Abstract
Colloidal magnetic nanoparticles are candidates for application in biology, medicine and nanomanufac-turing. Understanding how these particles interact collectively in fluids, especially how they assemble and aggregate under external magnetic fields, is critical for high quality, safe, and reliable deployment of these particles. Here, by applying magnetic forces that vary strongly over the same length scale as the colloidal stabilizing force and then varying this colloidal repulsion, we can trigger self-assembly of these nanoparticles into parallel line patterns on the surface of a disk drive medium. Localized within nanometers of the medium surface, this effect is strongly dependent on the ionic properties of the colloidal fluid but at a level too small to cause bulk colloidal aggregation. We use real-time optical diffraction to monitor the dynamics of self-assembly, detecting local colloidal changes with greatly enhanced sensitivity compared with conventional light scattering. Simulations predict the triggering but not the dynamics, especially at short measurement times. Beyond using spatially-varying magnetic forces to balance interactions and drive assembly in magnetic nanoparticles, future measurements leveraging the sensitivity of this approach could identify novel colloidal effects that impact real-world applications of these nanoparticles.
Highlights
Colloidal magnetic nanoparticles are candidates for application in biology, medicine and nanomanufacturing
While colloidal NPs are suspended by local forces that change over lengths comparable to the particle diameter, typically 10 nm, the field gradients used for magnetic separation, filtering, or trapping of nano-to-micro scale particles with wires[14,15,16], patterned magnetic films[17,18], or domain walls[19] are effectively constant over many NP diameters
The magnetic fields generated by the bits used to store information in a magnetic disk drive have gradients that vary over 10 nm in length
Summary
Onto a grating template recorded on a magnetic disk, we trigger self-assembly by increasing the IS of the fluid, studying the resulting dynamics as a function of both time and IS. For these large IS suspensions, the DE jumps immediately after NPs are introduced into the fluid cell (initially larger than the triggered DE) but at 200 s crosses under the triggered DE curve and is weaker at long times This observation suggests that while bulk fluid aggregation happens at larger IS, the aggregates assemble into a lower quality diffraction grating than we obtain with triggered self-assembly. Properly designed local magnetic forces, when combined with solution triggering, offer an exciting new means to control and enhance directed self-assembly of NPs
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