Shape evolution and splitting of ferrofluid droplets on a hydrophobic surface in the presence of a magnetic field

Abstract

We elucidate the phenomena of dynamic wetting, shape evolution and splitting of ferrofluid (FF) droplets on a hydrophobic surface under the influence of a magnetic field. In the case of a FF droplet interacting with a magnetic field, both surface energy and magnetic energy contribute to the total Gibb's free energy and hence the wetting phenomena. The nanoparticles in the FF droplet migrate and get accumulated at the apex of the droplet which enhances the magnetic interaction causing large deformation of the droplet. The FF droplet deformation and subsequent splitting are governed by the interplay between the magnetic Fm and surface tension Fs forces. The ratio of the forces km = (Fm/Fs) was found to be a function of the magnetic Bond number Bom and non-dimensional gap g* as km ∼ (Bom)0.3(g*)-0.86. Splitting of the FF droplets was observed for km > 1 and for km < 1, an equilibrium droplet shape was observed. The wetting behavior of the FF droplets was found to be strongly dependent on the FF concentration c - concentrated (c = 1.2%) FF droplets exhibit contact line (CL) pinning and decrease in contact angle (CA) θ with time throughout, while diluted (c = 0.6%) FF droplets show a mixed mode (CL pinning followed by constant CA). In splitting of FF droplets, the ratio of the volume of the daughter droplet to that of the parent droplet i.e. (Vd/Vp), was found to decrease with an increase in the parent droplet size Vp.

Microfluidics for Biosynthesizing: from Droplets and Vesicles to Artificial Cells

“…This droplet manipulation device promotes the development of autonomous open‐channel fluidic device. Sen and co‐workers reported the deformation and splitting of the ferrofluid droplets under the magnetic condition . This method can be applied in the open surface microfluidics.…”

“…[77] In this work, they used a device which consists of two T-shaped junctions and a Y-junction. These two parts were used to form droplets and to mix the Small 2020, 16,1903940 Creation, mixing, incubation, sorting An integrated droplet-digital microfluidic system [103] Sorting Superhydrophobic interdigitated array chips [104] Floating Movable electrode biased with a high voltage [105] Mixing Open and closed digital microfluidic platforms [106] Coalescence, migrating Polypyrrole whelk-like arrays [107] Generation, transportation, splitting Ionic-surfactant-mediated electro-dewetting digital microfluidics [108] Magnetic Sorting Magnetophoretic sorting system [101] Splitting, dispensing, exchange, trapping, release, demulsification An integrated magnetic repulsion-actuated microfluidic system [111] Mixing, migrating, transport, release, coalescence Ferrofluid-containing liquid-infused porous surfaces [113] Transport, sorting, mixing A magnetic digital microfluidics platform [114] Shape evolution, splitting A device with hydrophobic surface under the magnetic field [115] Transport, splitting, motion Magnetic tubular microactuators [116] Motion, transport, fusion A deformable paramagnetic liquid substrate [117] Acoustic Coalescence A straight channel with a narrow beam surface acoustic wave [119] Splitting, steering A disposable parallel-type SAW-based acoustofluidic device [120] Gravity Transport, collision, fusion, mixing, stopping A gravity-actuated droplet microfluidics device [67] Movement, mixing Polydopamine microfluidic system [121] Trapping, exchange, transfer, fusion A microstructures device possesses microwell arrays and straight microchannel [122] Optical Motion A device with a single focused laser [124] Motion, patterned writing A device with photoelectric cooperative-responsive slippery surface [125] Sorting, dispensing Printed droplet microfluidics [126] Transportation, merging, splitting Light-driven flexible opto-electrowetting devices [127] Mixing, motion, coalescence A pyroelectrotrapping superhydrophobic surface platform [128] Mechanical Mixing, movement, Mechanical-activated digital microfluidics…”

“…Sen and co-workers reported the deformation and splitting of the ferrofluid droplets under the magnetic condition. [116] This method can be applied in the open surface microfluidics. Furthermore, the manufacturing of magnetic tubular microactuators [117] and magnetic liquids [118] for the manipulation of droplets has also attracted much attention.…”

Flexible Ferrofluids: Design and Applications

“…When a droplet of ferrofluid is placed on a hydrophobic surface and exposed to an increasing magnetic field by a permanent magnet, it would undergo a deformation process into a spiked cone and split into daughter conical droplets ( Figure a,b) . This division could be attributed to the normal‐field instability of ferrofluids, which is also named as Rosensweig instability .…”

Experimental and numerical studies on freezing of droplets under effects of nanoparticles and magnetic fields

“…Specifically, the second effect is associated with the enhanced magnetic force due to the redistribution of Fe3O4 particles and the elongated ferrofluid droplet. Under the magnetic life conditions, the magnetic force pulls Fe3O4 particles to be accumulated in the upper region of the droplet [276]. Such particle accumulation increases the concentration of particles, which according to Eq.…”