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Field Directed Assembly of Aerosol Nanoparticles in Free-molecular and Transition Regime
PRITHWISH BISWAS, Pankaj Ghildiyal, George Mulholland, Michael Zachariah, University of California, Riverside
Abstract Number: 524
Working Group: Aerosol Physics
Abstract
Manipulating the fractal dimension of nanostructures in continuum regime to obtain different architectures has been widely explored. However, ballistic dynamics in the free molecular regime, typically observed in aerosol phase, causes nanoparticles synthesized through high temperature aerosol routes, to form aggregates with the universal fractal dimension (Df) of 1.8. In this study, it has been demonstrated that external field induced interparticle interactions can be utilized to linearly assemble particles and hence tune the Df of particle aggregates even in the free-molecular regime. Through performing a hybrid ensemble/cluster-cluster aggregation Monte Carlo simulation, we have demonstrated that an ensemble of particles can be made to interact through directional attractive and repulsive forces by inducing dipole moments from an externally applied field, thereby leading to an aggregate structure transition. The fractal dimension of the aggregates can be tuned by changing primary particle size, temperature and the applied field strength and linear chain-like aggregates with Df~1 can be obtained when the applied field strength is above a certain threshold value for a particular primary particle size and temperature. It has been demonstrated that the threshold magnetic field strength required to linearly assemble 10–500 nm particle sizes are practically achievable whereas the field required to assemble sub-100 nm particles by inducing electrical dipoles, are beyond the breakdown strength of most gases. A correction factor to free molecular and transition regime coagulation kernel, based on magnetic dipolar interactions, has been derived to theoretically account for the enhanced coagulation rates due to attractive interactions. A comparison has been made between the coagulation time-scales estimated by theory and simulation, with the estimated polarization time-scales of the primary particles and the oscillation time period of the magnetic field, to demonstrate that sub-50 nm superparamagnetic primary particles can be magnetized and assembled at any temperature, while below the Curie temperature ferromagnetic particles of all sizes can be magnetized and assembled, given the applied field is higher than the threshold.