Metal-coated Particles Research Articles

Active microparticle propulsion pervasively powered by asymmetric AC field electrophoresis

HypothesisSymmetry breaking in an electric field-driven active particle system can be induced by applying a spatially uniform, but temporally non-uniform, alternating current (AC) signal. Regardless of the type of particles exposed to sawtooth AC signals, the unevenly induced polarization of the ionic charge layer leads to a major electrohydrodynamic effect of active propulsion, termed Asymmetric Field Electrophoresis (AFEP). ExperimentsSuspensions containing latex microspheres of three sizes, as well as Janus and metal-coated particles were subjected to sawtooth AC signals of varying voltages, frequencies, and time asymmetries. Particle tracking via microscopy was used to analyze their motility as a function of the key parameters. FindingsThe particles exhibit field-colinear active propulsion, and the temporal reversal of the AC signal results in a reversal of their direction of motion. The experimental velocity data as a function of field strength, frequency, and signal asymmetry are supported by models of asymmetric ionic concentration-polarization. The direction of particle migration exhibits a size-dependent crossover in the low frequency domain. This enables new approaches for simple and efficient on-chip sorting. Combining AFEP with other AC motility mechanisms, such as induced-charge electrophoresis, allows multiaxial control of particle motion and could enable development of novel AC field-driven active microsystems.

Dielectric relaxation in metal-coated particles: the dramatic role of nano-scale coatings

Insulating materials filled with conducting particles permit tailoring of electrical, electromagnetic and thermal properties of the resulting composite. When the filler particles are small and metallic, a dielectric relaxation due to interfacial polarization is commonly observed at optical or smaller wavelengths. Here, experimental results are presented in which the dielectric relaxation is shifted to microwave frequencies as a result of using metal-coated dielectric particles with a nano-scale coating thickness. The results are analysed in the context of effective medium theory adapted for multi-layer particles. Such a large shift in relaxation frequency, compared with that for a similar composite with solid metal filler particles, is shown to be a function of both the coating geometry and a thin-film-related reduction in the conductivity of the metal. The observed broadening of the relaxation peak is attributed to non-uniformity of the coating thickness and a consequent distribution of coating conductivity.

Synthesis and Self-Assembly of Metal-Coated Nanoparticles

AbstractWe report theoretical and experimental studies on the metal-dielectric photonic crystal structure, which is constructed by the self-assembly of metal-coated nanoparticles. The finite- difference time-domain (FDTD) simulations were carried out to predict the width and position of the 3D photonic bandgap. For fabrication, we first prepared gold-embedded silica nanoparticles containing extremely small gold nanoparticles. The core particles are then immersed in a gold- containing solution in which the gold shells were formed by subsequent reduction. Formation of continuous shells was confirmed by scanning electron microscope (SEM) and the optical response due to the surface plasmons. We also performed self-assembly of both the gold- embedded core particles and metal-coated particles. SEM micrographs showed the formation highly ordered structures. The optical reflectance spectra exhibited shifts of the Bragg reflection peak due to the change in refractive index produced by gold nanoparticles.

Peculiarities of absorption and optical fields inside hollow metallic and metalcoated particles

Peculiarities of absorption and optical fields inside hollow metallic and metalcoated particles

EXAFS Study of Metal-Coated Particles Produced by Ball Milling

EXAFS measurements have been carried out on mixtures of Sn with Ge and Si, and Pt with SiO2 which have been ball milled for extended periods. In these systems the brittle component (Ge, Si, or SiO2) is ground to nanocrystalline dimensions while the ductile metal is found to coat the outer surface of the particles. Analysis shows that in the Sn-Ge and Sn-Si systems, mixing at the interface is found to form apparently cubic SnGe and SnSi alloys respectively. In the Pt-SiO2 system while no mixing is observed, the Pt is found to be highly dispersed on the surface of SiO2 particles.

Preparation of nickel-alumina composite using the metal-coated particles.

Al2O3 particles (mean diameter: 250μm) were coated with fine Ni particles (mean diameter: 5μm) by an agglomeration coating technique. The Ni content was 40-50vol%. The compact of the coated particles was sintered at a temperature of 1200°-1500°C. The use of the coated particles resulted in uniform dispersion of Al2O3 particles in Ni matrix. The pre-coating of Al2O3 particles with fine W particles (mean diameter: 1μm) was effective to improve the adhesion and wettability of Ni to Al2O3. Although the relative density of the sintered body was 70-80%, the dense composite was produced by the infiltration of molten Ni into the porous sintered body.

Measurement of the falling velocity of particles in opaque fluids

An apparatus is described for the measurement of the falling velocity of small metallic or metal-coated particles through opaque fluids. An electromagnetic method of detecting the particles is employed.