Light Transmission Measurements Research Articles

Effect of organic anion with multiple hydrophobic sites on gelation and phase separation in aqueous methylcellulose solution: Beyond simple salting-in effect

We studied addition effect of sodium tetraphenylborate (NaBPh4), an organic salt with multiple hydrophobic sites, on thermoresponsive behavior of aqueous methylcellulose (MC) solution. By means of simultaneous measurements of viscosity and light transmittance during heating, it was clarified that the concentration of added NaBPh4 determines which of gelation and phase separation occurs first. Aqueous MC solution without NaBPh4 formed gels below the phase separation temperature, and therefore the phase separation remained incomplete. With increasing the concentration of added NaBPh4, the gelation temperature monotonously increased, whereas the phase separation exhibited complicated behavior. The phase separation temperature in gel phase drastically increased by addition of small amount of NaBPh4 in similar fashion to the gelation temperature, although the phase separation in gel phase faded away when the concentration of NaBPh4 exceeded ca. 0.05 M. Alternatively, liquid-liquid phase separation appeared. In contrast to the phase separation in gel phase, the liquid-liquid phase separation occurred essentially below the gelation temperature and proceeded completely to form macroscopic binary domains. Temperature of the liquid-liquid phase separation decreased initially, but it turned upward when the concentration of added NaBPh4 exceeded ca. 0.15 M and increased drastically with the further addition. In other words, the addition effect of NaBPh4 to the liquid-liquid phase separation switched from salting-out to salting-in. The liquid-liquid phase separation in progress was immobilized by application of additional heating above the gelation temperature. These various structure developments depending on the concentration of added NaBPh4 were observed by optical microscopy (OM) and small-angle light scattering (SALS). On the basis of the experimental results, we proposed a mechanism of the addition effects of organic anion with multiple hydrophobic sites on the gelation and the phase separation of aqueous MC.

Preparation, characterization and antibacterial properties of cellulose membrane containing N-halamine

Polyethylenimine (PEI), a type of water-soluble chain polymer containing a great number of primary, secondary and tertiary amine groups in the molecule, was used as an N-halamine precursor. It was grafted onto dialdehyde cellulose membrane (DCM) followed by chlorination to prepare a novel N-halamine antibacterial cellulose membrane (Cl–PEI–DCM). The appropriate conditions affecting the aldehyde content of DCM and the active chlorine (Cl+) content of Cl–PEI–DCM were systematically studied at oxidation, grafting and chlorination stages, respectively. The structure and properties of the samples were investigated by SEM, ATR–FTIR, XRD, XPS, light transmittance measurements and tensile tests. The stability and rechargeability of the Cl–PEI–DCM were evaluated, and its antibacterial activity was tested against S. aureus and E. coli. Results showed that the Cl+ content of the chlorinated samples was strongly dependent on the aldehyde content of DCM and PEI concentration. Under the optimum testing condition, the Cl+ content of the Cl–PEI–DCM reached 1.30 wt%. The hydroxyl groups on the cellulose membrane were changed to aldehyde groups and the successful introduction of PEI and N–Cl bond were confirmed. The 1.30 wt% Cl–PEI–DCM exhibited high antibacterial activity against both S. aureus and E. coli, which can completely inactivate two bacterial pathogens within 5 min even after 15 days of storage. Moreover, the Cl–PEI–DCM membrane displayed good stability, rechargeability, transparency and high mechanical strength. These results demonstrated that the Cl–PEI–DCM can be considered as a visualized wound dressing material or antibacterial shoe insole.

Device for Simultaneous Measurements of Viscosity and Light Transmittance with Example of Application

Device for Simultaneous Measurements of Viscosity and Light Transmittance with Example of Application

Growth, characterization, and analysis of the nanostructures of ZnO:B thin films grown on ITO glass substrates by a LPCVD: a study on the effects of boron doping

In this work, boron-doped ZnO (ZnO:B) thin films grown by low-pressure chemical vapor deposition (LPCVD) on ITO glass substrates with film thicknesses of 1, 2, and 3 µm are analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic-force microscopy (AFM), cathodoluminescence (CL), and light transmittance measurements. The effects of boron doping on the grain crystallization, grain sizes, film strains, surface texture, and light transmittance of ZnO:B thin films are studied from the analysis of these measurements. XRD results show that ZnO:B and undoped ZnO thin films both have the largest percentage of the (110) crystalline grains which form ridge-like structures at the surfaces of the thin films. These ridge-like structures of (110) grains provide sharper surface textures than the polygon structures of (100) grains. Hence, ZnO:B and undoped ZnO thin films grown on ITO glass substrates both have good-quality surface textures which can provide light-trapping functions for solar-cell applications. In addition, the surfaces of ZnO:B thin films are less sharp and rough than those of undoped ZnO thin films due to their smaller grain sizes. Boron-doping decreases the ZnO grain sizes and reduces the surface textures of ZnO thin films grown on ITO glass substrates. Since ZnO:B thin films have smaller average grain sizes than undoped ZnO thin films, ZnO:B thin films have larger quantum-size effects and hence larger band-gap energies and shorter CL emission wavelengths. Furthermore, thicker films have more ZnO(110) grains with ridge-like structures and have better surface textures which can provide better light trapping than thinner ones. However, thicker films are shown to have poorer optical transparency. The results of this work provide useful information for growth of textured well-faceted ZnO:B films on ITO glass substrates that can potentially be utilized for various applications, such as solar cells.

Synthesis of High-Refractive-Index Epoxy-Modified Vinyl Methyl Phenyl Silicone Resins for Encapsulation of LEDs

A series of high-refractive-index epoxy-modified vinyl methyl phenyl silicone resins with different content of vinyl groups have been synthesized by a combination of hydrolytic and nonhydrolytic sol–gel processes to improve their adhesion. The products were characterized by Fourier-transform infrared spectroscopy and 1H nuclear magnetic resonance spectroscopy. The results showed that epoxy and vinyl groups were successfully introduced into the products while the content of hydroxyl groups was low. Then, epoxy-modified silicone materials were manufactured by addition reaction between the epoxy-modified vinyl methyl phenyl silicone resins and hydrogen-containing phenyl silicone resin. The epoxy-modified silicone materials were characterized by thermal aging (250°C for 2 h) experiments, light transmittance measurements, thermogravimetric analysis, red dye penetration tests, reflow soldering (270°C for 10 cycles), and lumen depreciation (50 mA for 165 h) tests. The results showed that the epoxy-modified silicone materials exhibited high transmittance (> 90%) and good thermal stability. The epoxy-modified silicone materials showed better resistance to high temperature and ultraviolet (UV) radiation than commercial epoxy resin (6103). The introduced epoxy groups greatly improved the adhesion of the silicone resin. The epoxy-modified silicone material prepared using epoxy-modified vinyl methyl phenyl silicone resin with higher content of vinyl groups showed better mechanical performance (50 Shore D). The epoxy-modified silicone material EVMPS-3′ is expected to be applicable in packaging of high-power light-emitting diodes (LEDs).

Phenomenological modelling of light transmission through nanowires arrays

A phenomenological model has been developed and discussed. The model is able to describe the experimentally measured light transmission of nanowires arrays. Two representative samples were studied in this report. A well aligned nanowires sample was first modelled by an effective layer model with surface roughness added. Then a slightly tilted nanowires sample with round top was phenomenologically modelled by a two layers model with a first top layer of an effective medium which includes a volume scattering contribution (Mie theory). Roughness and waviness are added to account for the nanowires small and large scale fluctuation in height. This phenomenological description was proven to be feasible by fitting experimental data. As a conclusion, light transmitted through randomly distributed nanowires can be explained by the combination of volume and surface scatterings using respectively Mie theory and rough Fresnel coefficients at the interfaces.

Evaluation of measurements of light transmission for the determination of black carbon on filters from different station types

We compared two approaches (denoted as Virkkula” and “Drinovec” equations) for the determination of black carbon via attenuation measurements with a transmissometer OT-21 (Magee Scientific) on quartz fiber filters from different station types (urban-traffic, rural-background and remote) within the Austrian air monitoring network. First, we evaluated the entire data set to compare the performance of the models in the determination of the loading parameter k and the mass attenuation cross section σ. Then we investigate k and σ for different station types. The data set comprises 299 filters, all representative for 24 h samples collected with High-Volume-Samplers. Mass concentrations ranged from approximately 5-100 μg m−3 and covered all seasons.Both approaches give statistically indistinguishable results of k with values of 1.00*10−3 ( ±2*10−4) and 0.97*10−3 ( ±1*10−4), respectively, if the entire data set is evaluated. The “Drinovec” approach gives slightly lower values for σ than “Virkkula” with values of 32.1 m2 g−1 ( ±1.1 m2 g−1) and 35.3 m2 g−1 ( ±1.9 m2 g−1). If the data set is restricted to ATN<300 only, no statistically relevant changes are obtained for k and σ using the “Virkkula” equation. Applying the “Drinovec” equation both parameters, k and σ increase when the data set is restricted.The investigation of the different station types yields values of k statistically not different from zero, whereas σ values are different for the respective station types. Highest values of σ = 33.7 m2 g−1 ( ±1.4 m2 g−1) were observed for the remote, followed by σ = 31.8 m2 g−1 ( ±0.5 m2 g−1) for the rural-background station and lowest values of σ = 24.4 m2 g−1 ( ±0.5 m2 g−1) for the urban-traffic stations, possibly reflecting the respective aerosol type.Based on the results for k and σ using either overall values or station type specific values we performed an extensive validation, discussing the application of the transmissometer measurements for a quick, easy and non-destructive quantification of eBC on either fresh sampled or archived filters of different station types. Applying the method for stations not classified as urban, rural or remote a deviation of eBC and EC values up to 23% can be expected. If the station type is known and the respective k and σ values are used this deviation goes down to 10% or lower. This would allow users to obtain long-term time-series going back to times when no on-line instruments were existing, and to obtain these data sets for sites where no on-line instruments are installed even now.

Preparation of polyacrylic acid-grafted-acryloyl/hemicellulose (PAA-g-AH) hybrid films with high oxygen barrier performance

Developing high-performance oxygen barrier films using biomass-based materials is crucial for the development of green and sustainable society. Herein, we develop a strategy to synthesize polyacrylic acid-grafted-acryloyl/hemicellulose composites (PAA-g-AH) as film materials with excellent oxygen barrier property. The as-synthesized films were extensively characterized by Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), gel permeation chromatography (GPC), 1H nuclear magnetic resonance (1H NMR), mechanical test, light transmittance and oxygen transmission rate (OTR) measurement. All these testing results showed that the optimum film exhibit great oxygen barrier property with a low OTR value of 0.25 ± 0.01 cm3* μm/(m2*d*kPa), which was much lower than that of pure PAA film. Moreover, the hybrid films possess well mechanical strength, light transmittance and recycle usage properties. Experimental results indicated that the as-synthesized films have great potential applications in several fields as packaging materials, such as drug, food and electronic products.

Secondary Particle Formation during the Nonaqueous Synthesis of Metal Oxide Nanocrystals.

This study aims to elucidate the aggregation and agglomeration behavior of TiO2 and ZrO2 nanoparticles during the nonaqueous synthesis. We found that zirconia nanoparticles immediately form spherical-like aggregates after nucleation with a homogeneous size of 200 nm, which can be related to the metastable state of the nuclei and the reduction of surface free energy. These aggregates further agglomerate, following a diffusion-limited colloid agglomeration mechanism that is additionally supported by the high fractal dimension of the resulting agglomerates. In contrast, TiO2 nanoparticles randomly orient and follow a reaction-limited colloid agglomeration mechanism that leads to a dense network of particles throughout the entire reaction volume. We performed in situ laser light transmission measurements and showed that particle formation starts earlier than previously reported. A complex population balance equation model was developed that is able to simulate particle aggregation as well as agglomeration, which eventually allowed us to distinguish between both phenomena. Hence, we were able to investigate the respective agglomeration kinetics with great agreement to our experimental data.

Synthesis of Thermosensitive Poly(N-vinylamide) Derivatives Bearing Oligo Ethylene Glycol Chain for Kinetic Hydrate Inhibitor

Thermosensitive poly(N-vinylamide) derivatives bearing an oligo ethylene glycol (OEG) chain at the N-position were designed for development of a kinetic hydrate inhibitor (KHI). Novel N-vinylamide monomers bearing an OEG chain at the N-position were synthesized and copolymerized with N-vinylformamide (NVF) or methyl N-vinylacetamide (MNVA) by free radical polymerization. Then, thermosensitive behaviors, such as lower critical solution temperature (LCST), of the synthesized poly(N-vinylamide) derivatives were investigated by light transmittance and DSC measurement. The LCST values were observed over a wide temperature range from 45 to 90 °C due to the ethoxy OEG chain at the N-position. Their phase transition properties were investigated under dilute condition by light scattering measurement. Furthermore, KHI values of the synthesized polymers were evaluated by the THF hydrate crystal growth method. On the basis of these results, it is expected that the polymers of synthesized poly(N-vinylamide) derivative...

Chemical Mechanical Polishing and Direct Bonding of YAG

Current limitations in both single crystal and polycrystalline (ceramic) solid state laser technologies for high power applications stem from thermal effects that cause degradation in both lasing efficiency and beam quality. YAG and Y2O3 have favorable material properties for producing these high power lasers. We demonstrate chemical mechanical polishing (CMP) processes for YAG and Y2O3 resulting in smooth surfaces, (< 1 nm RMS roughness), defect free, and subsequently suitable for direct bonding. The CMP process has been used in conjunction with surface activation to form single crystal YAG-YAG and polycrystalline Y2O3-Y2O3 bonded elements showing a proof of concept for the fabrication of composite laser elements. YAG single crystals were successfully polished with a 70-nm colloidal silica solution containing NaOH (pH 9.9). X-ray photoelectron spectroscopy measurements provided insights to the chemical impact of the NaOH. After NaOH exposure, YAG showed complete removal of Al from tetrahedral sites and a change in the Y3/2 and Y5/2 peak area ratios, indicating that the surface of YAG was sufficiently modified to allow the silica particles to abrade the byproduct surface layer. The final surface roughness after polishing was measured at 0.1 nm RMS roughness with no scratches deeper than 0.3 nm. YAG single crystals were also polished with a 70-nm Al2O3 slurry containing NaOCl (pH 11.4), however AFM measurements showed that the surface produced, 0.5 nm RMS roughness with no scratches deeper than 6.0 nm, was not as good as polishing with the colloidal silica slurry which was attributed to the effect of the chemical action of NaOH with the YAG surface. Polishing polycrystalline Y2O3 required a slurry with less aggressive chemical reactions, so the NaOCl/Al2O3 slurry was used to successfully polish the substrates to RMS roughness values of 0.5 nm and scratches no deeper than 1.0 nm. Triple axis diffraction rocking curves and double crystal x-ray diffraction imaging were used to demonstrate that the CMP process also removed subsurface damage that was present in the as-supplied material. The triple axis technique was also successfully employed for the first time to demonstrate that the polycrystalline Y2O3 subsurface damage was also significant reduced after the CMP process. After CMP and surface treatment, YAG-YAG and Y2O3-Y2O3 were bonded together and annealed at 1425 °C with no applied pressure. Cross section and plan view high resolution transmission electron images of the YAG-YAG bonded interface revealed a defect free bonding interface. Light transmission measurements showed that 90% of the interface area was bonded at room temperature contact and was further strengthened with annealing at 1425 °C for 72 hours. These conditions possess a much lower thermal budget than those previously shown to be required to bond YAG without a CMP step. The CMP and surface treatment processes that have been developed are applicable towards the creation of solid state laser composite elements and enable further progress and development for high power laser applications.

Concentration fluctuations in the early stages of LLPS and partial dissolution of melt-memory in broadly distributed ethylene copolymers

The mechanism of liquid-liquid phase separation (LLPS) is investigated in a random ethylene 1-hexene copolymer (E/H) with a broad comonomer content distribution. SANS measurements were first carried out using the original resin with deuterium added via direct isotope exchange to preserve the original branching distribution. Upon lowering the melt temperature, the upshift of SANS profile observed only below a critical scattering vector is consistent with the signature of nucleation and growth during LLPS in the metastable region. Differences in branching distribution between the original broad resin and a broad multi-components blend of narrow E/H copolymers studied previously, result in slower LLPS dynamics in the former that has a more continuous change of comonomer content between molecules. Time-resolved light scattering and light transmittance measurements in the metastable region of the broad resin, reveal the nucleation and growth stages of LLPS. Dissolution of crystalline melt-memory, associated with an inversion of the crystallization rate with decreasing the temperature of the initial melt, is found to occur mainly within the LLPS induction period and is associated with concentration fluctuations prior to the LLPS nucleation step. The typical sea-island morphology associated with LLPS via nucleation and growth is observed under bright-field optical microscopy after one-day annealing in the metastable region.

Optical measurement of local permeability of flax fiber fabrics before liquid composite molding

Tracking the variability of natural fiber-based fabrics properties, such as local areal weight, fiber volume fraction, and therefore permeability, is crucial to optimize the parts processing of the bio-composites. This paper aims at developing a cost-effective and efficient optical method in order to predict the permeability of flax fabrics used in liquid composite molding processes. This method using an LCD monitor as light source and a reflex camera as a measurement device is based on light transmission measurement through fabric thickness. The raw data given by the camera are gray scale maps, transformed into areal weight maps. FEM software based on levelset method is finally used to highlight the influence of the local variability of the fiber volume fraction, and of the related fabrics porosity and permeability on the mold filling time. The proposed method can be directly implemented on the manufacturing line of the composites. It can be used to optimize, part-to-part, the resin consumption by predicting the resin flow through perform. Interestingly, this novel optical method is auto-calibrated and does not depend on picture resolution.

The effect of particle size of body components on the processing parameters of semi transparent porcelain

The effect of the particle size of quartz and K- feldspar on the sintering behaviour and technical properties of semi transparent porcelain were investigated. This study was implemented by “the Factorial Experimental Design Method” to determine the effect of factors on semi vitreous porcelain production. The sintering behaviour of the samples were examined by using an optical dilatometer. The phases and their respective amounts in the microstructure were determined by way of Rietvelt X- ray diffraction (XRD) technique. Furthermore, the values such as shrinkage, water absorption, bulk density, porosity, thermal expansion, light transmission measurements were taken on the samples and the microstructures were studied by SEM. It was observed that as the particle size of quartz and K- feldspar decreased; the viscosity of the liquid phase fell down, the relative amount of secondary mullite crystals increased and the large pores were removed among the particles and the densification rate increased. Data were analysed by Minitab 13.20 software, and assessed in relation to the amount of glassy phase and its viscosity. The effect of K- feldspar particle size on the densification of porcelain is less than the change of heat treatment and quartz particle size.

Sandstorm erosion testing of anti-reflective glass coatings for solar energy applications

Components for concentrating solar power applications can suffer from optical performance loss due to their permanent exposure to the environment. There is still lack of experience regarding the destructive effects of sand- and duststorms on glass envelope materials for receiver tubes of parabolic-trough collectors for concentrating solar power (CSP) plants. So far no accelerated aging guideline is formulated yet to account for the performance loss of optical components due to particle erosion to a realistic extent. Within this study 4 different anti-reflective (AR) coatings deposited on borosilicate glass are subjected to an artificial sandstorm test and their resistance towards erosion is evaluated. An uncoated borosilicate glass is also tested as reference. Noticeable differences were obtained depending on the type of coating. Microscope analysis and light transmission measurements in the spectrophotometer were undertaken and it could be concluded that the selection of an AR coating should not only be based on the initial optical performance but also in accordance with meteorological data, especially when erosive sandstorms are expected for the chosen site of the CSP plant.

Optical properties of healthy and rotten onion flesh from 700 to 1000 nm

To facilitate the design of an optical system for detecting internal rot in onions, the light propagation properties in healthy and rotten onions were studied in the 700 to 1000 nm range. The absorption and reduced scattering coefficients of internal onion tissues were measured using a combination of measurements on excised onion slices, using the Inverse-Adding Doubling method (IAD), and light transmittance measurements on extracted onion juice. Standard NIRS transmittance measurements were made on intact onions, revealing a transmission window in the range of 670 to 950 nm. From 700 to 750 nm, more severely rotten onions had lower transmittance, whereas from 850 to 900 nm onions with larger rotten tissue had higher transmittance. A multi-laser light transmittance system was implemented to make accurate measurements for different source-detector positions around intact onions, enabling calculations of attenuation coefficients at 636, 700, 728 and 804 nm. The results showed the onion juice measurements yielded much more accurate absorption coefficients than the IAD method on the onion slices. The attenuation coefficients calculated using accurate optical properties were generally matched with the light transmission characteristics, and were in good agreement (6% on average) with the measured attenuation coefficients for both healthy and rotten onions at 700, 728 and 804 nm.

Oxygen surface exchange kinetics measurement by simultaneous optical transmission relaxation and impedance spectroscopy: Sr(Ti,Fe)O3-x thin film case study

We compare approaches to measure oxygen surface exchange kinetics, by simultaneous optical transmission relaxation (OTR) and AC-impedance spectroscopy (AC-IS), on the same mixed conducting SrTi0.65Fe0.35O3-x film. Surface exchange coefficients were evaluated as a function of oxygen activity in the film, controlled by gas partial pressure and/or DC bias applied across the ionically conducting yttria-stabilized zirconia substrate. Changes in measured light transmission through the film over time (relaxations) resulted from optical absorption changes in the film corresponding to changes in its oxygen and oxidized Fe (~Fe4+) concentrations; such relaxation profiles were successfully described by the equation for surface exchange-limited kinetics appropriate for the film geometry. The kchem values obtained by OTR were significantly lower than the AC-IS derived kchem values and kq values multiplied by the thermodynamic factor (bulk or thin film), suggesting a possible enhancement in k by the metal current collectors (Pt, Au). Long-term degradation in kchem and kq values obtained by AC-IS was also attributed to deterioration of the porous Pt current collector, while no significant degradation was observed in the optically derived kchem values. The results suggest that, while the current collector might influence measurements by AC-IS, the OTR method offers a continuous, in situ, and contact-free method to measure oxygen exchange kinetics at the native surfaces of thin films.

Laser-driven ion acceleration via target normal sheath acceleration in the relativistic transparency regime

We present an experimental study investigating laser-driven proton acceleration via target normal sheath acceleration (TNSA) over a target thickness range spanning the typical TNSA-dominant regime (∼1 μm) down to below the onset of relativistic laser-transparency (<40 nm). This is done with a single target material in the form of freely adjustable films of liquid crystals along with high contrast (via plasma mirror) laser interaction (∼2.65 J, 30 fs, W cm−2). Thickness dependent maximum proton energies scale well with TNSA models down to the thinnest targets, while those under ∼40 nm indicate the influence of relativistic transparency on TNSA, observed via differences in light transmission, maximum proton energy, and proton beam spatial profile. Oblique laser incidence (45°) allowed the fielding of numerous diagnostics to determine the interaction quality and details: ion energy and spatial distribution was measured along the laser axis and both front and rear target normal directions; these along with reflected and transmitted light measurements on-shot verify TNSA as dominant during high contrast interaction, even for ultra-thin targets. Additionally, 3D particle-in-cell simulations qualitatively support the experimental observations of target-normal-directed proton acceleration from ultra-thin films.

Electrochemical Deposition of Transparent p‐Type Semiconductor NiO

SUMMARYNiO is a p‐type semiconductor having a large band gap (&gt;3 eV). In this study, thin films containing Ni‐O were deposited by the cathodic electrochemical deposition method, and characteristics change by heat treatment was investigated. An aqueous solution containing Ni(NO3)2 was used as a deposition solution. X‐ray photoelectron spectroscopy showed that the sample before annealing was predominantly Ni(OH)2. After heat treatment in air at temperatures higher than 300 °C, the NiO phase was observed by X‐ray diffraction, and the p‐type response was confirmed by the photoelectrochemical measurement. The band gap obtained from the light transmittance measurement was around 3.5 eV.

Direct monitoring of erythrocytes aggregation under the effect of the low-intensity magnetic field by measuring light transmission at wavelength 800 nm

Abstract Interacting electromagnetic field with the living organisms and cells became of the great interest in the last decade. Erythrocytes are the most common types of the blood cells and have unique rheological, electrical, and magnetic properties. Aggregation is one of the important characteristics of the erythrocytes which has a great impact in some clinical cases. The present study introduces a simple method to monitor the effect of static magnetic field on erythrocytes aggregation using light transmission. Features were extracted from the time course curve of the light transmission through the whole blood under different intensities of the magnetic field. The findings of this research showed that static magnetic field could influence the size and the rate of erythrocytes aggregation. The strong correlations confirmed these results between the static magnetic field intensity and both the time of aggregation and sedimentation of erythrocytes. From this study, it can be concluded that static magnetic field can be used to modify the mechanisms of erythrocytes aggregation.