Theory for hierarchical assembly with dielectrophoresis and the role of particle anisotropy

Nonuniform electric fields cause polarizable particles to move through an effect known as dielectrophoresis (DEP). Additionally, the particles themselves create nonuniform fields due to their induced dipoles. When the nonuniform field of one particle causes another to move, it represents a path to hierarchical assembly termed mutual DEP (mDEP). Anisotropic particles potentially provide further opportunities for assembly through intense and intricate local field profiles. Here, we construct a theoretical framework for describing anisotropic particles as templates for assembly through mDEP by considering the motion of small nanoparticles near larger anisotropic nanoparticles. Using finite element analysis, we study eight particle shapes and compute their field enhancement and polarizability in an orientation-specific manner. Strikingly, we find a more than tenfold enhancement in the field near certain particle shapes, potentially promoting mDEP. To more directly relate the field intensity to the anticipated assembly outcome, we compute the volume experiencing each field enhancement versus particle shape and orientation. Finally, we provide a framework for predicting how mixtures of two distinct particle species will begin to assemble in a manner that allows for the identification of conditions that kinetically bias assembly toward specific hierarchical outcomes.     

Microrheology of chemically crosslinked polymer gels by diffusing-wave spectroscopy

The Brownian motion of probe particles in aqueous solutions of poly(vinyl alcohol)(PVA) and in chemically crosslinked PVA gels has been studied by diffusing-wave spectroscopy (DWS). At long time scales the measurements allow us to determine the effect of the crosslinking ratio on the macroscopic viscosity of sols and the shear modulus of gels. The local shear modulus of gels as obtained from the characteristic length of the Brownian cage was found to agree with that measured by classical rheometry and dynamic light scattering (DLS). These microrheological techniques were applied to two polymer gel systems. Substrate induced gradient structure of hydrogels was studied from a microrheological point of view using DLS. It is clearly seen that hydrophobic substrate induces weakly crosslinked network formation at the interface region up to a few millimeters as expected from other experimental facts. Magnetic particle motion in gels under external magnetic field was investigated by DWS. The translational motion of the magnetic particles in gels due to the alternating magnetic force can be detected and found to be superimposed on the relaxation due to the thermal motion.     

Controlled particle placement through convective and capillary assembly

A wide variety of methods are now available for the synthesis of colloidal particle having controlled shapes, structures, and dimensions. One of the main challenges in the development of devices that utilize micro- and nanoparticles is still particle placement and integration on surfaces. Required are engineering approaches to control the assembly of these building blocks at accurate positions and at high yield. Here, we investigate two complementary methods to create particle assemblies ranging from full layers to sparse arrays of single particles starting from colloidal suspensions of gold and polystyrene particles. Convective assembly was performed on hydrophilic substrates to create crystalline mono- or multilayers using the convective flow of nanoparticles induced by the evaporation of solvent at the three-phase contact line of a solution. On hydrophobic surfaces, capillary assembly was investigated to create sparse arrays and complex three-dimensional structures using capillary forces to trap and organize particles in the recessed regions of a template. In both methods, the hydrodynamic drag exerted on the particle in the suspension plays a key role in the assembly process. We demonstrate for the first time that the velocity and direction of particles in the suspension can be controlled to perform assembly or disassembly of particles. This is achieved by setting the temperature of the colloidal suspension above or below the dew point. The influence of other parameters, such as substrate velocity, wetting properties, and pattern geometry, is also investigated. For the particular case of capillary assembly, we propose a mechanism that takes into account the relative influences of these parameters on the motion of particles and that describes the influence of temperature on the assembly efficiency.     

Infrared studies of small azobenzene peptides: unexpectedly slow reactions on the time range of minutes

Infrared absorption experiments on light-triggered azobenzene peptides have been performed below and above the freezing point of the solvent dimethyl sulfoxide (DMSO). Even 20 K below the freezing point, illumination of the azobenzene chromophore resulted in IR absorption changes indicative of light-induced structural rearrangements of the peptide. In addition, new conformational states could be found at low temperature, which involve the formation of additional hydrogen bonds. In one sample the new low-temperature state survived melting and reheating and disappeared at 298 K only on the time scale of 10 min. The observations indicate that at low temperatures the peptide, together with traces of water present in the sample, forms a shell in the vicinity of the chromophore that facilitates internal motion even in the rigid cage of frozen DMSO.     

Microscale fish bowls: a new class of latex particles with hollow interiors and engineered porous structures in their surfaces

Microscale fish bowls, hollow particles with engineered holes in their surfaces, were prepared using two different methods. In the first method, commercial latex beads suspended in water were swollen with a good solvent of the polymer, followed by freezing with liquid nitrogen and evaporation of the solvent below 0 degrees C. While one big hole was generated when the amount of solvent used for the swelling was relatively low, small holes could be produced in the outer surface of each bowl by increasing the degree of swelling. The porosity and pore structure show a similar dependence on the degree of swelling for both amorphous and semicrystalline polymers even though they are supposed to exhibit different phase behaviors during the freezing and solvent evaporation processes. In the second method, a polymer emulsion in water was prepared and then frozen with liquid nitrogen, followed by solvent evaporation below 0 degrees C. The porosity and pore structure could be controlled by adjusting the concentration of the polymer solution used to prepare the emulsion. As for encapsulation, the bowl-shaped particles could be transformed back into solid beads via thermal annealing at a temperature near the glass transition temperature of the polymer or by adding a good solvent of the polymer to the colloidal suspension. In a proof-of-concept experiment, microscale fish bowls were fabricated from poly(caprolactone), quickly loaded with a fluorescent dye, and sealed through thermal annealing. The encapsulated dye could then be slowly released in a phosphate buffered saline, suggesting their potential use as a new class of microscale capsules for drug delivery.     

Synthesis of indium nanoparticles: digestive ripening under mild conditions

Here we report the synthesis of monodispersed indium nanoparticles by evaporation/condensation of indium shot using the solvated metal atom dispersion (SMAD) technique, followed by digestive ripening in low boiling point (BP 38 °C) methylene chloride and in a high boiling point (BP 110 °C) toluene solvent. The as-prepared SMAD indium nanoparticles are polydispersed with particle size ranging from 25 to 50 nm, but upon digestive ripening (heating of colloidal material at the boiling point of solvent in presence of excess surface active ligands) in methylene chloride, a remarkable reduction of particle size was achieved. In higher boiling solvent (toluene), where the indium nanoparticles at reflux temperature are probably melted, it does not allow the best result, and less monodispersity is achieved. We employed different surface active ligands (amine, phosphine, and mixed ligands) to passivate these indium nanoparticles. The temporal evolution of the surface plasmon of indium nanoparticles was monitored by in situ UV-vis spectroscopy, and particles were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The merits of this synthesis procedure are the use of bulk indium as starting material, tuning the particle size in low boiling point solvent, particle size adjustment with the choice of ligand, and a possible scale up.     

Computer simulation to investigate magnetic and wave-absorbing characteristics of iron nanoparticles

Computer simulations were carried out to investigate the magnetic and wave-absorbing characteristics of iron nanoparticles. The magnetic properties were found to depend on the shape, aggregation, and array of the nanoparticles. Nanoparticle systems were simulated using a molecular dynamics (MD) method, and the resulting configurations were used to compute the magnetic properties of the systems. In this work, microdot magnetic dipoles were assumed to localize on the iron atoms of nanoparticles. The dynamics of these magnetic dipoles under an external magnetic field were simulated by solving the Landau-Lifshitz-Gilbert equation numerically. The energy loss of a system under an external magnetic field was computed from the hysteresis curve and was correlated with the wave-absorbing characteristic of the system. The findings suggested that the disk-shaped iron particles had a greater hysteresis loss energy than the ball-shaped iron particles. It was also found that the aggregation of nanoparticles apparently reduced the wave-absorbing characteristic of the system. All the outcomes were in good agreement with the experimental measurements.     

Crystallization, melting, and structure of water nanoparticles at atmospherically relevant temperatures

Water nanoparticles play an important role in atmospheric processes, yet their equilibrium and nonequilibrium liquid-ice phase transitions and the structures they form on freezing are not yet fully elucidated. Here we use molecular dynamics simulations with the mW water model to investigate the nonequilibrium freezing and equilibrium melting of water nanoparticles with radii R between 1 and 4.7 nm and the structure of the ice formed by crystallization at temperatures between 150 and 200 K. The ice crystallized in the particles is a hybrid form of ice I with stacked layers of the cubic and hexagonal ice polymorphs in a ratio approximately 2:1. The ratio of cubic ice to hexagonal ice is insensitive to the radius of the water particle and is comparable to that found in simulations of bulk water around the same temperature. Heating frozen particles that contain multiple crystallites leads to Ostwald ripening and annealing of the ice structures, accompanied by an increase in the amount of ice at the expense of the liquid water, before the particles finally melt from the hybrid ice I to liquid, without a transition to hexagonal ice. The melting temperatures T(m) of the nanoparticles are not affected by the ratio of cubic to hexagonal layers in the crystal. T(m) of the ice particles decreases from 255 to 170 K with the particle size and is well described by the Gibbs-Thomson equation, T(m)(R) = T(m)(bulk) - K(GT)/(R - d), with constant K(GT) = 82 ± 5 K·nm and a premelted liquid of width d = 0.26 ± 0.05 nm, about one monolayer. The freezing temperatures also decrease with the particles' radii. These results are important for understanding the composition, freezing, and melting properties of ice and liquid water particles under atmospheric conditions.     

A simple solvothermal synthesis of MFe2O4 (M=Mn, Co and Ni) nanoparticles

Nanoparticles of MFe₂O₄ (M=Mn, Co and Ni), with diameters ranging from 5 to 10 nm, have been obtained through a solvothermal method. In this synthesis, an alcohol (benzyl alcohol or hexanol) is used as both a solvent and a ligand; it is not necessary, therefore, to add a surfactant, simplifying the preparation of the dispersed particles. We have studied the influence of the synthetic conditions (temperature, time of synthesis and nature of solvent) on the quality of the obtained ferrites and on their particle size. In this last aspect, we have to highlight that the solvent plays an important role on the particle size, obtaining the smallest diameters when hexanol was used as a solvent. In addition, the magnetic properties of the obtained compounds have been studied at room temperature (RT). These compounds show a superparamagnetic behaviour, as was expected for single domain nanoparticles, and good magnetization values. The maxima magnetization values of the MFe₂O₄ samples are quite high for such small nanoparticles; this is closely related to the high crystallinity of the particles obtained by the solvothermal method.     

Structural properties of charge-stabilized ferrofluids under a magnetic field: a Brownian dynamics study

We present Brownian dynamics simulations of real charge-stabilized ferrofluids, which are stable colloidal dispersions of magnetic nanoparticles, with and without the presence of an external magnetic field. The colloidal suspensions are treated as collections of monodisperse spherical particles, bearing point dipoles at their centers and undergoing translational and rotational Brownian motions. The overall repulsive isotropic interactions between particles, governed by electrostatic repulsions, are taken into account by a one-component effective pair interaction potential. The potential parameters are fitted in order that computed structure factors are close to the experimental ones. Two samples of ferrofluid differing by the particle diameter and consequently by the intensity of the magnetic interaction are considered here. The magnetization and birefringence curves are computed: a deviation from the ideal Langevin behaviors is observed if the dipolar moment of particles is sufficiently large. Structure factors are also computed from simulations with and without an applied magnetic field H: the microstructure of the repulsive ferrofluid becomes anisotropic under H. Even our simple modeling of the suspension allows us to account for the main experimental features: an increase of the peak intensity is observed in the direction perpendicular to the field whereas the peak intensity decreases in the direction parallel to the field.     

Molecular dynamics simulation of optically trapped colloidal particles at an oil-water interface

Using molecular dynamics simulations, we calculate the net force on a colloidal particle trapped by an optical tweezer and confined within a particle monolayer which is in motion relative to the trapped particle. The calculations are compared with recent experimental data on polystyrene particles located at an oil-water interface. Good agreement between theory and experiment is obtained over the investigated range of lattice constants for an interaction mechanism between the polystyrene particles which is dominated by an effective dipole-dipole potential. The assumed interaction mechanism is consistent with the formation of surface charge dipoles at the particle-oil interface due to the dissociaton of the hydrophilic sulfate headgroups at the surface of the polystyrene particles. A possible physical mechanism for the formation of the surface charge dipoles, involving a diffuse cloud of fully hydrated counterions, is described, and the fraction of surface groups contributing to the formation of surface charge dipoles is estimated to be of the order of 10(-1) for the present system.     

Preparation of composite particles with magnetic silica core and fluorescent polymer shell

Dual functions of magnetic and fluorescent properties were created in composite particles that incorporated magnetite (Fe₃O₄) nanoparticles in particle cores of silica and fluorescent pyrene in particle shells of polystyrene. The Fe₃O₄ nanoparticles were prepared with a conventional homogeneous precipitation method and surface modified with a coupling agent of carboxyethylsilanetriol. The silica particles incorporating Fe₃O₄ nanoparticles were synthesized with a modified Stöber method in which the Fe₃O₄ nanoparticles were added to a system of tetraethylorthosilicate (TEOS)/ammonia/water/ethanol. Then, the magnetite/silica composite particles were coated with the pyrene/polystyrene shell in a soap-free emulsion polymerization, which was conducted in the presence of pyrene in a mixed solvent of water/ethanol. The composite particles prepared in the mixed solvent had both magnetic and fluorescent properties. The fluorescent spectrum of the particles with Fe₃O₄ was very similar to that without Fe₃O₄, indicating that the magnetic component within the core particles scarcely interfered with the fluorescent emission from the polymer shell.     

Effects of ionic strength on the size and compactness of chitosan nanoparticles

In this work, chitosan nanoparticles were prepared by ionotropic gelation of chitosan with tripolyphosphate (TPP). The effects of the ionic strength of the solvent employed in the particle preparation on the average size and compactness of the particles were investigated. In addition, the effects of the chitosan concentration and the crosslinker to polymer ratio on the particle characteristics were studied. The chitosan–TPP nanoparticles were characterized by dynamic light scattering, zeta potential, and turbidity measurements. The compactness of the nanoparticles was estimated with a method based on the size of the nanoparticles and the turbidity of the nanoparticle suspension. All the investigated preparation parameters, i.e., the ionic strength of the solvent, the chitosan concentration, and the TPP to chitosan ratio, affected the particle characteristics. For instance, smaller and more compact particles were formed in saline solvents, compared to particles formed in pure water. Further, the addition of monovalent salt rendered it possible to prepare particles in the nanometer size range at a higher polymer concentration. Solvent salinity is thus an important parameter to address in the preparation of chitosan nanoparticles crosslinked with TPP.     

On Nonlinear Orientational Effects in the Ferrocolloid Rotating in a Magnetic Field

The orientational dynamics of a system of noninteracting hard-magnetic ferroparticles in a fluid matrix rotating in a transverse magnetic field small as compared with the effective anisotropy field of a particle was considered. The problem of the dynamics of an individual particle with a magnetic moment incompletely frozen in the crystalline matrix was solved. An orientational microkinetic equation was formulated with allowances made for the Brownian motion of particles for high rotation frequencies. It was established that the presence of a small orientational freedom of the moment in the particle body dramatically changes the behavior of the system. A particular result is a giant retardation of the mechanical and magnetic orientational relaxation in the colloid preliminary magnetized along the rotation axis.     

Physical and Chemical Properties of Magnetite and Magnetite-Polymer Nanoparticles and Their Colloidal Dispersions

The properties of polymer-coated magnetite nanoparticles, which have the potential to be used as effective magnetic resonance contrast agents, have been studied. The magnetite particles were synthesized by using continuous synthesis in an aqueous solution. The polymer-coated magnetite nanoparticles were synthesized by seed precipitation polymerization of methacrylic acid and hydroxyethyl methacrylate in the presence of the magnetite nanoparticles. The particle size was measured by laser light scattering. It was shown that the particle size, variance, magnetic properties, and stability of aqueous magnetite colloidal dispersion strictly depend on the nature of the stabilizing agent. The average hydrodynamic radius of the magnetite particles was found to be 5.7 nm in the stable aqueous colloidal dispersion. An inclusion of the magnetite particle into a hydrophilic polymeric shell increases the stability of the dispersion and decreases the influence of the stabilizing agent on the magnetic and structural properties of the magnetite particles as was shown by X-ray diffraction and M?ssbauer and IR spectroscopy, as well as by vibrating sample magnetometry. The variation in the polymeric shell size and the polymer net density can be useful tools for evaluation of the polymer-coated magnetite particles as effective contrast agents. Copyright 1999 Academic Press.     

水溶液法制备CeF3纳米颗粒

氟化铈(CeF3)具有很高的离子电导率［1］和独特的光学特性［2］, 可用于制作化学传感器和光学元器件. CeF3还是良好的固体润滑剂［3］. 近年来的研究结果表明, 与氟化物粗晶材料相比, 纳米尺度的氟化物的性能有显著的提高， 因此, 其制备方法也很受关注. 目前, 制备氟化物纳米颗粒的方法有蒸发法［4］、 微乳液法［2, 5~8］、 水醇混合溶液法［3,9］和微波固相氧化还原合成法［10］等. 但是, 水溶液直接沉淀法作为一种操作简单, 成本低, 适合批量生产的制备方法尚未见用于制备纳米CeF3颗粒.     

Permanent antistatic phthalocyanine/epoxy nanocomposites - Influence of crosslinking agent, solvent and processing temperature

Cross-linked epoxy matrices containing small amounts of semi-conductive phthalocyanine (Phthalcon) nanoparticles were prepared using different crosslinking agents and processing temperatures. A starting mixture containing an optimum dispersion of these nanoparticles and with an almost equal and large Hamaker constant was always used. Nevertheless large differences in the relation between the volume conductivity σ_v and the particle concentration φ were found and this relation appeared to be sensitive to small changes in processing temperature and the application of a post-cure. Also the amine crosslinker chosen and the initial amount of solvent (catalyst) in the starting dispersion had a major effect. It was shown that these changes influence strongly the formation of and the final conductive fractal particle network morphology through the polymer matrix. During processing a local relaxation of the initially formed fractal particle network into another fractal particle network was often observed, which introduced or enlarged the amount of isolating material between the particles of the conductive network and changed the fractality and structure of the conductive backbone of the particle network. This local relaxation lowered the σ_v at each phthalcon concentration and enlarged φ_c by several orders of magnitude. The occurrence of local relaxation is dependent on the rate of viscosity change during the crosslinking of the polymer matrix components, the way the fractal conductive particle network is formed during processing (universal or non-universal) and the amount of solvent present. Local relaxation may even occur after the gel point of the polymer matrix. A severe post-cure may be needed to stop this local relaxation. To our knowledge local relaxation of a (fractal) nanoparticle network in a polymer matrix during processing is a new phenomenon, not reported before for polymer composites containing (conductive) nanoparticles.     

Calorimetric effects in rapidly frozen micellar solutions below the macroscopic melting point

Small samples of aqueous solutions of three cetyltrimethylammonium (CTA) salts and of sodium dodecyl sulfate (SDS) were rapidly frozen by plunging into liquid Nitrogen. The frozen samples were heated and recooled in a differential scanning calorimeter between −50 and −5°C, i.e., below the macroscopic melting point. Upon first heating an exothermic process was observed in CTA salts (but not in SDS) at between −20 and −30°C. The process was assigned to crystallization of micellar microdomains that had not taken place upon rapid freezing. The mobility for crystallization in the frozen environment was possible because of local melting point depression. In the presence of sufficient sodium chloride the local melting was proven by according endo- and exothermic DSC peaks upon heating and recooling, respectively.     

Superparamagnetic behavior and AC-losses in NiFe2O4 nanoparticles

Crystallographic, microstructural and magnetic properties of NiFe₂O₄ nanoparticles synthesized by precipitation from nonaqueous solutions have been studied in the work. The transmission electron microscopy studies reveal particle sizes ∼5 nm for the as-prepared particles which increase up to ∼20 nm upon annealing at 500 °C. Quasistatic magnetic measurements show superparamagnetic behavior with blocking temperature below room temperature for both the as-prepared and annealed particles. Characteristic magnetic parameters of the particles including average magnetic moment of an individual nanoparticle and effective anisotropy constant have been determined. The specific loss power which is released on the exposure of an ensemble of synthesized particles to an electromagnetic field is calculated and measured experimentally.     

Switching between laser-induced thermophoresis and thermal convection of liquid suspension in a microgap with variable dimension

Phoretic motion of particles along a temperature gradient formed in a fluid, known as thermophoresis, often takes place under the influence of bulk motion caused by thermal convection. In this paper, using a laser heating method, the significance of two competing effects, that is, thermophoresis and thermal convection, for the particle transport in a liquid phase confined in a microgap is investigated experimentally by changing the gap size as a control parameter. It is found that there is a threshold of the gap size, above which the particles tend to accumulate around the heated spot, forming a ring-like particle distribution. On the contrary, if the gap size is below the threshold, the particles are depleted from the heated spot. Switching between these accumulation and depletion modes is expected to develop novel manipulation techniques.