Magnetic field is widely used in the separation of magnetic nanoparticles. 1,2 The magnetic separation is a result of magnetophoresis where various forces by magnetic field gradient, viscous drag, sedimentation, and interactions of magnetic dipoles are involved. 3,4 Magnetic nanoparticles usually agglomerate reversibly under magnetic field. Magnetophoretic transport demonstrates the possibilities of the biomedical applications such as intercellular manipulation of magnetic nanoparticles for imaging or drug delivery. 5-7 Magnetization of ferrofluid occurs by Neel and Brownian mechanism 5 and aggregation of nanoparticles. 8 We have recently reported the magnetization of magnetite ferrofluid studied by measuring the change of magnetic weight. 9 As the magnetic nanoparticles agglomerate and the structural relaxation occurs to form stable aggregates, the magnetic weight increases with the stretched exponential time dependence, m(t) = m(∞) + [m(0) − m(∞)] exp[−(t/τ) β ] where the exponent β is a positive constant less than one and τ is the characteristic time constant. The stretched exponential function H(t) = exp[−(t/τ) β ] results from the distribution of the energy barriers. 10,11 In our case, the formation of stable aggregates is considered to have different activation energies depending on the shape, size, and packing geometry of aggregates. The stretched exponential behaviors have been observed for the samples of different concentrations under various magnetic fields. When we study the transmission change of the fluid as like in Refs 2 and 12, the single or double exponential decay is observed, which suggests that the particle movement in the fluid is affected by diffusion but the structural relaxations of aggregates cause the stretched exponential dynamics. 13

Magnetic properties of magnetite ferrofluid are studied by measuring magnetic weights under different magnetic fields with a conventional electronic balance. Magnetite nanoparticles of 11 nm diameter are synthesized to make the ferrofluid. Magnetization calculated from the magnetic weight reveals the hysteresis and deviates from the Langevin function at high magnetic fields. Magnetic weight shifts instantaneously with magnetic field change by Neel and Brown mechanism. When high magnetic field is applied to the sample, slower change of magnetic weight is accompanied with the instantaneous shift via agglomeration of nanoparticles. The slow change of the magnetic weight shows the stretched exponential kinetics. The temporal change of the magnetic weight and the magnetization of the ferrofluid at high magnetic fields suggest that the superparamagnetic sample turns into superspin glass by strong magnetic interparticle interactions.

Magnetic nanoparticles capped with proper surfactants are dispersed well in the aqueous solution. Solution of magnetic nanoparticles called ferrofluid shows the well-known interesting morphological behaviors under magnetic field. 1Magnetic nanoparticles of relatively low concentration agglomerate by magnetic field.2,3 Agglomeration of magnetite nanoparticles under low magnetic field (<100 T/m) is used in the protein purification or the separation of magnetic impurities for water purification. 3,4 Magnetic weight of magnetic nanoparticle, which appears by magnetization of nanoparticle, can be measured with a conventional electronic balance. 5,6 When the magnetic field from a small disc magnet is applied to the superparamagnetic magnetite solution, the magnetic weight of the sample jumps instantaneously (<<1 s) by the Neel and Brown mechanism. Thereafter the magnetic weight increases slowly as the nanoparticles agglomerate at the bottom of the sample holder where the magnetic field gradient is the greatest.Figure 1(a) shows the slow growth of magnetic weight of magnetite solution with a fitting curve. The 1 mL aqueous solution contains 0.7 wt % magnetite nanoparticles. The fitting curve of Figure 1(a) is a stretched exponential as followingwhere the exponent β is 0 < β < 1 and τ is the relaxation time. The initial magnetic weight, M(0), is 8.008 g, which includes the weight of the cuvette and the sample before applying magnetic field, 7.363 g. Increase of the magnetic weight by the Neel and Brown mechanism is 0.645 g. The equation of magnetic force, F = mg = VMrB suggests that the magnetic weight of the sample by the magnetic field gradient at the bottom of the cuvette, 31 T/m is 2.23 g. The saturation magnetization of magnetite (M) is known 4 as 4.7 × 10 5 A/m and the volume of the sample (V) is estimated as 1.5 × 10 −9 m 3 from the concentration of solution and the density of magnetite. If the magnetite sample is thoroughly magnetized, the magnetic weight would be 9.593 g, which is much greater than the final magnetic weight of the fitting curve M(∞), 8.257 g. There is a large difference between the structures of the agglomerate and the crystalline magnetite. The agglomerate can be seen as a polycrystal with very fine, loosely combined domains. Therefore, the magnetic weight of the agglomerate in solution cannot be close to that of solid magnetite. In addition to the structural differences of the agglomerate and the crystalline solid, the error in the estimation of the sample concentration and the magnetic field gradient as well as the inaccuracy of the fitting parameters is attributed to the discrepancy of the final magnetic weights.The stretched exponential growth of the magnetic weight indicates that the energy barrier for agglomeration is not a single value but has some distribution. 7,8 There is a relation of the inverse Laplace transform between the distribution function of the energy barrier and the temporal response function of the dynamics. The distribution function with an asymmetric bell shape can be d...

Steady-state and time-resolved fluorescence anisotropy for nile red and oxazine 725 in the isotropic phase of the liquid crystal p-methoxybenzylidene-n-butylaniline (MBBA) are measured in the temperature range 50−80 °C. The fluorescence characteristics of the dye molecules in the isotropic MBBA agree well with the Perrin equation over the investigated temperature range. The decay of the fluorescence anisotropy of the dye molecules in the isotropic liquid crystal follows the simple hydrodynamic (Debye−Stokes−Einstein) model rather than the Landau−de Gennes model for the orientational dynamics of neat isotropic liquid crystals. These results imply that the microscopic anisotropy, which is important in the dynamics of neat isotropic liquid crystals, has no effects on the reorientation dynamics of the dopant dye molecules. Attractive interactions among MBBA molecules are so strong that the dopant molecules are not embedded in the pseudonematic domains of the isotropic phase. The dielectric friction affects the reorientation dynamics to give small deviations from the simple hydrodynamic model.

Agglomeration of magnetite nanoparticles in the aqueous solution is studied at the low magnetic field gradients of 23-34 T/m by monitoring the temporal change of magnetic weight. A conventional electronic balance is used to measure the magnetic weight that is the magnetic force on the magnetic sample by the magnetic field gradient. The magnetic weight grows slowly following the stretched exponential after the instantaneous jump by the Neel and Brown relaxation. Magnetization of the magnetite nanoparticles is estimated from the magnetic weight and compared with the Langevin function. The magnetization is close to the saturation in the studied magnetic field range and the saturation magnetization of the agglomerate of nanoparticles is about 60% of that of the bulk magnetite. Kinetic parameters of the stretched exponential show little the magnetic field dependence in the investigated range. Complex energy landscape is involved in the agglomeration as the stretched exponential dynamics indicates. The half-life of the response function for the magnetic weight change suggests that the pathways of low energy barriers are activated by magnetic field at the early stage of agglomeration.