Soto Aquino, Denisse

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    Ferrohydrodynamics studies through Brownian dynamics simulations of spherical nanoparticle suspensions
    (2012) Soto Aquino, Denisse; Rinaldi, Carlos; College of Engineering; Briano, Julio; Acevedo, Aldo; Leonardi, Stefano; Department of Chemical Engineering; Sundaram, Paul
    In this contribution, Brownian dynamics simulations of spherical, thermally blocked, magnetic nanoparticles under applied shear and magnetic fields were studied. Equilibrium and dynamical properties as well as rheological properties and energy dissipation rates of a dilute system were obtained. The algorithm describing the change in the magnetization and magnetoviscosity of the suspension was derived from the stochastic angular momentum equation. Simulation results were compared with the predictions of suspension-scale models based on three magnetization relaxation equations for different situations: i) constant magnetic field and shear flow, ii) transient response of magnetic and shear flow, iii) oscillatory shear flow with constant magnetic fields, and iv) alternating magnetic field for energy dissipation rate determination. For all the conditions studied, excellent agreement is observed between simulation results and the predictions of an equation due to Martsenyuk, Raikher, and Shliomis. From simulation results at constant magnetic field over a wide range of conditions, master curves were obtained using a newly defined Mason number based on the balance of hydrodynamic and magnetic torques. From the transient response studies, both simulations and analysis show that the transient approach to a steady state magnetoviscosity can be either monotonic or oscillatory depending on the relative magnitudes of the applied magnetic field and shear rate. Simulations for the dynamic properties of ferrofluids under oscillatory shear and constant magnetic fields show an apparent elastic character to the rheology of these suspensions. Energy dissipation rates were obtained from the dynamical magnetization properties and compared with Rosensweig’s energy dissipation model. Results show that Rosensweig’s original analysis is strictly limited to low magnetic field amplitude and frequency. Finally, a Brownian dynamics simulation algorithm for interacting particles was developed. Simulation results for the equilibrium properties of magnetized particles show agreement with theoretical models, but fail to predict dynamic properties.