Numerical analysis of Pickering emulsion stability: insights from ABMD simulations

The issue of the stability of Pickering emulsions is tackled at a mesoscopic level using dissipative particle dynamics simulations within the Adiabatic Biased Molecular Dynamics framework. We consider the early stage of the coalescence process between two spherical water droplets in decane solvent. The droplets are stabilized by Janus nanoparticles of different shapes (spherical and ellipsoidal) with different three-phase contact angles. Given a sufficiently dense layer of particles on the droplets, we show that the stabilization mechanism strongly depends on the collision speed. This is consistent with a coalescence mechanism governed by the rheology of the interfacial region. When the system is forced to coalesce sufficiently slowly, we investigate at a mesoscopic level how the ability of the nanoparticles to stabilize Pickering emulsions is discriminated by nanoparticle mobility and the associated caging effect. These properties are both related to the interparticle interaction and the hydrodynamic resistance in the liquid film between the approaching interfaces.Comment: 15 pages (12 pages and Supplemental Information), 11 figures, to be published in Faraday Discussions (Royal Society of Chemistry

Interaction of particles with a cavitation bubble near a solid wall

Hard particle erosion and cavitation damage are two main wear problems that can affect the internal components of hydraulic machinery such as hydraulic turbines or pumps. If both problems synergistically act together, the damage can be more severe and result in high maintenance costs. In this work, a study of the interaction of hard particles and cavitation bubbles is developed to understand their interactive behavior. Experimental tests and numerical simulations using computational fluid dynamics (CFD) were performed. Experimentally, a cavitation bubble was generated with an electric spark near a solid surface, and its interaction with hard particles of different sizes and materials was observed using a high-speed camera. A simplified analytical approach was developed to model the behavior of the particles near the bubble interface during its collapse. Computationally, we simulated an air bubble that grew and collapsed near a solid wall while interacting with one particle near the bubble interface. Several simulations with different conditions were made and validated with the experimental data. The experimental data obtained from particles above the bubble were consistent with the numerical results and analytical study. The particle size, density and position of the particle with respect to the bubble interface strongly affected the maximum velocity of the particles

Inertial dynamics of air bubbles crossing a horizontal fluid–fluid interface

The dynamics of isolated air bubbles crossing the horizontal interface separating two Newtonian immiscible liquids initially at rest are studied both experimentally and computationally. High-speed video imaging is used to obtain a detailed evolution of the various interfaces involved in the system. The size of the bubbles and the viscosity contrast between the two liquids are varied by more than one and four orders of magnitude,respectively, making it possible to obtain bubble shapes ranging from spherical to toroidal. A variety of flow regimes isobserved,including that of small bubbles remaining trapped at the fluid–fluid interface in a film-drainage configuration.In most cases, the bubble succeeds in crossing the interface without being stopped near its undisturbed position and, during a certain period of time, tows a significant column of lower fluid which sometimes exhibits a complex dynamics as it lengthens in the upper fluid. Direct numerical simulations of several selected experimental situations are performed with a code employing a volume of-fluid type formulation of the incompressible Navier–Stokes equations. Comparisons between experimental and numerical results confirm the reliability of the computational approach in most situations but also points out the need for improvements to capture some subtle but important physical processes, most notably those related to film drainage. Influence of the physical parameters highlighted by experiments and computations, especially that of the density and viscosity contrasts between the two fluids and of the various interfacial tensions, is discussed and analysed in the light of simple models and available theories

Bubble size, coalescence and particle motion in flowing foams

In minerals processing, froth flotation is used to separate valuable metal minerals from ore. The efficiency of a froth to recover these valuable minerals is closely related to the bubble size distribution through the depth of the froth. Measurement of the bubble size entering the froth and at the froth surface has been achieved previously; however measurement of the bubble size within the froth is extremely difficult as the mineral laden bubble surfaces are opaque and fragile. This work developed a flowing foam column to enable new measurement techniques, in particular visual measurement of the bubble size distribution and velocity profile throughout the depth of the foam. Two phase foam systems share their structure with three phase froth flotation systems, but are transparent in a thin layer. A foam column was constructed to contain a monolayer of overflowing and coalescing foam. This enabled direct measurement of the dynamic bubble size and coalescence through image analysis. The results showed a strong link between column geometry and the foam behaviour. In addition, the measured bubble streamlines closely matched simulated results from a foam velocity model. Positron Emission Particle Tracking (PEPT) is the only existing technique to measure particle behaviour inside froths. In this work, tracer particles with different size and hydrophobicity were tracked in a foam flowing column with PEPT. The particle trajectories were verified with image analysis, thereby increasing confidence in PEPT measurements of opaque flotation systems. The results showed that as hydrophilic tracer particles passed through the foam, their trajectory was determined by the local structure and changes of the foam, such as coalescence events. A hydrophobic tracer particle was involved in drop–off and reattachment events, however in the majority of cases still overflowed with the foam. The tracer particle did not always follow the bubble streamlines of the flowing foam, taking instead the shortest path to overflow which cut across streamlines. This work has developed an experimental methodology to validate flowing foam and coalescence models and has developed the necessary techniques to interpret PEPT trajectories in froth flotation

Analysis of Granular Flow in a Pebble-Bed Nuclear Reactor

Pebble-bed nuclear reactor technology, which is currently being revived around the world, raises fundamental questions about dense granular flow in silos. A typical reactor core is composed of graphite fuel pebbles, which drain very slowly in a continuous refueling process. Pebble flow is poorly understood and not easily accessible to experiments, and yet it has a major impact on reactor physics. To address this problem, we perform full-scale, discrete-element simulations in realistic geometries, with up to 440,000 frictional, viscoelastic 6cm-diameter spheres draining in a cylindrical vessel of diameter 3.5m and height 10m with bottom funnels angled at 30 degrees or 60 degrees. We also simulate a bidisperse core with a dynamic central column of smaller graphite moderator pebbles and show that little mixing occurs down to a 1:2 diameter ratio. We analyze the mean velocity, diffusion and mixing, local ordering and porosity (from Voronoi volumes), the residence-time distribution, and the effects of wall friction and discuss implications for reactor design and the basic physics of granular flow.Comment: 18 pages, 21 figure

Modeling capillarity and two-phase flow in granular media: from porescale to network scale

Numerical simulations at the pore scale are a way to study the behavior of multiphase flows encountered in many natural processes and industrial applications. In this work, liquid morphology and capillary action are examined at the pore-scale by means of the multicomponent Shan-Chen lattice Boltzmann method (LBM). The accuracy of the numerical model is first contrasted with theoretical solutions. The numerical results are extended to complex microstructures beyond the pendular regime. The LBM has been employed to simulate multiphase flow through idealized granular porous media under quasi-static primary drainage conditions. LBM simulations provide an excellent description of the fluid-fluid interface displacement through the grains. Additionally, the receding phase trapped in the granular media in form of pendular bridges or liquid clusters is well captured. Unfortunately, such simulations require a significant computation time. A 2D model (Throat-Network model) based on analytical solutions is proposed to mimic the multiphase flow with very reduced computation cost, therefore, suitable to replace LBM simulations when the computation resources are limited. The approach emphasizes the importance of simulating at the throat scale rather than the pore body scale in order to obtain the local capillary pressure - liquid content relationships. The Throat-Network model is a starting point for a hybrid model proposed to solve 3D problems. The hybrid model combines the efficiency of the pore-network approach and the accuracy of the LBM at the pore scale to optimize the computational resources. The hybrid model is based on the decomposition of the granular assembly into small subsets, in which LBM simulations are performed to determine the main hydrostatic properties (entry capillary pressure, capillary pressure - liquid content relationship and liquid morphology for each pore throat). Despite the reduction of computation time, it is still not negligible and not affordable for large granular packings. Approximations by the Incircle and the MS-P method, which predict hydrostatic properties, are contrasted with the results provided by LBM and the hybrid model. Relatively accurate predictions are given by the approximations.Per tal d’estudiar els fluxos multifàsics presents a molts processos naturals i industrials és indispensable entendre les propietats físiques dels sistemes multifàsics a escala microscòpica. La morfologia dels fluids i les forces capil·lars s’investiguen a l’escala del porus mitjançant el ”multicomponent Shan-Chen lattice Boltzmann method (LBM)”. La precisió del model numèric ha estat contrastada amb solucions teòriques. Els resultats numèrics s’han estès a microestructures líquides complexes més enllà del règim pendular. El LBM ha estat emprat per simular fluxos multifàsics a través de medis porosos sota condicions quasi-estàtiques de drenatge. Les simulacions dutes a terme mitjançant el LBM proporcionen una descripció excel·lent del moviment de la interfície entre fluids a través de les partícules sòlides. Durant el drenatge, les simulacions numèriques són capaces de reproduir l’efecte del fluid atrapat dins el medi granular en forma de ponts o estructures líquides complexes. Malauradament, aquestes simulacions requereixen un temps de computació molt elevat. Per tal d’optimitzar els recursos de computació, proposem un model 2D (model Throat-Network) basat en solucions analítiques que permet reproduir fluxos multifàsics a través d’un conjunt de discs amb un temps de computació molt reduït. Per tant, aquest mètode és una alternativa que pot substituir les simulacions LBM quan els recursos de computació són escassos. El model Throat-Network destaca la importància de tractar el problema a l’escala de la gola del porus per tal d’obtenir les relacions pressió capil·lar - volum locals. Aquest enfocament és un punt de partida pel model híbrid que es presenta per resoldre els problemes en 3D. El model híbrid combina l’eficàcia del model ”Pore-Network” i la precisió del LBM a l’escala del porus. El model híbrid es basa en la descomposició d’una mostra granular en subdominis més petits, els quals corresponen a les goles dels porus (la gola dels porus és l’espai que connecta dos porus adjacents). Les simulacions LBM s’executen per a cada un dels subdominis per tal de determinar les propietats hidroestàtiques més rellevants (pressió capil·lar d’entrada, la corba de pressió capil·lar - grau de saturació i la morfologia líquida per cada una de les goles del porus). Malgrat la reducció significativa en el cost computacional del model híbrid, els temps de càlcul no són menyspreables i poc realistes per mostres granulars de grans dimensions. Les aproximacions donades pels mètodes de l’”Incircle” i el MS-P, que permeten estimar les propietats hidroestàtiques, han estat contrastades amb els resultats obtinguts amb LBM i el model híbrid.Les simulations numériques à l’échelle du pore sont fréquemment utilisées pour étudier le comportement des écoulements multiphasiques largement rencont des structures liquides et l’actiorés dans phénomènes naturels et applications industrielles. Dans ce travail, la morphologien capillaire sont examinées à l’échelle des pores par la méthode de Boltzmann sur réseau (LBM) à plusieurs composants selon le modèle de Shan-Chen. Les résultats numériques obtenus sont en bon accord avec les solutions théoriques. Les simulations numériques sont étendues à microstructures complexes au-delà du régime pendulaire. La LBM a été utilisée pour modéliser l’écoulement multiphasique à travers un milieu poreux idéalisé dans des conditions de drainage primaire quasi-statique. Les simulations LBM ont fourni une excellente description du déplacement de l’interface fluide-fluide à travers les grains. Pendant le drainage, les simulations LBM sont capables de reproduire la déconnexion d’une phase dans le milieu granulaire sous la forme de ponts pendulaires ou structures liquides complexes. Malheureusement, le temps de calcul nécessaire pour ce type de simulations est assez élevé. Afin d’optimiser les ressources de calcul, nous présentons un modèle 2D (modèle Throat-Network) basé sur des solutions analytiques pour décrire l’écoulement biphasique à travers un ensemble de disques dans un temps de calcul très réduit, donc le modèle 2D est susceptible de remplacer les simulations LBM lorsque les ressources de calcul sont limitées. L’approche souligne l’importance de simuler le problème a l’échelle de la gorge du pore pour obtenir les relations volume - pression capillaire locales. Le modèle Throat-Network est un point de départ pour le modèle hybride proposé pour résoudre les problèmes en 3D. Le modèle hybride combine l’efficacité de l’approche réseau de pores et la précision du LBM à l’échelle des pores. Le modèle hybride est basé sur la décomposition de l’échantillon en petits sous-domaines, dans lesquels des simulations LBM sont effectuées pour déterminer les propriétés hydrostatiques principales (pression capillaire d’entrée, courbe de drainage primaire et morphologie du liquide pour chaque gorge du pore). Malgré la réduction significative des temps de calcul obtenus avec le modèle hybride, le temps n’est pas négligeable et les modélisations numériques d’échantillons de grandes tailles ne sont pas réalistes. Les approximations données par les méthodes Incircle et MS-P, qui prédisent les propriétés hydrostatiques, sont comparées à celles de LBM et du modèle hybride

Modeling capillarity and two-phase flow in granular media: from porescale to network scale

Tesi en modalitat de cotutela: Universitat Politècnica de Catalunya i Université Grenoble Alpes. Resums extesos en francès i català a l'apendeix de la tesi.Numerical simulations at the pore scale are a way to study the behavior of multiphase flows encountered in many natural processes and industrial applications. In this work, liquid morphology and capillary action are examined at the pore-scale by means of the multicomponent Shan-Chen lattice Boltzmann method (LBM). The accuracy of the numerical model is first contrasted with theoretical solutions. The numerical results are extended to complex microstructures beyond the pendular regime. The LBM has been employed to simulate multiphase flow through idealized granular porous media under quasi-static primary drainage conditions. LBM simulations provide an excellent description of the fluid-fluid interface displacement through the grains. Additionally, the receding phase trapped in the granular media in form of pendular bridges or liquid clusters is well captured. Unfortunately, such simulations require a significant computation time. A 2D model (Throat-Network model) based on analytical solutions is proposed to mimic the multiphase flow with very reduced computation cost, therefore, suitable to replace LBM simulations when the computation resources are limited. The approach emphasizes the importance of simulating at the throat scale rather than the pore body scale in order to obtain the local capillary pressure - liquid content relationships. The Throat-Network model is a starting point for a hybrid model proposed to solve 3D problems. The hybrid model combines the efficiency of the pore-network approach and the accuracy of the LBM at the pore scale to optimize the computational resources. The hybrid model is based on the decomposition of the granular assembly into small subsets, in which LBM simulations are performed to determine the main hydrostatic properties (entry capillary pressure, capillary pressure - liquid content relationship and liquid morphology for each pore throat). Despite the reduction of computation time, it is still not negligible and not affordable for large granular packings. Approximations by the Incircle and the MS-P method, which predict hydrostatic properties, are contrasted with the results provided by LBM and the hybrid model. Relatively accurate predictions are given by the approximations.Per tal d’estudiar els fluxos multifàsics presents a molts processos naturals i industrials és indispensable entendre les propietats físiques dels sistemes multifàsics a escala microscòpica. La morfologia dels fluids i les forces capil·lars s’investiguen a l’escala del porus mitjançant el ”multicomponent Shan-Chen lattice Boltzmann method (LBM)”. La precisió del model numèric ha estat contrastada amb solucions teòriques. Els resultats numèrics s’han estès a microestructures líquides complexes més enllà del règim pendular. El LBM ha estat emprat per simular fluxos multifàsics a través de medis porosos sota condicions quasi-estàtiques de drenatge. Les simulacions dutes a terme mitjançant el LBM proporcionen una descripció excel·lent del moviment de la interfície entre fluids a través de les partícules sòlides. Durant el drenatge, les simulacions numèriques són capaces de reproduir l’efecte del fluid atrapat dins el medi granular en forma de ponts o estructures líquides complexes. Malauradament, aquestes simulacions requereixen un temps de computació molt elevat. Per tal d’optimitzar els recursos de computació, proposem un model 2D (model Throat-Network) basat en solucions analítiques que permet reproduir fluxos multifàsics a través d’un conjunt de discs amb un temps de computació molt reduït. Per tant, aquest mètode és una alternativa que pot substituir les simulacions LBM quan els recursos de computació són escassos. El model Throat-Network destaca la importància de tractar el problema a l’escala de la gola del porus per tal d’obtenir les relacions pressió capil·lar - volum locals. Aquest enfocament és un punt de partida pel model híbrid que es presenta per resoldre els problemes en 3D. El model híbrid combina l’eficàcia del model ”Pore-Network” i la precisió del LBM a l’escala del porus. El model híbrid es basa en la descomposició d’una mostra granular en subdominis més petits, els quals corresponen a les goles dels porus (la gola dels porus és l’espai que connecta dos porus adjacents). Les simulacions LBM s’executen per a cada un dels subdominis per tal de determinar les propietats hidroestàtiques més rellevants (pressió capil·lar d’entrada, la corba de pressió capil·lar - grau de saturació i la morfologia líquida per cada una de les goles del porus). Malgrat la reducció significativa en el cost computacional del model híbrid, els temps de càlcul no són menyspreables i poc realistes per mostres granulars de grans dimensions. Les aproximacions donades pels mètodes de l’”Incircle” i el MS-P, que permeten estimar les propietats hidroestàtiques, han estat contrastades amb els resultats obtinguts amb LBM i el model híbrid.Les simulations numériques à l’échelle du pore sont fréquemment utilisées pour étudier le comportement des écoulements multiphasiques largement rencont des structures liquides et l’actiorés dans phénomènes naturels et applications industrielles. Dans ce travail, la morphologien capillaire sont examinées à l’échelle des pores par la méthode de Boltzmann sur réseau (LBM) à plusieurs composants selon le modèle de Shan-Chen. Les résultats numériques obtenus sont en bon accord avec les solutions théoriques. Les simulations numériques sont étendues à microstructures complexes au-delà du régime pendulaire. La LBM a été utilisée pour modéliser l’écoulement multiphasique à travers un milieu poreux idéalisé dans des conditions de drainage primaire quasi-statique. Les simulations LBM ont fourni une excellente description du déplacement de l’interface fluide-fluide à travers les grains. Pendant le drainage, les simulations LBM sont capables de reproduire la déconnexion d’une phase dans le milieu granulaire sous la forme de ponts pendulaires ou structures liquides complexes. Malheureusement, le temps de calcul nécessaire pour ce type de simulations est assez élevé. Afin d’optimiser les ressources de calcul, nous présentons un modèle 2D (modèle Throat-Network) basé sur des solutions analytiques pour décrire l’écoulement biphasique à travers un ensemble de disques dans un temps de calcul très réduit, donc le modèle 2D est susceptible de remplacer les simulations LBM lorsque les ressources de calcul sont limitées. L’approche souligne l’importance de simuler le problème a l’échelle de la gorge du pore pour obtenir les relations volume - pression capillaire locales. Le modèle Throat-Network est un point de départ pour le modèle hybride proposé pour résoudre les problèmes en 3D. Le modèle hybride combine l’efficacité de l’approche réseau de pores et la précision du LBM à l’échelle des pores. Le modèle hybride est basé sur la décomposition de l’échantillon en petits sous-domaines, dans lesquels des simulations LBM sont effectuées pour déterminer les propriétés hydrostatiques principales (pression capillaire d’entrée, courbe de drainage primaire et morphologie du liquide pour chaque gorge du pore). Malgré la réduction significative des temps de calcul obtenus avec le modèle hybride, le temps n’est pas négligeable et les modélisations numériques d’échantillons de grandes tailles ne sont pas réalistes. Les approximations données par les méthodes Incircle et MS-P, qui prédisent les propriétés hydrostatiques, sont comparées à celles de LBM et du modèle hybride.Postprint (published version

Drop coalescence in technical liquid/liquid applications : a review on experimental techniques and modeling approaches

The coalescence phenomenon of drops in liquid/liquid systems is reviewed with particular focus on its technical relevance and application. Due to the complexity of coalescence, a comprehensive survey of the coalescence process and the numerous influencing factors is given. Subsequently, available experimental techniques with different levels of detail are summarized and compared. These techniques can be divided in simple settling tests for qualitative coalescence behavior investigations and gravity settler design, single-drop coalescence studies at flat interfaces as well as between droplets, and detailed film drainage analysis. To model the coalescence rate in liquid/liquid systems on a technical scale, the generic population balance framework is introduced. Additionally, different coalescence modeling approaches are reviewed with ascending level of detail from empirical correlations to comprehensive film drainage models and detailed computational fluid and particle dynamics