Theory and simulation of anisotropic pair correlations in ferrofluids in magnetic fields

Abstract

Articles you may be interested inMagnetic field role on the structure and optical response of photonic crystals based on ferrofluids containing Co0.25Zn0.75Fe2O4 nanoparticles J. Appl. Phys. 115, 193502 (2014) Anisotropic pair correlations in ferrofluids exposed to magnetic fields are studied using a combination of statistical-mechanical theory and computer simulations. A simple dipolar hard-sphere model of the magnetic colloidal particles is studied in detail. A virial-expansion theory is constructed for the pair distribution function (PDF) which depends not only on the length of the pair separation vector, but also on its orientation with respect to the field. A detailed comparison is made between the theoretical predictions and accurate simulation data, and it is found that the theory works well for realistic values of the dipolar coupling constant (λ = 1), volume fraction (ϕ ≤ 0.1), and magnetic field strength. The structure factor is computed for wavevectors either parallel or perpendicular to the field. The comparison between theory and simulation is generally very good with realistic ferrofluid parameters. For both the PDF and the structure factor, there are some deviations between theory and simulation at uncommonly high dipolar coupling constants, and with very strong magnetic fields. In particular, the theory is less successful at predicting the behavior of the structure factors at very low wavevectors, and perpendicular Gaussian density fluctuations arising from strongly correlated pairs of magnetic particles. Overall, though, the theory provides reliable predictions for the nature and degree of pair correlations in ferrofluids in magnetic fields, and hence should be of use in the design of functional magnetic materials.

Pair correlations in a bidisperse ferrofluid in an external magnetic field: theory and computer simulations

“…Следуя [32,33,41] в каждой функции Майера может быть отдельно выделен вклад, связанный со взаимодействием твердых сфер f hs (ij) = exp(−βu…”

“…Соответственно, СФ также становится функцией компонент волнового вектора q = (q ⊥ , q || ), перпен-дикулярной и параллельной направлению внешнего магнитного поля. Изучение анизотропии ПФР в поле для монодисперсной феррожидкости проведено в работах [32,33], где подробно изложена процедура усредне-ния диаграмм по ориентациям магнитного момента · · · i и положению третьей частицы dr 3 . В случае присутствия поля в ПФР появляются вклады, линейные по λ γ , поскольку диаграммы I 2b , I 3b и I 3c на Рис.…”

“…В работе [32], основываясь на монодисперсной моде-ли дипольных твердых сфер, анизотропия СФ в магнитном поле впер-вые теоретически была объяснена наличием межчастичных корреляций в феррожидкостях с умеренными концентрациями и невысокой интен-сивностью диполь-дипольных взаимодействий. Сравнение построенной теории с результатами компьютерного моделирования [33] (метод Мон-те Карло) и [34] (метод молекулярной динамики) позволили определить область применимости теории. Поскольку реальные феррожидкости яв-ляются полидисперсными по своему гранулометрическому составу, сле-дующим шагом развития теории микроструктуры феррожидкостей по-служила работа [35], где было изучено влияние полидисперсности на СФ в отсутствие внешнего магнитного поля.…”

Microstructure and magnetic properties of magnetic fluids consisting of shifted dipole particles under the influence of an external magnetic field

“…The investigation of ferrofluid magnetic properties started more than 50 years ago, and comprises an extreme variety of experimental, 2-10 theoretic, [11][12][13][14][15][16][17][18][19][20][21][22][23] and computer simulations [24][25][26][27][28][29][30][31][32][33][34][35] studies. The findings of these studies can be summarised as follows: (1) there are various mechanism with which the magnetic particle might respond to an external magnetic field, namely, Neél (the dipole rotate within the particle to coalign with an applied field, inherent to smaller particles, grows with particle size and depends on the particle material) and Brownian (the particle rotates as a whole to adjust its dipole moment to the direction of the field, a) weeber@icp.uni-stuttgart.de b) marco.klinkigt@me.com c) sofia.kantorovich@univie.ac.at d) holm@icp.uni-stuttgart.de inherent to bigger particles, grows with the volume of the particle and carrier viscosity); (2) the characteristic magnetic relaxation time of the ferrofluid depends on the magnetic particle size distribution; (3) there is no ferrofluid which exhibits an initial susceptibility lower than the one of the Langevin law; 36 (4) the stronger the inter-particle interaction is, the higher is the magnetic susceptibility of the system; (5) an external magnetic field strongly enhances the chain formation, and for higher fields even magnetic fluids with moderately interacting particles become strongly aggregated.…”

“…The findings of these studies can be summarised as follows: (1) there are various mechanism with which the magnetic particle might respond to an external magnetic field, namely, Neél (the dipole rotate within the particle to coalign with an applied field, inherent to smaller particles, grows with particle size and depends on the particle material) and Brownian (the particle rotates as a whole to adjust its dipole moment to the direction of the field, a) weeber@icp.uni-stuttgart.de b) marco.klinkigt@me.com c) sofia.kantorovich@univie.ac.at d) holm@icp.uni-stuttgart.de inherent to bigger particles, grows with the volume of the particle and carrier viscosity); (2) the characteristic magnetic relaxation time of the ferrofluid depends on the magnetic particle size distribution; (3) there is no ferrofluid which exhibits an initial susceptibility lower than the one of the Langevin law; 36 (4) the stronger the inter-particle interaction is, the higher is the magnetic susceptibility of the system; (5) an external magnetic field strongly enhances the chain formation, and for higher fields even magnetic fluids with moderately interacting particles become strongly aggregated. All these features influence viscous, 7, 37-41 optical, 2-4, 42-44 diffusion, 6,45 scattering, [45][46][47][48][49][50][51][52] thermodynamic, 22 and acoustic 53 properties of ferrofluids. These systems are widely used in medicine, e.g., actuators [55][56][57] or sensors for the monitoring of anti-body reactions.…”