Transition from liquid droplet to solid particle investigated by ultrasonic spectroscopy.

The size distribution and elastic modulus of micron-sized particles dispersed in liquid can be quantitatively evaluated by ultrasonic spectroscopy at a megahertz frequency range combined with a scattering theory. Conventional theories dealing with the wavelength comparable with the micron-sized particles consider viscosity for liquid droplets in emulsions and elasticity for solid particles in suspension, but very few studies have simultaneously considered viscosity and elasticity for the dispersed phase. In this study, a toluene (Tol) solution of polystyrene (PS) was dispersed in a continuous phase (water), and the ultrasonic properties of the PS-Tol/water emulsion were investigated.

Nanoparticle sizing by focused-beam dynamic ultrasound scattering method.

When nanoparticles in Brownian motion in liquid are irradiated with ultrasonic waves in the megahertz frequency range, scattering from the particles occurs, albeit at a very low intensity. The diffusion coefficient and the corresponding particle size can be calculated by analyzing the time correlation function of the ultrasound pulses. Since ultrasonic waves with long wavelengths in comparison with the particle size are unfavorable for detecting such small particles, increasing the energy of the ultrasonic waves is a primary solution.

Patterning Silver Nanowires by Inducing Transient Concentration Gradients in Reaction Mixtures.

Patterning nanocrystals in polymer films is essential for the widespread use of nanocrystals in various fields from optics to electronics; therefore, the development of patterning methods for nanocrystals is an important task. Here, we report a unique approach for patterning silver nanowires (AgNWs) using a thermodynamic driving force induced by transient concentration gradients in reaction mixtures. The procedure starts with the preparation of a photocurable monomer solution containing homogeneously dispersed AgNWs.

Latex agglutination analysis by novel ultrasound scattering techniques.

The latex agglutination test is employed to visualize antigen-antibody reactions through the aggregation of antibody-coated particles in the presence of an antigen. In the present study, we developed an ultrasound scattering technique to detect latex agglutination in an optically turbid media. However, the ultrasonic technique had less sensitivity to the dilute particle suspension than the optical techniques because of its wavelength.

Interfacial structures of particle-stabilized emulsions examined by ultrasonic scattering analysis with a core-shell model.

Pickering emulsions comprising liquid droplets stabilized by solid microparticles have gained much attention in the field of cosmetics, inks, and drug delivery systems. To ensure that microparticles in Pickering emulsions are localized at the surface of liquid droplets, ultrasonic spectroscopy analysis combined with scattering function theory was conducted in this study. Two specific cases were investigated: (1) silica particles and liquid droplets independently dispersed in liquid and (2) silica particles effectively localized at the surface of the droplets.

Viscoelastic ECAH: Scattering analysis of spherical particles in suspension with viscoelasticity.

Ultrasonic scattering method is a promising technique to evaluate the particle size distribution and/or the elastic properties of particle suspended in liquid. Among the wide variety of scattering theories, the ECAH theory proposed by Epstein-Carhart-Allegra-Hawley is one of the most relevant acoustic scattering theories to reproduce the ultrasonic spectroscopy data for the particle suspensions. However, the original theory assumes that the shear contribution is provided for either elastic solid or viscous liquid.

Particle size distribution analysis of oil-in-water emulsions using static and dynamic ultrasound scattering techniques.

Dynamic ultrasound scattering allows us to investigate the particle motion and its average size via the time-evolution analysis of the scattering amplitude in optically turbid media. Recently, we proposed a novel particle sizing method that simultaneously analyzes the depth dependences on the sedimentation velocity and the scattered intensity without prior knowledge about the shape of the size distribution (Ultrasonics, 65 (2016) 59-68). In this study, the applicability of the technique to Fluorinert/water dilute and concentrated emulsions (up to 40 vol%) was examined.

Nanocrystals Assembled by the Chemical Reaction of the Dispersion Solvent.

The development of methods to pattern nanocrystals with different sizes and shapes remains a challenge. In this study, we demonstrate a unique class of bottom-up approaches to assemble nanocrystals into patterns. Our approach for patterning nanocrystals focuses on the utilization and control of the chemical reaction of solvents surrounding nanocrystals.

Simultaneous measurements of ultrasound attenuation, phase velocity, thickness, and density spectra of polymeric sheets.

The size distribution and mechanical properties of microparticle dispersed in liquid can be characterized by ultrasonic spectroscopy with the aid of acoustic scattering theories. In order to carry out the accurate analysis of the particles, the basic properties, such as the density, viscosity, longitudinal and shear velocities and intrinsic attenuation coefficient of the particle must be known prior to the analysis. Particularly, for soft elastomers or rubbers which exhibit complex mechanical properties with comparable real and imaginary parts, such fundamental information should be provided prior to the particle analysis to minimize the uncertainty of estimation associated with the number of adjustable parameters.

Effects of Nanowire Length on Charge Transport in Vertically Aligned Gold Nanowire Array Electrodes.

In this study, we demonstrate that vertically aligned gold nanowire array electrodes provide rapid ion and electron transport to the electrode-electrolyte interface. The charge-transport properties of the nanowire electrodes were investigated through cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy under a constant-volume device configuration. The total charge stored in the corresponding devices increases monotonically with the length of the nanowires owing to the concomitant increase in the electroactive real surface area of the electrode.

Ultrasound attenuation and phase velocity of moderately concentrated silica suspensions.

Ultrasound attenuation coefficient and phase velocity of moderately concentrated suspensions of charged silica particles were measured as a function of frequency. The attenuation coefficients were found to be significantly smaller than the theoretical prediction, and such a difference did not appear in the neutral particle suspensions under the corresponding concentrations. In this study, we have investigated the acoustic spectra of silica particles with different particle sizes, concentrations as well as salt concentrations.

Structures and dynamics of carbon-black in suspension probed by static and dynamic ultrasound scattering techniques.

Carbon black (CB) suspension exhibits various structures depending on the properties of solvent and dispersant as well as the preparation process of suspension. In most cases, CB particles do not exist as independent nanoparticles but as aggregates or agglomerates. In order to evaluate the size distribution at different level of hierarchal structure, we carried out static/dynamic ultrasound scattering analysis for the CB suspensions in alcohol and/or water with or without Nafion, a perfluorinated polymer.

Metal Nanowire-Based Hybrid Electrodes Exhibiting High Charge/Discharge Rates and Long-Lived Electrocatalysis.

Nanostructured electrodes are at the forefront of advanced materials research, and have been studied extensively in the context of their potential applications in energy storage and conversion. Here, we report on the properties of core-shell (gold-polypyrrole) hybrid nanowires and their suitability as electrodes in electrochemical capacitors and as electrocatalysts. In general, the specific capacitance of electrochemical capacitors can be increased by faradaic reactions, but their charge transfer resistance impedes charge transport, decreasing the capacitance with increasing charge/discharge rate.

Size distribution and elastic properties of thermo-responsive polymer gel microparticles in suspension probed by ultrasonic spectroscopy.

The size distribution and the elasticity of thermo-responsive gel particles dispersed in water were examined in-situ by ultrasonic spectroscopy (US). Poly(N-isopropylacrylamide) (PNIPAM) gel particles, undergoing volume phase transition at 34°C were synthesized by redox polymerization in emulsion, and the temperature dependence of the acoustic properties was examined. The longitudinal velocity and the attenuation coefficient of particle exhibited a drastic change at the transition temperature, indicating the increase in the mechanical properties, such the longitudinal, bulk and shear moduli, due to the shrinkage of particles upon temperature stimulation.

Ultrasound attenuation and phase velocity of micrometer-sized particle suspensions with viscous and thermal losses.

The attenuation coefficient and the phase velocity of micrometer-sized polydivinylbenzene particles in water were investigated by ultrasound spectroscopy equipped with 10-30MHz longitudinal wave transducers. While the surrounding liquid could be assumed to be inviscid for large particles with the size comparable to the wavelength of longitudinal ultrasound, the viscous and thermal waves were considered to have important roles with decreasing the particle size because the particle size becomes comparable with those wavelengths. In this study, these contributions were systematically investigated by changing the particle-size.

Metal-Organic Coaxial Nanowire Array Electrodes Combining Large Energy Capacity and High Rate Capability.

Pseudocapacitors have been widely studied in the context of their potential applications in portable electronics and energy regeneration. However, the internal resistance within these devices hampers charge transport and limits their performance. As a result, maximum charge/discharge rates are typically limited to a few hundred mV s for pseudocapacitors.

Dynamic sound scattering: Field fluctuation spectroscopy with singly scattered ultrasound in the near and far fields.

Dynamic sound scattering (DSS) is a powerful acoustic technique for investigating the motion of particles or other inclusions inside an evolving medium. In DSS, this dynamic information is obtained by measuring the field autocorrelation function of the temporal fluctuations of singly scattered acoustic waves. The technique was initially introduced 15 years ago, but its technical aspects were not adequately discussed then.

Investigating the Existence of Bulk Nanobubbles with Ultrasound.

Nanobubbles are expected to dissolve in milliseconds. Experimental evidence of nanobubbles that were stable for days had thus been first received with circumspection. If the large number of experimental confirmations has now made clear that surface nanobubbles could exist, bulk nanobubbles are still subject to debate.

Polymer networks with bicontinuous gradient morphologies resulting from the competition between phase separation and photopolymerization.

Poly(ethyl acrylate)/poly(methyl methacrylate) (PEA/PMMA) polymer networks (IPNs) with spatially graded bicontinuous morphology were designed and controlled by taking advantage of the spinodal decomposition process induced by photopolymerization of the MMA monomer. Spatial gradients of the quench depth, induced by the gradients of light intensity, were generated along the path of the excitation light travelling through the mixture. Bicontinuous structures with uniaxial gradient of characteristic length scales were obtained by two different methods: simply irradiating the mixture with strong light intensity along the Z-direction and using the so-called computer-assisted irradiation (CAI) method with moderate intensity to generate the light intensity gradient exclusively in the XY plane.

Dynamics of micron-sized particles in dilute and concentrated suspensions probed by dynamic ultrasound scattering techniques.

A novel ultrasound technique called Frequency-Domain Dynamic ultraSound Scattering (FD-DSS) was employed to determine sedimentation velocities and the diameters of microparticles in a highly turbid suspension. The paper describes the importance of the scattering vector q for dynamic scattering experiments using broadband ultrasound pulses because q (or frequency) corresponds to the spatial length scale whereas the pulses involve inevitable uncertainty in the time domain due to the frequency distribution of broadband pulse. The results obtained from Stokes velocity of monodispersed silica and polydivinylbenzene (PDVB) particles were compared to those obtained by a Field Emission Scanning Electron Microscope (FE-SEM).

Effects of the positive feedback loop in polymerization on the reaction-induced phase separation of polymer mixtures.

Phase separation of mixtures composed of a guest polymer dissolved in monomer of a host polymer was induced by photopolymerization. From the polymerization kinetics, it was found that for the host polymer with the glass transition temperature (Tg) higher than the experimental temperature, the Trommsdorff-Norrish effect emerges, providing a method to terminate the phase separation at various intermediate stages of the process and thereby a variety of morphologies can be constructed. For the host polymers with (Tg) lower than the experimental temperature, the freezing process of morphology is not complete, leading to the secondary phase separation inside the domains already generated by the primary process.

Sound velocity and attenuation coefficient of hard and hollow microparticle suspensions observed by ultrasound spectroscopy.

Size and elastic properties of micro-particles suspended in liquid can be acoustically determined by ultrasound attenuation and velocity measurements with the aid of elastic scattering theories and a dispersion relation. While quantitative evaluation for hard micron-sized spheres using the theories is available in literature, that for hollow particles is not yet achieved. In this study, we show that the shell thickness and the elastic modulus of hollow particles can be quantitatively evaluated by ultrasound spectroscopy.

Origin of the anomalous decrease in the apparent density of polymer gels observed by multi-echo reflection ultrasound spectroscopy.

Multi-echo reflection ultrasound spectroscopy (MERUS), which enables one to simultaneously evaluate the attenuation coefficient α, the sound velocity v and the density ρ, has been developed for measurements of elastic moduli. In the present study, the technique was further developed to analyze systems undergoing gelation where an unphysical decrease in the apparent density was previously observed after polymerization. The main reason for this problem was that the shrinkage accompanying the gelation led to a small gap between the cell wall and the sample, resulting in the superposition of the reflected signals which were not separable into individual components.

Collective motion of microspheres in suspensions observed by phase-mode dynamic ultrasound scattering technique.

Compared with a nano-sized particle, dynamics of a micron-sized particle in a liquid is often associated with sedimentation (or floating) due to its relatively large mass. The motion of more than two particles is dominated by the hydrodynamic interactions, which are known to persist over a fairly long range, e.g.

Simultaneous evaluation of ultrasound velocity, attenuation and density of polymer solutions observed by multi-echo ultrasound spectroscopy.

Ultrasound spectroscopy is a powerful tool to investigate the viscoelastic properties of materials. The longitudinal elastic moduli M' and M(″), or the adiabatic compressibility κ(S) can be evaluated from ultrasound velocity v and attenuation coefficient α via the relation M'=ρv(2) and M(″)=2ραv(3)/ω, where ρ is the density and ω is the angular frequency. So far, the density was independently measured by other equipments or its variation during the chemical reaction has been ignored in the previous literatures.