Magnetic Field-Assisted Fission of a Ferrofluid Droplet for Large-Scale Droplet Generation

On-Demand Coalescence of Ferromagnetic Droplets in Microchannels Using an Oscillating Magnetic Field

Droplet-based microfluidics exhibit remarkable potential in achieving high-throughput chemical reactions with minimal reagent consumption. However, a pivotal challenge lies in the selective coalescence of droplets for precise process control, particularly when dealing with droplets of varying amounts and volumes, which are difficult to trap and coalesce due to their tiny dimensions and incessant movement. Hence, we proposed a method for on-demand coalescence of ferromagnetic droplets using an oscillating magnetic field. Experimental results show that the ferromagnetic droplets can be trapped in different positions in the microchannels according to the applied magnetic field intensity. A high-intensity pulsed amplitude of the magnetic field enables the migration of trapped droplets toward the same position, facilitating their mutual contact and interaction. By programmable modulation of the oscillating magnetic field, a controllable reciprocation of droplets in microchannels was successfully realized, which enabled us to dynamically capture, coalesce, and release two or more (≥3) droplets on demand. The integrated ferromagnetic droplet-based microfluidic platform allows contact-free, easily monitored, and on-demand coalescence of ferromagnetic droplets in microchannels, which holds promise for a wide range of applications, such as microfluidic-based drug synthesis, biosensing, reaction kinetics, and paracrine signaling, particularly.

NIR light-triggered bursting of double-emulsion drops (DEDs) for microdroplet generation

Microdroplets have significant applications in microbiology, pharmaceuticals, cosmetics, and synthetic materials. Herein, we present for the first time, a near-infrared (NIR)-light-triggered double-emulsion drop (DED) bursting method for generating a large number of micro-droplets with a size of several microns. Under the irradiation of NIR light, the inner water phase of the DED containing a trace amount of Prussian blue (PB) rapidly heats up and evaporates rapidly to generate microbubbles due to the photothermal property of PB. By controlling the light intensity, the DED could be inflated by the constant coalescence of microbubbles, which then burst immediately and tear the middle oil phase to form a large number of microdroplets. The performance of the microdroplets generated by NIR-light-triggered DED bursting was investigated by varying the oil shell thickness (HO), oil phase viscosity (ηO) and oil type. HO and ηO were the key factors affecting the generation of microdroplets. DEDs with lower HO and ηO generated lower polydispersity and a large number of microdroplets via NIR-triggered DED bursting. The proportion of microdroplets of sizes below 10 μm reached up to 95%. Furthermore, camellia oil, as the middle oil phase of the DEDs, generated lower polydispersity and a large number of microdroplets measuring several microns. The as-developed bursting method has great potential to generate micro-droplets for micro-/nano- and biotechnology applications.

Impact Deposition of a Single Droplet of Low-Melting-Point Alloy as the Top Electrode for Organic Photovoltaics

Top electrodes of organic photovoltaics (OPVs) are usually thermally evaporated in the vacuum, which is non-continuous and time-consuming and has been the bottleneck for the OPV fabrication process. Printable top electrodes that are free of vacuum, high temperature, and solvents will make OPVs more attractive. Low-melting-point alloys (LMPAs) are promising candidates for printable OPV electrodes thanks to the merits of matching work functions, high electron conductivity, high environment stability, and no need for post-treatment. Here, LMPA electrodes are directly deposited on OPVs by simply falling a single LMPA droplet onto the substrate. The LMPA droplet spreads to form a thin film with a smooth interface intimately contacting the substrate. The electrode area can be tailored by adjusting the droplet diameter or the Weber number, which is the ratio of inertia to surface tension. The interface morphology is mainly affected by the contact temperature. The degree of oxidation and charges on the droplet can also influence the electrode area and interface morphology. OPVs with droplet-impacted LMPA electrodes exhibit power conversion efficiencies of up to 16.17%. This work demonstrates the potential of single-droplet impact deposition as a simple method for printing OPV electrodes for scalable manufacturing.

Magnetic field effects on the surfactant concentration over ferrofluid droplet surfaces in shear flows

We investigate the impact of a magnetic field on surfactant concentration and interfacial forces across droplet surfaces within shear flows. Our analysis centers on a single two-dimensional ferrofluid droplet covered with surfactants, suspended in an immiscible, non-magnetizable liquid. The model combines incompressible Navier-Stokes equations and Maxwell's equations in the superparamagnetic limit in the single-fluid formulation, augmented by terms accounting for Marangoni, capillary, and magnetic forces at the droplet interface. We solve the surfactant convection-diffusion equation at the surface, while a non-linear Langmuir equation of state relates surfactant concentration to surface tension. The model is numerically solved using finite differences, a level-set method for multiphase flow computation, and the closest-point method for concentration equation. Our findings reveal that even though the surfactant is magnetically neutral, the presence of a magnetic field significantly modifies its distribution at the interface. A magnetic field perpendicular to the primary flow direction shifts the maximum concentration zone from the droplet tips toward the flow vorticity direction, while a parallel field produces the opposite effect. Alterations in surfactant distribution directly impact the surface tension field, offering a potential wireless means of controlling droplet dynamics.

Switchable Wettability States of a Ferrofluid Droplet atop a Hydrophobic Interface

Magnetically tuned soft machines offer great promise in performing a wide variety of programmable tasks via their dynamic shape adaptation and alteration. Despite dramatic recent advancements in this regard, selective reconfiguration of the wetting behavior of a ferrofluid droplet atop a hydrophobic interface adapted as a magnetically modulated micromachine remained elusive when the applied field intensity exceeds the saturation magnetization. Here we unveil a strategy to unsettle this perspective by harnessing a magnetic field-dependent magnetization phenomenon that may be exploited exclusively to arrive at highly controllable dynamic switchable wetting states of ferrofluid droplets, including the realization of wide ranges of contact angles for a given applied magnetic field. We arrive at a physical law from the resulting interplay of forces that quantifies the time dependence of the contact angle variation for a given magnetic field. Substantiated by experimental findings, our multiphysics-based simulations further evidence the possibilities of realizing switchable wetting states of soft magnetic matter over a wide range of physical parameters, delving into this principle. Disrupting the established notion of a trivially unique wetting phenomenon as governed by the droplet-substrate combination and the applied field alone, this paradigm may thus benefit a wide variety of practical applications, ranging from digital microfluidics to recombination chemistry.

Magnetic Control of Water Droplet Impact onto Ferrofluid Lubricated Surfaces

Controlling the impact process of a droplet impacting a liquid film has remained a wide-open challenge. The existing passive techniques lack precise on-demand control of the impact dynamics of droplets. The present study introduces a magnet-assisted approach to control water droplets' impact dynamics. We show that by incorporating a thin, magnetically active ferrofluid film, the overall droplet impact phenomena of the water droplets could be controlled. It is found that by modifying the distribution of the magnetic nanoparticles (MNPs) present inside the ferrofluid using a permanent magnet, the spreading and retraction behavior of the droplet could be significantly controlled. In addition to that, we also show that by altering the impact Weber number (We_i), and the magnetic Bond number (Bo_m), the outcomes of droplet impact could be precisely controlled. We reveal the role of the various forces on the consequential effects of droplet impact with the help of phase maps. Without the magnetic field, we discovered that the droplet impact on ferrofluid film results in no-splitting, jetting, and splashing regimes. On the other hand, the presence of magnetic field results in the no-splitting and jetting regime. However, beyond a critical magnetic field, the ferrofluid film gets transformed into an assembly of spikes. In such scenarios, the droplet impact only results in no-splitting and splashing regimes, while the jetting regime remains absent. The outcome of our study may find potential applications in chemical engineering, material synthesis, and three-dimensional (3D) printing where the control and optimization of the droplet impact process are desirable.

Advances in Microscale Droplet Generation and Manipulation

Microscale droplet generation and manipulation have widespread applications in numerous fields, from biochemical assays to printing and additive manufacturing. There are several techniques for droplet handling. Most techniques, however, can generate and work with only a limited range of droplet sizes. Furthermore, there are constraints regarding the workable variety of fluid properties (e.g., viscosity, surface tension, mass loading, etc.). Recent works have focused on developing techniques to overcome these limitations. This feature article discusses advances in this area that cover a wide range of droplet sizes from subpicoliter to microliter.