Self Assembly of Magnetic Nanoparticles at Silicon Surfaces

Published

June 21, 2015

Author(s)

Katharina Theis-Brohl, Philipp Gutfreund, Alexei Vorobiev, Max Wolff, Boris P. Toperverg, Joseph Dura, Julie Borchers

Abstract

Neutron reflectometry was used to study the assembly of magnetite nanoparticles in a water-based ferrofluid close to a silicon surface, Under three conditions, static, under shear and with a magnetic field, the depth profile is extracted. The particles have an average diameter of 11 nm and a volume density of 5 % in a D₂O/H₂O mixture. They are surrounded by a 4 nm thick bilayer of carboxylic acid for steric repulsion. The reflectivity data were fitted to a model using a least square routine based on the Parratt formalism. From the scattering length density depth profiles the following behavior is concluded: The fits indicate that excess carboxylic acid covers the silicon surface. Under constant shear the wetting layer persists but a depletion layer forms between the wetting layer and the moving ferrofluid. Once the flow is stopped, the wetting layer becomes more pronounced with dense packing and is accompanied by a looser packed second layer. In the case of an applied magnetic field the prolate particles experience a torque and align with their long axes along the silicon surface which leads to a higher particle density.

Citation

Soft Matter

Volume

11

Pub Type

Journals

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Keywords

neutron reflectometry, ferrofluid, colloid, Magnetic interactions, self assembly, shear, rheology

Citation

Theis-Brohl, K. , Gutfreund, P. , Vorobiev, A. , Wolff, M. , Toperverg, B. , Dura, J. and Borchers, J. (2015), Self Assembly of Magnetic Nanoparticles at Silicon Surfaces, Soft Matter, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=918187 (Accessed October 9, 2025)