Refractive Index Matching Technique Research Articles

Utilizing APTV to investigate the dynamics of polydisperse suspension flows beyond the dilute regime

We present a methodology that allows to measure the dynamics of polydisperse suspension flows by means of Astigmatism Particle Tracking Velocimetry (APTV). Measurements are successfully performed with tridisperse suspensions flows in a square duct of up to varPhi =9.1% particle volume fraction. Using a refractive index matching technique, a small amount of the particles (varPhi =0.08%) is labeled with fluorescent dye to be visible to the camera during the particle tracking procedure. Calibration measurements are performed for ten different particles diameters d_p ranging from d_p= {15}upmu mathrm{m} to d_p= {260},upmu mathrm{m}. It is shown that Euclidean calibration curves of different d_p overlap outside the focal planes, which induces ambiguities in a polydisperse APTV measurement. In the present approach, this ambiguity can be overcome utilizing the light intensity of a particle image which increases sharply with d_p. In this way, extended Euclidean calibration curves can be generated for each particle group which are spatially separated through the light intensity which serves as an additional calibration parameter (Brockmann et al. in Exp Fluids 61(2):67, 2020). The extended Euclidean calibration allows to simultaneously differentiate particles of different sizes and determine their 3D location. This facilitates to investigate the migration behavior of mono- and tridisperse suspension flows which we demonstrate here for square duct flows with cross-sectional areas of 0.6times 0.6,mathrm{mm}^2 and 0.4times 0.4,mathrm{mm}^2 at bulk Reynolds numbers of mathrm{Re}_b approx 20 and mathrm{Re}_b approx 40 for particle volume fractions of varPhi =0.08% and varPhi =9.1%. At varPhi =0.08% and mathrm{Re}_b=20, we observe particles to arrange themselves in a ring-like formation inside the capillary, henceforth referred to as Pseudo Segré Silberberg Annulus (PSSA), with no significant differences between mono- and polydisperse suspension particle distributions. At varPhi =9.1%, particles in monodisperse suspensions scatter around the PSSA. This scattering decreases when d_p increases or Re_b increases from 20 to 40. Striking differences are observed in polydisperse suspensions. Large particles ({60},upmu mathrm{m}) scatter significantly less around the PSSA in the polydisperse case compared to a monodisperse suspension of the same overall volume fraction. In contrast, small and intermediate particles ( {30},upmu mathrm{m}, {40},upmu mathrm{m}) are repelled by larger particles resulting in regions of high concentration close to the channel walls which can be only observed in the polydisperse case.Graphical abstract

Combined three-dimensional flow field measurements and motion tracking of freely moving spheres in a turbulent boundary layer

A combination of time-resolved tomographic particle image velocimetry, refractive index matching technique and machine vision algorithms was used to measure the translational and rotational motion of freely moving, nearly neutrally buoyant spheres in a fully developed turbulent boundary layer (TBL). Located in the buffer and logarithmic layers, the hydrogel spheres ( $\sim$ 70 inner wall units in diameter) were refractive index matched with the water and tagged by ‘spokes’. Besides translational motion, the spheres exhibited significant rotation. The spheres were surrounded by typical coherent structures observed in TBLs, among them hairpin packets and transverse and longitudinal vortices that induced ejections and sweeps. While the majority of instantaneous sphere Reynolds numbers did not exceed 100, and vortex shedding was not observed, the results showed that the spheres may affect the evolution of hairpin packets in TBLs due to their finite size. The instantaneous rotation-, wall- and shear-induced lift forces, as well as the drag forces, acting on the spheres were estimated using available correlations for the lift and drag coefficients. Results hinted at negative shear-induced lift due to flow separation at a smaller critical Reynolds number than incorporated in the correlations that do not include the effect of ambient turbulence. The results indicated further that the drag force aided by the rotation-induced lift force was instrumental in keeping one of the spheres aloft. For the wall-ward moving spheres, lift forces opposed sphere motion. As a result, the spheres approached the wall with velocities lower than their quiescent settling velocity.

A conveyor belt experimental setup to study the internal dynamics of granular avalanches

This paper shows how a conveyor belt setup can be used to study the dynamics of stationary granular flows. To visualise the flow within the granular bulk and, in particular, determine its composition and the velocity field, we used the refractive index matching (RIM) technique combined with particle tracking velocimetry and coarse-graining algorithms. Implementing RIM posed varied technical, design and construction difficulties. To test the experimental setup and go beyond a mere proof of concept, we carried out granular flow experiments involving monodisperse and bidisperse borosilicate glass beads. These flows resulted in stationary avalanches with distinct regions whose structures were classified as: (i) a convective-bulged front, (ii) a compact-layered tail and, between them, (iii) a breaking size-segregation wave structure. We found that the bulk strain rate, represented by its tensor invariants, varied significantly between the identified flow structures, and their values supported the observed avalanche characteristics. The flow velocity fields’ interpolated profiles adjusted well to a Bagnold-like profile, although a considerable basal velocity slip was measured. We calculated a segregation flux using recent developments in particle-size segregation theory. Along with vertical velocity changes and high expansion rates, segregation fluxes were markedly higher at the avalanche’s leading edge, suggesting a connection between flow rheology and grain segregation. The experimental conveyor belt’s results showed the potential for further theoretical developments in rheology and segregation-coupled models.Graphic

Highly transparent antioxidant high internal phase emulsion gels stabilized solely by C-phycocyanin: Facilitated formation through subunit dissociation and refractive index matching

In the work, we report a kind of novel highly transparent oil-in-water (O/W) HIPE gels stabilized solely by c -phycocyanin (CPC; an important biliprotein pigment), with a great potential to act as outstanding containers for heat or UV labile lipophilic bioactives. The CPC was solubilized in 6 M urea solution containing 40 wt% glycerol, in order to achieve the improved emulsification performance of the protein, as well as the transparency of the resultant HIPE gels through the refractive index (RI) matching between the aqueous and dispersed oil (dodacane) phases. Using 6 M urea containing 40 wt% glycerol as the aqueous phase, the transparent HIPE gels could be formed at the CPC concentration ( c ) as low as 0.1 wt%, and oil fractions in the range 0.6–0.86. In the protein-poor regime, e.g., c < 0.5 wt%, increasing the c led to a progressive decrease in emulsion size, as well as strengthening of gel network, while in the protein-rich regime ( c = 0.5–1.0 wt%), the emulsion droplet and gel stiffness were independent of the c . The as-fabricated transparent HIPEs (gels) exhibited an outstanding oxidative protection to encapsulated β -carotene against degradation upon extensive heating or UV irradiation. • A kind of novel transparent antioxidant HIPE gels stabilized by c -phycocyanin was reported. • The transparency of HIPE gels was achieved by a RI matching technique with urea and glycerol. • The emulsification performance of c -phycocyanin was remarkably improved by the presence of 6 M urea. • The chemical stability and antioxidant capacities of c-phycocyanin were unaffected by the usage of urea and glycerol. • The as-fabricated transparent HIPE gels exhibit a great potential in cosmetic and other formulations.

On the dynamics of unconfined and confined vortex rings in dense suspensions

Abstract

Velocity Field and Scalar Field Measurements of Turbulent Buoyant Round Jets in a Two-Layer Stratified Environment

Results reported in the literature have shown that the turbulence models currently implemented in computational fluid dynamics (CFD) commercial codes (e.g., ANSYS-CFX, STAR-CCM+, and FLUENT) have a tendency to overestimate thermal stratification and underestimate turbulent mixing when buoyancy effects become dominant with respect to momentum effects. Also, standard large eddy simulation models cannot fully capture the behavior of jets interacting with stratified environments because the assumption of turbulence isotropy of the smaller scales breaks down. Because of light diffraction and image distortion, it is challenging to apply nonintrusive optical flow measurements, like particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF), to get experimental data for CFD validations when there are density variances involved in the flow. However, a refractive index matching (RIM) technique that has been recently developed in our Experimental and Computational Multiphase Flow Laboratory allows us to perform high-resolution measurements of velocity fields and scalar fields for turbulent buoyant jet flow in the presence of density differences as high as 8.6%. To form a fully turbulent round free jet, an experimental facility was designed with a jet nozzle diameter of 2 mm, located at the bottom of a cubic tank with 30-cm side length. The jet flow is established by a servo-engine-driven piston to eliminate possible fluctuations introduced by the motor. A high-fidelity synchronized PIV/PLIF system was utilized in conjunction with RIM to measure the velocity and concentration fields in the self-similar regions of a jet flow with a density difference of 3.16% for aqueous solutions. With Reynolds numbers of 4000 and 10 000, the jet impinging with a two-layer stably stratified environment is compared to the positively buoyant jet with lighter fluid injected into denser surroundings. Detailed quantifications of the measurement uncertainties are also carried out. The experimental results are presented in terms of turbulent statistics and the analysis of jet penetration depths.

Generalized Shields criterion for weakly cohesive granular materials

The erosion of natural sediments by a superficial fluid flow is a generic situation in many usual geological or industrial contexts. However, there is still a lack of fundamental knowledge about erosional processes, especially concerning the role of internal cohesion and adhesive stresses on issues such as the critical flow conditions for the erosion onset or the kinetics of soil mass loss. This contribution investigates the influence of cohesion on the surface erosion by an impinging jet flow based on laboratory tests with artificially bonded granular materials. The model samples are made of spherical glass beads bonded either by solid bridges made of resin or by liquid bridges made of a highly viscous oil. To quantify the intergranular cohesion, the capillary forces of the liquid bridges are here estimated by measuring their main geometrical parameters with image-processing techniques and using well-known analytical expressions. For the solid bonds, the adhesive strength of the materials is estimated by direct measurement of the yield tensile forces and stresses at the particle and sample scales, respectively, with specific traction tests developed for this purpose. The proper erosion tests are then carried out in an optically adapted device that permits a direct visualization of the scouring process at the jet apex by means of the refractive index matching technique. On this basis, the article examines qualitatively the kinetics of the scour crater excavation for both scenarios, namely, for an intergranular cohesion induced by either liquid or solid bonds. From a quantitative perspective, the critical condition for the erosion onset is discussed specifically for the case of the solid bond cohesion. In this respect, we propose here a generalized form of the Shields criterion based on a common definition of a cohesion number from yield tensile values, derived at both micro- and macroscales. The article finally shows that the proposed form manages to reconcile the experimental data for cohesive and cohesionless materials, the latter in the form of the so-called Shields curve along with some previous results of the authors which have been appropriately revisited.

Experimental study on the boiling criterion of the fuel film formed from spray/wall impingement

The vaporization of the fuel film after spray/wall impingement dramatically affects the combustion and emission behaviors in engines. In this study, the formation and vaporization characteristics of the fuel film were investigated by injecting diesel fuel from a single-hole injector onto a high-temperature, rough wall. The film thickness was measured using the refractive index matching (RIM) technique. The influence of fuel injection duration, injection pressure, and wall temperature on the film vaporization process was discussed based on the measurements. The results indicated that when the wall temperature reached a critical value, the nucleate boiling of the fuel film appeared. The boiling intensity dramatically increased and the boiling duration decreased with the increasing wall temperature. It was also found that the boiling phenomenon of the wall film was closely associated with the thickness of the residual fuel film on the wall and the film temperature, while it was nearly independent on the fuel injection conditions. Based on the theoretical analysis, it was revealed that there was a critical bubble radius for the bubble nuclei growing in a certain thickness of the fuel film within a conical pit.

Effect of engine conditions and injection timing on piston-top fuel films for stratified direct-injection spark-ignition operation using E30

Mid-level ethanol/gasoline blends can provide knock resistance benefits for stoichiometric spark-ignition engine operation, but previous studies have identified challenges associated with spray impingement and wall wetting, leading to excessive particulate matter emissions. At the same time, stratified-charge spark-ignition operation can provide increased thermal efficiency, but care has to be exercised to avoid excessive in-cylinder soot formation. In support of the use of mid-level ethanol/gasoline blends in advanced spark-ignition engines, this study presents spray and fuel-film measurements in a direct-injection spark-ignition engine operated with a 30 vol.%/70 vol.% ethanol/gasoline blend (E30). Crank-angle resolved fuel-film measurements at the piston surface are conducted using two different implementations of the refractive index matching technique. A small-angle refractive index matching implementation allows quantification of the wetted area, while a large-angle refractive index matching implementation enables semi-quantitative measurements of fuel-film thickness and volume, in addition to fuel-film area. The fuel-film measurements show that both the amount of fuel deposited on the piston and the shape of the fuel-film patterns are strongly influenced by the injection timing, duration, intake pressure, and coolant temperature. For combinations of high in-cylinder gas density and long injection duration, merging of the individual spray plumes, commonly referred to as spray collapse, can cause a dramatic change to the shape and thickness of the wall fuel films. Overall, the study provides guidance to engine designers aiming at minimizing wall wetting through tailored combinations of injection timings and durations.

Impact of coolant temperature on piston wall-wetting and smoke generation in a stratified-charge DISI engine operated on E30 fuel

A late-injection strategy is typically adopted in stratified-charge direct injection spark ignition (DISI) engines to improve combustion stability for lean operation, but this may induce wall wetting on the piston surface and result in high soot emissions. E30 fuel, i.e., gasoline with 30% ethanol, is a potential alternative fuel that can offer a high Research Octane Number. However, the relatively high ethanol content increases the heat of vaporization, potentially exacerbating wall-wetting issues in DISI engines. In this study, the Refractive Index Matching (RIM) technique is used to measure fuel wall films in the piston bowl. The RIM implementation uses a novel LED illumination, integrated in the piston assembly and providing side illumination of the piston-bowl window. This RIM diagnostics in combination with high-speed imaging was used to investigate the impact of coolant temperature on the characteristics of wall wetting and combustion in an optical DISI engine fueled with E30. The experiments reveal that the smoke emissions increase drastically from 0.068 FSN to 1.14 FSN when the coolant temperature is reduced from 90 °C to 45 °C. Consistent with this finding, natural flame luminosity imaging reveals elevated soot incandescence with a reduction of the coolant temperature, indicative of pool fires. The RIM diagnostics show that a lower coolant temperature also leads to increased fuel film thickness, area, and volume, explaining the onset of pool fires and smoke.

研磨パッド・アスペリティ内のミクロなスラリー循環流れ

Chemical mechanical polishing (CMP) is used in a wide range of processes in semiconductor production. However, the polishing mechanism of CMP is not completely understood. The behavior of the abrasive particles and chemical species in the slurry is believed to play a critical role in the polishing mechanism. Therefore, in this study, we observed the slurry flow occurring in the polishing pad asperities experimentally. Because in situ observation of the slurry flow is exceptionally difficult, an enlarged visualization model of the pad asperities was made, based on the scaling laws of fluid dynamics. A refractive index matching technique and a particle suspension method were applied to visualize the scaled-up slurry flow. As a result, the circulation flows in the vertical direction to the wafer surface have been observed at the openings of the pad pores, and at the valleys of the undulations formed by the pad conditioning. The size of the biggest circulation flow observed in this study is approximately 200 μm. This result suggests that, similar to the pad pores, the valleys of undulations of the pad have the function of aiding the slurry transport to the entire wafer surface.

Measurement of single phase flow in porous media using PIV technique

Porous media are widely used in the production and living, and also in science and technology. With the development of energy, chemical industry, metallurgy, atomic energy and also with the progress of the modern industrial and agricultural production technology, a large number of heat and mass transfer problems in porous media gradually appear. Further promoting the development of the discipline about the formation and development of porous media becomes one of hot research points in the modern science and technology. It is expected that the accurate experimental picture and data can be obtained through the experiment, and the fluid flow picture and experimental data are analyzed in depth by using the corresponding software, so that the reliable data are obtained and the theory is supported intuitively, making the research of porous media more perfect. The experiment combined with particle image velocimetry technology and refractive index matching technique is conducted to test the transformation process of liquid flow in a random ball porous medium filled bed, and to extract the data. The extracted data are processed by using Tecplot software, and the transformation process of liquid flow mechanism is obtained. Experimental solid phase is a 25 mm-diameter crystal glass ball stacked bed, and liquid phase is the matching liquid prepared with the mixture of the 65% benzyl alcohol and 35% anhydrous ethanol. The refractive indexes of liquid phase and solid phase are both 1.477, which can successfully eliminate the laser light bending caused by the nismaching of refractive indexes. The flow field diagram in the pebble bed with Reynolds number Re in a range 4.7 Re 1000 is obtained experimentally. The comparisons of variations of flow field and flow lines among the different Reynolds numbers reveal that with the increase of Reynolds number, flow lines become more and more disorder: When the Reynolds number Rep exceeds 220, stable swirl flow inside the bed changes suddenly, and manifests a random feature in location and configuration, which forebodes its entrance into stable turbulence phase.

Experimental study on the dispersed phase macro-mixing in an immiscible liquid–liquid stirred reactor

The spatial nonuniformity of solute concentration in the dispersed phase of immiscible liquid–liquid dispersion is smeared out by coalescence and breakup of drops, which has a very significant effect on the productivity of relevant chemical processes. Novel experiments have been designed to visualize and quantify accurately the macro-mixing within the dispersed phase using the planar laser induced fluorescence (PLIF) method combined with a refractive index matching technique in this work. Batch experiments are carried out in a stirred reactor for an electrolytic solution dispersed in silicone oil. It is revealed that the dispersed phase macro-mixing behavior differs much from that of the continuous phase of multiphase systems and is heavily dependent upon the drop interaction rate and thus power consumption. The dispersed phase mixing time decreases with the increase of the dispersed phase volume fraction. Dispersed phase mixing time varies inversely with the power consumption per unit volume dispersion. The effect of impeller type on the dispersed phase mixing time is greatly different from that on the continuous/single phase mixing time. The effect of radial impeller clearance on the dispersed phase mixing time is contrary to that on the continuous/single phase mixing time.

J0510205 非軸対称エンクロージャ内の積層回転ディスクによる複雑せん断流れの可視化を用いた解明([J051-01]流れの高次元化可視化に向けて(2),流体工学部門)

We report on the complex behavior of the shear flow induced by corotating disks stacked in a non-axisymmetric enclosure. The present study aims to investigate the flow structure leading to the fatal crash of a hard disk drive (HDD) for data storage. We developed a simplified HDD model which represents features commonly observed in real HDD products. The model is equipped with an imitating read-and-write arm and a shroud opening. The flow is driven by disk rotations and affected by the arm blockage and the partly opened shroud. A series of flow visualization was performed by applying refractive index matching technique, where the refractive index of the working fluid was matched to that of the transparent model. Flow behavior was observed on three different planes parallel to the disks at different Reynolds numbers and arm insertion angles. Polygonal structures were observed near the hub in a certain range of the Reynolds numbers and the vertices were observed to decrease with the Reynolds numbers. Variation of the flow behavior was identified near the arm and the shroud opening area. Generation of three-dimensional shear was observed near in the arm wake region, which would create complex flow behavior of the inner flow inside the whole enclosure.

Liquid–liquid mixing using micro-fluidised beds

This study experimentally investigates the application of a solid–liquid micro-fluidised bed as a micro-mixing device. The experiments were performed in a borosilicate capillary tube with an internal diameter of 1.2mm (i.e. near the upper-limit dimension of a micro-fluidic system) using borosilicate particles with a mean diameter of 98μm. Refractive index matching technique using sodium iodide solution was employed to achieve a transparent fluidised bed. Mixing performance of the micro-fluidised bed in terms of mixing time was investigated using a dye dilution technique. Experiments were carried out in the creeping flow regime at Reynolds numbers ranging between 0.27 and 0.72. It was demonstrated that the micro-fluidised bed mixing time sharply decreases as the Reynolds number increases. That is because at relatively high Reynolds numbers, the particle oscillation is stronger creating larger disturbances in the flow. The energy dissipation rate in micro fluidised bed was estimated to be four orders of magnitude less than other passive micro mixers which operate in the turbulent regime. It was also demonstrated that the ratio of mixing time and the energy dissipation rate for fluidised bed micro-mixer was comparable to K-M, Tangential IMTEK, and interdigital micro-mixers. However, the fluidised bed micro-mixer was found to operate at much lower Reynolds numbers compared to other passive mixers, with a mixing time of the order of few seconds.

Application of the Monte-Carlo refractive index matching (MCRIM) technique to the determination of the absolute light yield of a calcium molybdate scintillator

The use of 40Ca100MoO in experimental searches for neutrinoless double beta decay (0νDBD) relies on knowledge of fundamental scintillation properties of the material. In this work we determine the absolute light yield of calcium molybdate using Monte-Carlo refractive index matching technique (MCRIM). The MCRIM technique is a combination of experiment and simulations that allows the absolute light yield of scintillators to be determined by taking into account effects of refraction, scattering and absorption in the material. The light collection efficiency of the scintillator-detector assembly was simulated using the ZEMAX ray-tracing software. By tuning the optical parameters of the scintillation crystal, a model was derived that gives good agreement with the experimental results. It is shown that the light collection efficiency of scintillators increases with transmittance and scattering due to an enhanced probability for photons to escape the crystal volume. Using MCRIM, the absolute light yield for the 40Ca100MoO4 scintillator was found to be 7.5±1.2 ph/keV at room temperature. Comparative measurements using a CaWO4 scintillator as a reference show good agreement with this result. In that way, the study demonstrated the potential of the MCRIM technique as a tool for quantitative characterization of scintillation materials.

Experimental study of entrainment and interface dynamics in a gravity current

The special case of entrainment in a stratified flow, relevant to many geophysical flows such as oceanic overflows, so far has not been studied experimentally in terms of small-scale aspects around the turbulent/non-turbulent interface. In view of the fact that existing engineering concepts perform unsatisfactorily in practice, a new gravity current facility was designed with the goal to gain understanding of how stratification affects interfacial physics. Here, we present the design of the new setup and give details on the turbulence enhancement in the inflow and the refractive index matching technique used. Validation measurements ensure that there is negligible backflow and an essentially irrotational flow outside the current. Measurements via particle image velocimetry of a flow with inflow Reynolds and Richardson numbers of \(Re_0\approx \hbox{4,000}\) and Ri 0 = 0.22 are reported. An analysis in a laboratory frame agrees well with flow features reported in the literature, i.e., a streamwise invariant top-hat velocity scale and a Reynolds stress distribution are matched closely by a mixing length model. In a second step, the instantaneous interface position is determined based on a threshold on the normal enstrophy component. An investigation in a frame of reference conditioned on the interface position reveals a strong interfacial shear layer that is much more pronounced than the one observed in jet flows. Its thickness is about two times the Taylor microscale. The data moreover suggest the existence of a fairly strong interfacial density jump across the shear layer. The entrainment parameter is estimated at \(E \approx 0.04\) congruently from the evaluations in laboratory and conditioned frame, respectively.

Particle image velocimetry in a foam-like porous structure using refractive index matching: a method to characterize the hydrodynamic performance of porous structures

We present a method to measure two-dimensional velocity fields inside an artificial foam-like porous structure using particle image velocimetry and a refractive index matching technique to avoid optical distortion. The porous structure is manufactured by stereolithography with the epoxy resin WaterShed® XC 11122 as solid material, and anisole is used as refractive index-matched fluid. It was found that the direction of build-up of the stereolithographic structure plays an important role for the quality of the recorded images. The velocity fields measured in this study and the turbulent statistics derived thereof allow to characterize the hydrodynamic performance of the artificial foam-like structure and clarify the mechanisms of mixing. Results from this study compare well to results from a large eddy simulation reported by Hutter et al. (Chem Eng Sci 66:519–529, 2011b) and hence reinforce these simulations.

Use of PIV to measure turbulence modulation in a high throughput stirred vessel with the addition of high Stokes number particles for both up- and down-pumping configurations

A refractive index matching technique combined with particle image velocimetry (PIV) was used to measure turbulent properties of solid–liquid suspensions in a small high throughput scale cylindrical vessel of 45 mm diameter agitated with a 45° pitched blade turbine (PBT) for up-pumping (U) and down-pumping (D) configurations. This study analyses the effect of large 1.5 mm diameter particles (Stokes number>1), on liquid mean velocities, turbulent kinetic energy (TKE) and energy dissipation ( ε) at particle concentrations of 0%, 1.5% and 5% by volume. Only small changes in the time-averaged liquid velocities were observed with increasing particle concentration. However, maximum TKE near the impeller decreased up to 40% with increasing particle concentration for both configurations. The Smagorinsky SGS method was used to estimate local energy dissipation rate near the impeller and the maximum value was found to decrease by 50% between 0% and 5% concentration for the (U) configuration. A lesser but still significant drop of 30% was observed for the (D) configuration. These data confirm that large Stokes number particles can suppress turbulence, in agreement with some previous experimental studies, but in contradiction with existing theories.

Mechanism of seeded dispersion polymerization of methyl methacrylate using submicron polystyrene seed particles

AbstractSubmicron polystyrene (PS) latex particles were used as seed in seeded dispersion polymerization of methyl methacrylate (MMA) to investigate the particle nucleation and aggregation behavior in this type of polymerization. The PS seed particles were located and tracked during the reaction using a refractive index matching technique. The number of PS seed particles present in the poly(methyl methacrylate) (PMMA) particles was investigated in detail throughout the reaction. The change in the distribution of PMMA particle populations containing different numbers of seed particles indicated that intensive nucleation and aggregation occurred during the early stage of the reaction until a transition point of 8.7% conversion was attained under the reaction conditions studied. The size of the large particles at this point was around 1 μm. These particles were regarded as mature particles that did not aggregate with other mature particles. Meanwhile, immature particle were still generated continuously from the continuous phase. These immature particles could not survive the aggregation process to grow to become mature particle, but instead, were captured by the mature particles. Therefore, the total number of the mature particles remained constant from this point until the end of the reaction. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011