Suspensions Of Latex Microspheres Research Articles

Solution of the multiple light scattering problem in turbid suspensions via cross-correlation processing

The original scheme of the cross-correlation technique for assessing the size of colloid particles in suspensions of high turbidity without any dilution is proposed. In the designed experimental scheme the illumination with two in-phase coherent laser beams are used. The scattered radiation signalsare registered by the square-law photodetectors and processed without any special correlators on a computer. It allowed us to simplify the experimental setup. Parameters of the setup were established by calibrating experiments. Cross-correlation functions were obtained for monodisperse suspensions of latex microspheres, and their sizes were assessed.

Concentration nonlinearity of a suspension of transparent microspheres under the action of a gradient force in a periodically modulated laser field

Based on a one-dimensional Smoluchowski equation we have developed the theory of concentration nonlinearity of a suspension of transparent microspheres under the action of a gradient force in an interference laser field. The numerical solution of a system of recurrence equations resulting from the Smoluchowski equation after expansion of the microsphere concentration in the harmonic series has allowed us to determine the dependence of the concentration nonlinearity settling time on the intensity of the incident radiation. In the diffusion limit, we have derived the expression for the optical Kerr coefficient, which is found to be for an aqueous suspension of latex microspheres with a radius of and a concentration of . Diffraction of a probe wave on a light-induced concentration grating is considered as a method for studying a nonlinear concentration response of an artificial highly efficient nonlinear medium for laser radiation of long pulse duration.

Particle Displacement in Aqueous Suspension Arising from Incident Radiant Energy

Colloidal particles in aqueous suspension generally sediment uniformly. By contrast, we found that suspensions of latex microspheres in polystyrene Petri dishes deviated sharply from the expected pattern when various objects were positioned immediately outside those dishes. When small coin-like metal discs were positioned immediately beneath the Petri dish, the microspheres sedimented to a point just above those discs. Other materials, including glass and wood, produced similar results, though less pronounced. After the microspheres had sedimented, shifting the metal to another position beneath the dish caused the microspheres to follow. Various control experiments ruled out trivial explanations. In concordance with earlier results, it appears that the infrared energy generated by the various materials draws microspheres, resulting in the unusual sedimentation patterns. The results have significant implications for the mechanism of sedimentation, particularly for the role of charge in that process.

Polarized Enhanced Backscattering Spectroscopy for Characterization of Biological Tissues at Subdiffusion Length Scales

Since the early 1980's, the enhanced backscattering (EBS) phenomenon has been well-studied in a large variety of non-biological materials. Yet, until recently the use of conventional EBS for the characterization of biological tissue has been fairly limited. In this work we detail the unique ability of EBS to provide spectroscopic, polarimetric, and depth-resolved characterization of biological tissue using a simple backscattering instrument. We first explain the experimental and numerical procedures used to accurately measure and model the full azimuthal EBS peak shape in biological tissue. Next we explore the peak shape and height dependencies for different polarization channels and spatial coherence of illumination. We then illustrate the extraordinary sensitivity of EBS to the shape of the scattering phase function using suspensions of latex microspheres. Finally, we apply EBS to biological tissue samples in order to measure optical properties and observe the spatial length-scales at which backscattering is altered in early colon carcinogenesis.

An AC Electrokinetic Technique for Collection and Concentration of Particles and Cells on Patterned Electrodes

We report an electrohydrodynamic effect arising from the application of alternating electric fields to patterned electrode surfaces. The AC fields were applied to dilute suspensions of latex microspheres enclosed between a patterned silicon wafer and an ITO-coated glass slide in a small chamber. The latex particles became collected in the center of the conductive "corrals" on the silicon wafer acting as bottom electrode. The particle collection efficiency and speed depended only on the frequency and strength of the field and were independent of the material properties of the particles or the electrodes. The leading effect in the particle collection process is AC electrohydrodynamics. We discuss how the electrohydrodynamic flows emerge from the spatially nonuniform field and interpret the experimental results by means of electrostatic and hydrodynamic simulations. The technique allows three-dimensional microfluidic pumping and transport by the use of two-dimensional patterns. We demonstrate on-chip collection of latex particles, yeast cells, and microbes.

Changes in fractal dimension during aggregation

Experiments were performed to evaluate temporal changes in the fractal dimension of aggregates formed during flocculation of an initially monodisperse suspension of latex microspheres. Particle size distributions and aggregate geometrical information at different mixing times were obtained using a non-intrusive optical sampling and digital image analysis technique, under variable conditions of mixing speed, coagulant (alum) dose and particle concentration. Pixel resolution required to determine aggregate size and geometric measures including the fractal dimension is discussed and a quantitative measure of accuracy is developed. The two-dimensional fractal dimension was found to range from 1.94 to 1.48, corresponding to aggregates that are either relatively compact or loosely structured, respectively. Changes in fractal dimension are explained using a conceptual model, which describes changes in fractal dimension associated with aggregate growth and changes in aggregate structure. For aggregation of an initially monodisperse suspension, the fractal dimension was found to decrease over time in the initial stages of floc formation.

Spontaneous density grating formation in suspensions of dielectric nanoparticles

Experimental evidence for nonlinear optical behaviour due to the spontaneous formation of wavelength-scale density modulations or gratings in suspensions of dielectric particles is presented. A collection of dielectric particles pumped by a coherent radiation field may simultaneously form a density grating on the scale of the radiation wavelength and a coherently backscattered radiation field. The particle density grating is generated as a result of a periodic ponderomotive potential formed by the interference of the pump and backscattered fields. The experiment used a water suspension of latex microspheres (radius ≈ 56nm) pumped by a green CW laser (532nm, power ≤ 5W). A theoretical model of collective scattering of light from dielectric particles has been extended to include the effects of viscous and Brownian forces on the particles. This model predicts a small degree of particle bunching from which coherent backscattering of the pump occurs. The results of the theoretical model compare favourably with the experimental evidence. The relation between the results presented here and the phenomenon of Collective Rayleigh Scattering (CRS) is discussed.

Flow generated around particle clusters in a rotating ultrasonic waveguide.

A chamber cavity, which has a square cross section and pressure-release walls, is used to produce a well-defined, 160-kHz standing ultrasonic field. A suspension of latex microspheres in aqueous metrizamide fills the chamber. The chamber rotates about a horizontal axis producing the centripetal force necessary to contain the buoyant spheres in the axial region. At low particle concentrations, clusters of microspheres form at half-wavelength intervals near the axial positions of acoustic pressure amplitude (p0) minima, as expected because of rotational and acoustic radiation forces. At higher concentrations, additional particle distributions are often seen that suggest the presence of flow. When high concentrations of larger particles are used, small clusters also form at axial positions of p0 maxima. Theory for acoustic streaming in a rotating fluid predicts flow speeds that are too small to account for the observed flow. Reasonable agreement with observations is obtained using a theory for flow generated by the buoyant gravitational force acting on the clusters.

New Method for Sampling Groundwater Colloids under Natural Gradient Flow Conditions

This paper describes a new method for the passive sampling of groundwater colloids using a multilayer sampler (MLS). It is based on the use of dialysis cells with large pore size (10 μm) membranes that are in dynamic equilibrium with the mobile colloid and liquid phases in the aquifer. Under laboratory conditions, the dialysis cells reached equilibrium with a suspension of latex microspheres (5 mg/L) after 44−100 h and with a suspension of kaolinite (16−41 NTU; 20−50 mg/L) after 50−180 h. No fractionation was detected in the particle-size distribution between the kaolinite suspensions inside and outside the dialysis cells. Field profiles, obtained under natural gradient flow conditions in a sand and sandstone aquifer, showed large variability (up to 1 order of magnitude) in the colloid content within profiles (e.g., variation of 7 NTU (∼45 mg/L) between cells located at a vertical distance of 40 cm) and between them. The predominant colloidal particles found in the cells were aluminosilicates, CaCO3, sili...

Real time optical coefficients evaluation from time and space resolved reflectance measurements in biological tissues

Quantitative and accurate determination of turbid media optical coefficients was achieved using time and space resolved measurements of diffuse backscattered light. The global analysis of the reflectance map registered on a streak camera was based on the diffusion approximation. This fast and practical method was tested on suspensions of latex microspheres in water, and the results were in good agreement with the Mie theory. Experiments were then performed on biological tissues. The total time required for the determination of the optical coefficients of a sample, essentially limited by the instrument integration time, consisted of a few seconds.

The attenuation of a coherent field by scattering

The intensity of a coherent field attenuated by scattering is described by an ordinary differential equation. This differential equation is derived in the same way as Lambert-Beer law of absorption, but the coherent forward scattering is taken into account. The analytical solution of the differential equation depends on two quantities of single scattering, concentration of scatterers and thickness of the medium. Measurements of coherent light transmitted through suspensions of latex microspheres confirm the resulting equation.

Streaming generated by aggregations in a rotating ultrasonic waveguide

A chamber cavity, which has a square cross section and pressure‐release walls, is used to produce a well‐defined, 160‐kHz standing ultrasonic field. A suspension of latex microspheres in aqueous metrizamide fills the chamber. The chamber rotates about a horizontal axis producing the centripetal force necessary to contain the buoyant spheres in the axial region. At low particle concentrations, aggregations of microspheres form at half‐wavelength intervals near the axial positions of acoustic pressure amplitude (PA) minima, as expected because of rotational and acoustic radiation forces. At higher concentrations, additional particle distributions are often seen that suggest the presence of flow. When high concentrations of larger particles are used, small aggregations also form at axial positions of PA maxima. To account for the flow, theory for acoustic streaming in a rotating fluid was developed and applied to flow near the larger aggregations. Reasonable agreement with observations was obtained when a term was added arising from drag on the aggregations. [Work supported by NIH Grant CA42947.]

Filtration rates of mosquito larvae in suspensions of latex microspheres and yeast cells

AbstractFiltration rates of fourth instars of Aedes aegypti L., Anopheles albimanus Wiedemann, Anopheles quadrimaculatus Say and Culex quinquefasciatus Say (Diptera: Culicidae) were determined by quantifying removal rates of suspended latex microspheres or yeast cells. Average filtration rates were 33–34 μl/larva/h (An. quadrimaculatus), 49–55 (An. albimanus), 490–590 (C. quinquefasciatus) or 590–690 μl/larva/h (Ae. aegypti) for larvae exposed to latex beads suspended in phagostimulant yeast extract solutions. In suspensions of yeast cells, filtration rates of Ae. aegypti and C. quinquefasciatus were not significantly different from filtration rates in latex bead suspensions. Larval density, ranging from 0.3 to 2.4 individuals/ml in tests with Ae. aegypti and C. quinquefasciatus and up to 4.8 larvae/ml in tests with Anopheles, did not influence filtration rates.ZUSAMMENFASSUNGFiltrierraten von Stechmückenlarven in Suspensionen von Hefezellen und LatexpartikelnDie Filtrierraten von Viertlarven der Stechmückenarten Aedes aegypti, Anopheles albimanus, Anopheles quadrimaculatus und Culex quinquefasciatus wurden in Laborversuchen bestimmt. Dabei wurde die Filtrierrate definiert als ein Wasservolumen, welches pro Stunde von einer Larve von den Testpartikeln (Hefezellen oder Latexkugeln mit einem Durchmesser von 2 μm) befreit wurde. Nach Exposition der Larven in Dichten zwischen 0.15 und 2.4 Larven/ml (Ae. aegypti und C. quinquefasciatus), oder zwischen 0.6 und 4.8 Larven/ml (Anopheles) wurde der Partikelgehalt der Suspensionen in einem elektronischen Partikelzähler bestimmt. Die Filtrierraten wurden über die Verringerung der Partikeldichte mit zunehmender Larvendichte entsprechend einer in der Literatur angegebenen Formel berechnet.Suspensionen von Hefezellen wurden von Ae. aegypti Larven mit einer Leistung von 680±220 μl/Larve/h gefiltert. Bei Larven von C. quinquefasciatus wurden Filtrierraten von 600±120 μl/Larve/h gemessen. Die Filtrierraten beider Arten waren unabhängig von der Larvendichte. In Suspensionen von Latexpartikeln (zur Phago‐stimulation der Larven wurden diese Partikel in Lösungen von Hefeextrakt angeboten) wurden die folgenden Filtrierraten festgestellt: An. quadrimaculatus: 33–34, An. albimanus: 49–55, C. quinquefasciatus: 490–590, und Ae. aegypti: 590–690 μl/Larve/h. Die Larvendichte hatte auch hier keinen Einfluss auf die Filtrierrate. Hefezellen und Latexpartikel wurden von Ae. aegypti und C. quinquefasciatus mit statistisch nicht signifikant verschiedenen Filtrierraten aufgenommen. Die Filtrierraten der Anopheles Larven waren um mehr als eine Zehnerpotenz kleiner als die Filtrierraten von Culex und Aedes, und auch untereinander signifikant verschieden. Der Einfluss der Filtrierraktivität von Stechmückenlarven auf das Seston der Brutgewässer wird diskutiert.

Picosecond light scattering measurements of cataract microstructure

A picosecond range-gated light scattering technique is used to measure the microstructure of cataracts in the living eye, with a resolution of the order of the wavelength of light. The cataract is illuminated with ultrashort pulses of light derived from a mode-locked Nd:glass laser. The pulses backscattered by the opacity are collected at several angles simultaneously by an array of fiber-optic light guides whose outputs are sequentially sampled by a multichannel picosecond Kerr shutter. This shutter transmits only that light which is scattered from the desired depth in the eye, while blocking from the detector any light scattered by foreground or background tissue and reducing multiple scattering effects. The size distribution of scatterers in the cataract is deduced from the angular distribution of scattering intensities by application of Mie theory. The results of experiments with suspensions of latex microspheres and with rabbit cataracts in vivo are presented. Cataract microstructure down to 0.5 microm has been measured in a rabbit eye and verified with electron microscopy.