Abstract
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.
Highlights
Evaporation of liquid drops and the pattern formation on substrates after evaporation have gathered a lot of attention in the last few decades
The drying behavior of magnetic nanofluid was studied under ambient conditions in the absence of external magnetic field
Equidistant noninteracting empty lines were observed, which were visible during 30% of initial evaporation time and later merged. These lines seem to be Marangoni instability loops created by a tangential temperature gradient, which is later stabilized as the contact angle decreases
Summary
Evaporation of liquid drops and the pattern formation on substrates after evaporation have gathered a lot of attention in the last few decades. To compensate the loss of fluid, the liquid inside the drop is forced to move toward the contact line, resulting in an outward flow This flow is responsible for the ring formation at the boundary of the evaporated drop. Ferrofluids[28] are known to show interesting behavior in the presence of external magnetic field These fluids are colloids containing magnetic nanoparticles coated with the surfactant and suspended in a carrier liquid.[29] Many recent applications were developed based on its field dependent behavior.[30,31,32,33] In the presence of magnetic field, colloidal particles can be arranged in the desired direction and many macroscale structures can be formed.[34] field-induced effects have been largely studied, the drying behavior of ferrofluid droplets under ambient conditions has not been explored as per our knowledge. A similar study under the influence of the external magnetic field will be discussed in the future
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