Aqueous Suspensions Of Polystyrene Spheres Research Articles

Fabrication of a microball lens array for OLEDs fabricated using a monolayer microsphere template

We report on a method for fabricating microball lens arrays with high outcoupling efficiency based on the replica molding process, which involves casting liquid polymethylmethacrylate (PMMA) into concave microball lens molds. Concave microball lens molds were fabricated by casting a poly-dimethyl-siloxane (PDMS) mixture onto a monolayer microsphere template, which was prepared by spin-coating an aqueous suspension of polystyrene spheres. Microball lenses were fabricated using microspheres with a diameter of 90 µm. After attaching the microball lens array to organic light-emitting diodes (OLEDs), the luminance was enhanced by approximately 108%.

Hydrodynamic interactions between colloidal particles in a planar pore

The correlation between the motion of pairs of colloidal particles confined in a planar pore is measured using optical microscopy. The systems studied here are aqueous suspensions of polystyrene spheres of diameter 1.9 μm, interacting as effective hard spheres, confined between two parallel planar plates separated by 2.9 μm. The lateral motion, along the plane parallel to the plates, of the particles is recorded with a time resolution of 30 frames s−1. From the short-time motion, the hydrodynamic diffusion coefficients are determined as functions of the interparticle distance for various particle concentrations. At low concentrations, when the static correlation between particles is also low, the diffusion coefficients exhibit some symmetry, and at higher concentrations they are modulated by the structure of static correlation.

Observation of Phase Conjugation of Light Arising from Enhanced Backscattering in a Random Medium

We measure the fundamental phase conjugation of a light field arising from enhanced backscattering in a multiple scattering medium. The measurements employ a two-window, time-resolved heterodyne method to suppress specularly reflected light and to determine the transverse Wigner function of the field, yielding joint amplitude and phase information. Using this method, a light field backscattered from an aqueous suspension of polystyrene spheres is found to reverse curvature relative to that of the incident field.

Electric field Monte Carlo simulation of polarized light propagation in turbid media

A Monte Carlo method based on tracing the multiply scattered electric field is presented to simulate the propagation of polarized light in turbid media. Multiple scattering of light comprises a series of updates of the parallel and perpendicular components of the complex electric field with respect to the scattering plane by the amplitude scattering matrix and rotations of the local coordinate system spanned by the unit vectors in the directions of the parallel and perpendicular electric field components and the propagation direction of light. The backscattering speckle pattern and the backscattering Mueller matrix of an aqueous suspension of polystyrene spheres in a slab geometry are computed using this Electric Field Monte Carlo (EMC) method. An efficient algorithm computing the Mueller matrix in the pure backscattering direction is detailed in the paper.

Angular change in backscattering of light from the human lens with nuclear cataract.

To determine how many meridians are needed to optimise detection of lens nuclear opacities in Scheimpflug photography utilising the Nidek EAS-1000 system. Ten eyes with nuclear cataract from 10 patients (age 53-83 years) were selected in this study. For each eye, the lens was imaged with the Nidek EAS-1000 system at the 0 degree, 30 degrees, 60 degrees, 90 degrees, 120 degrees and 150 degrees meridians, respectively. The intensity of the backscattering of light within a common lens nuclear area on the lens image was measured. In addition an aqueous suspension of polystyrene spheres was employed as a standard to evaluate the possible angular change caused by the camera system. Analysis of variance and the polynomial regression was used for data analysis. The intensity of the backscattered light differs significantly among meridians. The highest intensity is found at the 90 degrees meridian. The intensity decreases towards the nasal and temporal side. There is also an angular variation caused by the camera system. There is a regular change in the backscattered light documented by the Nidek EAS-1000 system. It is sufficient to take one meridian to estimate the lens nuclear cataract. The variation in the backscattered light at different angles is caused partly by the variation of the camera.

Resonance energy transfer in dye molecules adsorbed two dimensionally upon aqueous suspensions of polystyrene spheres

The time-resolved photoluminescence in Rhodamine B dye embedded in aqueous suspensions of polystyrene spheres shows an anomalous fast nonexponential decay in addition to the slow decay that is intrinsic to dye molecules. When the volume fraction of spheres is increased to a critical value, the relative intensity of the fast component increases while the decay rate remains constant at 15 ps. A further increase of the volume fraction lengthens the lifetime of this component to the order of 1 ns. The quantitative behavior of the time dependence of the luminescence is explained well when we consider that dye molecules are adsorbed two dimensionally upon spheres and that the fast decay originates in the resonance energy transfer between the dye molecules.

Static structure and colloidal interactions in partially quenched quasibidimensional colloidal mixtures

Partially quenched quasibidimensional colloidal suspensions are obtained by confining a bidisperse aqueous suspension of polystyrene spheres between two glass plates. The larger particles are fixed between the plates, forming a disordered bidimensional matrix of obstacles, with respect to which the colloidal suspension of the smaller species of particles equilibrates. Digital video microscopy is used to measure the static structure of both the colloidal suspension and the matrix for various fixed particles concentrations. The effective pair potentials between the mobile particles ${u}_{11}(r)$ and between mobile and fixed particles ${u}_{12}(r)$ are obtained by deconvoluting the structural information of the system via the Ornstein-Zernike equation. The measured pair potential ${u}_{11}(r)$ exhibits two attractive components. One of them, at intermediate distances, is present at all fixed particle concentrations, while the second one, of longer range, develops as the number of obstacles increases. The pair potential ${u}_{12}(r)$ also has an attractive component at intermediate distances.

Equivalence between three scattering formulations for ultrasonic wave propagation in particulate mixtures

The aim of this paper is to present a unified approach to the calculation of the complex wavenumber for a randomly distributed ensemble of homogeneous isotropic spheres suspended in a homogeneous isotropic continuum. Three classical formulations of the diffraction problem for a compression wave incident on a single particle are reviewed; the first is for liquid particles in a liquid continuum (Epstein and Carhart), the second for solid or liquid particles in a liquid continuum (Allegra and Hawley), and the third for solid particles in a solid continuum (Ying and Truell). Equivalences between these formulations are demonstrated and it is shown that the Allegra and Hawley formulation can be adapted to provide a basis for calculation in all three regimes. The complex wavenumber that results from an ensemble of such scatterers is treated using the formulations of Foldy (simple forward scattering), Waterman and Truell, and Lloyd and Berry (multiple scattering). The analysis is extended to provide an approximation for the case of a distribution of particle sizes in the mixture. A number of experimental measurements using a broadband spectrometric technique (reported elsewhere) to obtain the attenuation coefficient and phase velocity as functions of frequency are presented for various mixtures of differing contrasts in physical properties between phases in order to provide a comparison with theory. The materials used were aqueous suspensions of polystyrene spheres, silica spheres, iron spheres, pigment (AHR), droplets of 1-bromohexadecane, and a suspension of talc particles in a cured epoxy resin.

Quantitative broadband near-infrared spectroscopy of tissue-simulating phantoms containing erythrocytes

We report the use of steady-state diffuse reflectance spectroscopy (SSDRS) to measure the near-infrared absorption spectrum of liquid phantoms containing human erythrocytes in aqueous suspensions of polystyrene spheres which simulate the scattering properties of tissue. The absorption spectra obtained from these SSDRS measurements of intact red cells under oxygenated and deoxygenated conditions are compared with several published spectra of `stripped' haemoglobin prepared from lysed cells. Two fitting algorithms (nonlinear least squares and singular value decomposition) which exploit the broad spectral range provided by these measurements (170 data points spanning 164 nm in a single acquisition) are used to determine haemoglobin oxygen saturation from SSDR spectra collected over a wide range of measured oxygen partial pressures. The validity of these algorithms is assessed by comparing literature values of (the oxygen tension at which haemoglobin is 50% saturated) and the Hill coefficient to values of these parameters determined from the estimates. The singular value decomposition algorithm can also be used to reconstruct the non-haemoglobin background absorption spectrum without a priori assumptions regarding its constituent chromophores or their concentrations. Using this technique, the absorption spectrum of a small amount of India ink (maximum absorption coefficient added to a phantom containing red cells was reconstructed over a full range of oxygen saturations. The implications of these measurements for detection of weakly absorbing chromophores (such as cytochrome ) in the presence of haemoglobin are discussed.

Angular distribution of diffusely backscattered light

We demonstrate that the angular distribution of light diffusely backscattered from an opaque slab depends not only on sample boundary reflectivity but also, in contrast to previous results for diffusely transmitted light, on the anisotropy of the scattering events. This influence of scattering anisotropy is modeled within diffusion theory by a discontinuity in the photon concentration at the source point that is proportional to the average cosine of the scattering angle. The resulting predictions are compared with random walk simulations and with measurements of transmitted and backscattered intensity versus angle for glass frits and aqueous suspensions of polystyrene spheres held in air or immersed in a water bath. Predicted distributions capture the features of experimental and simulation data to within 1% for the best case of high reflectivity and weak anisotropy and to within 10% for the worst case of low reflectivity and strong anisotropy.

Effective forces between macroions: a Monte Carlo study

The effective forces between two spherical highly charged colloidal macroions are calculated within the primitive model of strongly asymmetric electrolytes using Monte Carlo simulations. For typical parameters corresponding to aqueous suspensions of polystyrene spheres, the forces are found to be repulsive over a broad range of distances between the macroions. Our results are in semi-quantitative agreement with different variants of linear screening theory. A recently developed cumulant expansion, however, fails in predicting the correct sign of the effective forces.

Acoustically driven collection of suspended particles within porous media

A method for collecting fine particles using a porous medium subjected to a standing ultrasonic wave field is described. Proof-of-concept experiments using aqueous suspensions of polystyrene spheres demonstrated the collection of particles up to two orders of magnitude smaller than the size of pores within the porous medium. Two types of porous media were studied; unconsolidated beds formed from glass spheres, and aluminum foam meshes. Removal of particles from the porous media is readily accomplished by deactivating the acoustic field and flushing the particles from the porous medium through continued liquid flow. The effects of processing conditions (suspension flow rate, acoustic power, and feed concentration) on the retention of particles were assessed. Explanations of the mechanism by which the collection occurs are postulated and an assessment of the practicality of the concept for suspension processing is presented.

Static properties of confined colloidal suspensions.

The static structure of aqueous suspensions of polystyrene spheres, confined between two parallel glass plates, is measured by means of digital video microscopy. The concentration of colloidal particles is varied in a wide range within the liquid phase. The effective interparticle pair potential between the colloidal particles in this geometry is obtained from the measured static properties of the system via the two-dimensional version of the Ornstein-Zernike equation. The effective pair potential obtained in this way is found to be of short range with a well defined attractive component, features that are qualitatively reproduced in the whole range of concentrations studied.

Rescaled mean spherical approximation structure factor for an aqueous suspension of polystyrene spheres

Rescaled mean spherical approximation (RMSA) has been used to calculate the structure factor for the aqueous suspension of polystyrene macroions with the interaction potential taken according to Derjaguin and Landau (1941) and Verwey and Overbeek (1948) (DLVO) model. The effects of charge over the macroion and size on the surface potential and therefore, the structure factor have been studied. The breakdown of the DLVO potential with an excess charge over the macroion (⩾800e) has been reported. The oscillation in the first peak height of structure factor versus wave vector curve with size has been correlated with the Debye length.

Yield stresses in magnetic suspensions

Static yield stresses are measured on magnetic suspensions as a function of the applied magnetic field. Two differents systems are used: One is an aqueous suspension of polystyrene spheres containing iron oxide inclusions and the second one is of ordinary polystyrene spheres dispersed in a ferrofluid. The experimental results are analyzed with the help of a model based on the calculation of forces between two particles in an effective medium. This model is discussed and tested against experimental measurements of forces on centimetric iron spheres placed in a magnetic field.

Thermal diffuse scattering from colloidal crystals

Measurement of the Bragg diffraction and thermal diffuse scattering from colloidal crystals consisting of aqueous suspensions of polystyrene spheres with and without strongly absorbing dye molecules show that dynamical diffraction theory is valid even for absorbing colloidal crystals. In addition, we show that thermal diffuse scattering from colloidal crystals may be described by the simple Debye model. We determine the effective penetration depth of the incident electric field for cases where the Bragg condition is satisfied.

Electrokinetic properties of aqueous suspensions of polystyrene spheres in the gas and liquid-like phase

Previous results on the electrophoretic mobility of aqueous suspensions of polystyrene latices at low ionic strengths are controversial. We have reexamined and extended mobility measurements as a function of polyion concentration down to salt concentrations smaller than n S = 10 −6 M, where the polyions are liquid-like ordered due to screened Coulomb forces. A significant increase in mobility is found below n S ∼ 10 −4 M. To discuss mobilities as a function of small ion concentration, for the first time electrophoretic mobility and electric conductivity data are combined to estimate salt content. Static light scattering is used to support the latter results and to characterize the structure of the suspensions. The weak polyion concentration dependence and the increase in mobility are discussed in the light of recently published calculations.

Observation of inhibited spontaneous emission in a periodic dielectric structure.

Inhibition of the radiative decay of dye molecules embedded in an ordered aqueous suspension of polystyrene spheres has been measured using picosecond pump-probe techniques. The inhibition is interpreted in terms of excluded vacuum modes and compared with results based on the Wigner-Weisskopf formalism and a three-dimensional scattering model for the fcc colloidal structures.

Decorrelation of multiple scattering for an arbitrary scattering angle

Multiple scattering currently presents a major limitation for intensity correlation spectroscopy of quasi-elastically scattered light from concentrated colloidal systems. Theoretical treatments have been restricted to low orders of multiple scattering. Experimentally, suppression of multiple scattering by dual-beam cross-correlation setups has been restricted to a 90-deg fixed scattering angle. We propose a variable scattering angle cross-correlation scheme and present the results of a successful experiment on a highly concentrated aqueous suspension of polystyrene spheres.

Theoretical Analysis of the Crystalline Colloidal Array Filter

The major parameters of a crystalline colloidal array optical filter have been investigated theoretically, and the results compared to published experimental data. This filter consists of an aqueous suspension of polystyrene spheres in a lattice which produces Bragg diffraction of incident light. The development of expressions for the filter bandwidth and attenuation utilized both dynamical x-ray diffraction theory and light scattering theory. The theoretical attenuation function indicates that extremely high absorbance values are achievable with relatively thin filters.