Kyle J. Solis

Isothermal Magnetic Advection: Creating functional fluid flows for heat and mass transfer

Natural convection has been of interest for over a century due to its rich nonlinear dynamics and applications to heat transfer. However, convection occurs only when both gravity and a destabilizing thermal gradient exist. We have discovered a unique class of vigorous, emergent fluid flows that have the full functionality of natural convection but can be stimulated regardless of gravity or thermal gradients, simply by subjecting a platelet suspension to certain time-dependent biaxial magnetic fields of modest strength. This enigmatic phenomenon may facilitate cooling in microgravity environments and in other circumstances where convection fails.

Field-structured magnetic platelets as a route to improved thermal interface materials

The development of high-performance thermal interface materials (TIMs) is crucial to enabling future generations of microelectronics because the TIM is usually the limiting thermal resistance in the heat removal path. Typical TIMs achieve modest thermal conductivities by including large volume fractions of randomly-dispersed, highly-conductive, spherical particles in a polymer resin. This paper explores field-structured magnetic platelet composites as a new approach to more effective TIMs. The motivation for this approach is rooted in shape functional theory, which shows that when the particle material has a significantly higher thermal conductivity than that of the polymer, the particle shape and orientation are the factors that limit conductivity enhancement. Oriented platelets are highly effective for heat transfer and if these are magnetic, then magnetic fields can be used to both orient and agglomerate these into structures that efficiently direct heat flow. In this paper we show that such field-stru...

Field-structured, multilayered platelets enable high performance, dielectric thermal composites

Moldable, thermally conductive polymer composites have broad applications as thermal interface materials and encapsulants. These thermal composites are generally comprised of single-phase particles that are randomly oriented and dispersed. Magnetic platelets have been shown to give exceptionally high thermal conductivities when magnetically aligned along the intended direction of heat flow, but produce composites that are electrically conductive. We have designed precision multilayered platelets that enable the development of high performance thermal composites that are electrically insulating. These platelets consist of a thin Ni core that permits field alignment, Al or Cu coatings that facilitate heat transport, and dielectric layers of MgF2 or SiO2 that ensure that the final composite is electrically insulating. These platelets can be made flat or corrugated, square or irregular, and the thickness of the various layers can be varied over a wide range. Thermal conductivity data for a variety of platelet...

Multiaxial fields drive the thermal conductivity switching of a magneto-responsive platelet suspension

We demonstrate the ability to change the thermal conductivity of a magnetic platelet suspension from insulating to conducting by using either uniaxial or multiaxial ac magnetic fields to control the suspension structure and dynamics. The equivalent thermal conductivity of the suspension can be modified either by creating static particle structures that facilitate or block heat transfer, or by using multiaxial ac fields to drive emergent particle dynamics that create vigorous, organized, non-contact flow. The equivalent thermal conductivity of a single suspension can be varied over a 100-fold range, and an equivalent thermal conductivity as high as 18.3 W m−1 K−1 has been achieved in an aqueous suspension containing only 2.0 vol% platelets. This value is more than twice the conductivity of liquid mercury.

Controlling the column spacing in isothermal magnetic advection to enable tunable heat and mass transfer

Isothermal magnetic advection (IMA) is a recently discovered method of inducing highly organized, non-contact flow lattices in suspensions of magnetic particles, using only uniform ac magnetic fields of modest strength. The initiation of these vigorous flows requires neither a thermal gradient nor a gravitational field, and so can be used to transfer heat and mass in circumstances where natural convection does not occur. These advection lattices are comprised of a square lattice of antiparallel flow columns. If the column spacing is sufficiently large compared to the column length and the flow rate within the columns is sufficiently large, then one would expect efficient transfer of both heat and mass. Otherwise, the flow lattice could act as a countercurrent heat exchanger and only mass will be efficiently transferred. Although this latter case might be useful for feeding a reaction front without extracting heat, it is likely that most interest will be focused on using IMA for heat transfer. In this pape...

Quantifying vorticity in magnetic particle suspensions driven by symmetric and asymmetric multiaxial fields

We recently reported two methods of inducing vigorous fluid vorticity in magnetic particle suspensions. The first method employs symmetry-breaking rational fields. These fields are comprised of two orthogonal ac components whose frequencies form a rational number and an orthogonal dc field that breaks the symmetry of the biaxial ac field to create the parity required to induce deterministic vorticity. The second method is based on rational triads, which are fields comprised of three orthogonal ac components whose frequency ratios are rational (e.g., 1 : 2 : 3). For each method a symmetry theory has been developed that enables the prediction of the direction and sign of vorticity as functions of the field frequencies and phases. However, this theory has its limitations. It only applies to those particular phase angles that give rise to fields whose Lissajous plots, or principal 2-d projections thereof, have a high degree of symmetry. Nor can symmetry theory provide a measure of the magnitude of the torque density induced by the field. In this paper a functional of the multiaxial magnetic field is proposed that not only is consistent with all of the predictions of the symmetry theories, but also quantifies the torque density. This functional can be applied to fields whose Lissajous plots lack symmetry and can thus be used to predict a variety of effects and trends that cannot be predicted from the symmetry theories. These trends include the dependence of the magnitude of the torque density on the various frequency ratios, the unexpected reversal of flow with increasing dc field amplitude for certain symmetry-breaking fields, and the existence of off-axis vorticity for rational triads, such as 1 : 3 : 5, that do not have the symmetry required to analyze by symmetry theory. Experimental data are given that show the degree to which this functional is successful in predicting observed trends.

Mesmerizing magnetic fields

The behavior displayed by a suspension of magnetic particles exposed to a rapidly varying multiaxial magnetic field is both dazzling and practically applicable.

Can symmetry transitions of complex fields enable 3-d control of fluid vorticity?

Methods of inducing vigorous noncontact fluid flow are important to technologies involving heat and mass transfer and fluid mixing, since they eliminate the need for moving parts, pipes and seals, all of which compromise system reliability. Unfortunately, traditional noncontact flow methods are few, and have limitations of their own. We have discovered two classes of fields that can induce fluid vorticity without requiring either gravity or a thermal gradient. The first class we call Symmetry-Breaking Rational Fields. These are triaxial fields comprised of three orthogonal components, two ac and one dc. The second class is Rational Triad Fields, which differ in that all three components are alternating. In this report we quantify the induced vorticity for a wide variety of fields and consider symmetry transitions between these field types. These transitions give rise to orbiting vorticity vectors, a technology for non-contact, non-stationary fluid mixing.