Dominant Deposition Mechanism Research Articles

Relationship between saccadic eye movements and formation of the Krukenberg's spindle-a CFD study.

In this research, a series of numerical simulations for evaluating the effects of saccadic eye movement on the aqueous humour (AH) flow field and movement of pigment particles in the anterior chamber (AC) was performed. To predict the flow field of AH in the AC, the unsteady forms of continuity, momentum balance and conservation of energy equations were solved using the dynamic mesh technique for simulating the saccadic motions. Different orientations of the human eye including horizontal, vertical and angles of 10° and 20° were considered. The Lagrangian particle trajectory analysis approach was used to find the trajectories of pigment particles in the eye. Particular attention was given to the relation between the saccadic eye movement and potential formation of Krukenberg's spindle in the eye. The simulation results revealed that the natural convection flow was an effective mechanism for transferring pigment particles from the iris to near the cornea. In addition, the saccadic eye movement was the dominant mechanism for deposition of pigment particles on the cornea, which could lead to the formation of Krukenberg's spindle. The effect of amplitude of saccade motion angle in addition to the orientation of the eye on the formation of Krukenberg's spindle was investigated.

Wax Deposition in the Presence of Suspended Crystals

Wax deposition in pipelines is one of the most relevant flow assurance problems faced by the petroleum industry. Molecular diffusion of dissolved paraffin has been considered the dominant deposition mechanism in simulation models available in the literature. In case the pipeline operational conditions are such that the fluid temperature is below the wax appearance temperature (WAT), wax crystals could be present in the bulk of the flowing solution and particle deposition mechanism may play a relevant role. In the present research, bench-scale, well-controlled deposition experiments were conducted for laminar channel flow of a solution with an inlet temperature below the WAT, so that wax crystals were available for deposition. In the experiments, visualizations of the deposition process were sought for three distinct heat flux conditions at the channel boundary: negative, zero, and positive heat flux. Detailed information on the temporal and spatial distributions of the wax deposited along the channel wall...

Deposition of Particles in Human Mouth-Throat Replicas and a USP Induction Port.

Oral inhalation is the common route of drug delivery to pulmonary airways. In general, deposition in the oropharyngeal airways from a drug-delivery device makes up a substantial portion of the emitted dose, which affects the dose delivered to the lung. Studies with airway replicas made from cadaver or magnetic resonance imaging scans show that for micrometer-sized particles, impaction is the dominant deposition mechanism. Several deposition studies in oropharyngeal replicas found that the deposition efficiency can be correlated with the mouth inlet velocity and inlet mouthpiece diameter. Other studies show that the deposition efficiency is best correlated with the mean diameter of internal geometry and the mean velocity based on the mean diameter. We investigated the mouth inlet diameter, as well as internal airway dimensions and their influence on oropharyngeal deposition based on experimental data from this study. Several human oropharyngeal replicas with different mouth inlet diameters and the USP induction port were used. We found that the aerosol deposition increased with decreasing mouth inlet diameter. Several mathematical expressions were tried to correlate the deposition efficiency with the Stokes number calculated based on (1) mouth inlet diameter and inlet velocity, (2) mean diameter of internal geometry and mean velocity, (3) mouth inlet velocity and mean diameter, and (4) mouth inlet velocity and minimum diameter in the oropharyngeal replica. The best correlation was obtained in case 4. This correlation could explain the intra-subject variation when deposition was found to vary with mouth inlet diameter, such as in some aerosol drug-delivery devices. It could also explain the intersubject variability in oropharyngeal deposition when human volunteers with different airway geometries and mouth openings were studied.

Particle Tracking in Large Eddy Simulated Turbulent Channel Flow Using Generalized Lattice Boltzmann Method

The deposition of spherical solid particles with 0.00192 ≤ τ+ ≤ 160.56 in three-dimensional turbulent vertical and horizontal channel flows with friction Reynolds number was studied. A generalized lattice Boltzmann equation (GLBE) was used for computation of instantaneous turbulent flow field for which large eddy simulation (LES) was employed. The subgrid scale (SGS) turbulence effects were simulated through a shear-improved Smagorinsky model (SISM), which can predict turbulent near wall region without any wall function. The Lagrangian particle tracking approach was used, and qualitative distribution of particles in a vertical channel center plane and in a near wall region was presented. It was shown that, in horizontal channels, the rate of deposition is higher than in vertical channels since gravitational sedimentation is the dominant mechanism of deposition on the lower wall. Combination of SISM in GLBE with particle tracking analysis in turbulent channel flow is the novelty of the present work. Since the accuracy of the flow field is essential to correctly predict particles' behavior, a reliable method should be used to simulate the flow field. In the proposed method, the flow field equations are solved locally. The collaboration of this method and particle tracking can be considered a good candidate for simulation of particulate flow.

Metal oxide aerosol dry deposition in laminar pipe flow at high thermal gradients and comparison with SOPHAEROS module of ASTEC reactor accident analysis code

During a severe nuclear reactor accident involving core melt down conditions, the deposition of fission product aerosols inside the reactor coolant system affects the final source term available to the containment and subsequently to the environment. Towards quantifying the aerosol deposition under varying flow conditions and thermal gradients, as may be encountered in the heat transport systems, experiments were performed to investigate the dry deposition behavior of metal oxide aerosols in a 3.6m long stainless steel piping test assembly. This assembly consisted of divergent and convergent sections, horizontal and vertical sections and right angle bends. Tin oxide aerosols, generated by a plasma torch aerosol generator, were transported into the test assembly using argon carrier gas. Temperature sensors coupled to data loggers were used to record the pipe inner wall and carrier gas temperatures. The experimental deposition results were found to be within 8% of those estimated by the SOPHAEROS module of the accident analysis code ASTEC (Accident Source Term Evaluation Code). Code results for experimental input parameters showed that for sections at higher temperature gradients the dominant deposition mechanism was thermophoresis, while in sections for low thermal gradients, gravitational settling dominated. The micrographs obtained using Transmission Electron Microscopy (TEM) showed that the deposited Tin oxide particles were mostly spherical and bimodal in nature. The X-ray diffraction (XRD) analysis showed that plasma torch generated aerosols exhibit tetragonal SnO and SnO2 phases.

Effects of growth conditions on the formation of self-assembled InAs quantum dots grown on (1 1 5)A GaAs substrate

Effects of growth conditions on the formation of InAs quantum dots (QDs) grown on GaAs (115)A substrate were investigated by using the reflection high-energy electron diffraction (RHEED) and photoluminescence spectroscopy (PL). An anomalous evolution of wetting layer was observed when increasing the As/In flux ratio. This is attributed to a change in the surface reconstruction. PL measurements show that QDs emission was strongly affected by the InAs deposited amount. No obvious signature of PL emission QDs appears for sample with 2.2 ML InAs coverage. Furthermore, carrier tunneling from the dots to the non-radiative centers via the inclination continuum band is found to be the dominant mechanism for the InAs amount deposition up to 4.2 MLs.

An Experimental Investigation of Permeability Impairment Under Dynamic Flow Conditions Due to Natural Depletion in an Iranian Oilfield

Asphaltene deposition is an issue that has received much attention since it has been shown to be the cause of major production problems. It leads to permeability reduction under the processes of natural depletion as well as hydrocarbon gas/CO2 injection. Though a great deal of researches have focused on studying permeability impairment in reservoir rocks, little is known about the asphaltene deposition mechanisms that control the permeability reduction for Iranian reservoirs. In this work, an experimental effort is made to investigate the permeability impairment of core samples of Iranian oil reservoirs. The experiments are performed on both sandstone and carbonate rock types at reservoir temperature and pressure. The mass balance was used for evaluating of porosity reduction during the experiments. The results indicate that the dominant deposition mechanism changes as production proceeds. In addition, it has been found that the primary mechanism in permeability impairment is surface deposition. On the other hand, entrainment of asphaltene particles is manifested when outlet pressure drops from 4,200 to 3,800 Psig for both sandstone and carbonate samples. It can be drawn that asphaltene entrainment dependence to pressure is much more than that to the injected pore volume. This research illuminates the deposition mechanisms and determines dynamic parameters of asphaltene deposition, which are necessary to devise reliable prevention strategies.

Anisotropy antireflection TiO2 nanoparticle films fabricated with directed cluster beam deposition

AbstractAntireflection TiO2 nanoparticle film was fabricated on silica glass substrate by using directed cluster beam deposition method. Self‐shadowing becomes a dominant growth mechanism for nanoparticle deposition at sufficiently large incident angles, which results in extremely porous nanoparticle stacking and yields excellent control over the refractive index profile. Inclined columnar and porous microstructure was confirmed with SEM and AFM. The TiO2 nanoparticle films show ultra‐high transmittance, broadband and omnidirectional characters, which can be attributed to its porous structure and modulated refractive index. We also found that the film displays an angular selective optical transmittance. The highest antireflection enhancement of the light transmission can be obtained for light incidence along the nanoparticle columns. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Flame spray pyrolysis synthesis and aerosol deposition of nanoparticle films

AbstractThe assembly of nanoparticle films by flame spray pyrolysis (FSP) synthesis and deposition on temperature‐controlled substrates (323–723 K) was investigated for several application‐relevant conditions. An exemplary SnO2 nanoparticle aerosol was generated by FSP and its properties (e.g., particle size distribution), and deposition dynamics were studied in details aiming to a simple correlation between process settings and film growth rate. At high precursor concentrations (0.05–0.5·mol/L), typically used for FSP synthesis, the nanoparticles agglomerated rapidly in the aerosol leading to large (>100 nm) fractal‐like structures with low diffusivity. As a result, thermophoresis was confirmed as the dominant nanoparticle deposition mechanism down to small (≈40 K) temperature differences (ΔT) between the aerosol and the substrate surface. For moderate‐high ΔT (>120 K), thermal equilibrium was rapidly obtained yielding a constant thermophoretic flux and film growth rate. A model was developed to predict the nanoparticle deposition rates by FSP synthesis at moderate‐high ΔT that does not require detailed analysis of the aerosol composition. Comparison with previous studies having similar nozzle geometries showed that the deposition rates of FSP‐made aerosols can be reasonably well predicted for various materials and flame conditions. © 2012 American Institute of Chemical Engineers AIChE J, 2012

In vitro deposition measurement of inhaled micrometer-sized particles in extrathoracic airways of children and adolescents during nose breathing

An in vitro study was conducted with the goal of developing empirical correlations to predict the deposition of particles with an aerodynamic diameter of 0.5–5.3 μm in nasal airways of children with ages 4–14 years. CT images of nasal airways of thirteen healthy subjects and one with congested nasal airways were used for fabricating the replicas by rapid prototyping. Replicas included nasal airways to the level of the upper trachea as well as the face. Four physiological breathing patterns (sinusoidal waves) of children were simulated by a pulmonary waveform generator. Using an Electrical Low Pressure Impactor (ELPI) we measured deposition of sunflower oil particles generated by a Collison atomizer. Moreover, using the same setup, particle deposition in five adult replicas fabricated using MRI images was measured for direct comparison with the child replicas and in vivo data available for adults. Deposition increased by increasing particle size and flow rate, indicating impaction as the dominant deposition mechanism. Existing correlations for adults were unable to reduce the scatter in the deposition data for children. A correlation was developed for prediction of deposition including the relevant non-dimensional numbers (Stokes and Reynolds numbers) that were calculated considering the dimensions of the airways. The corrected correlations should be useful for exposure estimation of children and for efficient pediatric drug delivery using face masks.

Aerosol deposition characteristics in distal acinar airways under cyclic breathing conditions

Although the major mechanisms of aerosol deposition in the lung are known, detailed quantitative data in anatomically realistic models are still lacking, especially in the acinar airways. In this study, an algorithm was developed to build multigenerational three-dimensional models of alveolated airways with arbitrary bifurcation angles and spherical alveolar shape. Using computational fluid dynamics, the deposition of 1- and 3-μm aerosol particles was predicted in models of human alveolar sac and terminal acinar bifurcation under rhythmic wall motion for two breathing conditions (functional residual capacity = 3 liter, tidal volume = 0.5 and 0.9 liter, breathing period = 4 s). Particles entering the model during one inspiration period were tracked for multiple breathing cycles until all particles deposited or escaped from the model. Flow recirculation inside alveoli occurred only during transition between inspiration and expiration and accounted for no more than 1% of the whole cycle. Weak flow irreversibility and convective transport were observed in both models. The average deposition efficiency was similar for both breathing conditions and for both models. Under normal gravity, total deposition was ~33 and 75%, of which ~67 and 96% occurred during the first cycle, for 1- and 3-μm particles, respectively. Under zero gravity, total deposition was ~2-5% for both particle sizes. These results support previous findings that gravitational sedimentation is the dominant deposition mechanism for micrometer-sized aerosols in acinar airways. The results also showed that moving walls and multiple breathing cycles are needed for accurate estimation of aerosol deposition in acinar airways.

Characterization of Atmospheric Dry Deposition of Polychlorinated Dibenzo-p-dioxins/Dibenzofuran in a Rural Area of Taiwan

The characteristics of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and the variation of the gas-particle partitioning of PCDD/Fs near two municipal solid waste incinerators (MSWIs) located in southern Taiwan were investigated. In order to better understand the mechanism of dry deposition, the atmospheric dry deposition flux and velocity of PCDD/Fs were calculated. It was found that the mean atmospheric PCDD/F concentrations (0.0386-0.106 pg I-TEQ/Nm^3) were comparable to those detected in the vicinity of MSWIs in Taiwan, but significantly lower than those in a highly industrialized urban area (0.15 pg I-TEQ/Nm^3) located in southern Taiwan. The relatively higher atmospheric PCDD/F concentrations was found in winter than in summer, probably because of several loss process including photolysis, chemical reactivity, wet and dry deposition, and scavenging by vegetation. The calculated total dry deposition flux of PCDD/Fs ranged from 0.0274-0.718 ng I-TEQ/m^2-month, and the atmospheric deposition flux in winter tended to be higher than those in summer. The results also indicated that dry deposition velocities of atmospheric particles for each month ranged from 0.52-0.91 cm/s (mean=0.63 cm/s) and 0.48-0.73 cm/s (mean=0.55 cm/s) in sites A and B, respectively, which were similar to that for the ambient air near two MSWIs in Taiwan, but slightly higher than those in urban area of Korea. In addition, the dry deposition of PCDD/Fs was mainly contributed by particle-phase at both sampling areas during the estimated period. The above results demonstrated that the dominant mechanism of dry deposition was particle phase deposition.

Thermophoretic deposition of aerosol particles in laminar mixed-convection flow in a channel with two heated built-in square cylinders

The thermophoretic deposition of aerosol particles in laminar mixed-convection flow in a channel with two heated built-in square cylinders was studied numerically. The objective of this research was to study the effect of free convection and the distance between cylinders, on deposition of particles. Continuity, momentum and energy equations were solved to determine the velocity and temperature profiles in the channel. The particle trajectories were evaluated by solving the Lagrangian equation of motion that included the drag, Brownian diffusion and thermophoresis forces. It was found that the temperature gradient near the channel wall, in mixed flow regime, is higher than the temperature gradient in forced convection regime. Increasing the temperature gradient increased the effect of thermophoresis on deposition of particles. It was observed that the deposition was increased with the Richardson number. The distance between cylinders is a parameter that influences the deposition of particles. Temperature gradient decreases with increasing the cylinders’ distance; on the other hand, the length of the high temperature gradient zone, which is located in the region between the cylinders where the most deposition occurs, will be increased. These two opposite phenomena cause the fact that at a distance which is four times longer than the cylinders’ length, a maximum cumulative deposition fraction occurs. It was eventually concluded that the thermophoresis and the inertial impaction are dominant deposition mechanisms of particles on the channel wall.

The origin of an unusual tuff ring of perlitic rhyolite pyroclasts: The last explosive phase of the Ramadas Volcanic Centre, Andean Puna, Salta, NW Argentina

A thick sequence of bedded pyroclastic deposits, comprised largely of crystal poor, partially flow-banded perlite fragments defines the remains of a tuff ring around the eastern margin of the Miocene Ramadas Volcanic Centre (RVC), Central Andes, NW Argentina. In numerous quarry exposures, planar bed-forms dominate, but low-angle cross-stratification, lensoidal truncations and lateral pinching and swelling of cm–dm scale bed-forms occur, consistent with pyroclastic surge as the dominant transport and depositional mechanism. Intercalated are mantling, very fine grained, well-sorted, mm–cm scale planar ash layers that represent deposition from pyroclastic fall out and are most likely the products of co-surge ash clouds. Also observed are thick m-scale, laterally continuous, poorly-sorted horizons that are interpreted as pyroclastic flow deposits. Grainsize variations within the > 70 m thick succession range from fine ash to coarse lapilli, with occasional large blocks reaching 20 cm. Clast vesicularities are typically very low. The sequence constitutes a rhyolitic tuff ring around the proximal margins of the RVC. Stratigraphic relationships indicate that the tuff ring was developed following cessation of the major plinian eruption phase of the RVC. A series of pyroclastic density currents and associated ash clouds is inferred to have resulted in the construction of the rhyolitic tuff ring, with deposition focussed on the eastern and southern margins of the central vent. The fine-grained nature of the deposits and low clast vesicularity are consistent with some degree of magma:water interaction during fragmentation. Pervasive perlitic fracturing of clasts found within the tuff deposits also indicates hydration and an extended, post-depositional hydration of the pyroclastic sequence, due to the influence of meteoric water, is likely to have occurred, with deposit permeability, clast specific surface and climate influential in facilitating additional textural modification of pyroclasts.

Surface processes recorded by rocks and soils on Meridiani Planum, Mars: Microscopic Imager observations during Opportunity's first three extended missions

The Microscopic Imager (MI) on the Mars Exploration Rover Opportunity has returned images of Mars with higher resolution than any previous camera system, allowing detailed petrographic and sedimentological studies of the rocks and soils at the Meridiani Planum landing site. Designed to simulate a geologist's hand lens, the MI is mounted on Opportunity's instrument arm and can resolve objects 0.1 mm across or larger. This paper provides an overview of MI operations, data calibration, and analysis of MI data returned during the first 900 sols (Mars days) of the Opportunity landed mission. Analyses of Opportunity MI data have helped to resolve major questions about the origin of observed textures and features. These studies support eolian sediment transport, rather than impact surge processes, as the dominant depositional mechanism for Burns formation strata. MI stereo observations of a rock outcrop near the rim of Erebus Crater support the previous interpretation of similar sedimentary structures in Eagle Crater as being formed by surficial flow of liquid water. Well‐sorted spherules dominate ripple surfaces on the Meridiani plains, and the size of spherules between ripples decreases by about 1 mm from north to south along Opportunity's traverse between Endurance and Erebus craters.

Amorphous particle deposition and product quality under different conditions in a spray dryer

Deposition of amorphous particles, as a prevalent problem particularly in the spray drying of fruit and vegetable juices, is due to low-molecular-weight sugars and is strongly dependent on the condition of the particles upon collision with the dryer wall. This paper investigates the condition of the amorphous particles impacting the wall at different drying conditions with the aim of elucidating the deposition mechanism and physical phenomena in the drying chamber. A model sucrose-maltodextrin solution was used to represent the low-molecular-weight sugar. Particle deposits were collected on sampling plates placed inside the dryer for analyses of moisture content, particle rigidity (using SEM) and size distribution. Moisture content was adopted as a general indicator of stickiness. Product particles collected at the bottom of the experimental dryer were found to have higher moisture than particle deposits on samplers inside the dryer. Moisture content profile in the dryer shows that apart from the atomizer region, where particles are relatively wet, particle deposits at other regions exhibit similar lower moisture content. At the highest temperature adopted in the experiments, particles became rubbery suggesting liquid-bridge formation as the dominant deposition mechanism. Further analysis on particles size distribution reveals a particle segregation mechanism whereby smaller particles follow preferentially to the central air stream while larger particles tend to re-circulate in the chamber, as predicted in past CFD simulation. The findings from this work will form the basis and provide validating data for further modeling of wall deposition of amorphous particles in spray drying using CFD.

Numerical study of fibre deposition in a human nasal cavity

The inhalation of toxic particles such as asbestos fibres through the nasal airway has been found to cause harmful damage to the respiratory system. This study made use of CFD techniques to investigate deposition of fibrous particles in a human nasal cavity. A 3D computational model was created from CT scans which provided the framework to study the flow and deposition of fibres at a constant flow rate of 7.5 L/min. The effects of the fibres’ elongated shape, density and size were incorporated into empirical drag correlations and the fibre trajectories were recorded through a Lagrangian tracking scheme. In general, good agreement was found in the right cavity and an overprediction in the left cavity. The major cause of deposition differences was in the geometrical variations between subjects as well as the left and right cavities. The dominant mechanism of deposition was by inertial impaction, with a majority of the particles depositing in the anterior region. It was found that asbestos had a very low deposition, ≈ 14 % , and was independent of fibre length. In comparison, the carbon fibre exhibited increases in deposition as the fibre length increased. A parameter ρ A cross which represents the mass per unit length was used to equate the d ae for different fibre lengths.

Simulating Wax Deposition in Pipelines for Flow Assurance

A method is presented for simulating wax deposition in pipelines in which the wax phase is represented as a continuous distribution of n-paraffin components. The thermodynamic properties of the wax are predicted using a previously developed thermodynamic model. We have adopted a mass transfer model to predict the likely rate of wax deposition. Predictions from the model are compared with observations of flow-loop experiments, and predictions for full-scale pipelines are presented. The impact of the removal of deposited wax by shearing is also investigated. We demonstrate the effect of assuming molecular diffusion to be the dominant mechanism of wax deposition, and we also show that not representing the wax with its full n-paraffin distribution leads to serious distortions of the pipeline simulations. Practical software development issues in the implementation of the model are outlined.

Combined inertial and gravitational deposition of microparticles in small model airways of a human respiratory system

Focusing on relatively small airways in terms of the medium-size bronchial generations G6–G9, the interplay of impaction and sedimentation on micron particle transport and deposition has been simulated. A commercial finite-volume code, enhanced with user-supplied programs, has been employed. Although impaction is still a dominant deposition mechanism for microparticle in medium-size airways under normal breathing conditions (say, Q in = 15 – 30 L / min ), sedimentation may play a role as well. In turn, that can influence the local particle deposition patterns, efficiencies and fractions for a realistic range of Stokes numbers ( 0.001 ⩽ St ⩽ 0.33 ) . However, deposition due to sedimentation is significantly amplified during slow inhalation; for example, the gravitational deposition may become dominant in the ninth bifurcation (i.e., generations G8–G9) for relatively large microparticles (say, d p > 5 μ m ) at Q in = 3.75 L / min . The occurrence of sedimentation changes the location of the deposition “hot spots” and reduces the order of the maximum deposition enhancement factor. The use of analytical formulas based on inclined tube models for predicting gravitational deposition in local bronchial airway segments as well as the combination of deposition by sedimentation and impaction has to be carefully examined. As shown, more prudent is the use of curve-fitted correlations generated from experimentally validated computer simulation results as a function of Stokes number and sedimentation parameter.

Bacteria transport and deposition under unsaturated conditions: The role of the matrix grain size and the bacteria surface protein

Unsaturated (80% water saturated) packed column experiments were conducted to investigate the influence of grain size distribution and bacteria surface macromolecules on bacteria ( Rhodococcus rhodochrous) transport and deposition mechanisms. Three sizes of silica sands were used in these transport experiments, and their median grain sizes were 607, 567, and 330 μm. The amount of retained bacteria increased with decreasing sand size, and most of the deposited bacteria were found adjacent to the column inlet. The deposition profiles were not consistent with predictions based on classical filtration theory. The experimental data could be accurately characterized using a mathematical model that accounted for first-order attachment, detachment, and time and depth-dependent straining processes. Visual observations of the bacteria deposition as well as mathematical modelling indicated that straining was the dominant mechanism of deposition in these sands (78–99.6% of the deposited bacteria), which may have been enhanced due to the tendency of this bacterium to form aggregates. An additional unsaturated experiment was conducted to better deduce the role of bacteria surface macromolecules on attachment and straining processes. In this case, the bacteria surface was treated using a proteolitic enzyme. This technique was assessed by examining the Fourier-transform infrared spectrum and hydrophobicity of untreated and enzyme treated cells. Both of these analytical procedures demonstrated that this enzymatic treatment removed the surface proteins and/or associated macromolecules. Transport and modelling studies conducted with the enzyme treated bacteria, revealed a decrease in attachment, but that straining was not significantly affected by this treatment.