Experimental evidence for reversible zippering of chains in magnetic nanofluids under external magnetic fields

Abstract

We study the time-dependent transmitted intensity and the scattered pattern from magnetic nanofluids at constant ramping of uniform external magnetic field. The nanofluid used is the dispersion of magnetite particles with an average diameter of 6.5 nm with a protective surfactant coating. We observe several critical fields at which the transmitted light intensity decreases drastically followed by the formation of a ringlike pattern on a screen placed perpendicular to the field direction. Interestingly, the critical fields occur at a regular interval of 20 G. The observed critical fields are attributed to zippering transitions of the chains due to attractive energy well when the chains are of different lengths or shifted with respect to one another. Interaction energy calculations show a decrease in the energy of the system due to dipolar interactions at different critical fields confirming the lowering of the system energy through lateral coalescence. The observed zippering phenomenon is perfectly reversible.

Features of Formation and Transformation of Fe$_{3}$O$_{4}$-Nanoparticle Clusters in a Magnetic Fluid under the Long-Term Magnetic-Field Pulse Action

“…В останньому методі після вмикання магнетного поля в тонких шарах йонних МР [29,30] та в МР, стабілізованих за допомогою су-рфактанта [16,23,31], спостерігалася інверсія напрямку оптичної екстинкції (ІНОЕ): спочатку зменшення оптичної прозорости МР одразу після ввімкнення імпульсу магнетного поля протягом пев-ного відрізку часу, а потім самовільне збільшення оптичної прозо-рости МР ще під час дії імпульсу магнетного поля.…”

Direct observations of field-induced assemblies in magnetite ferrofluids

“…Light scattering studies show that this is enough time to reach stable state before cryo-freezing is initiated. 42 Then, the grid was immediately plunged into liquid ethane.…”

“…50,51 Accordingly, the lateral interaction energy per unit chain length is given by U $ ðm=aÞ hH 2 i 1=2 $ vHðl 0 k B TÞ 1=2 a 5=2 q 2 , where q is the separation between the chains. 42 Compared to the head-to-tail aggregation, the process of zippering occurs over a larger timescale and is observed only at sufficiently high field strengths where the interaction energy U, overcomes the potential energy barrier for lateral aggregation of the chains. Once formed, the zipped-superstructures are stabilized by the van der Waals interactions between the magnetic particles.…”

“…1,[36][37][38][39][40] A variety of experimental techniques has been used in order to observe formation of structures. Amongst these techniques are SEM (Scanning Electron Microscopy), TEM (Transmission Electron Microscopy) and Cryo-TEM (Cryogenic Electron Microscopy), Optical Microscopy, small-angle neutron scattering (SANS), 41 light scattering and scattering dichroism, Raman spectroscopy, 42 and X-ray scattering (SAXS), 2,3,27,[45][46][47] each with its advantages and disadvantages. For instance, it is argued that TEM method may not give an accurate feature of the evolution of dipolar structures in liquid phase because during evaporation of the solvent distortions and sometimes cracks are formed by surface tension.…”

Optical Properties of Fe3O4Magnetic Fluid from Iron Sand

“…First, the head-tail of the magnetic particles oriented randomly knits each other as long as the field direction by forming a number of chains. This chain-like structure is the result of the competition between the magnetic dipolar interaction and thermal interaction [8]. After that, these chains experience aggregation again through lateral smelting forming a bundle of the chain [9].…”