Template-based synthesis of nanotube and nanowire ferrofluids and their magnetoviscosity
Zusammenfassung der Projektergebnisse
Current models for magnetoviscosity suggest that replacing the spherical nanoparticles of a conventional ferrofluid with magnetic nanorods would lead to a stronger field-induced viscosity enhancement - this could make new applications of the so-called magnetoviscous effect (MVE) feasible, such as in adaptive damping systems. It is a tall task, however, to produce the constituent nanorods in sufficient quantity for sample volumes in excess of a few microliters. And even when one succeeds in this regard, the magnetoviscosity turns out to be disappointingly weak. Over the course of a prior joint project, we discovered a promising alternative strategy: instead of relying on nanorods to interact directly with the externally applied magnetic field, we can attempt to have them rearrange the spherical magnetic nanoparticles of a conventional ferrofluid into quasilinear aggregates. If the latter act as rod-shaped magnets under the applied field, then a strongly enhanced MVE might be expected at much smaller nanorod volume fractions than in a “pure” nanorod ferrofluid. Investigation of the magnetorheological properties of such “hybrid” ferrofluids (nanorods + nanoparticles) was a central focus of this just-completed research effort between scientists at the Universities of Hamburg and Ulm. In the prior Abschlussbericht we reported a strongly enhanced MVE in hybrid ferrofluids containing Ni nanorods of size 400 x 40 nm prepared by templating. Owing to their ferromagnetism, it is not surprising that even small volume fractions of Ni nanorod additives had a major impact on the viscosity of the base ferrofluid. However, the templated synthesis route is not amenable to being upscaled to industrially relevant ferrofluid volumes. Consequently, our subsequent investigations of hybrid ferrofluids had the aim of finding a lessexpensive, mass-produceable nanorod species that could induce a similarly strong enhancement in magnetoviscosity when dispersed in a conventional ferrofluid. The example of Tobacco mosaic virus (TMV) additives, which we had previously shown to raise the MVE by more than an order of magnitude, suggested that nonmagnetic nanorods could do the job just as well as Ni. In the end, we settled on nanorods of Au, which can be purchased commercially in a wide range of geometries, and silica, which our colleagues in the chemistry department of Ulm University produce in gram quantities. The results obtained for the magnetoviscosity of 85 nm-long Au nanorods dispersed in a conventional cobalt ferrite-based ferrofluid were initially promising: at volume fractions similar to those studied for Ni nanorod additives, the MVE was larger than that of the base ferrofluid by up to a factor of four. Surprisingly, the nanorod diameter - we tested both 25 and 45 nm - had no significant impact on the MVE. Moreover, moving to higher nanorod volume fractions turned out to be counterproductive, as the MVE leveled off and even decreased! Puzzled by this behavior, we attempted to promote the adsorption of magnetic nanoparticles on the surface of Au nanorods by varying the pH level of the ferrofluid, but the results were inconclusive. More consistent results were obtained from samples containing nanorods of silica (450 x 130 nm). In this case, our chemist colleagues are able to functionalize the nanorod surface with carboxyl groups, which are highly conducive to the adsorption of a species like cobalt ferrite. Indeed, the MVE was enhanced by a factor of 5 to 10 in every hybrid ferrofluid made with carboxy-functionalized silica nanorods, reaching values comparable to those of Ni-based hybrid ferrofluids (albeit at a much higher volume fraction). On the other hand, without surface functionalization the silica nanorods had no discernible impact on the MVE whatsoever. We conclude that surface chemistry exerts an inordinately strong influence on the ability of nanorod additives to manipulate the internal structure of a ferrofluid. Further studies will exploit the chemical and morphological variability of silica nanorods as well as their ease of mass production to unravel the competing effects of particle charge, surface chemistry, nanorod geometry and volume fraction on the magnetoviscosity of hybrid ferrofluids.
