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. Our study reveals that the migration phenomenon is governed by the advection (τ_a) and magnetophoretic (τ_m) time scales. Experimental data show that the dimensionless equilibrium cross-stream migration distance δ^* and the length ${L_{\delta }}^{*}$ required to attain equilibrium cross-stream migration depend on the Strouhal number, St = (τ_a/τ_m), as δ^* = 1.1 St^0.33 and ${L_{\delta }}^{*}=5.3S{t}^{-0.50}$ , respectively. We find that the droplet-stream coalescence phenomenon is underpinned by the ratio of the sum of magnetophoretic (τ_m) and film-drainage time scales (τ_fd) and the advection time scale (τ_a), expressed in terms of the Strouhal number (St) and the film-drainage Reynolds number (Re_fd) as ξ = (τ_m + τ_fd)/τ_a = (St⁻¹ + Re_fd). Irrespective of the flow rates of the coflowing streams, droplet size, and magnetic field, our study shows that droplet-stream coalescence is achieved for ξ ≤ 50 and ferrofluid stream width ratio w^* < 0.7. We utilize the phenomenon and demonstrated the extraction of microparticles and HeLa cells from aqueous droplets to an aqueous stream.

Generating multi-layer cargo using conventional methods is challenging. We hypothesize that incorporating a Y-junction compound droplet generator to encase a target core inside a second liquid can circumvent the kinetic energy dependence of the impact-driven liquid-liquid encapsulation technique, enabling minimally restrictive multi-layer encapsulation. Stable wrapping is obtained by impinging a compound droplet (generated using Y-junction) on an interfacial layer of another shell-forming liquid floating on a host liquid bath, leading to double-layered encapsulation. The underlying dynamics of the liquid-liquid interfaces are captured using high-speed imaging. To demonstrate the versatility of the technique, we used various liquids as interfacial layers, including magnetoresponsive oil-based ferrofluids. Moreover, we extended the technique to triple-layered encapsulation by overlaying a second interfacial layer atop the first floating interfacial layer. The encapsulating layer(s) effectively protects the water-soluble inner core (ethylene glycol) inside the water bath. A non-dimensional experimental regime is established for successful encapsulation in terms of the impact kinetic energy, interfacial layer thickness, and the viscosity ratio of the interfacial layer and the outer core liquid. Using selective fluorescent tagging, we confirm the presence of individual shell layers wrapped around the core, which presents a promising pathway to visualize the internal morphology of final encapsulated droplets.

Due to microscale effects, the segmented microchannels or micro pin fin heat sinks emerged as a high thermal management solution. In this context, the present work analyzes the influence of different heights of square micro pin fins with an aligned array and investigates their influence on pressure drop and heat transfer behavior. The HFE-7100 is used as the working fluid, and the pressure drop and surface temperature behavior are analyzed for different mass fluxes and inlet subcooling. The single-phase flow was analyzed numerically using the computational fluid dynamics (CFD) software ANSYS FLUENT^® for comparing the simulation results with the experimental data, showing that the highest micro pin fins configuration provides a more uniform and lowest wall temperature distribution compared to the lowest configuration. There is a good agreement between the experimental results and the numerical analysis, with a mean absolute error of 6% for all the considered parameters. For the two-phase flow condition, experimental tests were performed, and for the highest subcooling, an increase in mass flux causes an enhancement in the heat transfer for low heat flux; by increasing heat flux, there is a gradual predominance of boiling heat transfer over convection as the heat transfer mechanism. The pressure drop drastically increases with the vapor amount flowing into the system, regardless of the pin fin height; the boiling curves for the higher fin height show a much smaller slope and a smaller wall superheat than the fin with the smallest height, and consequently, a high heat transfer performance. A larger region of the heat sink is filled with vapor for lower inlet subcooling temperatures, degrading the heat transfer performance compared to higher inlet subcooling temperatures.

This study visualized the complex flow dynamics around the encapsulated droplets generated by impact-driven liquid-liquid encapsulation. In our experiment, a core drop is dispensed from a needle tip and penetrates the interfacial layer formed on a host liquid, resulting in the formation of a drop encapsulated by the interfacial layer. While the hydrodynamics of the encapsulation process significantly influence the morphology and stability of the droplet, detailed knowledge of the surrounding flow field remains scarce. To fill this gap, we conducted comprehensive flow measurements using two-dimensional particle image velocimetry, capturing the flow field during the encapsulation process—impact, interfacial penetration and necking, and cargo separation. Our findings reveal that both velocity and vorticity distributions exhibit similar trends, regardless of the presence of the interfacial layer. Furthermore, we explored the effect of the impact Weber number, revealing that the flow field in encapsulation cases can be categorized into three distinct types based on time series vorticity data obtained from the drop's wake. These insights advance the understanding of the encapsulation process and offer a new framework for analyzing flow behaviors in liquid-liquid systems.