Collaborative Research: How magnetic nanoparticles organize in extreme force gradients

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Non-Technical AbstractMagnetic nanoparticles in fluids can click-together to make miniature stir sticks, scaffolds for building artificial bones, and even barriers to trap cancer cells and prevent their traveling through blood. To realize such structures, one must understand the forces that control how these particles come together. The research team is studying a novel method for connecting nanoparticles together under extreme magnetic forces that are many orders of magnitude larger than naturally occurring ones. Systematic real-time experiments and theoretical simulations are compared to understand the new physics of these systems and their potential applications in nanotechnology. Women and minority PhD and undergraduate students will be recruited for this work that takes place within two Physics departments. In the US, Physics has the lowest female and minority percentage of graduating PhDs of any discipline so this is of critical importance. Through their close interaction, the two groups will design a 30 minute long Magnetic Nanoparticle Show with videos and simple demonstrations, and then perform the show to school groups. This topic is very visual so students will be able to understand the results and the implications for exciting applications such as drug delivery, heating to kill cancer tumors, and lab-on-a-chip. Finally, the two PIs will give talks on nanotechnology at local "Science Cafes," where members of the public can come to discuss science. Technical AbstractMassive magnetic force gradients can be created at the nanoscale and used to make virtually any self-assembled shape from magnetic nanoparticles in fluids. However, assembly in these extreme gradients shows novel behaviors that remain unexplained, including vastly different results when a minute amount of salt is added to the suspending fluid. This project seeks to understand how the immediate environment surrounding a nanoparticle (ionic, magnetic, ligand chemistry) affects its ability to assemble into geometric arrangements in extreme force gradients, an understanding that could transform how and where nanoparticles can be used in real-world applications. The specific aims of this project focus on three parameters that affect nanoparticle assembly: (i) external field gradient, (ii) ionic strength of colloidal suspensions, and (iii) interparticle magnetic interactions. The research team is applying controllable forces - that vary by orders of magnitude over just nanometers - to nanoparticles as their colloidal environment is modified, with a transformative approach involving magnetic recording media submerged in liquid. These parameters are studied by observing nanoparticles self-assemble into a diffraction grating, as well as by performing torque-mixing magnetic resonance spectroscopy measurements to study few-nanoparticle assemblies. These experiments are closely tied with theoretical calculations and Langevin dynamics simulations that explain how changes in nanoscale interactions modify the emergent self-assembled structures they produce. Multi-timescale, finite-temperature simulations allow predicted large assemblies to be compared to experiments within reasonable computation times.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.