Biblioteca Digital

The evaluation of heat production from gold nanoparticles (AuNPs) irradiated with radio-frequency (RF) energy has been problematic due to Joule heating of their background ionic buffer suspensions. Insights into the physical heating mechanism of nanomaterials under RF excitations must be obtained if they are to have applications in fields such as nanoparticle-targeted hyperthermia for cancer therapy. By developing a purification protocol that allows for highly stable and concentrated solutions of citrate-capped AuNPs to be suspended in high-resistivity water, we show herein, for the first time, that heat production is only evident for AuNPs of diameters ≤10 nm, indicating a unique size-dependent heating behavior not previously observed. Heat production has also shown to be linearly dependent on both AuNP concentration and total surface area and was severely attenuated upon AuNP aggregation. These relationships have been further validated using permittivity analysis across a frequency range of 10 MHz–3 GHz as well as static conductivity measurements. Theoretical evaluations suggest that the heating mechanism can be modeled by the electrophoretic oscillation of charged AuNPs across finite length scales in response to a time-varying electric field. It is anticipated these results will assist future development of nanoparticle-assisted heat production by RF fields for applications such as targeted cancer hyperthermia.

In the present work homogeneous mullite and mullite–alumina composites reinforced with carbon nanofibers (CNFs) were prepared. The stability of aqueous mullite and mullite–CNFs suspensions was studied through zeta potential and rheological behaviour of concentrated slurries. Slurries were optimised in terms of dispersant concentration and solids content and further used for the preparation of homogeneous mixtures of powders by freeze drying. The obtained powders were characterised through the determination of particle size distribution and morphology by Field Emission Scanning Electron Microscopy (FSEM) and Laser Scattering. The freeze dried powders were used to obtain dense composites by spark plasma sintering. Their microstructure and mechanical properties were evaluated.

Using nonequilibrium molecular dynamics simulations, we study the non-Newtonian rheology of a microscopic sample of simple fluid. The calculations were performed using a configurational thermostat which unlike previous nonequilibrium molecular dynamics or nonequilibrium Brownian dynamics methods does not exert any additional constraint on the flow profile. Our findings are in agreement with experimental results on concentrated "hard sphere"-like colloidal suspensions. We observe: (i) a shear thickening regime under steady shear; (ii) a strain thickening regime under oscillatory shear at low frequencies; and (iii) shear-induced ordering under oscillatory shear at higher frequencies. These results significantly differ from previous simulation results which showed systematically a strong ordering for all frequencies. They also indicate that shear thickening can occur even in the absence of a solvent.; J.D. acknowledges support from the Research School of Chemistry ~ANU! through a visiting fellowship.

Preparation of Reversibly Inactivated (R.I.) Phage.— If B. megatherium phage (of any type, or in any stage of purification) is suspended in dilute salt solutions at pH 5–6, it is completely inactivated; i.e., it does not form plaques, or give rise to more phage when mixed with a sensitive organism (Northrop, 1954). The inactivation occurs when the phage is added to the dilute salt solution. If a suspension of the inactive phage in pH 7 peptone is titrated to pH 5 and allowed to stand, the activity gradually returns. The inactivation is therefore reversible. Properties of R.I. Phage.— The R.I. phage is adsorbed by sensitive cells at about the same rate as the active phage. It kills the cells, but no active phage is produced. The R.I. phage therefore has the properties of phage "ghosts" (Herriott, 1951) or of colicines (Gratia, 1925), or phage inactivated by ultraviolet light (Luria, 1947). The R.I. phage is sedimented in the centrifuge at the same rate as active phage. It is therefore about the same size as the active phage. The R.I. phage is most stable in pH 7, 5 per cent peptone, and may be kept in this solution for weeks at 0°C. The rate of digestion of R.I. phage by trypsin, chymotrypsin, or desoxyribonuclease is about the same as that of active phage (Northrop...

The nido-carborane species K[nido-7-CH3(CH2)15-7,8-C2B9H11] has been synthesized for use as an addend for the bilayer membrane of liposomes. Small unilamellar vesicles, composed of distearoylphosphatidylcholine/cholesterol, 1:1, and incorporating K[nido-7-CH3(CH2)15-7,8-C2B9H11] in the bilayer, have been investigated in vivo. The time-course biodistribution of boron delivered by these liposomes was determined by inductively coupled plasma-atomic emission spectroscopy analyses after the injection of liposomal suspensions in BALB/c mice bearing EMT6 mammary adenocarcinomas. At the low injected doses normally used (approximately 5-10 mg of boron per kg of body weight), peak tumor boron concentrations of approximately 35 micrograms of boron per g of tissue and tumor/blood boron ratios of approximately 8 were achieved. These values are sufficiently high for the successful application of boron neutron capture therapy. The bilayer-embedded boron compound may provide the sole boron source or, alternatively, a concentrated aqueous solution of a hydrophilic boron compound may also be encapsulated within the liposomes to provide a dose enhancement. Thus, the incorporation of both K[nido-7-CH3(CH2)15-7,8-C2B9H11] and the hydrophilic species, Na3[1-(2'-B10H9)-2-NH3B10H8]...

Cholesterol oxidase (CO) and choline phosphohydrolase (CPH) exoenzymes were isolated from culture supernatants of Rhodococcus equi ATCC 33701 and their hemolytic and cytotoxic activities examined. The purifications involved differential ammonium sulphate precipitation, ion exchange and gel filtration chromatography. A purification of 32.8-fold and a yield of 0.3% of CO were determined by synergistic hemolysis of sheep red blood cells (SRBC) presensitized with Staphylococcus aureus beta toxin. The enzymatic activity of CO was also demonstrated by oxidation of aqueous cholesterol suspensions. The activity of CO was reversibly inhibited by concentration. A purification of 412.4-fold and a yield of 1.7% of CPH were determined by hydrolysis of p-nitrophenyphosphorylcholine. Purity of both exoenzymes was confirmed by immunoblotting. On sodium dodecyl sulphate polyacrylamide gel electrophoresis, the CO had a molecular mass (Mr) of 60 kd and the CPH a Mr of 65 kd. Choline phosphohydrolase did not hydrolyse sphingomyelin. Sphingomyelinase C (SMC) activity was however demonstrated in concentrated culture supernatants. This dissociation of SMC from CPH activity indicates that R. equi produces two distinct phospholipase C exoenzymes, a CPH and a SMC. Both CO and CPH combined...

Lanthanum phosphate (LaPO4) nanorods dispersed in the non-aqueous solvent of ethylene glycol form a system exhibiting large intrinsic birefringence, high colloidal stability and the ability to self-organize into liquid crystalline phases. In order to probe the electro-optical response of these rod dispersions we study here the electric-field-induced birefringence, also called Kerr effect, for a concentrated isotropic liquid state with an in-plane a.c. sinusoidal electric field, in conditions of directly applied (electrodes in contact with the sample) or externally applied (electrodes outside the sample cell) fields. Performing an analysis of the electric polarizability of our rod-like particles in the framework of Maxwell-Wagner-OKonski theory, we account quantitatively for the coupling between the induced steady-state birefringence and the electric field as a function of the voltage frequency for both sample geometries. The switching time of this non-aqueous transparent system has been measured, and combined with its high Kerr coefficients and its features of optically isotropic offstate and athermal phase behavior, this represents a promising proof-of-concept for the integration of anisotropic nanoparticle suspensions into a new generation of electro-optical devices.; Comment: To be published in "Soft Matter"

The change of the structure of concentrated colloidal suspensions upon addition of non-adsorbing polymer is studied within a two-component, Ornstein-Zernicke based liquid state approach. The polymers' conformational degrees of freedom are considered and excluded volume is enforced at the segment level. The polymer correlation hole, depletion layer, and excess chemical potentials are described in agreement with polymer physics theory in contrast to models treating the macromolecules as effective spheres. Known depletion attraction effects are recovered for low particle density, while at higher densities novel many-body effects emerge which become dominant for large polymers.; Comment: 7 pages, 4 figures; to be published in Europhys. Lett

The dominant modes of charge transport in variant polar liquid based nanoparticulate colloidal dispersions (dilute) have been theorized. Theories formulating electrical characteristics of colloids have often been found to over or under predict charge transport in dilute suspensions of nanoparticles in polar fluids owing to grossly different mechanistic behavior of concentrated systems. Three major interacting modes with independent yet simultaneous existence have been proposed and found to be consistent with analyses of experimental data. Electric Double Layer (EDL) formation at nanoparticle fluid interface conjugated electrophoresis under the influence of the electric field has been determined as one important mode of charge transport. Nanoparticle polarization due to short range field non-uniformity caused by the EDL with consequent particle motion due to interparticle electrostatic interactions acts as another mode of transport. Coupled electrothermal diffusion arising out of Brownian randomization in presence of the electric field has been determined as the third dominant mode. An analytical model based on discrete interactions of the charged particle fluid domains explains the various behavioral aspects of such dispersions, as observed and validated from detailed experimental analysis. The analysis is also predictive of the dominance and behavior of the three modes with important nanocolloid parameters such as temperature and concentration.

In this article we report on a study of the near-wall dynamics of suspended colloidal hard spheres over a broad range of volume fractions. We present a thorough comparison of experimental data with predictions based on a virial approximation and simulation results. We find that the virial approach describes the experimental data reasonably well up to a volume fraction of $\phi=0.25$ which provides us with a fast and non-costly tool for the analysis and prediction of Evanescent Wave DLS data. Based on this we propose a new method to assess the near-wall self-diffusion at elevated density. Here, we qualitatively confirm earlier results [Michailidou et al., Phys. Rev. Lett., 2009, 102, 068302], which indicate that many-particle hydrodynamic interactions are diminished by the presence of the wall at increasing volume fractions as compared to bulk dynamics. Beyond this finding we show that this diminishment is different for the particle motion normal and parallel to the wall.; Comment: Submitted to Soft Matter; 12 pages, 9 figures

We present recent advances in the instrumentation and analysis methods for quantitative imaging of concentrated colloidal suspensions under flow. After a brief review of colloidal imaging, we describe various flow geometries for two and and three-dimensional (3D) imaging, including a `confocal rheoscope'. This latter combination of a confocal microscope and a rheometer permits simultaneous characterization of rheological response and 3D microstructural imaging. The main part of the paper discusses in detail how to identify and track particles from confocal images taken during flow. After analyzing the performance of the most commonly used colloid tracking algorithm by Crocker and Grier extended to flowing systems, we propose two new algorithms for reliable particle tracking in non-uniform flows to the level of accuracy already available for quiescent systems. We illustrate the methods by applying it to data collected from colloidal flows in three different geometries (channel flow, parallel plate shear and cone-plate rheometry).; Comment: 21 pages, 24 figures

Secondary homotopy groups supplement the structure of classical homotopy groups. They yield a track functor on the track category of pointed spaces compatible with fiber sequences, suspensions and loop spaces. They also yield algebraic models of homotopy types with homotopy groups concentrated in two consecutive dimensions.; Comment: We added further commets and references to make the paper more easily readable

The non-linear Poisson-Boltzmann equation for a circular, uniformly charged platelet, confined together with co- and counter-ions to a cylindrical cell, is solved semi-analytically by transforming it into an integral equation and solving the latter iteratively. This method proves efficient, robust, and can be readily generalized to other problems based on cell models, treated within non-linear Poisson-like theory. The solution to the PB equation is computed over a wide range of physical conditions, and the resulting osmotic equation of state is shown to be in fair agreement with recent experimental data for Laponite clay suspensions, in the concentrated gel phase.; Comment: 13 pages, 4 postscript figures

A nonlinear viscoelastic theory of nematodynamic type is developed for nematic liquid crystalline (LC) semi-flexible polymers. A measure of transient elastic strain due to the change in length of macromolecular strands under stress, and the director of unit length are employed in the theory as hidden variables. In the marked contrast to the common theoretical approaches to low molecular nematics, the effect of directors space gradient is neglected in the present theory. Nevertheless, the theory allows describing nonlinear anisotropic viscoelasticity and evolution equation for the director in flows of LC polymers. When LC macromolecules are relatively rigid or when they are soft but the flow is slow, a weakly nonlinear viscoelastic anisotropic behavior is described by few temperature dependent parameters. In the infinitesimal case the evolution equation for director reminds the Ericksens equation, but with an additional relaxation term. The present theory can also be applied for analyzing flows of concentrated polymer suspensions and nano-composites filled with uniaxially symmetric particles.; Comment: 26 pages

A new theory for dynamics of concentrated colloidal suspensions and the colloidal glass transition is proposed. The starting point is the memory function representation of the density correlation function. The memory function can be expressed in terms of a time-dependent pair-density correlation function. An exact, formal equation of motion for this function is derived and a factorization approximation is applied to its evolution operator. In this way a closed set of equations for the density correlation function and the memory function is obtained. The theory predicts an ergodicity breaking transition similar to that predicted by the mode-coupling theory, but at a higher density.; Comment: to be published in PRL

We study concentrated colloidal suspensions, a model system which has a glass transition. Samples in the glassy state show aging, in that the motion of the colloidal particles slows as the sample ages from an initial state. We study the relationship between the static structure and the slowing dynamics, using confocal microscopy to follow the three-dimensional motion of the particles. The structure is quantified by considering tetrahedra formed by quadruplets of neighboring particles. We find that while the sample clearly slows down during aging, the static properties as measured by tetrahedral quantities do not vary. However, a weak correlation between tetrahedron shape and mobility is observed, suggesting that the structure facilitates the motion responsible for the sample aging.; Comment: Submitted to Solid State Communications

Often experimentalists study colloidal suspensions that are nominally monodisperse. In reality these samples have a polydispersity of 4-10%. At the level of an individual particle, the consequences of this polydispersity are unknown as it is difficult to measure an individual particle size from microscopy. We propose a general method to estimate individual particle radii within a moderately concentrated colloidal suspension observed with confocal microscopy. We confirm the validity of our method by numerical simulations of four major systems: random close packing, colloidal gels, nominally monodisperse dense samples, and nominally binary dense samples. We then apply our method to experimental data, and demonstrate the utility of this method with results from four case studies. In the first, we demonstrate that we can recover the full particle size distribution {\it in situ}. In the second, we show that accounting for particle size leads to more accurate structural information in a random close packed sample. In the third, we show that crystal nucleation occurs in locally monodisperse regions. In the fourth, we show that particle mobility in a dense sample is correlated to the local volume fraction.; Comment: 7 pages, 5 figures

Self-sustained turbulent structures have been observed in a wide range of living fluids, yet no quantitative theory exists to explain their properties. We report experiments on active turbulence in highly concentrated 3D suspensions of Bacillus subtilis and compare them with a minimal fourth-order vector-field theory for incompressible bacterial dynamics. Velocimetry of bacteria and surrounding fluid, determined by imaging cells and tracking colloidal tracers, yields consistent results for velocity statistics and correlations over two orders of magnitude in kinetic energy, revealing a decrease of fluid memory with increasing swimming activity and linear scaling between energy and enstrophy. The best-fit model parameters allow for quantitative agreement with experimental data.; Comment: 5 pages, 4 figures

We analyze the dynamics of concentrated polymer solutions modeled by a 2D Smoluchowski equation. We describe the long time behavior of the polymer suspensions in a fluid. \par When the flow influence is neglected the equation has a gradient structure. The presence of a simple flow introduces significant structural changes in the dynamics. We study the case of an externally imposed flow with homogeneous gradient. We show that the equation is still dissipative but new phenomena appear. The dynamics depend on both the concentration intensity and the structure of the flow. In certain limit cases the equation has a gradient structure, in an appropriate reference frame, and the solutions evolve to either a steady state or a tumbling wave. For small perturbations of the gradient structure we show that some features of the gradient dynamics survive: for small concentrations the solutions evolve in the long time limit to a steady state and for high concentrations there is a tumbling wave.; Comment: Minor typos fixed. References added

We use X-Ray Photon Correlation Spectroscopy to investigate the structural relaxation process in a metallic glass on the atomic length scale. We report evidence for a dynamical crossover between the supercooled liquid phase and the metastable glassy state, suggesting different origins of the relaxation process across the transition. Furthermore, using different cooling rates we observe a complex hierarchy of dynamic processes characterized by distinct aging regimes. Strong analogies with the aging dynamics of soft glassy materials, such as gels and concentrated colloidal suspensions, point at stress relaxation as a universal mechanism driving the relaxation dynamics of out-of-equilibrium systems.; Comment: 6 pages, 4 figure. To be published in Phys. Rev. Lett