Alicia Williams

Dispersion of Ferrofluid Aggregates in Steady Flows

Using focused shadowgraphs, we investigate steady flows of a magnetically non-susceptible fluid interacting with ferrofluid aggregates comprised of superparamagnetic nanoparticles. The ferrofluid aggregate is retained at a specific site within the flow channel using two different applied magnetic fields. The bulk flow induces shear stresses on the aggregate, which give rise to the development of interfacial disturbances, leading to Kelvin-Helmholtz (K-H) instabilities and shedding of ferrofluid structures. Herein, the effects of bulk Reynolds number, ranging from 100 to 1000, and maximum applied magnetic fields of 1.2 × 105 and 2.4 × 105 A/m are investigated in the context of their impact on dispersion or removal of material from the core aggregate. The aggregate interaction with steady bulk flow reveals three regimes of aggregate dynamics over the span of Reynolds numbers studied: stable, transitional, and shedding. The first regime is characterized by slight aggregate stretching for low Reynolds numbers...

The Dynamics of Accumulating Ferrofluid Aggregates

The physics of steady and pulsatile flows laden with superparamagnetic nanoparticles in a square channel accumulating under the influence of a 0.5 Tesla permanent magnet are studied by means of focused shadowgraphs. The accumulation physics of these nanoscale particles is explored as functions of the flow type (steady and unsteady) and accumulation type (injected from the top of channel versus bottom of channel). Ferrofluid is accumulated by the steady injection of a streakline that enters the test section upstream of the magnet, where an aggregate forms. The interfacial phenomena resulting from the interaction of the ferrofluid with the bulk flow is resolved using shadowgraph imaging, where a digital camera captures the side view of the aggregate. Ferrofluid aggregate physics is examined both visually in the raw frames as well as by post-processing to determine the aggregate size evolution in time and couple that bulk information with interfacial behavior using the Proper Orthogonal Decomposition (POD). The shadowgraphs show that the aggregate exhibits different regimes based on bulk flow Reynolds number, which is varied between 100 and 1000, based on the mean flow rate. The aggregate exhibits stable behavior at low Reynolds numbers, where it stretches as it grows and minimal decay of the aggregate occurs. At moderate Reynolds numbers above 400, inertial forces dominate the dynamics, and aggregates do not attain the same size and height as in low Reynolds number cases. Therefore, the interaction of the aggregate with the bulk flow is diminished. The accumulation of ferrofluids is positively impacted by increased magnetic field gradients for some Reynolds numbers, while very high or low magnetic field gradients result in smaller, unstable aggregates. This work is the first to study the accumulation of ferrofluid aggregates over such a large parameter space, which reveals many physics that were previously unexplored in ferrohydrodynamics.Copyright

Spatiotemporally-Resolved Dynamics of Dispersing Ferrofluid Aggregates

Ferrohydrodynamics research has been approached predominantly from either numerical or basic experimental techniques. However, to date, these experimental techniques have been limited to ultrasonic point measurements or shadowgraphs due to the opacity of the ferrofluids. As a result, the complete dynamics of many ferrohydrodynamics flows have remained unexplored. In this work, Time Resolved Digital Particle Image Velocimetry (TRDPIV) is employed to fully resolve the dynamic interaction of ferrofluid aggregates with bulk nonmagnetic fluids. This topic is hydrodynamically rich, where shearing between the aggregate and bulk flow develop into the Kelvin-Helmholtz instability. Ferrofluid aggregates are mixed with fluorescent particles in order to enable visualization of the internal flow structure of the aggregate and generate quantitative velocity measurements. The TRDPIV measurements are made in a 15 mm square channel where ferrofluid retained by a 0.5 Tesla permanent magnet is studied as it disperses. The effects of both steady and pulsatile flows are quantified, as are the impact of varying the magnetic field gradients. In both steady and pulsatile flows, a recirculation region is observed within the ferrofluid, driven by the shear layer between the bulk flow and aggregate interface. The interaction of the aggregate with the flow is also governed by the aggregate height relative to that of the test section. Higher, larger aggregates are less stable, and therefore, more likely to be dispersed by the bulk flow. As the aggregate diminishes in size, it is both more stable and is less subject to shearing forces from the flow. Flow pulsatility enriches the dynamics of the flow and generates complex flow structures resulting from interaction between the aggregate and bulk flow. This work is the first to explore the rich spatiotemporal behavior of dispersing ferrofluid aggregates interacting with steady and unsteady bulk flows.Copyright

Active Laminar Mixing Induced by Surface Disturbance

Traditionally, passive mixers and chaotic advection have played a key role in the field of microfluidics by stretching and folding the fluid. Some examples of these passive mixing techniques include channels with riblets or grooves on the walls, or serpentine curved channels. In this work, we explore an actuated wall for use as a closed-chamber micromixer. Although active mixers for low Reynolds number flows have been previously studied, a significant portion of that work has involved piezoelectrics. This work presents a novel polymer that offers many advantages over conventional piezoelectric stacks, including very small thickness and inexpensive construction cost. Moreover, these actuators are water-based, eliminating the need for coating or electrical isolation. The polymer used to create this moving boundary or “active skin” is a flexible surface that deforms mechanically in response to an electrical signal. This was accomplished with a polymer that was approximately 560 mm2 divided into nine sections. Every other section of the polymer was wired to actuate simultaneously under the influence of an applied voltage cycling between 1 and 70 hertz with a 2 volt amplitude. The flow over the active skin was measured via TRDPIV (Time Resolved Digital Particle Image Velocimetry) using an Nd:YAG laser. The result of this research indicates that flow stirring is generated by the use of an active skin. Moreover, even though the deflection of the polymer is on the order of microns, its influence extends to the entire area of the channel studied in this work (approximately 13 mm height and 43 mm length).Copyright

Magnetic Drug Targeting: Drug Delivery in Large Vasculature

Magnetic drug targeting (MDT) is a novel drug delivery method with potential to dramatically revolutionize clinical approaches of the treatment of many diseases. In fact, MDT has been proposed for ailments ranging from vascular disease to cancer [1, 2]. Conventional drug delivery methods utilize large doses of medication to account for the dispersion of the drug throughout the body in the hope that a sufficient concentration of medicine arrives at the diseased site. Unfortunately, many medications can have caustic effects on healthy systems leaving patients with discomfort, weakened immunity or lowered quality of life. Alternatively, MDT aims to reduce potentially harmful global dosage levels by localizing medication at the diseased site. Additionally, magnetic drug targeting not only reduces chemicals seen by healthy areas of the body, it may provide a higher concentration of drug capable of remaining at the damaged location for a longer duration than typically seen for current treatment practices. Possibly the most important advantage of MDT is the method’s ability to enhance delivery while providing no additional invasive procedures.Copyright

Mixing at Low Reynolds Numbers by Vibrating Cantilevered Ionic Polymers

The objective of this research was to develop a novel active device for laminar mixing. The mixing device developed herein capitalized on Nafion ionic polymers, which are a class of active materials that are thin, flexible, inexpensive, and readily deployable in an aqueous medium and offer strains up to 5% under a small (<2V) applied voltage. The effect of these deflections on an incident laminar flow is the mixing mechanism explored in this effort. To the author’s knowledge, this high-risk effort presented herein is the first attempt to exploit ionic polymers as an active mixing device. Several different configurations of ionic polymers were tested and Digital Particle Image Velocimetry (DPIV) measurements were obtained. Resulting analysis using a quantitative mixing metric shows that the polymer creates differences in the flow under some of the configurations. Namely, in some actuation cases, a clear increase in mixing potential is observed in the near-polymer regions. Compared to a simple channel increases in mixing potential that ranged from 150% to over 300% were measured. Although these differences are present, they do not appear consistently in the results. However, only a partial set of flow information was obtained from DPIV, and an improved understanding of the effect of these polymers could be developed from additional experiments. Using ionic polymers for laminar mixing could foster the development of a new generation of efficient micromixing devices, which will improve the capabilities and effectiveness of numerous microfluidic technologies that range across biomedical, lab-on-a-chip, separation and sorting technologies and many more.Copyright

Dynamics of Magnetic Drug Targeting in Cardiovascular Flows

In spite of the level of sophistication and development of medicine, the effective delivery of drugs remains an unsolved problem. The conventional method to deliver drugs is to inject them intravenously to the patient, or administer a pill capsule. The medicine diffuses through the bloodstream without regard to the location of the malady. For many types of diseases, the inefficiency of this method leads to increased dosage requirements, which can cause the patient to experience adverse side effects.Copyright

Magnetic Targeting of Particle Transport Under Pulsatile Flow

Magnetic Drug Targeting (MDT) has been shown to be a promising technique to effectively deliver medicinal drugs via functionalized [1] magnetic particles to target sites during the treatment of cancer and other diseases [2,3,4]. In this paper, we investigate the interaction of steady and pulsatile flows injected with a ferrofluid, which is a colloidal suspension of superparamagnetic nanoparticles in a glass tube under the influence of a magnetic field. Ferrofluids are colloidal suspensions of single domain magnetic nanoparticles that are of the order of 10 nm in diameter. In this experiment, the ferrofluid particles were directed to a particular region of interest within a 10 mm diameter glass vessel by means of an applied localized magnetic field that originated outside of the vessel. The magnetic field was generated using a rare earth sintered permanent magnet which produced the magnetic field gradient required for inducing a body force on the volume of the ferrofluid. The experimental results reveal flows with rich dynamical phenomena. The aggregation of the ferrofluid produces a self-assembled hemispherical structure which dynamically interacts with the host flow. The aggregation generates an occlusion creating a flow field that is similar to that past an obstruction. However, since the structure itself is of a fluidic nature, it is subject to shear forces caused by the host fluid. In addition, the wake of the flow behind the aggregation creates vortices which are critical to study the stability of the ferrofluid aggregate. This paper presents a detailed investigation of the dynamics of the flow using Time-Resolved Digital Particle Image Velocimetry. To the best of the authors’ knowledge, these are the first quantitative, spatiotemporally resolved measurements documenting the interaction of a host fluid with a ferrofluid aggregate under steady or pulsatile flow conditions.Copyright