Open surface Magnetic manipulation:
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.
Soft Matter, 2018.
Droplets can be used as carrier vehicles for the transportation of biological and chemical reagents. Manipulation of water- and oil-based ferromagnetic droplets in the presence of a magnetic field has been well-studied. Here, we elucidate the transport of a sessile aqueous (diamagnetic) droplet placed over spikes of oil-based ferrofluid (FF) in the presence of a nonuniform magnetic field. An oil-based FF droplet, dispensed over a rigid oleophilic surface, interacts with a magnetic field to get transformed into an array of spikes which then act as a carrier for the transportation of the aqueous droplet.
Langmuir, 2019.
In-Channel Magnetic manipulation:
We report the influence of stationary and moving permanent magnets on the magnetocapillary flow of ferrofluid in a microchannel. With the stationary magnet placed at the microchannel exit, a sudden increase in the meniscus velocity was observed when the ferrofluid meniscus comes under the influence of the magnet. In case of the moving magnet, for a fixed initial magnet location, the meniscus velocity remains constant when the meniscus is closer to the magnet and is directly proportional to the magnet velocity. For a given magnet velocity, irrespective of the initial location of the magnet, a fixed steady state meniscus velocity is achieved. We demonstrated forward motion, stopping and reverse motion of the ferrofluid meniscus by controlling the motion of the magnet, which could find applications in biological and chemical analysis.
Sensors and Actuators B : Chemical, 2017.
In summary, we reported the physics behind the manipulation of aqueous ferrofluid droplets (FFD) of different sizes and concentrations at the interface of coflowing immiscible liquids in a microchannel in the presence of the magnetic field. The interplay between magnetic force, non-inertial lift force, and interfacial tension force governs the interfacial migration of FFD. Three different regimes, namely, “no migration,” “partial migration,” and “migration” were observed depending on the force ratios. Finally, sorting of FFDs of various sizes and concentrations is elucidated. It was observed that the migration range of FFDs is widened in the case of magnetic manipulation as compared to non-magnetic manipulation.
Applied Physics Letters, 2018.
Manipulation of aqueous droplets in microchannels has great significance in various emerging applications such as biological and chemical assays. Magnetic-field based droplet manipulation that offers unique advantages is consequently gaining attention. However, the physics of magnetic field-driven cross-stream migration and the coalescence of aqueous droplets with an aqueous stream are not well understood. Here, we unravel the mechanism of cross-stream migration and the coalescence of aqueous droplets flowing in an oil based ferrofluid with a coflowing aqueous stream in the presence of a magnetic field.
Physics of Fluids, 2019.
The manipulation of aqueous droplets has a profound significance in biochemical assays. Magnetic field-driven droplet manipulation, offering unique advantages, is consequently gaining attention. However, the phenomenon relating to diamagnetic droplets is not well understood. Here, we report the understanding of trapping and coalescence of flowing diamagnetic aqueous droplets in a paramagnetic (oil-based ferrofluid) medium using negative magnetophoresis.
Langmuir, 2020.
Handling and manipulation of particle-encapsulating droplets (PED) have profound applications in biochemical assays. Herein we report encapsulation of microparticles in aqueous ferrofluid droplets in a primary continuous phase (CP) and sorting of PED from empty droplets (ED) at the interface of the CP in co-flow with a second continuous phase using a magnetic field. We find that the encapsulation process results in a size contrast between the PED and ED that depends on the flow regime - squeezing, dripping, or jetting - which in turn is governed by the ratio of the discrete phase to the continuous phase capillary number. The difference between the volume fractions of ferrofluid in the PED and ED, is utilized for sorting, and is found to depend on the ratio of the capillary numbers.
Soft Matter, 2021.
Encapsulated magnetic microdroplets are of paramount importance in drug targeting and therapeutic applications. However, conventional techniques for generating encapsulated magnetic microdroplets suffer from several challenges, including lack of monodispersity, inflexibility in core-shell combinations, and complex device architecture to achieve encapsulation. Herein, a facile magnet-assisted framework to controllably wrap ferrofluid (FF) droplets inside polydimethylsiloxane (PDMS) floating on a water bath is developed. A permanent magnet placed at the bottom of a static glass cuvette pulls the ferrofluid droplet across the PDMS-water interface, which results in the wrapping of the FF droplet by a thin PDMS layer. The deformation of the FF-PDMS interface and the encapsulation of FF inside PDMS thereof is attributed to the interplay of magnetic force and force due to PDMS-water interfacial tension. Based on the experimental observations, three regimes are identified, namely, stable encapsulation, unstable encapsulation, and no encapsulation, which depends on the magnetic Bond number (Bom) and the thickness of the PDMS layer (δ). The versatility of the technique is demonstrated further by showing stable wrapping of multiple ferrofluid droplets inside the same encapsulated cargo and successful underwater manipulation of the encapsulated droplets, which finds relevance in the encapsulation and magnet-assisted actuation of novel encapsulated materials.
Advanced Materials Interfaces, 2022.