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Influence of magnetic field on the orientation of anisotropic magnetic particles at liquid interfaces

Newton, Bethany J.; Brakke, Kenneth A.; Buzza, D. Martin A.

Authors

Bethany J. Newton

Kenneth A. Brakke

Abstract

We study theoretically the influence of an external magnetic field on the orientation of an ellipsoidal magnetic particle adsorbed at a liquid interface. Using the finite element program Surface Evolver, we calculate the equilibrium meniscus shape around the ellipsoidal particle and its equilibrium tilt angle with respect to the undeformed interface θt when a magnetic field B is applied perpendicular to the interface. We find that as we increase field strength, θt increases and at a critical magnetic field Bc1 and tilt angle θc1, the particle undergoes a discontinuous transition to the ‘perpendicular’ orientation (θt = 90°). Our results agree qualitatively with the simplified theory of Bresme and Faraudo [F. Bresme and J. Faraudo, J. Phys.: Condens. Matter, 2007, 19, 375110] which assumes that the liquid interface is flat, while they agree quantitatively with recent lattice-Boltzmann simulations of Davies et al. [G. Davies et al., Soft Matter, 2014, 10, 6742] which account for the deformation of the liquid meniscus. We also show for the first time that upon reducing the external magnetic field, at a critical magnetic field Bc2 < Bc1, the particle undergoes a second discontinuous transition from the perpendicular orientation to a finite tilt angle θc2 < θc1. In other words, for micron-sized particles where the thermal energy kBT is negligible compared to the interfacial energy, the tilt angle vs. magnetic field curve exhibits hysteresis behaviour. Due to the higher degree of accuracy of the Surface Evolver method, we are able to analyse the behaviour of the particles near these orientational transitions accurately and study how the critical quantities Bc1, Bc2, θc1 and θc2 vary with particle aspect ratio and contact angle.

Journal Article Type Article
Publication Date Nov 13, 2014
Journal Phys. Chem. Chem. Phys.
Print ISSN 1463-9076
Electronic ISSN 1463-9084
Publisher Royal Society of Chemistry
Peer Reviewed Peer Reviewed
Volume 16
Issue 47
Pages 26051-26058
Institution Citation Newton, B. J., Brakke, K. A., & Buzza, D. M. A. (2014). Influence of magnetic field on the orientation of anisotropic magnetic particles at liquid interfaces. Physical chemistry chemical physics : PCCP, 16(47), 26051-26058. doi:10.1039/c4cp04270k
DOI https://doi.org/10.1039/c4cp04270k
Publisher URL http://pubs.rsc.org/en/content/articlelanding/2014/cp/c4cp04270k#!divAbstract
Copyright Statement ©2018 The authors.
Additional Information : This document is CrossCheck deposited; : The PCCP Owner Societies have an exclusive publication licence for this journal; OPEN ACCESS: The accepted version of this article will be made freely available after a 12 month embargo period; : Received 23 September 2014; Accepted 27 October 2014; Accepted Manuscript published 28 October 2014; Advance Article published 31 October 2014; Version of Record published 13 November 2014

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