A new multi-axial particle shape factor--application to particle sampling

Because for a given sample size the sampling uncertainty increases with increasing particle mass, the mass of a representative sample depends on the particle mass during chemical, physical and biological analysis. Sampling theory can be used to formulate the quantitative relationship between the particle mass and the corresponding mass or weight of a representative sample. But in practice, especially for small particles, it is often easier to evaluate the particle size in dimension of length (e.g. μm) rather than in dimension of mass (e.g. μg). In order to be able to apply sampling theory to predict the mass or weight of a representative sample, a well-defined methodology that relates the mass of a particle to its size is required. We here propose a new multi-axial shape factor which requires information of multiple sizes of the particle of interest, whereas a uniaxial shape factor only needs one. In view of the information loss that is implicit in the use of a one-dimensional shape factor like the Brunton shape factor, the here-proposed new multi-axial shape is expected to perform better. Experimental data confirm the better performance of the new shape factor. A multi-segment generalisation of the new multi-axial shape factor is proposed.     

Magnetic particle nanorheology

We investigate the employment of AC susceptometry as a novel method to extract frequency- and scale-dependent rheological properties of soft materials. The approach makes use of the interrelation between the magnetic susceptibility of magnetically blocked tracer particles and the mechanical modulus of the matrix as experienced on the particle scale. We report susceptibility measurements on aqueous solutions of ethylene glycol, triethylene glycol, or poly-(ethylene glycol), using CoFe₂O₄ nanoparticles as tracer particles, in the frequency range of 1 Hz–250 kHz. Frequency-dependent rheological properties, including viscosity and loss moduli, of the solutions were obtained by applying an extended Debye relaxation model and a method analogous to the Gemant-DiMarzio-Bishop approach.     

Studies of particle drying using non-invasive Raman spectrometry and particle size analysis

The evaporation of methanol from needle-shaped particles of cellobiose octaacetate (COA) has been studied directly in a jacketed vacuum drier using in situ measurements by Raman spectrometry. A design of experiments (DoE) approach was used to investigate the effects of three parameters (method of agitation, % solvent loss on drying and jacket temperature), with the intention of minimising the drying time and extent of particle attrition. Drying curves based on Raman signals for methanol and COA in the spectra of the wet particles indicated the end of drying and revealed three stages in the drying process that could be used to monitor the progress of solvent removal in real time. Off-line particle size measurements based on laser diffraction were made to obtain information on the extent of attrition, to compare with the trends revealed by the Raman drying curves. The study demonstrated that non-invasive Raman spectrometry can be used to study the progress of drying during agitation of particles in a vacuum drier, allowing optimisation of operating conditions to minimise attrition and reduce drying times. Although a correlation between particle size and off-line Raman measurements of COA was demonstrated, it was not possible to derive equivalent information from the in situ Raman spectra owing to the greater effects of particle motion or bulk density variations of the particles in the drier.     

Superhydrophobic polymer‐particle composite films produced using various particle sizes

Hydrophilic alumina (Al₂O₃) nanoparticles (25, 35, and 150 nm) are dispersed in different concentrations in solutions of a commercial hydrophobic poly(alkyl siloxane) (Silres BS‐290), and the suspensions are sprayed on glass surfaces. Static contact angles (θ_S), measured on surfaces of siloxane‐nanoparticle composite films that were prepared from dilute dispersions, increase rapidly with particle concentration. Composite films prepared from concentrated dispersions exhibit a maximum, constant θ_S (at saturation θ_S is 160°), which is not affected by the size of the particles. These films exhibit also very small contact angle hysteresis (5°), which is also independent of the particle size. Consequently, the same superhydrophobic character can be induced in siloxane films using nanoparticles, which can range from a few up to several tenths of nanometers. However, the particle size and more precisely the particle specific surface area affects dramatically the minimum critical particle concentration, which must be used in the dispersions to induce superhydrophobicity on the surface of the composite films, that is, to achieve θ_S = 150°. It is shown that critical particle concentration decreases exponentially with specific surface area. This result can be important for manufacturers of superhydrophobic surfaces who are interested in having a good control on the wettability of the composite films. Copyright © 2012 John Wiley & Sons, Ltd.     

Single aerosol particle studies

This paper surveys the research carried out on single aerosol particles in the micron and submicron size range with emphasis on the work performed by the authors. The principles and design of the electrodynamic and electrostatic balances are reviewed, and experimental data for evaporating droplets in a stagnant gas at various total pressures and various temperature are compared with theoretical results for Knudsen aerosol evaporation and are used to determine Lennard-Jones interaction parameters, diffusivities and vapor pressures for relatively nonvolatile compounds. The use of the electrodynamic balance or “picoblance” developed to study aerosol particles of the order of a piogram is illustrated for diffusion-controlled droplet evaporation measurements, and new data and an analysis for binary dorplet evaporation are presented.     

Dielectrophoresis of nanosized particle

The theory of dielectrophoresis is constructed with allowance for electroosmotic perturbances. Changes in the flow regime of electrolyte in the diffuse part of electrical double layer under the action of quadratic (with respect to external field) electric forces are considered. The expression for low-frequency limit of the dielectrophoretic velocity of a spherical particle, which is valid at the arbitrary thickness of electrical double layer and fairly small value of ζ-potential (< 50 mV), is derived. It is shown that electroosmotic perturbances appeared to be rather significant for nanosized particles and their dielectrophoretic velocity under the effect of electroosmotic perturbances changes by several times.     

Single particle powder patterns

A-19-mm diameter insert has been built for a Gandolfi X-ray powder diffraction camera. The camera makes it possible to obtain good X-ray diffraction patterns on a single 2-μm particle in 20 hr and in a few hours time with larger particles.     

Dye-labeled silver nanoshell-bright particle

Silica beads with average diameters of 40-600 nm were prepared, and Ru(bpy)3(2+) complexes were incorporated into the beads. These beads were coated by silver layer by layer to generate porous but continuous metal nanoshells. The thicknesses of these metal shells were 5-50 nm. The emission band from the dyes in the silica cores was more narrow and the intensity was enhanced with growth of silver shell thickness due to coupling of the emission light from Ru(bpy)3(2+) in the cores with the metal plasmon from the silver shells. The enhancement of emission intensity was also dependent on the size of the silica core, showing that the enhancement efficiency decreased with an increase in the size of the silica beads. Lifetime measurements support the coupling mechanism between the dye and metal shell. This study can be used to develop novel dye-labeled metal particles with bright and narrow emission bands.     

A multi-ion particle sensor

The first sub-micron polyacrylic sensor containing two independent ion-sensing systems is shown, that uses a single excitation wavelength and separates signals by using quantum dot donors to form FRET pairs with other fluorophores.     

Exploring the complexity of aerosol particle properties and processes using single particle techniques

The complex interplay of processes that govern the size, composition, phase and morphology of aerosol particles in the atmosphere is challenging to understand and model. Measurements on single aerosol particles (2 to 100 μm in diameter) held in electrodynamic, optical and acoustic traps or deposited on a surface can allow the individual processes to be studied in isolation under controlled laboratory conditions. In particular, measurements can now be made of particle size with unprecedented accuracy (sub-nanometre) and over a wide range of timescales (spanning from milliseconds to many days). The physical state of a particle can be unambiguously identified and its composition and phase can be resolved with a high degree of spatial resolution. In this review, we describe the advances made in our understanding of aerosol properties and processes from measurements made of phase behaviour, hygroscopic growth, morphology, vapour pressure and the kinetics of water transport for single particles. We also show that studies of the oxidative aging of single particles, although limited in number, can allow the interplay of these properties to be investigated. We conclude by considering the contributions that single particle measurements can continue to make to our understanding of the properties and processes occurring in atmospheric aerosol.     

A molecular theory for particle nucleation: primary particle formation in emulsion polymerization

Homogeneous nucleation is fundamentally important in emulsion polymerization. A molecular theory is proposed to quantify primary particle formation in the process. The proposed model divides a polymerization system into three portions: the domains formed by growing radicals, their surrounding aqueous solution, and dispersed monomer droplets. In general, the total free energy of the domains is contributed from the mixing among the molecules including monomer and water, the elasticity of oligomeric radical chains, and the transferable free energy of electrolytes; while that of the outer solution is from the mixing of monomer and water molecules and the transferable energy of electrolytes. Application of this theory to vinyl acetate emulsion polymerization has shown that the critical degree of polymerization (j(cri)) predicted is in a good agreement with the value derived from experimental data reported in literature. Furthermore, this model can also estimate the concentrations of VAc and water in the domain at the degrees of polymerization around j(cri).     

Visualization of microscale particle focusing in diluted and whole blood using particle trajectory analysis

Inertial microfluidics has demonstrated the potential to provide a rich range of capabilities to manipulate biological fluids and particles to address various challenges in biomedical science and clinical medicine. Various microchannel geometries have been used to study the inertial focusing behavior of particles suspended in simple buffer solutions or in highly diluted blood. One aspect of inertial focusing that has not been studied is how particles suspended in whole or minimally diluted blood respond to inertial forces in microchannels. The utility of imaging techniques (i.e., high-speed bright-field imaging and long exposure fluorescence (streak) imaging) primarily used to observe particle focusing in microchannels is limited in complex fluids such as whole blood due to interference from the large numbers of red blood cells (RBCs). In this study, we used particle trajectory analysis (PTA) to observe the inertial focusing behavior of polystyrene beads, white blood cells, and PC-3 prostate cancer cells in physiological saline and blood. Identification of in-focus (fluorescently labeled) particles was achieved at mean particle velocities of up to 1.85 m s(-1). Quantitative measurements of in-focus particles were used to construct intensity maps of particle frequency in the channel cross-section and scatter plots of particle centroid coordinates vs. particle diameter. PC-3 cells spiked into whole blood (HCT = 45%) demonstrated a novel focusing mode not observed in physiological saline or diluted blood. PTA can be used as an experimental frame of reference for understanding the physical basis of inertial lift forces in whole blood and discover inertial focusing modes that can be used to enable particle separation in whole blood.     

Heat transfer to a particle under plasma conditions with vapor contamination from the particle

Heat transfer to a copper particle immersed into an argon plasma is considered in this paper, including the effects of contamination of the plasma (transport coefficients) by copper vapor from the particle. Except for cases of high plasma temperatures, the vapor content in the plasma is shown to have a considerable influence on heat transfer to a nonevaporating particle, and, to a lesser extent, on heat transfer to an evaporating particle. Evaporation itself reduces heat transfer to a particle substantially as shown in a previous paper [Xi Chen and E. Pfender, Plasma Chem. Plasma Process.,2, 185 (1982)]. Comparisons of the calculated results with those based on a method suggested in the above reference show that the simplified assumptions employed, i.e., that the surface temperature is equal to the boiling point and that plasma properties based on a fixed composition are applicable, can be employed to simplify calculations for many cases. This study reveals that a considerable portion of a particle must be vaporized before a steady concentration distribution is established around the particle.Nomenclature C _p specific heat at constant pressure - D diffusion coefficient of copper in the mixture - D _a diffusion coefficient of copper atoms in the mixture - D _i ambipolar diffusion coefficient of copper ions in the mixture - f mass fraction of copper in the mixture - f _a mass fraction of copper atoms in the mixture - f _i mass fraction of copper ions in the mixture - f mass fraction of copper in the plasma far away from the particle - f _s mass fraction of copper at the particle surface - G total mass flow rate due to evaporation - G _a mass flow rate of copper atoms - G _i mass flow rate of copper ions - H function defined in Eq. (19) - h specific enthalpy - h _s specify enthalpy at the particle surface - h specific enthalpy corresponding toT andf - k thermal conductivity - L latent heat of evaporation - M ₁ molecular weight of argon (M ₁=39.99) - M ₂ molecular weight of copper (M ₂=63.55) - p ₀ pressure of the gas mixture - p _s partial pressure of copper vapor at the particle surface - Q ₀ heat flux to a particle without evaporation - Q ₁ heat flux to a particle with evaporation - R gas constant - r radical coordinate - r _s particle radius - S heat conduction potential defined in Eq. (4) - S _s surface value ofS, corresponding toT _s andf _s - S free-stream value ofS, corresponding toT andf - T temperature - T _b boiling temperature of particle material - T _s particle surface temperature - T plasma temperature - density - T temperature step for numerical integration     

Roughness models for particle adhesion

The effects of different surface roughness models on a previously developed van der Waals adhesion model were examined. The van der Waals adhesion model represented surface roughness with a distribution of hemispherical asperities. It was found that the constraints used to define the asperity distribution on the surface, which were determined from AFM scans, varied with scan size and thus were not constant for all surfaces examined. The greatest variation in these parameters occurred with materials that had large asperities or with materials where a large fraction of the surface was covered by asperities. These rough surfaces were modeled with fractals and also with a fast Fourier transform algorithm. When the model surfaces generated using the Fourier transforms are used in the adhesion model, the model accurately predicts the experimentally observed adhesion forces measured with the AFM.     

Electrokinetic particle entry into microchannels

The fundamental understanding of particle electrokinetics in microchannels is relevant to many applications. To date, however, the majority of previous studies have been limited to particle motion within the area of microchannels. This work presents the first experimental and numerical investigation of electrokinetic particle entry into a microchannel. We find that the particle entry motion can be significantly deviated from the fluid streamline by particle dielectrophoresis at the reservoir-microchannel junction. This negative dielectrophoretic motion is induced by the inherent non-uniform electric field at the junction and is insensitive to the microchannel length. It slows down the entering particles and pushes them toward the center of the microchannel. The consequence is the demonstrated particle deflection, focusing, and trapping phenomena at the reservoir-microchannel junction. Such rich phenomena are studied by tuning the AC component of a DC-biased AC electric field. They are also utilized to implement a selective concentration and continuous separation of particles by size inside the entry reservoir.     

Electrokinetics of particle covered surfaces

This article reviews recent developments in the field of streaming current/potential determination for surfaces covered by particles, including polyelectrolytes and proteins. Special emphasis is focused on the new theoretical results enabling one to evaluate quantitatively the relationship between the streaming potential and surface coverage of particles. Experimental results obtained for model systems of monodisperse polymeric particles are discussed and interpreted in terms of the theoretical approach. These reference data are exploited for interpretation of more complex experimental results obtained for polyelectrolytes and proteins. A major utility of the streaming potential measurements for determination of submonolayer coverages of particles and proteins is pointed out.     

Channeling in charged particle analysis

When a matrix of interest is a single crystal the channeling phenomenon can be exploited to increase the scope of charged particle analyses. One application provides increased sensitivity and precision in the analysis of surface films; another a means of locating foreign atoms within the crystal lattice.     

Acoustofluidic platforms for particle manipulation

There is an increasing interest in acoustics for microfluidic applications. This field, commonly known as acoustofluidics involves the interaction of ultrasonic standing waves with fluids and dispersed microparticles. The combination of microfluidics and the so-called acoustic standing waves (ASWs) led to the development of integrated systems for contact-less on-chip cell and particle manipulation where it is possible to move and spatially localize these particles based on the different acoustophysical properties. While it was initially suggested that the acoustic forces could be harmful to the cells and could impact cell viability, proliferation, or function via phenotypic or even genotypic changes, further studies disproved such claims. This review is summarizing some interesting applications of acoustofluidics in the manipulations of biomaterials, such as cells or subcellular vesicles, in works published mainly within the last 5 years.