Sharpening the surface of magnetic paranematic droplets

Magnetowetting dynamics of sessile ferrofluid droplets: a review

The fascinating behavior of ferrofluids in a magnetic field has been intriguing researchers for many years. With the advancement in digital microfluidics, ferrofluid droplets have been extensively used in different applications ranging from biomedical to mechanical systems. Notably, the magnetic field can change the wetting dynamics of sessile ferrofluid droplets, leading to a plethora of interesting hydrodynamic phenomena. In the recent past, the spatiotemporal evolution of the droplet shape and contact line dynamics of a ferrofluid droplet in different magnetowetting scenarios has been explored widely. The relevant studies elucidate several critical aspects, such as the role of magnetic nanoparticles, carrier fluid, and the interaction of the magnetic fluid with the solid surface, among many others. Hence a systematic review of the progress made in understanding the fundamental and practical aspects of magnetowetting in the past decade (2010-2020) would be a helpful resource to the scientific community in the near future. Drawn by this motivation, an honest effort has been made in this Review to highlight the significant scientific findings concerning the sessile droplet magnetowetting phenomena within the timeline of interest. Several cutting-edge applications developed from the scientific findings in the purview of magnetowetting have also been discussed before outlining the conclusions and future areas of scope.

Magnetowetting dynamics of sessile ferrofluid drops on soft surfaces

We investigate the magnetowetting behaviour of sessile ferrofluid droplets on elastomeric surfaces with different stiffness. The non-uniform magnetic field engenders deformation and splitting of the ferrofluid droplet, which is greatly influenced by the softness of the substrate. We observe that the decrease in the dynamic contact angle is maximum on the softest substrate, while the contact line remains pinned. Again, for an apparently rigid substrate, the contact radius decreases almost linearly, whereas the decrease in the contact angle appears to be lower than that of soft substrates. The contact line experiences a transition from "stick-slip" on rigid surfaces to "pinned" motion on soft surfaces, which is favoured by the formation of a large wetting ridge and smaller receding contact angle on the latter. We also find that the splitting time and splitting ratio (the ratio of the volume of the daughter droplet to that of the residual droplet) increase with an increase in the softness of the underlying substrate.

Self-Sustained Coalescence-Breakup Cycles of Ferrodrops under a Magnetic Field

The self-sustained coalescence-breakup cycles of ferrodrops were investigated for the first time by a high-speed camera under various magnetic fields. Under an axial magnetic field, the upper ferrodrop would deform into a conic shape before coalescing with the bottom ferropeak. Within 0.2 ms after coalescence, the minimum width of the expanding neck obeys a power-law relationship with time, while the exponents increase with the magnetic field and deviate with a decreasing trend in the later coalescence. The cone angle of the upper ferrodrop before coalescence gradually decreases while it increases before breakup with the magnetic field. A critical magnetic field around 35 mT was reported, above which the ferrofluid column undergoes the periodic phenomenon of coalescence and breakup. The frequency for the whole coalescence-breakup cycle increases exponentially with the applied magnetic field. A simplified force balance allows capturing the periodic mechanism involved in this driven harmonic oscillator.

Magnetospinning of Nano- and Microfibers

Magnetospinning is a new method for spinning of continuous micro- and nano-fibers using a permanent revolving magnet. The method utilizes magnetic forces and the hydrodynamic features of stretched threads to produce highly loaded, fine magnetic nanofibers. The magnetospinning process is independent of the solution dielectric properties and requires no high voltages, in contrast to the more-traditional electrospinning technique.

A gradient field defeats the inherent repulsion between magnetic nanorods

When controlling the assembly of magnetic nanorods and chains of magnetic nanoparticles, it is extremely challenging to bring them together side by side while keeping a desired spacing between their axes. We show that this challenge can be successfully resolved by using a non-uniform magnetic field that defeats an inherent repulsion between nanorods. Nickel nanorods were suspended in a viscous film and a non-uniform field was used to control their placement. The in-plane movement of nanorods was tracked with a high-speed camera and a detailed image analysis was conducted to quantitatively characterize the behaviour of the nanorods. The analysis focused on the behaviour of a pair of neighbour nanorods, and a corresponding dynamic model was formulated and investigated. The complex two-dimensional dynamics of a nanorod pair was analysed analytically and numerically, and a phase portrait was constructed. Using this phase portrait, we classified the nanorod behaviour and revealed the experimental conditions in which nanorods could be placed side by side. Dependence of the distance between a pair of neighbour nanorods on physical parameters was analysed. With the aid of the proposed theory, one can build different lattices and control their spacing by applying different field gradients.