title: Direct observations of field-induced assemblies in magnetite ferrofluids.
abstract: Evolution of microstructures in magnetite-based ferrofluids with weak dipolar moments (particle size ≤ 10 nm) is studied with an emphasis on examining the effects of particle concentration (ϕ) and magnetic field strength (H) on the structures. Nanoparticles are dispersed in water at three different concentrations,ϕ= 0.15%, 0.48%, and 0.59% (w/v) [g/ml%] and exposed to uniform magnetic fields in the range ofH= 0.05-0.42 T. Cryogenic transmission electron microscopy is employed to providein-situobservations of the field-induced assemblies in such systems. As the magnetic field increases, the Brownian colloids are observed to form randomly distributed chains aligned in the field direction, followed by head-to-tail chain aggregation and then lateral aggregation of chains termed as zippering. By increasing the field in low concentration samples, the number of chains increases, though their length does not change dramatically. Increasing concentration increases the length of the linear particle assemblies in the presence of a fixed external magnetic field. Thickening of the chains due to zippering is observed at relatively high fields. Through a systematic variation of concentration and magnetic field strength, this study shows that both magnetic field strength and change in concentration can strongly influence formation of microstructures even in weak dipolar systems. Additionally, the results of two commonly used support films on electron microscopy grids, continuous carbon and holey carbon films, are compared. Holey carbon film allows us to create local regions of high concentrations that further assist the development of field-induced assemblies. The experimental observations provide a validation of the zippering effect and can be utilized in the development of models for thermophysical properties such as thermal conductivity.
authors.affiliation: Mechanical Engineering Department, Polytechnic School of Engineering, New York University , 6 Metrotech Center, Brooklyn, New York 11201, USA.
authors.forename: N S Susan
authors.initials: NS
authors.lastname: Mousavi
authors.affiliation: New York University Abu Dhabi , Saadiyat Island, PO Box 129 188, Abu Dhabi, United Arab Emirates.
authors.forename: Sachin D
authors.identifier: 0000-0002-4910-7381
authors.initials: SD
authors.lastname: Khapli
authors.forename: Sunil
authors.identifier: 0000-0002-7470-4225
authors.initials: S
authors.lastname: Kumar
references.citation: Phys Rev E Stat Nonlin Soft Matter Phys. 2004 Nov;70(5 Pt 1):051502
references.pmid: 15600619
references.citation: Small. 2009 Jul;5(14):1600-30
references.pmid: 19517482
references.citation: Phys Rev Lett. 2006 Nov 3;97(18):185702
references.pmid: 17155554
references.citation: Phys Rev Lett. 1990 Nov 26;65(22):2820-2823
references.pmid: 10042703
references.citation: Phys Rev E Stat Nonlin Soft Matter Phys. 2005 Jun;71(6 Pt 1):061203
references.pmid: 16089727
references.citation: Phys Rev Lett. 2011 May 20;106(20):208301
references.pmid: 21668267
references.citation: J Phys Condens Matter. 2008 May 21;20(20):204113
references.pmid: 21694243
references.citation: Phys Rev Lett. 1992 Sep 7;69(10):1524-1527
references.pmid: 10046244
references.citation: Phys Rev E Stat Nonlin Soft Matter Phys. 2002 Aug;66(2 Pt 1):021405
references.pmid: 12241176
references.citation: J Colloid Interface Sci. 1998 Jul 15;203(2):233-48
references.pmid: 9705760
references.citation: Langmuir. 2006 Jun 20;22(13):5611-6
references.pmid: 16768484
references.citation: Phys Rev E Stat Nonlin Soft Matter Phys. 2009 Oct;80(4 Pt 1):041401
references.pmid: 19905308
references.citation: J Am Chem Soc. 2004 Dec 29;126(51):16706-7
references.pmid: 15612692