Breakup of a pendant magnetic drop

Evaporation and drying characteristics of the sessile ferrofluid droplet under a horizontal magnetic field

In this study, the evaporation characteristics and drying patterns of various sessile ferrofluid droplets on certain substrate under horizontal magnetic fields of controlled intensities are reported. The effects of droplet concentration and magnetic field intensity on the duration of each evaporation stage and drying patterns of droplets have been systematically investigated. It turned out that a plateau appears at the initial stage of evaporation in the absence of magnetic field and it was found that the plateau value is positively correlated with the concentration of ferrofluid droplets. Under the external magnetic field, the evaporation time of droplets decreases, the stage of contact line retreat extends, the stage of late pinning mode shortens, and the deposition area of ferrofluid droplet decreases compared to that of without magnetics field. The deposition area increases gradually and becomes more uniform with the increase of magnetic field. The decrease of friction force which is due to the decrease of the number of nanoparticles at the contact line under external magnetic field is the main reason for the observed phenomena. We found that the coffee ring and the uniform deposition inside the droplet will be destroyed when the magnetic field intensity is higher than a critical value. Our work has a significant reference value for the evaporation of sessile magnetic fluid droplets under the applied magnetic field, especially when the drying pattern needs to be precisely controlled, such as in spray or biomedicine.

Bond Number Revisited: Axisymmetric Macroscopic Pendant Drop

The numbers Rb_θ and Rb_r introduced recently (Berim, G.O.; Ruckenstein, E. J. Phys. Chem. 2019, 123, 10294-10300) to characterize the cylindrical pendant drops hanging from the solid surface are modified for characterization of axisymmetric pendant drops which are more common in experiment. Similar to the case of cylindrical drops, Rb_θ and Rb_r are defined on the basis of rigorous solution of Young-Laplace equation for the drop profile. As a consequence, they contain only the input parameters of the drop-solid system such as the volume of the drop, gravitational acceleration, fluid densities, surface tension, and contact angle. This constitutes the main difference between Rb numbers and traditional Bond and Bond-like numbers which involve the dimensions of the drop unknown prior to the experiment (e.g., height of the drop or radius of curvature at the drop apex). It is shown, that the new numbers can be used, for example, for predicting the breakup conditions of the drop, determining the surface tension, and estimating the deviation of drop shape from the circular one. A good agreement between theoretical predictions and experimental results known from the literature was demonstrated.

Motion, deformation and pearling of ferrofluid droplets due to a tunable moving magnetic field

We demonstrate unique evolutions of the shape of ferrofluid droplets during their motion over horizontal and inclined planes assisted by an applied field. We substantially extend the basic study regarding the motion of the ferrofluid droplets to address state-of-the-art parametric variation. While doing so, we describe some uncommon phenomena such as pearling, which has not been reported to date in connection with the motion of ferrofluid droplets. The contact angle varies dynamically with the motion of the droplet over horizontal and inclined planes. The shape of the ferrofluid droplet evolves continuously with the variation of different parameters. The spheroidal shape transforms into oblate, pear shape and tear drop with a symmetric corner and finally tears into daughter globules, famously known as pearling. The field strength, magnet velocity and concentration of nanoparticles control the motion, deformation and pearling. We emphasize that pearling is predominant during horizontal translation and depreciates during the ascent of an inclined plane (due to the resolved component of gravity). From a scale analysis, we demonstrate that the deformation and pearling can be predicted in terms of the magnetic Bond number, magnetic Laplace number and Bond number.

Bond Number Revisited: Two-Dimensional Macroscopic Pendant Drop

The definition of the Bond number, which is widely used for the qualitative description of the shape and stability of a macroscopic pendant drop, is revisited. Two new numbers, named Rb_θ and Rb_λ, are introduced for two kinds of two-dimensional pendant drops, which have a constant contact angle and a constant width of drop-solid contact, respectively. These numbers contain only input parameters such as the volume of the drop, gravitational acceleration, fluid densities, surface tensions, and contact angles. This distinguishes them from the traditional Bond number, which involves, in addition to the input parameters, prior unknown experimentally derived characteristics of the drop, e.g., height of the drop or radius of the curvature at the drop apex. It is shown that, when properly defined, the new numbers can be used, in particular, for predicting the breakup conditions of the drop and estimating their shape.

An analog to Bond number for pendant nanodrops

A new dimensionless number is introduced which characterizes the shape and stability of a pendant nanodrop.

Interface manipulation for printing three-dimensional microstructures under magnetic guiding

Precisely controlled 3D microstructures are printed by 2D interface manipulation. The morphologies of 3D microstructures are deterministically dependent on the receding angles of droplets on the surfaces, and the exact relationship between 3D morphology and interface properties is clarified. Accurate-positioned and oriented-patterned 3D arrays are facilely printed, demonstrating high controllability and large-scale fabrication of uniform 3D microstructures.