Thermal motion of magnetic iron nanoparticles in a frozen solvent

The thermal rotation of iron nanoparticles dispersed in cyclohexane was studied by measuring the dynamic magnetic susceptibility above and below the freezing point of the solvent. Above the freezing point, the orientation of the magnetic dipoles changes mainly by reorientation of the entire particle. Below the freezing point, complete arrest of particle motion was expected, such that the magnetic dipoles would only be able to reorient themselves inside the nanoparticles (Neel relaxation). However, we find that thermal motion continues well below the temperature at which the bulk of the solvent is frozen. We ascribe this to local lowering of the freezing point, due to the presence of polymers in the close vicinity of the colloids. Furthermore, because strong dipole-dipole interactions result in the formation of dipolar chains, we have systematically studied the effect of particle size on dynamics in a frozen solvent. For the larger particles, our data indicate that local wiggling of the individual particles in a chain may become the dominating mode of thermal motion.     

Paramagnetic particles and mixing in micro-scale flows

Mixing in microscale flows with rotating chains of paramagnetic particles can be enhanced by adjusting the ratio of viscous to magnetic forces so that chains dynamically break and reform. Lattice Boltzmann (LB) simulations were used to calculate the interaction between the fluid and suspended paramagnetic particles under the influence of a rotating magnetic field. Fluid velocities obtained from the LB simulations are used to solve the advection diffusion equation for massless tracer particles. At relatively high Mason numbers, small chains result in low edge velocities, and hence mixing is slower than at other Mason numbers. At low Mason numbers, long, stable chains form and produce little mixing toward the center of the chains. A peak in mixing rate is observed when chains break and reform. The uniformity of mixing is greater at higher Mason numbers because more small chains result in a larger number of small mixing areas.     

An ultrashort mixing length micromixer: the shear superposition micromixer

We report for the first time a laminar high-performance continuous micromixing process of two fluids over a length of 200 microns in under 10 milliseconds achieved by an optimization of the control parameters amplitude and frequency in the mixing device denoted as 'Shear Superposition Micromixer'. We improve mixing time by approximately 5 orders of magnitude over diffusion-limited mixing. The data indicate that rapid mixing is a result of the combined action of Taylor-Aris dispersion in the main and secondary microchannels and unsteady vortex motion that occurs at finite Reynolds number, which occurs above a threshold amplitude and frequency. The mixing performance is quantified using micron-resolution particle image velocimetry (micro-PIV) and computational fluid dynamics (CFD) simulations.     

Interpretation of conservative forces from Stokesian dynamic simulations of interfacial and confined colloids

This paper presents Stokesian dynamics simulations of experiments involving one or two charged colloids near either a single charged wall or confined between parallel charged walls. Equilibrium particle-particle and particle-wall interactions are interpreted from dynamic particle trajectories in simulations involving (1) a single particle levitated above a wall, (2) two particles below a wall, and (3) two particles confined between two parallel walls. By specifying only repulsive electrostatic Derjaguin-Landau-Verwey-Overbeek (DLVO) potentials and including multibody hydrodynamics, we successfully recover expected potentials in some cases, while anomalous attraction is observed in other cases. Attraction inferred in the latter simulations displays quantitative agreement with literature measurements when particle dynamics are interpreted using reported analyses. Because anomalous attraction is reproduced in simulations using only electrostatic repulsive DLVO potentials, our results reveal the one-dimensional analyses to be invalid for configurations that are inherently multidimensional via multibody hydrodynamics. Parameters related to experimental sampling of particle dynamics are also found to be critical for obtaining accurate potentials. We explain the anomalous attraction in each experiment using effective potentials, which can be employed in an a priori fashion to assist the confident design of future experiments involving interfacial and confined colloids. Ultimately, our findings reveal the importance of dimensionality and multibody hydrodynamics for understanding nonequilibrium dynamics of colloids near surfaces.     

UV induced local heating effects in TiO2 nanocrystals

When isolated TiO(2) nanocrystals are subjected to UV light at 77 K and pressures below 10(-6) mbar, trapping of photogenerated hole centers occurs on the surface of the nanocrystals and can be tracked by time-resolved electron paramagnetic resonance spectroscopy. Irrespective of the selected UV irradiance used, the maximum concentration of trapped charges was found to be constant for a given number of nanocrystals ( approximately 10(15)) and corresponds to one electron-hole pair per particle. On a time scale of seconds to minutes the dynamics for the trapping process depend on the number of photons with supra band gap energy. A local temperature rise of the TiO(2) nanocrystals was observed for irradiances above 1.55 mW cm(-2) (10(15) photons cm(-2) s(-1)). This is attributed to enhanced nonradiative recombination of photogenerated charge carriers via heat production and points to a substantial contribution of thermal chemistry in photocatalytic reaction cycles.     

Core-shell emulsion copolymerization of styrene and acrylonitrile on polystyrene seed particles

Seeded emulsion copolymerization of an azeotropic composition of styrene (St) and an acrylinitrile (AN) comonomer mixture in polystyrene (PS) seed at different polymerization temperature of 55–75°C were investigated. The kinetic data showed a transition temperature at 65°C, above which the activation energy of polymerization is low, 6.1 Kcal/mol, compared with 9.8 Kcal/mol below it. The particle-size results and thin layer chromatographic (TLC) data showed two types of particle of different composition and morphology in the final latex system: a smaller size of (St–AN) copolymer and a larger size of core-PS and (St–AN) copolymer shell, with a zone of PS grafted (St–AN) copolymer in between. Various polymerization parameters, that is emulsifier concentration, type of seed particle and its size, and monomer/polymer ratio, were studied and their effects on particle size and particle morphology were examined. The percent of grafted core-PS was 10% below a polymerization temperature of 65°C and 40% above that temperature. By adjusting the size and number of the seed particles, monomer-polymer ratio, and emulsifier concentration conditions were established in which a final copolymer latex with “perfect” core-shell morphology was achieved.     

Polymer dynamics in responsive microgels: influence of cononsolvency and microgel architecture

The dynamics of polymers on the nm and ns scales inside responsive microgels was probed by means of Neutron Spin Echo (NSE) experiments. Four different microgels were studied: poly(N-isopropylacrylamide) (PNIPAM) and poly(N,N-diethylacrylamide) (PDEAAM) microgels, a P(NIPAM-co-DEAAM) copolymer microgel and a core-shell microgel with a PDEAAM core and a PNIPAM shell. These four different microgel systems were investigated in a D(2)O/CD(3)OD solvent mixture with a molar CD(3)OD fraction of x(MeOD) = 0.2 at 10 °C. The PNIPAM and the P(NIPAM-co-DEAAM) microgels are in the collapsed state under these conditions. They behave as solid diffusing objects with only very small additional contributions from internal motions. The PDEAAM particle is swollen under these conditions and mainly Zimm segmental dynamics can be detected in the intermediate scattering function at high momentum transfer. A cross-over to a collective diffusive motion is found for smaller q-values. The shell of the PDEAAM-core-PNIPAM-shell particle is collapsed, which leads to a static contribution to S(q,t); the core, however, is swollen and Zimm segmental dynamics are observed. However, the contributions of the Zimm segmental dynamics to the scattering function are smaller as compared to the pure PDEAAM particle. Interestingly the values of the apparent solvent viscosities inside the microgels as obtained from the NSE experiments are higher than for the bulk solvent. In addition different values were obtained for the PDEAAM microgel, and the PDEAAM-core of the PDEAAM-core-PNIPAM-shell particle, respectively. We attribute the strongly increased viscosity in the PDEAAM particle to enhanced inhomogeneities, which are induced by the swelling of the particle. The different viscosity inside the PDEAAM-core of the PDEAAM-core-PNIPAM-shell microgel could be due to a confinement effect: the collapsed PNIPAM-shell restricts the swelling of the PDEAAM-core and may modify the hydrodynamic interactions in this restricted environment inside the microgel.     

Single molecule probing of dynamics in supercooled polymers

Fluorescence experiments with single BODIPY molecules embedded in a poly(methyl acrylate) matrix have been performed at various temperatures in the supercooled regime. By using pulsed excitation, fluorescence lifetime and linear dichroism time trajectories were accessible at the same time. Both observables have been analyzed without data binning. While the linear dichroism solely reflects single particle dynamics, the fluorescence lifetime observable depends on the molecular environment, so that the dynamics from the polymer host surrounding a chromophore contributes to this quantity. We observe that the lifetime correlation decays slightly faster than polarization correlation, indicating the occurrence of large angular reorientations. Additionally, dichroism time trajectories have been adducted to reveal directly the geometry of rotational dynamics. We identify small but also significantly larger rotational jumps being responsible for the overall molecular reorientation.     

Transient behaviour of magnetic micro-bead chains rotating in a fluid by external fields

Magnetic micro-beads can facilitate many functions in lab-on-a-chip systems, such as bio-chemical labeling, selective transport, magnetic sensing and mixing. In order to investigate potential applications of magnetic micro-beads for mixing in micro fluidic systems, we developed a pin-jointed mechanism model that allows analysing the behaviour of rotating superparamagnetic bead chains. Our numerical model revealed the response of the chains on a rotating magnetic field over time. We could demonstrate that the governing parameters are the Mason number and number of beads in the chain. The results are in agreement with the simplified analytical model, assuming a straight chain, but also allow prediction of the transient chain shape. The modelled chains develop an anti-symmetric S-shape that is stable, if the Mason number for a given chain length does not surpass a critical value. Above that value, rupture occurs in the vicinity of the chain centre. However, variations in bead susceptibility can shift the location of rupture. Moreover, we performed experiments with superparamagnetic micro-beads in a small fluid volume exposed to a uniform rotating magnetic field. Our simulation could successfully predict the observed transient chain form and the time for chain rupture. The developed model can be used to design optimised bead based mixers in micro fluidic systems.     

Reorganization of small Co particles on Al2O3 surfaces monitored by ferromagnetic resonance

Changes of the magnetic properties of ferromagnetic Co particles deposited on the radical31 x radical31R +/- 9 degrees reconstructed alpha-Al2O3(0001) as well as on a thin alumina film grown on a NiAl(110) substrate were investigated as a function of thermal annealing. On the thin film changes of the magnetic response were found above 500 K which correlates with changes in the particle size distribution. Annealing to 870 K leads to a permeation of the metal though the oxide film which causes significant changes in the ferromagnetic resonance response. On the alpha-Al2O3 single crystal sintering of particles requires temperatures above 600 K being about 100 K higher as compared to the thin alumina film. For large clusters intraparticle redistribution takes place already below 600 K a phenomenon not observed for the small clusters. In addition, a significant dependence of the measured g values from the substrate as well as the thermal treatment is found which can be understood in terms of the structural properties of the systems.     

Complex ion effects on polypeptide conformational stability: chloride and sulfate salts of guanidinium and tetrapropylammonium

The effects of chloride and sulfate salts of tetrapropylammonium (TPA(+)) and guanidinium (Gdm(+)) on the conformational stabilities of tryptophan zipper (trpzip) and α-helical (alahel) peptides were measured by circular dichroism spectroscopy. Like Gdm(+), TPA(+) interacts with the planar tryptophan indole group, perturbing the conformational stability of trpzip peptides. TPA(+) effects are largely unaffected by sulfate, indicating an absence of the heteroion pairing that is observed in concentrated Gdm(2)SO(4) solutions. TPA(+) stabilizes helical conformations in alahel peptides, indicating exclusion from the peptide bond. The observations are broadly consistent with predictions of molecular dynamics simulations [Mason, P. E.; et al. J. Phys. Chem. B2009, 113, 3227-3234], indicating that the effects of complex ions on proteins are increasingly predictable in terms of ion hydration, complementary interactions with specific protein groups, and ion-pairing contributions.     

Electrochemical quartz crystal microbalance studies of the rectified quantized charging of gold nanoparticle multilayers

Electrochemical quartz crystal microbalance (EQCM) was employed to investigate the dynamics of rectified quantized charging of gold nanoparticle multilayers by in situ monitoring of the interfacial mass changes in aqueous solutions with varied electrolytes. EQCM measurements showed that interfacial mass changes only occurred at potentials more positive than the potential of zero charge (PZC), where nanoparticle quantized charging was well-defined, whereas in the negative potential regime where only featureless voltammetric responses were observed, the QCM frequency remained virtually invariant. This was ascribed to the fact that nanoparticle quantized charging was induced by the formation of ion-pairs between hydrophobic electrolyte anions (PF6-, ClO4-, BF4-, and NO3-) and positively charged gold nanoparticles. Based on the total frequency changes and the number of electrolyte anions adsorbed onto the particle layers, the number of water molecules that were involved in the ion-pairing processes was then quantitatively estimated at varied particle charge states, which was found to increase with increasing hydrophobicity of the anions. Additionally, the electron-transfer dynamics of the gold particle multilayers were also evaluated by electrochemical impedance measurements. It was found that the particle electron-transfer rate was about an order of magnitude slower than that of the ion diffusion and binding.     

Circular dichroism effect for linear molecules induced by a resonant circularly polarized pumping optical field

In this paper we propose and discuss the laser-induced circular dichroism (LICD) effect, which is expected to occur in linear molecules pumped by a strong circular resonant light beam. The effect is to be detected via the absorption of a weak circularly polarized probe beam on another transition. Analogous to the external magnetic field in magnetic circular dichroism the resonant circular polarized pumping optical field can induce the nonzero antisymmetric rotational polarizabilities of a linear molecule, and cause the LICD effect. LICD contains three distinct contributions from M-dependent splittings of the sublevels mid R:JM due to the ac Stark effect, from the differences of Boltzmann statistical distributions among the ground state sublevels mid R:JM due to the ac Stark splittings, and from the changes of occupation probability in rotational sublevels mid R:JM due to the pumping effect. The fundamental formulas for the above three terms of LICD have been deduced by the density matrix method. As an example, the LICD for CO molecules have been calculated. The results indicate that in comparison with the rotationally resolved magnetic vibrational circular dichroism experiment, LICD may be measurable and form a basis of a different kind of CD spectroscopy.     

The electron–nucleus coupling: A breakthrough in the investigation of paramagnetic metalloproteins

Water proton relaxation measurements as a function of magnetic field on solutions containing paramagnetic metalloproteins contain information on (i) number and distance of metal-coordinated water molecules, (ii) dynamics of water exchange, (iii) electron relaxation rates, and (iv) dynamics of the metal coordination sphere influencing electron relaxation. The use of appropriate theoretical tools permits us in many cases to learn about some or all of the above properties.     

Strong magnetic field effects on solid-liquid and particle-particle interactions during the processing of a conducting liquid containing non-conducting particles

The behavior of micrometer-sized weak magnetic insulating particles migrating in a conductive liquid metal is of broad interest during strong magnetic field processing of materials. In the present paper, we develop a numerical method to investigate the solid-liquid and particle-particle interactions by using a computational fluid dynamics (CFDs) modeling. By applying a strong magnetic field, for example, 10 Tesla, the drag forces of a single spherical particle can be increased up to around 15% at a creeping flow limit. However, magnetic field effects are reduced when the Reynolds number becomes higher. For two identical particles migrating along their centerline in a conductive liquid, both the drag forces and the magnetic interaction will be influenced. Factors such as interparticle distance, Reynolds number and magnetic flux density are investigated. Shielding effects are found from the leading particle, which will subsequently induce a hydrodynamic interaction between two particles. Strong magnetic fields however do not appear to have a significant influence on the shielding effects. In addition, the magnetic interaction forces of magnetic dipole-dipole interaction and induced magneto-hydrodynamic interaction are considered. It can be found that the induced magneto-hydrodynamic interaction force highly depends on the flow field and magnetic flux density. Therefore, the interaction between insulating particles can be controlled by applying a strong magnetic field and modifying the flow field. The present research provides a better understanding of the magnetic field induced interaction during liquid metal processing, and a method of non-metallic particles manipulation for metal/ceramic based materials preparation may be proposed.     

Sub-nm resolution depth profiling of the chemical state and magnetic structure of thin films by a depth-resolved X-ray absorption spectroscopy technique

Development and recent progress of a depth-resolved X-ray absorption spectroscopy (XAS) technique are presented, together with future prospects. The technique has been developed by controlling the probing depth of the electron-yield XAS data, which depends on the electron emission angle. This novel technique enables us to achieve depth profiling of the magnetic structure of thin films with a sub-nm depth resolution by using X-ray magnetic circular dichroism (XMCD) in X-ray absorption, which provides quantitative information on the element-specific spin and orbital magnetic moments. The chemical state and electronic structure at the surface and interface are also investigated by depth-resolved XAS analysis. As for future prospects, a three-dimensional micro XAS technique is being developed by combining an X-ray microbeam with depth-resolved XAS. Moreover, it is expected to manipulate magnetic anisotropy by using element-specific and depth-resolved magnetic anisotropy energies obtained from the depth-resolved XMCD to design thin films and multilayers with proper elements and proper thicknesses. The observation of the spin dynamics at the interface will be also possible in future by adopting the pump-probe method.     

Temperature-dependent relaxation of excitons in tubular molecular aggregates: Fluorescence decay and stokes shift

We report temperature-dependent steady-state and time-resolved fluorescence studies to probe the exciton dynamics in double-wall tubular J-aggregates formed by self-assembly of the dye 3,3'-bis(3-sulfopropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimidacarbocyanine. We focus on the lowest energy fluorescence band, originating from the inner cylindrical wall. At low temperatures, the experiments reveal a nonexponential decay of the fluorescence, with a typical time scale that depends on the emission wavelength. At these temperatures we also find a dynamic Stokes shift of the fluorescence spectrum and its nonmonotonic dependence on temperature under steady-state conditions. All these data indicate that below about 20 K the excitons in the lowest fluorescence band do not reach thermal equilibrium before emission occurs, while above about 60 K thermalization on this time scale is complete. By comparing the two lowest fluorescence bands, we also find indications for fast energy transfer from the outer to the inner wall. We show that the Frenkel exciton model with diagonal disorder, which previously has been proposed to explain the absorption and linear dichroism spectra of these aggregates, yields a quantitative explanation to the observed dynamics. To this end, we extend the model to account for weak phonon-induced scattering of the localized exciton states; the spectral dynamics are then described by solving a Pauli master equation for the exciton populations.     

Dipolophoresis of Janus nanoparticles in a microchannel

A nonlinear dipolophoretic analysis is applied to analytically explain the counterintuitive experimental results of Gangwal et al. [16, 17] that an uncharged micro/nanosize dielectric Janus particle is attracted to the wall of a microchannel when exposed to an AC‐uniform electric field in the direction parallel to the no‐slip boundaries. We employ the so‐called “weak” field assumption and consider a metallodielectric Janus colloid comprising two semispheres of distinct dielectric properties subject to an oscillating‐uniform electric field with moderate frequency (below the Maxwell–Wagner limit). The Debye scale (ratio of electric double layer thickness to particle size) is considered unrestricted. Under the low Reynolds number hypothesis, Faxén's theorem and the Green's function (Stokeslet) method of singularities, including appropriate images with respect to the no‐slip boundary, are applied under the remote‐field approximation to determine the dynamics and trajectory of a small colloid moving near a wall. When assuming maximum dielectric contrasts between hemispheres and relatively low Debye scale (compared to particle radius), a rather simple relation for the equilibrium position of the colloid (i.e. tilt angle and distance from the wall) is obtained and found to be in qualitative good agreement with the experimental observations of Gangwal et al. [16, 17] and the predictions of Kilic and Bazant [12].     

Influence of local chain dynamics on diffusion of gases in polymers as determined by pulsed field gradient NMR

Diffusion of gases in polymers below the glass transition temperature, T_g, is strongly modulated by local chain dynamics. For this reason, an analysis of pulsed field gradient (PFG) nuclear magnetic resonance (NMR) diffusion measurements considering the viscoelastic behavior of polymers is proposed. Carbon‐13 PFG NMR measurements of [¹³C]O₂ diffusion in polymer films at 298 K are performed. Data obtained in polymers with T_g above (polycarbonate) and below (polyethylene) the temperature set for diffusion measurements are analyzed with a stretched exponential. The results show that the distribution of diffusion coefficients in amorphous phases below T_g is wider than that above it. Moreover, from a PFG NMR perspective, full randomization of the dynamic processes in polymers below T_g requires long diffusion times, which suggests fluctuations of local chain density on a macroscopic scale may occur. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 231–235, 2010