We characterize how suspensions of magnetic particles in a liquid respond to a magnetic field in terms of the effective magnetic susceptibility χ ef f using inductance measurements. We test a model that predicts how χ ef f varies due to demagnetization, as a function of sample aspect ratio, particle packing fraction, and particle aspect ratio [1]. For spherical particles or cylindrical particles aligned with external magnetic field, the model can be fitted to the measured data with agreement within 17%. However, we find that the random alignment of particles relative to the magnetic field plays a role, reducing χ ef f by a factor of 3 in some cases, which is not accounted for in models yet. While suspensions are predicted to have χ ef f that approach the particle material susceptibility in the limit of large particle aspect ratio, instead we find a much smaller particle aspect ratio where χ ef f is maximized. A prediction that χ ef f approaches the bulk material susceptibility in the limit of the packing fraction of the liquid-solid transition also fails. We find χ ef f no larger than about 4 for suspensions of iron particles.
We demonstrate the applicability of extended Lagrangian Born-Oppenheimer quantumbased molecular dynamics (XL-BOMD) to model electron transfer reactions occurring on solidliquid interfaces. Specifically, we consider the reduction of O 2 as catalyzed at the interface of an N-doped graphene sheet and H 2 O at fuel cell cathodes. This system is a good testbed for nextgeneration computational chemistry methods since the electrochemical functionalities strongly depend on atomic-scale quantum mechanics. As opposed to prior iterations of first principles molecular dynamics, XL-BOMD only requires a full self-consistent-charge relaxation during the initial time step. The electronic ground state and total energy are stabilized thereafter through nuclear and electronic equations of motion assisted by an inner-product kernel updated with low-rank approximations. A species charge analysis reveals that the kernel-based XL-BOMD simulation can capture an electron transfer between the PGM-free catalyst and a solvated O 2 molecule mediated by H 2 O, which results in the molecular dissociation of O 2 .
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