Zhiping Wang

Magnetic Trapping of Bacteria at Low Magnetic Fields.

A suspension of non-magnetic entities in a ferrofluid is referred to as an inverse ferrofluid. Current research to trap non-magnetic entities in an inverse ferrofluid focuses on using large permanent magnets to generate high magnetic field gradients, which seriously limits Lab-on-a-Chip applications. On the other hand, in this work, trapping of non-magnetic entities, e.g., bacteria in a uniform external magnetic field was studied with a novel chip design. An inverse ferrofluid flows in a channel and a non-magnetic island is placed in the middle of this channel. The magnetic field was distorted by this island due to the magnetic susceptibility difference between this island and the surrounding ferrofluid, resulting in magnetic forces applied on the non-magnetic entities. Both the ferromagnetic particles and the non-magnetic entities, e.g., bacteria were attracted towards the island, and subsequently accumulate in different regions. The alignment of the ferrimagnetic particles and optical transparency of the ferrofluid was greatly enhanced by the bacteria at low applied magnetic fields. This work is applicable to lab-on-a-chip based detection and trapping of non-magnetic entities bacteria and cells.

Spreading of a ferrofluid core in three-stream micromixer channels

Spreading of a water based ferrofluid core, cladded by a diamagnetic fluid, in three-stream micromixer channels was studied. This spreading, induced by an external magnetic field, is known as magnetofluidic spreading (MFS). MFS is useful for various novel applications where control of fluid-fluid interface is desired, such as micromixers or micro-chemical reactors. However, fundamental aspects of MFS are still unclear, and a model without correction factors is lacking. Hence, in this work, both experimental and numerical analyses were undertaken to study MFS. We show that MFS increased for higher applied magnetic fields, slower flow speed of both fluids, smaller flow rate of ferrofluid relative to cladding, and higher initial magnetic particle concentration. Spreading, mainly due to connective diffusion, was observed mostly near the channel walls. Our multi-physics model, which combines magnetic and fluidic analyses, showed, for the first time, excellent agreement between theory and experiment. These results can be useful for lab-on-a-chip devices.

Droplet Merging on a Lab-on-a-Chip Platform by Uniform Magnetic Fields.

Droplet microfluidics offers a range of Lab-on-a-chip (LoC) applications. However, wireless and programmable manipulation of such droplets is a challenge. We address this challenge by experimental and modelling studies of uniform magnetic field induced merging of ferrofluid based droplets. Control of droplet velocity and merging was achieved through uniform magnetic field and flow rate ratio. Conditions for droplet merging with respect to droplet velocity were studied. Merging and mixing of colour dye + magnetite composite droplets was demonstrated. Our experimental and numerical results are in good agreement. These studies are useful for wireless and programmable droplet merging as well as mixing relevant to biosensing, bioassay, microfluidic-based synthesis, reaction kinetics, and magnetochemistry.

Control of Ferrofluid Droplets in Microchannels by Uniform Magnetic Fields

Magnetic droplets are versatile tools for a range of lab-on-a-chip (LoC) applications. The combination of a uniform magnetic field and magnetic droplet offers wireless and programmable manipulation. We performed LoC experiments and numerical studies on ferrofluid droplets under the influence of a uniform magnetic field. The dynamic behavior of flowing ferrofluid droplets was examined. The droplet size, shape, interdroplet spacing and velocity could be controlled by tuning the magnetic susceptibility of the ferrofluid, the viscosity of the carrier medium, and the flow rates. Our droplet-based micromagnetofluidic numerical model is in good agreement with our experiments. These studies are useful for magnetic droplet control and mixing in a LoC using a uniform magnetic field.

Magnetic Droplet Merging by Hybrid Magnetic Fields

Wireless and programmable manipulation of droplets is a challenge. We addressed this challenge by a combination of magnetic fluids and hybrid magnetic fields. We investigated the remote, wireless and programmable manipulation of ferrofluid droplets in a capillary microfluidic platform by a combination of uniform and non-uniform magnetic fields. The time-dependent motion of droplets under the influence of magnetic field was studied. Actuation and inter-droplet spacing of the droplets could be controlled by tuning the magnetic field strength. The influence of viscosity on the inter-droplet spacing was investigated. The time-dependent merging of (a) ferrofluid-ferrofluid and (b) ferrofluid-rhodamine droplets was demonstrated. Simulation and experimental results are in good agreement. The present work can be used for magnetically controlled, remote, wireless and programmable droplet actuation and merging relevant to biomedical assay, cell manipulation, tissue culture, drug efficacy, and synthesis of magnet-polymer composite particles.

Instability-Induced Mixing of Ferrofluids in Uniform Magnetic Fields

The advantages of ferrofluids in microfluidic lab-on-a-chip applications include remote control of the fluid flow within the chips, e.g., mixing of the species using an external uniform magnetic field. Hence, three-stream flow systems consisting of a ferrofluid core clad by two streams of diamagnetic silicone oil were studied. The instability of the ferrofluid, subjected to an external uniform magnetic field, was also studied. When the strength of this magnetic field was increased to a critical value, the ferrofluid was spread toward the silicone oil and a transient instability developed at the ferrofluid-silicone oil interface. Further increasing magnetic field resulted in periodic instability structures and permanent instability. The effect of magnetic field strength, flow rate, and flow rate ratio were determined. With a higher flow rate ratio, the permanent instability was observed only at the larger magnetic field strength. Our modeling results were consistent with these experimental results. Our work shows that an external uniform magnetic field of only a few millitesla can lead to instability and mixing, thus it is relevant to mixing in practical microfluidic devices.

Label-Free Alignment of Nonmagnetic Particles in a Small Uniform Magnetic Field

Label-free manipulation of biological entities can minimize damage, increase viability and improve efficiency of subsequent analysis. Understanding the mechanism of interaction between magnetic and nonmagnetic particles in an inverse ferrofluid can provide a mechanism of label-free manipulation of such entities in a uniform magnetic field. The magnetic force, induced by relative magnetic susceptibility difference between nonmagnetic particles and surrounding magnetic particles as well as particle-particle interaction were studied. Label-free alignment of nonmagnetic particles can be achieved by higher magnetic field strength (Ba), smaller particle spacing (R), larger particle size (rp1), and higher relative magnetic permeability difference between particle and the surrounding fluid (Rμr). Rμr can be used to predict the direction of the magnetic force between both magnetic and nonmagnetic particles. A sandwich structure, containing alternate layers of magnetic and nonmagnetic particle chains, was studied. This work can be used for manipulation of nonmagnetic particles in lab-on-a-chip applications.