Trapping and Coalescence of Diamagnetic Aqueous Droplets Using Negative Magnetophoresis

Directional Manipulation of Drops and Solids on a Magneto-Responsive Slippery Surface

The cloaking of the droplet and solid spheres by a thin ferrofluid layer forms a ferrofluid-wetting ridge, enabling the magnet-assisted directional manipulation of droplets and solid spheres on the magneto-responsive slippery surface. Understanding the interplay of various forces governing motion unravels the manipulation mechanism. The transportation characteristics of droplets and solid spheres on such surfaces enable their controlled manipulation in multiple applications. Here, we prepare magneto-responsive slippery surfaces by using superhydrophobic coatings on glass slides, creating a porous network and impregnating them with ferrofluid. Using a permanent magnet (and its translation) in the proximity of these surfaces, we manipulate the motion of liquid drops and solid spheres. Upon dispensing the droplet on the magneto-responsive slippery surface, the droplet is cloaked by a thin ferrofluid layer and forms a ferrofluid wetting ridge. Incorporating the magnetic field creates a magnetic-nanoparticle-rich zone surrounding the ferrofluid ridge. Thereafter, the motion of the magnet gives rise to the movement of the droplet. We found that the interplay of the magnetic force and viscous drag leads to the magnetic manipulation of droplets in a controlled fashion up to a certain magnet speed. Increasing the magnet speed further results in the uncontrolled motion of the droplet, where the droplet cannot follow the magnet trajectory. Moreover, we delineate multifunctional droplet manipulations, such as trapping, pendant droplet manipulation, coalescence, and microchemical reactions, which have wide engineering applications.

Radial Magnetic Levitation and Its Application to Density Measurement, Separation, and Detection of Microplastics

This work describes the development of radial magnetic levitation (MagLev) using two radially magnetized ring magnets to solve the problem of limited operational spaces in standard MagLev and the major shortcoming of a short working distance in axial MagLev. Interestingly and importantly, we demonstrate that for the same magnet size, this new configuration of MagLev doubles the working distance over the axial MagLev without significantly sacrificing the density measurement range, whether for linear or nonlinear analysis. Meanwhile, we develop a magnetic assembly method to fabricate the magnets for the radial MagLev, where multiple magnetic tiles with single-direction magnetization are used as assembly elements. On this basis, we experimentally demonstrate that the radial MagLev has good applicability in density-based measurement, separation, and detection and show its advantages in improving separation performance compared with the axial MagLev. The open structure of two-ring magnets and good levitation characteristics make the radial MagLev have great application potential, and the performance improvement brought by adjusting the magnetization direction of magnets provides a new perspective for the magnet design in the field of MagLev.

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.

Water-in-water droplet microfluidics: A design manual

Droplet microfluidics is utilized in a wide range of applications in biomedicine and biology. Applications include rapid biochemical analysis, materials generation, biochemical assays, and point-of-care medicine. The integration of aqueous two-phase systems (ATPSs) into droplet microfluidic platforms has potential utility in oil-free biological and biomedical applications, namely, reducing cytotoxicity and preserving the native form and function of costly biomolecular reagents. In this review, we present a design manual for the chemist, biologist, and engineer to design experiments in the context of their biological applications using all-in-water droplet microfluidic systems. We describe the studies achievable using these systems and the corresponding fabrication and stabilization methods. With this information, readers may apply the fundamental principles and recent advancements in ATPS droplet microfluidics to their research. Finally, we propose a development roadmap of opportunities to utilize ATPS droplet microfluidics in applications that remain underexplored.

Acoustic particle trapping driven by axial primary radiation force in shaped traps

We study particle trapping driven by the axial primary radiation force (A-PRF) in shaped traps exposed to standing bulk acoustic waves (S-BAW) using numerical simulations and experiments. The utilization of the stronger A-PRF as the main retention force is a consequence of standing-wave formation along the flow direction, instead of the orthogonal direction as in the case of traditionally used lateral-PRF S-BAW trapping setups. The study of particle dynamics reveals that the competition between A-PRF and viscous drag force governs particle trajectory. The ratio of the acoustic energy to the viscous work (β) provides a general criterion for particle trapping at a distinctive off-node site that is spatially controllable. Particles get trapped for β≥β_{cr} at some distance away from the nodal plane and the distance varies as β^{-c} (c=0.6-1.0). The use of A-PRF as the retention force could potentially allow traditional S-BAW trapping systems to envisage high-throughput advancements surpassing the current standards in cell-handling unit operations.

Design and operation of magnetophoretic systems at microscale: Device and particle approaches

Combining both device and particle designs are the essential concepts to be considered in magnetophoretic system development. Researcher efforts are often dedicated to only one of these design aspects and neglecting the interplay between them. Herein, to bring out importance of the idea of integration between device and particle, we reviewed the working principle of magnetophoretic system (includes both device and particle design concepts). Since, the magnetophoretic force is influenced by both field gradient and magnetization volume, hence, accurate prediction of the magnetophoretic force is relying on the availability of information on both parameters. In device design, we focus on the different strategies used to create localized high-field gradient. For particle design, we emphasize on the scaling between hydrodynamic size and magnetization volume. Moreover, we also briefly discussed the importance of magnetoshape anisotropy related to particle design aspect of magnetophoretic systems. Next, we illustrated the need for integration between device and particle design using microscale applications of magnetophoretic systems, include magnetic tweezers and microfluidic systems, as our working example. On the basis of our discussion, we highlighted several promising examples of microscale magnetophoretic systems which greatly utilized the interplay between device and particle design. Further, we concluded the review with several factors that possibly resulted in the lack of research efforts related to device and particle design integration.

Splitting dynamics of ferrofluid droplets inside a microfluidic T-junction using a pulse-width modulated magnetic field in micro-magnetofluidics

Micro-magnetofluidics offers a promising tool for better control over the ferrofluid droplet manipulation which has been vastly utilized in biomedical applications in recent years. In this study, the ferrofluid droplet splitting under an asymmetric Pulse-Width-Modulated (PWM) magnetic field in a T-junction is numerically investigated using a finite volume method and VOF two-phase model. By utilizing the PWM magnetic field, two novel regimes of ferrofluid droplet splitting named as Flowing through the Same Branch (FSB) and Double Splitting (DS) have been observed for the first time. In the FSB regime, the daughter droplets move out of the same microchannel outlet, and in the DS regime, the droplet splitting occurs two times which results in generating three daughter droplets. The main problem related to the asymmetric droplet splitting under a steady magnetic field is daughter droplet trapping. By using a PWM magnetic field, this issue is resolved and the trapped/escaped regions are obtained in terms of the duty cycle and dimensionless magnetic field frequency. The effects of six important dimensionless parameters on the splitting ratio, including magnetic Bond number, duty cycle, dimensionless magnetic field frequency, capillary number, dimensionless mother droplet length, and dimensionless dipole position are investigated. The results showed that the splitting ratio increases with increasing magnetic Bond number or duty cycle, or decreasing the dimensionless magnetic field frequency. Eventually, a correlation is offered for the splitting ratio based on the dimensionless variables with an average relative error of 2.67%.