Abstract
Improved knowledge of the magnetic field dependent flow properties of nanoparticle-based magnetic fluids is critical to the design of biomedical applications, including drug delivery and cell sorting. To probe the rheology of ferrofluid on a sub-millimeter scale, we examine the paths of 550 μm diameter glass spheres falling due to gravity in dilute ferrofluid, imposing a uniform magnetic field at an angle with respect to the vertical. Visualization of the spheres’ trajectories is achieved using high resolution X-ray phase-contrast imaging, allowing measurement of a terminal velocity while simultaneously revealing the formation of an array of long thread-like accumulations of magnetic nanoparticles. Drag on the sphere is largest when the applied field is normal to the path of the falling sphere, and smallest when the field and trajectory are aligned. A Stokes drag-based analysis is performed to extract an empirical tensorial viscosity from the data. We propose an approximate physical model for the observed anisotropic drag, based on the resistive force theory drag acting on a fixed non-interacting array of slender threads, aligned parallel to the magnetic field.
Highlights
Ferrofluids are materials designed so that a remotely applied magnetic field may drive flow, manipulate a surface or interface or control the fluid’s physical properties [1,2]
Ferrofluids are built from three basic components: magnetic nanoparticles, a carrier fluid and a dispersant, usually a surfactant, that adheres to the nanoparticles
As applications of ferrofluids are developed on micro- and nano-meter scales, as in biomedical applications [5], magnetorheological effects become more relevant to the prediction of flows and the design of new devices
Summary
Ferrofluids are materials designed so that a remotely applied magnetic field may drive flow, manipulate a surface or interface or control the fluid’s physical properties [1,2]. Ferrofluids are built from three basic components: magnetic nanoparticles, a carrier fluid and a dispersant, usually a surfactant, that adheres to the nanoparticles. This coating enhances steric repulsion between nearby particles and, along with Brownian effects, colloidally stabilizes commercial ferrofluids. As applications of ferrofluids are developed on micro- and nano-meter scales, as in biomedical applications [5], magnetorheological effects become more relevant to the prediction of flows and the design of new devices. The formation of large particle aggregates may have a significant impact of the use of ferrofluids in vivo [6]
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