Alignment of Magnetic Nanorods in Solidifying Films

Abstract

Magnetic nanorods offer many opportunities in manufacturing of multifunctional composites with unprecedented magnetic and mechanical properties. [1][2][3][4][5][6][7] Coating fi lms with ordered magnetic nanorods are especially attractive for different hightech applications. These fi lms provide additional functionality, for example, enabling magnetic recording or magneto-optic features. [ 4,[8][9][10] However, the strategy for nanorod alignment in the macroscopic samples has not been developed so far. In the processing of magnetic fi lms, one important step lacking understanding of the behavior of nanorods is related to the complexity of rheological behavior of solidifying fi lm during the sol-gel processing. Many liquids used for sol-gel processing rapidly react on the environmental conditions by changing their rheological properties. [ 11,12 ] The time-dependent viscosity of many practically important carriers is typically described by the following equation η ( t ) = η 0 exp( t / τ 0 ), where η 0 is the initial viscosity of the carrier, t is the time, and τ 0 is the characteristic time of polymerization. [ 11,12 ] Recent experiments showed that the spinning behavior of a single nanorod in a fi lm with this type of viscosity variation is drastically different from the spinning behavior of nanorods in fl uids with constant viscosity. [ 13 ] However, alignment of an assembly of magnetic nanorods in a solidifying fi lm has not been discussed in the literature.In this paper, we study the behavior of an orientational distribution function of nanorods in the fi lm when its viscosity exponentially increases with time. Nanorods are assumed to have a large diameter to length ratio and are confi ned within the fi lm, keeping their longer axes parallel to the fi lm plane. The concentration of nanorods is assumed to be small, so that the dipole-dipole interactions between nanorods are insignifi cant. We also assume that the torque caused by the external fi eld is strong and that only viscous friction forces can balance this torque; the thermal fl uctuations are insignifi cant. These conditions are of practical interest in nanocomposite manufacturing; recent experiments on Newtonian fi lms confi rmed the validity of these assumptions. This paper sets up the basic theory needed for interpretation of experiments on non-Newtonian fi lms. [ 14 ] Assuming that in the fi rst moment of time the nanorods are randomly distributed in the fi lm, we show that the kinetics of their alignment cannot be explained using the theory for a single nanorod. The analysis of the time dependence of orientational distribution function reveals different regimes of nanorod alignment. We focus mainly on the specifi cation of a window of materials parameters, where the nanorods can be completely aligned along the fi eld during a specifi c time period. . Rotation of a Single Nanorod in a Solidifying FilmConsider a single nanorod suspended in fi lm. At time t = 0, one applies an external uniform fi eld B acting in the fi lm plane. In thin fi lms, the na...

Ferromagnetic Nanorods in Applications to Control of the In‐Plane Anisotropy of Composite Films and for In Situ Characterization of the Film Rheology

“…The images of the nanorods were taken at t = 0, 1, 2, 3, and 4 s and then processed to obtain the histograms and to compare them with the theory (solid curves). Adapted with permission . Copyright 2013, Wiley‐VCH.…”

“…Using this rheological equation of state, one can generalize the theory of nanorod rotation in a solidifying film by replacing the constant viscosity in the drag coefficient γ with the exponentially increasing one . The time τ 0 sets up a time scale for the nanorod rotation.…”

“…The quantitative analysis of these two cases given in ref. leads to the phase diagram shown in Figure . This diagram specifies the conditions for the in‐plane alignment of nanorods in solidifying films.…”

“…This diagram specifies the conditions for the in‐plane alignment of nanorods in solidifying films. The diagrams help to determine the process parameters such as η 0 /( τ 0 B ) and l / d which would ensure the complete alignment of different ferromagnetic nanorods into the field direction …”

Dynamics of ferromagnetic nanowires in a rotating magnetic field

“…Manipulating rod-like nanoparticles, such as nanowires, with external magnetic fields has attracted extensive interests in a myriad of engineering applications in micro/nanofluidics and biomedical engineering. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] For instance, in a study of nanowire alignment in solidifying films where the fluid viscosity increased with time, it was found that a large portion of the nanowires cannot properly align due to the viscous force. Hence, by analyzing the relaxation time of a single nanowire under different viscosities, the total time required for the whole field to align can be extracted.…”