High concentration Brownian coagulation in jet flow using two enhancement formulations

Ultra-fine particle coagulation by Brownian motion at high concentration in planar jet flow is simulated. A Taylor-Series Expansion Method of Moments is employed to solve the particle general dynamic equation. The volume fraction gets high value, very closes to that at the nozzle exit. As the vortex pairing develops, the high volume fraction region rolls out and mixes with the low value region. The enhancement factor given by Trzeciak et al. will be less than one at some specific outer positions, which seems to be less accurate than the one given by Heine et al

Pairwise hydrodynamic interactions of permeable particles and flow-induced structuring in dilute suspensions

The study of particle-level interactions in suspension flows enables a better understanding and control of systems where suspensions play a key role. Examples include the physiology of blood flow microcirculation, flow-induced segregation of polydisperse suspensions, particle aggregation in marine environments, and filtration processes, to name a few. In these systems, a detailed description of the hydrodynamic interactions between the particles and between the particles and fluid boundaries characterize the evolution of the suspension microstructure. Typically, the characteristic size of the particles is small compared to the imposed flow length scale, and hence low-Reynolds-number conditions usually apply. In the dilute regime, pair-interactions between smooth, rigid and spherical particles yield symmetric particle trajectories with zero net cross-flow displacements and thus no structuring. However, short-range phenomena including material specific forces, e.g., electrostatic repulsion and van der Waals attraction, and physical properties of the particles, e.g., particle permeability, surface roughness, and interface mobility, break the symmetry of particle trajectories resulting in net particle displacements and hence particle structuring. This thesis contains a detailed analysis of the near-contact motion of permeable particles in the limit of weak surface permeability where Darcy\u27s law is used to describe the flow in the permeable medium. A full set of resistance and mobility functions that relates particle motion to forces, torques, and stresslets acting on the particles are calculated. Results show that non-zero values of particle permeability qualitatively alter the near-contact particle motion, removing the classical lubrication resistance between approaching smooth impermeable spheres that prevents particle contact under the action of a finite force without the need for nonhydrodynamic interparticle forces (van der Waals attraction). Particle permeability also qualitatively alters the tangential motion of particles, providing access to non-singular rolling motion of particles along walls. This analysis may help to predict the capacity for crossflow filtration devices. Analytical closed-form expressions are derived for binary collision rates for permeable particles in Brownian motion, gravity sedimentation, uniaxial straining, and shear flow. Here, the solution of the analogous problem of binary collision rates of particles with small-amplitude surface roughness provide accurate approximations for the collision rates of permeable particles for all aggregation mechanisms considered herein. Finally, a pairwise hydrodynamic theory is presented for flow-induced particle distributions in dilute polydisperse suspensions. Diffusive fluxes and a drift velocity in non-homogeneous shear flows are obtained from a Boltzmann-like master equation. A boundary-layer analysis in regions of vanishing shear rates (e.g., centerline of a channel flow) overcomes the failure of the current theories that predict aphysical singular behavior. The analysis presented herein yields non-singular particle distributions that qualitatively resemble experimental results in the literature. Results for bidisperse suspensions show that size segregation occurs in Poiseuille flow leading to relative enrichment of larger or smaller species at the centerline, depending on the size ratio, relative number densities, and physical properties of the particles

Miniaturization of Electrical Ultrafine Particle Sizers

Nanoparticles, or ultrafine particles, have potential risks for human health, and the adverse health effects caused by ultrafine particles have been proven to be size related. To meet the increasing demanding for personal exposure monitoring and spatial distribution measurements of ultrafine particles, this dissertation studied the development and miniaturization of electrical ultrafine particle sizers (EUPS). There are three essential components for developing a EUPS unit: a charger to electrically charge the sample particles, an electrical mobility classifier to classify the charged particles, and a downstream particle count detector to measure the number concentrations. Two generations of EUPS were developed in this dissertation. The first generation was a precipitator–type (p–type EUPS, which was assembled with a miniature corona–discharge unipolar charger, a miniature disk–type precipitator, and a portable condensation particle counter. All three components were calibrated under the optimized operation conditions. By combining the component calibration results, a data inversion scheme was developed to retrieve particle size distribution from measured signals. Size distribution measurement performance of the p–type EUPS prototype was then evaluated with both laboratory generated aerosols and field ambient aerosols. Evaluation results solidly verified the size distribution measurement reliability and flexibility of the p–type EUPS. Several possible improvements were implied, for a more precise EUPS size distribution measurement, based on the p–type EUPS development. These improvements were realized in the second part of this dissertation, as the component development and evaluation for a second generation EUPS. A new corona–discharge based, miniature unipolar aerosol charger was developed and evaluated. The new charger design made significant improvements in both intrinsic and extrinsic charging efficiency, and it also maintained a more stable charging performance. To improve the electrical mobility classification resolution, a miniature electrostatic aerosol classifier (EAC) prototype, named the Dumbbell EAC, was designed as an improved replacement of the mini–disk precipitator for the next generation EUPS. It had a novel axial–symmetric dumbbell–shaped curved classification channel design, to achieve an extended classification length within the compact overall device size. The Dumbbell EAC classification performance was evaluated both numerically and experimentally. According to both evaluation results, this palm size device, with its higher aerosol to sheath flow ratio as up to 1:5, and extended detectable size range from 10 to 850 nm, provided an improved solution for more precise portable size distribution measurements by the next generation EUPS

Application of Particle Transfer by Dipping Using Polymer Binder

The demand for particle transfer is increasing in various industries, such as manufacturing, metal joining, microfluidics, roller lubrication process, fuel cells, super-capacitors, hybrid coating, and protective layer applications. As a result, the importance of efficient transfer of solid micron-size particles is becoming more crucial. Submicron-sized particles can easily adhere to solid substrates due to negligible gravitational force, while micron-sized or larger particles require a binder to overcome the gravitational effects. This thesis aims to investigate the interactions between microparticles and polymer thin film on cylindrical substrates using particle transfer methods. The process parameters are optimized and demonstrated two applications of this process: sorting particles based on their size from poly-disperse particle mixtures and controlling the friction force of the rods. To transfer particles into a substrate, a density-mismatching heterogeneous suspension is utilized, where kinetic energy is supplied by a magnetic stirrer’s rotation to keep the particles suspended during transfer. Initially, the effect of magnetic stirrer rotation and binder concentration on the optimal particle transfer was investigated. As a result of optimizing process parameters, a novel technique was developed for filtering poly-disperse particles from density mismatching heterogeneous mixtures at the solid-liquid interface (submerged condition) using entrapment instead of the conventional entrainment approach used in dip-coating processes. The polymer layer thickness formed over the substrate is controlled by controlling the binder concentration in the suspension. The binder concentration is varied from ϕb = 1% to ϕb = 13% at different intervals and the particle concentration is kept fixed ϕp = 10%. The viscosity is measured at room temperature (25 ºC) to observe the behavior of the suspension using a rotational rheometer. The variation in the polymer layer thickness controls the size of the entrapped particles. This work successfully showed the size-based separation of particles from a poly-disperse particle mixture. Another aspect of this thesis involved the systematic control of frictional forces between elastic rods in contact by transferring particles via dip-coating. Non-spherical particles adhere to the rods using a polymeric binder. A custom continuous dip-coating setup was constructed in the laboratory to coat the elastic rods. The particle delivery over the rods is regulated by controlling the concentration of particles in the suspension. Particle concentration in the suspension is varied from ϕp = 1% to ϕp = 13% at different intervals to observe the effect of variation of particle concentration keeping the binder concentration fixed (ϕb = 5%). The coated rods are dried in the oven to overcome the effect of the solvent during the friction force measurement. Table-top experimental setup with a push-pull digital force gauge is used to measure the variation friction force at different pulling lengths of overhand knots with a variety of unknotting numbers. This work successfully demonstrates a novel method of controlling the friction force of elastic rods by controlling the particle concentration in the suspension

Filtration of NaCl and WOx Nanoparticles using Wire Screens and Nanofibrous Filters

Airborne nanoparticle filtration is essential for the protection of public health and the environment. The principles and fundamentals of air filtration have been validated with respect to micron particles; however, the mechanisms for airborne nanoparticle filtration are still uncertain. Conventional filtration theory states that diffusion dominates the behavior of submicron particles and that filtration efficiency increases inversely with the size of fine particles. This theory implies that nanoparticles can be effectively captured by properly designed air filters. However, some researchers have pointed out that single-digit nanosized airborne particles may behave like gas molecules upon impacting the filter media, if the kinetic energy is greater than the adhesion energy. As a result, such small nanoparticles may rebound from the filter media upon collision, in what is called thermal rebound. However, this phenomenon has not yet been observed in experimental studies, so uncertainties are still associated with the concept of thermal rebound, which itself has yet to be either proven or disproven. Despite the large amount of work done on nanoparticle filtration, there is still a gap between theory and experiments. This research aims to understand the interaction between nanoparticles and various filter media. The following tasks were done to achieve this goal: 1) Determining the performance of a scanning mobility particle sizer coupled with a Faraday cup electrometer (SMPS+E) for sizing airborne nanoparticles, 2) Implementing the nanoparticle filtration tests using wire screens for various particle number concentration distributions, 3) Developing a new thermal rebound model to determine the particle size at which thermal rebound occurs, 4) Characterizing PVA nanofibrous filters for nanoparticle removal, 5) Evaluating the effects of particle concentration on the filtration of PVA nanofibrous filters. Before conducting any filtration efficiency measurement, the performance of the scanning mobility particle sizer coupled with a Faraday cup electrometer (SMPS+E, GRIMM 7.860) was analyzed for NaCl and WOx particles at various particle number concentration distributions. The effects of instrument parameters, including the sheath air flow rate and sample air flow rate on particle number concentration distribution and on the lower and upper particle size detection limits were investigated theoretically and experimentally. For both types of nanoaerosol particles, the measurement of particle number concentration distribution depended on the selection of sheath air and sample air flow rate ratio, which depended on the initial particle concentrations and aerosol flow rate. Due to the high resolution of GRIMM SMPS+E for particle classification, no mobility shift was observed. The filtration efficiencies of nanoparticles with a broad size range and concentrations were then determined for uncharged micron-sized stainless-steel wire screens. Results showed that the filtration efficiency of WOx nanoparticles depends on particle number concentration distribution. For particles smaller than the mode size, the filtration efficiency followed the conventional theoretical model; however, the filtration efficiency deviated from that conventional model for larger particles. This result is likely due to the different morphology of WOx particles, which affects both particle charging and measurement performance. The next step was to develop a new filtration model by considering the possibility of thermal rebound effect. Theoretical analysis showed that, when nanoparticles collide on a solid filter media, it is more likely for plastic deformation to occur than elastic deformation does. Therefore, the nanoparticle filtration model was developed based on the assumption of plastic deformation of nanoparticles upon impaction to the surface of the filter media. Furthermore, results showed that the probability of thermal rebound increases inversely with relative humidity, which attenuates capillary force. The interactions between nanoparticles and various filter media are characterized by surface loading. Electrospun PVA nanofibrous filters were thus fabricated, then characterized in terms of fiber size distribution by SEM analysis using an automated tool, and their filtration performances were evaluated using NaCl nanoparticles. The higher the applied voltage and tip-to-collector distance and the shorter the deposition time, the higher the quality factors of the nanofibrous filters. Furthermore, the filter quality factor can be greatly improved by stacking up single layer filters made in a short deposition time. The electrospun PVA nanofibrous filters were then tested for sub-4 nm WOx particles with triple modal particle number concentration distributions at a low relative humidity (RH=2.9%). The upstream particle concentration affected the performance of nanofibrous filters, as it was higher at the lower particle concentration. The filtration efficiency for sub-2 nm particles showed that the particle critical diameter, below which thermal rebound may happen, is in the range of 1.07-1.17 nm. An analytical model was developed to predict the effects of particle concentration. Comparison between the developed model and experiments showed qualitative agreement; but more research is needed to further improve the model

Computational modelling of thermal spraying processes

The main aim of this project is to model the effects of varied injection parameters on the gas dynamics and droplet dynamics of the HVSFS and SP- HVOFS processes for improving the droplet breakup and evaporation to enhance the nanoparticles heating and deposition efficiency. Thermal spraying processes are widely used to generate thermal-, corrosion-, and wear-resistant layers over the machine parts, to increase the durability of the equipment under severe environmental conditions. The liquid feedstock is used to achieve nanostructured coatings. It is used either in the form of a suspension or a solution precursor. The suspension is a mixture of solid nanoparticles suspended in a liquid medium consisting, for instance, of water, ethanol, or isopropanol. This dispersion mechanism in a liquid carrier provides adequate flowability to the nanoparticles, which cannot be handled by conventional gas- based feeding systems, whereas the solution precursor is mixed at the molecular level; hence, more uniform phase composition and properties are expected in the sprayed coatings as compared to the suspension and conventional powder spraying. Firstly, experiments are conducted to analyse the effects of different precursor concentrations, solvent types and injection nozzles on the size and morphology of synthesized nanoparticles. The results indicate that the particle size increased with increasing precursor concentration due to the variations in the physical properties of the mixture solution. The higher precursor concentrations had an adverse effect on the droplet atomization and evaporation process that led to bigger size particle formation. The use of aqueous solvent has some limits and with higher precursor concentration the surface tension increases that resulted in the reduction of droplets’ disintegration, and thus bigger size precursor droplets generate larger nanoparticles. A mixture of aqueous-organic solvents and pure organic precursors are preferred to improve the process efficiency of the nanoparticles size and morphology. Furthermore, the nanoparticles size can be controlled by using liquid feedstock atomization before injecting into the HVOF torch. A new effervescent injection nozzle is designed and compared to different types of existing injection nozzles, to see the variations in the droplet disintegration, and its effects on the performance of the HVOF torch processes. It is detected that the atomization would result in smaller size particles with homogeneous morphology. In a numerical study, different droplet injection types are analysed to see their effects on the gas and droplet dynamics inside the HVOF torch. The group-type injection (GTI) and effervescent-type atomization (ETI) are used effectively to overcome the heat losses and delays in the droplet evaporation. These approaches reduce the thermal and kinetic energy losses in the suspension-fed-HVOF torch, thereby improving the coating formation. The effects of using multicomponent water-ethanol mixture injection in the HVOF torch are also modelled, and its impact on the droplet breakup and evaporation are studied. The organic solvents have a low heat of vaporization and surface tension, and can effectively be used in the HVOF spraying process over the water-based solvents. Furthermore, nanoparticles are suspended in the liquid feedstock and injected into the HVOF torch. The effect of increasing nanoparticles’ concentration in the feedstock and its consequence on the gas dynamics, droplet breakup and evaporation are analysed. The augmentation in the nanoparticles loading in the suspension droplets can decrease the droplet breakup and evaporation rate because the required heat of vaporization increases significantly. Moreover, the size of injection droplet affects the droplet fragmentation process; bigger sized droplets observed a delay in their evaporation that resulted in coating porosity. The results suggest that smaller droplet sizes are preferred in coating applications involving a higher concentration of nanoparticles with high melting point. Further, the gas flow rates (GFRs) are regulated to control the droplet dispersion, atomization and evaporation inside the solution precursor fed-HVOF torch. The size of the droplet diameter is decreased by an increment in the GFR, as higher combustion rates increase the combustion flame enthalpy and kinetic energy. Moreover, the increase in the oxygen/fuel flow rates dilutes the injected precursor. It reduces ZrO2 concentration in the process and decreases the rate of particle collision; as a result, non-agglomerated nanoparticles can be obtained

Synthesis and application of colloids in soft matter systems

We explore two distinct domains in the field of soft matter. The first three experimental chapters concern the synthesis, characterisation and application of Janus particles fabricated by heterogeneous polymerisation techniques. Initially in Chapter 2 we describe an optimised one pot seeded emulsion polymerisation strategy to render submicron amphiphilic Janus particles exhibiting surface active behaviour which can be tuned by the variation of hydrophilic to hydrophobic lobe volume ratios. These particles have been shown to inhibit ice recrystallisation in aqueous systems. In Chapter 3 we explore the synthesis of hard-soft Janus particles comprising of respective high and low glass transition temperature lobes. Although the rate of polymerisation is unaffected by available seed particle surface area, particles with multiple soft lobes and secondary nucleation occur below a seed surface area threshold. We additionally demonstrate the ability to fabricate sub-micron hard-soft Janus particles. Chapter 4 utilises the particles made in the previous chapter as building blocks to fabricate ‘colloidal molecules’ and colloidosomes. In the former case, cluster morphology of particles is shown to be governed by surface area minimisation of the central soft domain. The final two experimental chapters explore two different strategies to emulsify water into chocolate whilst retaining the desirable physical characteristics of the confectionery. In Chapter 5 we utilise colloidal silica and a cationic polyelectrolyte to generate highly stable quiescent Pickering emulsions, allowing up to 50% of the fat content in chocolate to be replaced with water and fruit juice. Chapter 6 improves upon this work by allowing the replacement of up to 80% of the fat content in chocolate by the dispersion of aqueous hydrogels within the chocolate fat matrix. In both chapters we characterise the physical properties of the formulations and demonstrate their suitability for use in chocolate confectionery

Combined physical and oxidative stability of food Pickering emulsions

Many food products contain lipid droplets dispersed in an aqueous phase (e.g., milk, mayonnaise), thus are oil-in-water (O/W) emulsions. Food emulsions may be subjected to destabilization, both from a physical and a chemical perspective. Physical destabilization is generally prevented by the use of conventional emulsifiers such as surfactants and proteins. Chemical destabilization, in particular lipid oxidation, is a major concern in food products, especially when healthy polyunsaturated fatty acids are present, and this degradation is usually mitigated by the use of synthetic antioxidants, often in large amounts. The use of alternative ingredients for the formulation of food emulsions has been emerging, for example solid particles (so-called Pickering particles, that are very popular nowadays) that irreversibly adsorb to the interface and therewith provide high physical stability; or natural antioxidants such as tocopherols and rosemary extracts, which are attractive in the current clean-label trend to prevent lipid oxidation. The efficiency of these natural antioxidants is unfortunately often not optimal, which can be explained by their tendency to locate into the oil or water phase, whereas lipid oxidation is initiated at the oil-water interface, and thus is the place where antioxidants should be located to optimally exert their protective effect. The objective of this project was to develop food emulsions with a new and controlled architecture directed at yielding both excellent physical and oxidative stability. In these emulsions the oil droplets were covered by food-grade Pickering particles that exert a double role: they act as physical stabilizers, and as a reservoir for antioxidant molecules located close to the oil-water interface, therewith preventing the first lipid oxidation events, which is expected to drastically enhance antioxidant activity. The first part of this thesis focused on the preparation and characterization of a new food-grade lipid-based Pickering particles, referred to as colloidal lipid particles (CLPs). We prepared both surfactant-covered and protein-covered CLPs, and found that the type of emulsifier largely determined their morphology: protein-covered CLPs were roughly spherical, whereas surfactant-covered CLPs looked more lath-like (Chapters 3 and 6). We also showed that the lipid material alters the crystal polymorphism and subsequent CLP structure, which consequently influenced their performance as emulsion stabilizers (Chapter 3). For instance, surfactant-covered CLPs containing only high melting point lipids showed highly ordered crystalline structures, and formed jammed, cohesive interfacial layers once adsorbed onto oil droplets, whereas the ones containing a fraction of low melting point lipids showed less ordered crystalline structures and formed thin and bridged layers. Since protein-covered CLPs were particularly resilient to subsequent emulsification processes, these particles were used to study the formation of emulsion droplets in a microfluidic device and their stability to short-term coalescence (Chapter 4). We found a non-monotonic dependency of the droplet stability on the particle concentration: at low surface coverage, CLPs had a destabilizing effect as incompletely covered surfaces led to droplet-droplet bridging and subsequent coalescence, whereas at higher surface coverage, particles formed an effective barrier against droplet coalescence, resulting in physically stable emulsions over the time scales probed. As a next step, we investigated lipid oxidation in Pickering emulsions stabilized by protein-based CLPs that did not contain antioxidants (Chapter 5). We showed that these Pickering emulsions had a similar oxidative stability as conventional protein-stabilized emulsions for a similar composition of the oil droplets. Yet, when in both emulsions the same amount of solid lipids was present (either as stabilizing CLPs, or within the oil droplet core), a Pickering emulsion had a higher physicochemical stability. This shows that the location of crystallizable lipids influences lipid oxidation in O/W emulsions, and thus needs to be carefully considered in emulsion design. CLPs that did contain the lipophilic antioxidant α-tocopherol are presented in Chapter 6. The chemical stability of α-tocopherol was negatively influenced by lipid crystallization that probably promoted the localization of α-tocopherol close to the particle surface, which was further enhanced by emulsifiers that actively induce lipid crystallization. When applied as Pickering stabilizers in O/W emulsions (Chapter 7), lipid oxidation was reduced compared to control emulsions with the same composition and structure, but where the antioxidant was present in the core of the oil droplets. This confirmed that the interfacial localization of the antioxidant is crucial to prevent lipid oxidation in emulsions, and that the two main instability issues (i.e., physical and chemical instability) of emulsions can be mitigated through one single approach. After establishing the proof of concept with the CLPs, we used biobased particles (that may contain antioxidants) from various natural sources to stabilize O/W emulsions (Chapter 8). Emulsions stabilized by matcha tea powder or spinach leaf powder were both highly physically and oxidatively stable, which shows that the double functionality that we achieved using purposely built particles (CLPs) can also be achieved with naturally occurring particles. In the general discussion of the thesis (Chapter 9) we describe that the dual functionality of CLPs can also be reached using other food components, which makes this approach a generic one. We expect that the system could be further improved, for example, by increasing the residence time of antioxidants at the interface. To do so, we probably need to link the time scale at which the relevant oxidation events occur with those during which the antioxidant actually resides at the interface. Follow-up research on entrapment of antioxidants within particles is needed to reach long residence times at the interface while not compromising the ability of antioxidants to exert their chemical activity. To conclude: through our approach the highly-stable food emulsions of the future may come within reach

Self-assembly of granular particles

Granular particles are ubiquitous in nature and daily life, and have wide applications in various disciplines such as infrastructure engineering, architecture, agriculture, etc. Yet, their fundamentals have not been fully understood by scientists. This is mainly because the structure of granular particles, which determines their properties, is complicated and can experience critical changes from disorder to ordered state. In recent years, understanding the fundamentals of such critical structural transitions of granular materials has become a hot multidisciplinary research topic attracting both scientists and engineers. Generally the transition from disordered to ordered structure can be regarded as a self-assembly process, which happens at different scales. In the nucleation of crystals, atoms or molecules can self-assemble due to thermal energy. For such thermodynamics systems, the theory of self-assembly is well established and is dependent on the Gibbs free energy. However, granular particles are much bigger and can dissipate energy quickly with the collision between particles, so they are normally at athermal or low-thermal states. The granular packings are prone to be disordered in structure, whereas they can also self-assemble with the input of external energy via vibration or shear, which can densify the granular packings and hence improve their properties for different applications. This thesis is devoted to advancing the knowledge of the self-assembly of granular spheres, particularly in better understanding the effects of the energy input and the boundary shape. The thesis has revealed a rich and deep picture for the effect of various factors on the self-assembly of granular particles, including the vibration mode, the container shape, material properties, different wall motions and gravity. The obtained results can improve the current understanding of the structural evolution and phase transition of the granular packings with or without vibration. The findings of this study enhance the knowledge on the self-assembly of granular systems and help take a step forward toward stablishing the mechanism behind the phenomenon. Thorough comprehension of the structure of the granular particles are essential for controlling the behaviour and properties of the granular materials, which can be of paramount importance for both the science and technology and have sensible influence on the mankind’s life