient temperature, substrate’s thermal conductivity, particle size, and density. Ferrofluids are known to show a variety of magnetic field dependent properties. Controllable evaporation using ferrofluids can result in desired patterns of particles on a substrate. However, before studying the evaporation of these nanofluids in the presence of magnetic field, their drying behavior under ambient conditions needs to be studied. Here, kerosene-based ferrofluid droplets were allowed to evaporate under ambient conditions. Video analysis of particle motion showed a Marangoni flow inside the drop. At the early stages of evaporation, non-interacting Marangoni instability loops were observed with equidistant empty lines between them propagating in the radial direction. These lines merged in the later stages of evaporation. The particles moved from the center toward the contact line and reversed their direction at a very close distance from the contact line, moving toward the top of the drop through the liquid–air interface. The distance of the point of reverse motion, called the stagnation point, was measured from the contact line, and it agrees with an existing theory. Moreover, the measurements of contact angle and mass evolution indicate that this evaporation follows the model of thin droplets. After drying, the ring pattern was observed on the substrate with a central accumulation of particles. The region between the central accumulation and the outer ring was seen to be empty. The size of this empty region decreased with increasing droplet size and increasing volume fraction of the nanoparticles. This study may help in understanding the drying behavior of magnetic nanofluids under ambient conditions for self-assembly and inkjet printing applications. The drying behavior in the presence of external magnetic field will be discussed in the future.