Model Particle Systems Research Articles

A precise nanoparticle quantification approach using microfluidics and single-particle tracking

Due to the limited available amounts of components, especially of low water-soluble drugs, formulation development is often impeded by a careful characterization. The use of small batch sizes might solve this problem but requires also adequate analytics. Concentration of nanoparticulate formulations lack straightforward evaluation techniques. In this work, a precise and straight-forward method is established to individually count nanoparticles. A microfluidic chip with known dimensions was used to visualize single particles flowing through the channel (single-particle tracking (SPT)). A sequence of 10,000 images was analyzed to determine the mean particle concentration. The proposed method is independent of the particular flow rate through the microfluidic chip as long as there is no particle overlap and a continuous exchange of particles. Monodisperse Rhodamine B labeled poly (methyl methacrylate) (PMMA) nanoparticles (267.03 ± 9.79 nm) were used as a model and reference particle system for the evaluation process of SPT allowing for a gravimetric determination based on density analysis using analytical ultracentrifugation (AUC) and gas pycnometry. The SPT method was evaluated and compared to other techniques used for concentration measurement. Both approaches (SPT and gravimetry) provide very similar and comparable results indicating the applicability of this novel quantification approach. In contrast, multi angle dynamic light scattering (MADLS) could not yield a precision as good as SPT (number density rel. standard deviation SD nSPT = 11.67%; SD nMADLS = 49.45%). Finally, the measured particle number concentrations can be realized in low concentration ranges (0.8249 μg mL−1 – 0.08249 μg mL−1) not accessible for MADLS (0.08249 mg mL−1 – 0.008249 mg mL−1) and gravimetric analysis.

Mass effects for thermodiffusion in dilute aqueous solutions.

Thermodiffusion is the phenomenon by which molecules in a mixture present concentration gradients in response to an imposed temperature gradient. Despite decades of investigations, this effect remains poorly understood at a molecular level. A common, phenomenological approach is to individuate the molecular factors that influence the Soret coefficient, the parameter that quantifies the resulting concentration-gradient. Experimental studies, often performed on organic mixtures, as well as simulations of model particle systems have evidenced that the difference in masses between the mixture components has an important effect on the amplitude of the Soret coefficient. Here, we use molecular dynamics simulations of a thermophoretic setting to investigate the mass dependence of the Soret coefficient in dilute aqueous solutions. An advantage of simulation approaches is that they are not limited in the range of explored molecular masses, which is often limited to isotopic substitutions in the experiments. Our simulations reveal that the mass dependence of the Soret coefficient in these solutions is in agreement with previous experimental and simulation work on molecular-size systems. In particular, it is sensitive to the relative mass difference between the solute and the solvent, but not to their absolute mass. Adjusting the mass of the solvent and of the solute can turn a thermophobic solution into a thermophilic one, where solute accumulation is reversed. This demonstrates that the mass effect can indeed compensate for the other contributions to the Soret coefficient. Finally, we find that changing the molecular moments of inertia has a much more limited impact as compared to a change in the total molecular mass.

Magnetic particle imaging of particle dynamics in complex matrix systems

Abstract Magnetic particle imaging (MPI) is a young imaging modality for biomedical applications. It uses magnetic nanoparticles as a tracer material to produce three-dimensional images of the spatial tracer distribution in the field-of-view. Since the tracer magnetization dynamics are tied to the hydrodynamic mobility via the Brownian relaxation mechanism, MPI is also capable of mapping the local environment during the imaging process. Since the influence of viscosity or temperature on the harmonic spectrum is very complicated, we used magnetic particle spectroscopy (MPS) as an integral measurement technique to investigate the relationships. We studied MPS spectra as function of both viscosity and temperature on model particle systems. With multispectral MPS, we also developed an empirical tool for treating more complex scenarios via a calibration approach. We demonstrate that MPS/MPI are powerful methods for studying particle-matrix interactions in complex media.

The influence of filter cloth on cake discharge performances during backwashing into liquid phase

In industrial solid/liquid separation, the removal of fine particles in a suspension with a significant amount of solids content can be a challenging task. Cake filtration in particular is a useful principle for separation of such suspensions. This separation occurs in a wide array of industrial production applications and has some advantages compared to other separation principles. To achieve the required performance, different types of filters can be used. One commonly used type is operated semi-continuous and use backwashing to discharge the filter cake. The regeneration of the filter cloth is as important as the actual filtration step. In the case of a properly regenerated filter cloth, the filtrate flow at the beginning of each step is still the same, while the initial filtrate flow decreases for and inefficient regeneration.This regeneration of the filter cloth depends strongly on the interaction between the particle system and the filter cloth. In the case of backwashing filtration, a clear difference could be observed between multifilament and monofilament fabrics.For multifilament filter cloths, it is possible to calculate the minimum required cake thickness for a complete discharge by means of the total resistance, i.e. the product of the specific cake resistance multiplied by the cake thickness. For 3 investigated fabrics with mesh size ≤ 20 µm, a complete cake discharge could be achieved by all 3 model particle systems. It is noticeable that the minimum required cake thickness decreases with increasing specific resistance. As soon as the cake thickness with the respective specific cake resistance of the particle system exceeds the value 5∙1010 m−1, a complete discharge can be observed. The cake thicknesses found within the same particle system were very similar between the 3 fabrics investigated. Based on this observation, the release of cakes from multifilament fabrics appears to be dominated by cohesion of particles in the cake. The adhesion from the first particle layer to the fabric does not seem to be a significant influencing factor.The situation is different for monofilament fabrics. This behaviour is different from the multifilament fabrics in the cake thickness. Here, smaller cake minimum required cake thicknesses were observed for small specific cake resistances. This is due to the particle deposition mechanism within the fabric. A lower specific cake resistance is associated with a larger particle size and/or a different particle shape (greater deviation from the ideal sphericity). Larger particles relative to the mesh size of the filter fabric preferably form a clean interface between the particle layer and the filter fabric (surface deposition). This leads to a lower interference and better cake discharge. This behaviour could be estimated by dividing the filter media resistance (determined by a particle system whose particle size is significantly larger than the mesh size of the fabric), by the specific cake resistance multiplied by a particle shape-dependent factor (Eq. (2)). The shape-dependent factors were determined to be 0.03 for orthorhombic, 0.15 for flaky and 0.46 for needle-shaped particles and thus increases with increasing deviation from the ideal sphericity. By including the filter medium resistance, the influence of the fabric and the adhesion of particles to the fabric is to be expected. Validity of this model was proven with 6 filter clothes with ≤ 22 µm mesh (3 twill and 3 plain weave).Similar behaviour between multifilament fabric and monofilament fabric could only be observed with the required minimum backwashing volume for a complete discharge. The required necessary backwash volume is always ≤ 2 l/m2. In summary, this study shows the regeneration performance of different filter cloth types (multifilament and monofilament) can be predicted by different models.

How gangue particle size can affect the recovery of ultrafine and fine particles during froth flotation

In general, the poor flotation behavior of ultrafine (<10µm) particles is mainly associated with a low particle/bubble collision efficiency within the flotation process due to an unfavorable particle/bubble size ratio. In those considerations the size of the gangue particles is usually not considered. This study investigates the effect of gangue particle size on the recovery of ultrafine and fine (10–50µm) particles. Artificial, binary model particle systems, with magnetite as the target mineral and quartz as the gangue mineral, are used in this study in order to minimize reported issues associated with ultrafine gangue particles. Results indicate that ultrafine magnetite can be recovered similar to fine magnetite when the gangue particles are fine as well. In contrast, fine magnetite recovery drops significantly when ultrafine quartz is used as the gangue mineral system. This should thus open a discussion of a reconsideration of the collision efficiency models to incorporate the effect of the gangue particles.

Discrete Element Model for Suppression of Coffee-Ring Effect

When a sessile droplet evaporates, coffee-ring effect drives the suspended particulate matters to the droplet edge, eventually forming a ring-shaped deposition. Because it causes a non-uniform distribution of solid contents, which is undesired in many applications, attempts have been made to eliminate the coffee-ring effect. Recent reports indicated that the coffee-ring effect can be suppressed by a mixture of spherical and non-spherical particles with enhanced particle-particle interaction at air-water interface. However, a model to comprehend the inter-particulate activities has been lacking. Here, we report a discrete element model (particle system) to investigate the phenomenon. The modeled dynamics included particle traveling following the capillary flow with Brownian motion, and its resultant 3D hexagonal close packing of particles along the contact line. For particles being adsorbed by air-water interface, we modeled cluster growth, cluster deformation, and cluster combination. We found that the suppression of coffee-ring effect does not require a circulatory flow driven by an inward Marangoni flow at air-water interface. Instead, the number of new cluster formation, which can be enhanced by increasing the ratio of non-spherical particles and the overall number of microspheres, is more dominant in the suppression process. Together, this model provides a useful platform elucidating insights for suppressing coffee-ring effect for practical applications in the future.

Visualization of Battlefield Meteorological Environment and Battle Visual Effects Based on General Model Particle System

Battlefield environment information have multi-dimensional, multi-scale, multi-temporal state characteristics. Particle system is an effective method for three-dimensional battlefield meteorological environment and combat effects of visualization, can effectively organize, thereby efficiently Multidimensional Dynamic visualize the battlefield weather information. In this paper, the transmitter and influencers to build a common mode particle system, the model system consists of a particle emitter, particle renderer, particle control, particle composition manager, via a transmitter, controller interaction between each particle simulation kinds of changes in motion to improve the authenticity and versatility of the particle system, and through the model to achieve a three-dimensional battlefield meteorological environment and explosions and other special effects visualization.

Investigating the significance of coagulation kinetics on maintaining membrane permeability in an MBR following reactive coagulant dosing

In this study, the impact of kinetically controlled floc growth on sustaining membrane permeability following reactive coagulant dosing was determined using a model particle system. Floc formation was indicated to comprise of two stages following coagulant addition: (i) an initial destabilisation phase which encouraged complexation of protein and polysaccharide; and (ii) entrapment of the coarse model particles (3µm Firefli™ microspheres) in the polymeric complex during the floc growth phase. Floc growth was characterised by an expected time lag as with conventional flocculation systems and biopolymer aggregation was kinetically favoured. When coagulant was dosed during the filtration cycle, the intermediate biopolymer aggregates (comprised of protein and polysaccharide) were preferentially transported toward the membrane increasing fouling. However, when coagulant was dosed at the onset of filtration, membrane fouling was constrained. It is asserted that by dosing at the onset of filtration: (i) early development of biopolymer aggregation is initiated which inhibits transport of the individual biopolymers to the membrane; and (ii) by dosing coagulant in the absence of a developed polarised layer, formation of biopolymer complexes local to the membrane is obviated. However, when dosing coagulant at the onset of filtration, only limited floc growth occurred which can be explained by the low applied wall shear rate and the absence of a ‘polarised’ region which ostensibly promoted floc growth when coagulant was dosed mid-filtration. Based on results from the model particle system studied, it is proposed that reactive coagulant dosing is best undertaken when: (i) filtration is stopped; (ii) modest shear is applied within the bioreactor to promote coagulant dispersion; and (iii) sufficient contact time is allowed to promote floc growth before commencement of filtration.

Adsorption of Gemini surfactants onto clathrate hydrates

This work addresses the adsorption of two Gemini surfactants at the cyclopentane (CP) hydrate–water interface. The Gemini surfactants investigated here are Dowfax C6L and Dowfax 2A1 that have two anionic head groups and one hydrophobic tail group. The adsorption of these surfactants was quantified using adsorption isotherms and the adsorption isotherms were determined using liquid–liquid titrations. Even if the Gemini surfactant adsorption isotherms show multi-layer adsorption, they possess the first Langmuir layer with the second adsorption layer only evident in the 2A1 adsorption isotherm. Zeta potentials of CP hydrate particles in the surfactant solution of various concentrations of Dowfax C6L and Dowfax 2A1 were measured to further explain their adsorption behavior at the CP hydrate–water interface. Zeta potentials of alumina particles as a model particle system in different concentrations of sodium dodecyl sulfate (SDS), Dowfax C6L and Dowfax 2A1 were also measured to confirm the configuration of all the surfactants at the interface. The determination of the isotherms and zeta-potentials provides an understanding framework for the adsorption behavior of the two Gemini surfactants at the hydrate–water interface.

Quantification and Characterization of Micrometer and Submicrometer Subvisible Particles in Protein Therapeutics by Use of a Suspended Microchannel Resonator

The ability to characterize micrometer and submicrometer particles in solution is of fundamental importance to understanding the relationship between protein particles in biotherapeutics and concerns raised regarding immunogenicity. While a number of characterization methods are available for analyzing subvisible particle content in protein pharmaceuticals, counting and characterizing particles within the entire subvisible size range remains a significant challenge due to the properties of the proteinaceous particles themselves and to the limitations of the available techniques. Additionally, as silicone oil-lubricated prefilled syringes become a favored primary packaging for biotherapeutic products, proteinaceous subvisible particle characterization is further complicated by the presence of silicone oil droplets in solution. Here, we critically evaluate and apply a novel method for particle characterization that relies on differences in particle buoyant mass to characterize particle content in the range of ca. 0.5-5 μm. A model particle system was specifically designed to evaluate the ability of the suspended microchannel resonator (SMR) to distinguish between buoyant particles (e.g., silicone oil) and dense particles (e.g., protein particles) in aqueous solution. In addition, this emerging technique was successfully applied to high-concentration monoclonal antibody solutions stored in prefilled syringes in stressed stability studies. It is shown that the SMR system can potentially distinguish between silicone oil droplets and protein particles in a size range that is challenging for many subvisible particle characterization methods. Limitations of the SMR method are also discussed.

Widom Line for the Liquid–Gas Transition in Lennard-Jones System

The locus of extrema (ridges) for heat capacity, thermal expansion coefficient, compressibility, and density fluctuations for model particle systems with Lennard-Jones (LJ) potential in the supercritical region have been obtained. It was found that the ridges for different thermodynamic values virtually merge into a single Widom line at T < 1.1T(c) and P < 1.5P(c) and become practically completely smeared at T < 2.5T(c) and P < 10P(c), where T(c) and P(c) are the critical temperature and pressure. The ridge for heat capacity approaches close to critical isochore, whereas the lines of extrema for other values correspond to density decrease. The lines corresponding to the supercritical maxima for argon and neon are in good agreement with the computer simulation data for LJ fluid. The behavior of the ridges for LJ fluid, in turn, is close to that for the supercritical van der Waals fluid, which is indicative of a fairly universal behavior of the Widom line for a liquid-gas transition.

Analysis of the fragments of a ‘dead leaves’ mosaic via a puzzle-fitting function Φ (r, L 0)

The origin of homogeneous, randomly shaped particles embedded in a homogeneous medium can be analyzed by use of a puzzle-fitting function Φ. A well-defined range order L of the random medium is considered. The function Φ is defined in terms of the correlation function of the scattering experiment γ (r) of the whole particle ensemble and of the correlation function γ P (r) of the isolated puzzle particles. It can be written Φ (r,L) = γ”(r)/[γ ′(r)·γ P ′(r)]; (0 ≤ r < L). For fitting fragments (if all the puzzle particles belong to one and the same puzzle and fit together), Φ (0,L) = 1 is fulfilled, whatever the shape of the smooth grains of the ‘dead leaves’ tessellation which produces the fragments. These relations have been checked for several model particle systems, as well as for an alloy system for L 0 = 8 nm. Numerical details for a stable determination of Φ from the scattering experiment are included.

Real time detection of particulate heterogeneities in polymer extrusion processes using microphotometric measuring method

A novel inline measuring system can be applied to polymer extrusion to achieve real time data on melt quality. Disturbing particles such as gels, unmolten resins, black spots or gas bubbles can be detected within flowing transparent polymer melts continuously without any lag of time directly during extrusion processing. In the first part of the present paper the state of the art related to particle monitoring in polymer melts in terms of 'process analytical technology (PAT)' will be presented briefly. After this the microphotometric measuring principle of the used inline process sensors from polymer melt particle (PMP) type will be described as well as various kinds of sensor adaptation to extruder machines. First experiments were realised on labscale extruders applying model particle systems to achieve detailed information on sensor performance characteristics. As a next step industrial polymer systems have been examined on pilot plant extruders. Finally results of these experiments contributed to the transfer of inline particle measurements to production line extruders.

2-n-Pentyl-4-quinolinol produced by a marine Alteromonas sp. and its potential ecological and biogeochemical roles.

Bacterium-bacterium interactions occur at intimate spatial scales on the order of micrometers, but our knowledge of interactions at this level is rudimentary. Antagonism is a potential interaction in such microenvironments. To study the ecological role of antibiosis, we developed a model system involving an antibiotic-producing isolate (SWAT5) derived from a marine particle and its dominant antibiotic product, 2-n-pentyl-4-quinolinol (PQ). This system was used to address questions about the significance of this antibiotic for microbial ecology and carbon cycling on particles. We characterized the chemical and inhibitory properties of PQ in relation to the mechanisms used by particle-associated bacteria in interacting with particles and with other attached bacteria. PQ was produced by SWAT5 only on surfaces. When SWAT5 was grown in polysaccharide matrices, PQ diffused within the matrices but not into the surrounding seawater. SWAT5 might thus be able to generate a localized zone of high antibiotic concentration on particles suspended or sinking through seawater. Target bacterial respiration was most sensitive to PQ (75 nM), while inhibition of DNA synthesis, protein synthesis, and bacterial motility required higher (micromolar) PQ levels. The presence of PQ altered the composition of the bacterial community that colonized and developed in a model particle system. PQ also inhibited Synechococcus and phytoplankton growth. Our results suggest that antibiosis may significantly influence community composition and activities of attached bacterial and thus regulate the biogeochemical fate of particulate organic matter in the ocean.

Path integral molecular dynamics for Bose–Einstein and Fermi–Dirac statistics

In this paper, we propose a promising extension of the path integral molecular dynamics method to Bose–Einstein and Fermi–Dirac statistics. The partition function for the quantum statistics was rewritten in a form amenable to the molecular dynamics method with the aid of an idea of pseudopotential for the permutation of particles. Our pseudopotential, here, is a rigorous one describing the whole effect of Bose–Einstein and Fermi–Dirac statistics. For a model calculation, we chose a system consisting of three independent particles in a one-dimensional harmonic well. The calculation has been performed for the particles obeying Bose–Einstein and Fermi–Dirac statistics. The calculated kinetic and potential energies were in excellent agreement with the analytical results even near the ground state. It was found that the pseudopotential shows attractive and repulsive characters for the static properties of Bose–Einstein and Fermi–Dirac particles, respectively. For interacting model particle systems, we studied a bosonic triatomic cluster. The calculated thermodynamic quantities were in qualitative agreement with those obtained by Fourier path integral Monte Carlo calculation.

Particle stress in bioreactors.

In many biological processes, e.g. the fermentation of cells and sensitive microorganisms or bioconversion with immobilised enzymes, low shear stress is of crucial importance for the optimal course of processes. Starting with the causes of particle stress, the following report discussed the hydrodynamic principles of the most frequently used model reactors and bioreactors, which are required for an approximate calculation of stress. The main part of the report describes the results of systematic investigations into the hydrodynamic stress on particles in stirred tanks, reactors with dominating boundary-layer flow, shake flasks, viscosimeters, bubble columns and gas-operated loop reactors. These results for model and biological particle systems permit fundamental conclusions on particle stress and the dimensions and selection of suitable bioreactors according to the criterion of particle stress.

Coalescing Markov labelled partitions and a continuous sites genetics model with infinitely many types

Let Z be a Borel right process with Lusin state-space E. For any finite set S it is possible to associate with Z a processof coalescing partitions of S with components labelled by elements of E that evolve as copies of Z. It is shown that, subject to a weak duality hypothesis on Z, there is a Feller process X with state-space a certain space of probability measure valued functions on E. The process X has its “moments” defined in terms of expectations forin a manner suggested by various instances of martingale problem duality between coalescing Markov processes and voter model particle systems, systems of interacting Fisher-Wright and Fleming-Viot diffusions, and stochastic partial differential equations with Fisher-Wright noise. Some sample path properties are examined in the special case where Z is a symmetric stable process on ℝ with index 1 < α ≤ 2. In particular, we show that for fixed t > 0 the essential range of the random probability measure valued function X t is almost surely a countable set of point masses.

Modelluntersuchungen zur Partikelbeanspruchung in Reaktoren

AbstractThis paper reports systematic studies on hydrodynamic stress in agitated vessels with baffles, reactors with predominantly laminar flow, SERALE viscosimeters, bubble columns, and gas‐driven loop reactors. The stress on particles is determined by way of the destruction of model particle systems, with the kinetics of destruction, the equilibrium particle diameters, and the enzyme activity after a given time serving, respectively, for assessment of the stress. Thereof one obtaines an indication of the methodology necessary for determining the stress and it permits selection of appropriate reactor systems according to the criterion of particle stress. In the case of reactors with turbulent motion without dominant laminar flow or gas/liquid interfaces give analogous results, permitting the assumption that they are also applicable to other particle systems. Specifically for stirred reactors a geometric function is derived from the experimental results which permits prediction of stress caused by various types of impellers. Impellers with large blade areas relative to the dimensions of the tank produce less shearing forces owing to their uniform power input, in contrast to small and especially axial flow impellers. In laminar flow or reactors with gas/liquid interfaces the level of particle stress depends upon the proclivity of the particles to enter the boundary layers. It can therefore be deduced that particles with different surface properties, which lead to differing interactions with boundary layers and interfaces, will also experience different kinds of particle stress on laminar flow or in aeration processes. Thus the scope for application of the results obtained with model particle systems to other particle systems is limited in such reactors.

Microscopic shock structure in model particle systems: The Boghosian‐Levermore cellular automation revisited

AbstractWe carried out new computer simulations of the Boghosian‐Levermore stochastic cellular automaton for the Burgers equation. The existence of an extra “conservation law” in the dynamics—even and odd lattice sites exchange their contents at every time step—implies that the automaton decomposes into two independent subsystems; the simulations show that the density from each subsystem exhibits a “shock front” which does not broaden with time. The location of the shock in a particular microscopic realization differs from that predicted by the Burgers equation by an amount which depends only on the initial microscopic density of the particle system, that is, fluctuations in the stochastic dynamics do not affect the shock profile on the time scale considered. This is in complete accord with theoretical expectations. The apparent broadening of the shock in the original Boghosian‐Levermore simulations is shown to result from averaging the two subsystem densities.

Transverse particle trajectories in high-gradient magnetic separation

We have used human red blood cells as a model particle system for studying transverse particle trajectories relevant to high gradient magnetic separators. Trajectories measured around a single 50 μm diameter wire are compared with calculations based on potential and laminar flow models of the fluid motion. The magnetic velocity is determined in an independent experiment which precisely models potential flow. At the low Reynolds numbers (Re = 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ) used here it is found that the laminar flow modeI gives better agreement with experiment.