R. Skartlien

DPD Molecular Simulations of Asphaltene Adsorption on Hydrophilic Substrates: Effects of Polar Groups and Solubility

Adsorption of asphaltenes onto a polar substrate (e.g., a mineral) was modeled with dissipative particle dynamics (DPD) simulations, using continental asphaltene models. The adsorption mechanisms in 10–20% wt, of asphaltene in toluene/ heptane solutions were studied (well above the solubility limit). The structure in the adsorbed layer was highly sensitive to the presence of polar groups in the alkyl side chains and heteroatom content in the aromatic ring structure. Four types of asphaltene models were used: completely apolar (zero adsorption), apolar chains and polar heteroatoms, polar chains and no heteroatoms, and polar chains and heteroatoms (maximum adsorption). One hundred asphaltene monomers were distributed homogeneously in the solvent initially, in a ∼(10 nm)3 domain. Asphaltene monomers adsorbed irreversibly on the substrate via the polar group in the side chains, resulting in an average perpendicular orientation of the aromatic rings relative to the substrate. More frequent π–π stacking of the aromatic rings occurred for less solubility (more heptane), as in aggregates. With apolar side chains, only the heteroatoms in the aromatic ring structure had affinity to the substrate, but the ring plane did not have any preferred direction. An important finding is that the aromatic ring assemblies “shielded” the substrate and polar groups that were anchored to the substrate, resulting in an effective non-polar surface layer seen by asphaltenes in the bulk, leading to much lower adsorption probability of the remaining asphaltenes. This “adsorption termination” effect leads to mono-layer formation. Continued adsorption with multilayering and reversible nanoaggregate adsorption occurred when both side chains in the model asphaltene (located on opposite sides of the aromatic sheet) contained polar groups, with a higher probability of exposing further polar groups to the bulk asphaltene. The general conclusion is that the number and position of the polar groups in side chains determine to a large degree the adsorption and aggregation behavior/efficiency of (continental) asphaltenes, in line with experimental evidence. The heteroatoms in the aromatic ring structure plays a more passive role in this context, only by providing organization via more π–π stacking in the adsorbed layer, and in aggregates. GRAPHICAL ABSTRACT

Droplet size distributions in turbulent emulsions: Breakup criteria and surfactant effects from direct numerical simulations

Lattice Boltzmann simulations of water-in-oil (W/O) type emulsions of moderate viscosity ratio (≃1/3) and with oil soluble amphiphilic surfactant were used to study the droplet size distribution in forced, steady, homogeneous turbulence, at a water volume fraction of 20%. The viscous stresses internal to the droplets were comparable to the interfacial stress (interfacial tension), and the droplet size distribution (DSD) equilibrated near the Kolmogorov scale with droplet populations in both the viscous and inertial subranges. These results were consistent with known breakup criteria for W/O and oil-in-water emulsions, showing that the maximum stable droplet diameter is proportional to the Kolmogorov scale when viscous stresses are important (in contrast to the inviscid Hinze-limit where energy loss by viscous deformation in the droplet is negligible). The droplet size distribution in the inertial subrange scaled with the known power law ~d(-10/3), as a consequence of breakup by turbulent stress fluctuations external to the droplets. When the turbulent kinetic energy was sufficiently large (with interfacial Péclet numbers above unity), we found that turbulence driven redistribution of surfactant on the interface inhibited the Marangoni effect that is otherwise induced by film draining during coalescence in more quiescent flow. The coalescence rates were therefore not sensitive to varying surfactant activity in the range we considered, and for the given turbulent kinetic energies. Furthermore, internal viscous stresses strongly influenced the breakup rates. These two effects resulted in a DSD that was insensitive to varying surfactant activity.

Coalescence kinetics in surfactant stabilized emulsions: evolution equations from direct numerical simulations.

Lattice Boltzmann simulations were used to study the coalescence kinetics in emulsions with amphiphilic surfactant, under neutrally buoyant conditions, and with a significant kinematic viscosity contrast between the phases (emulating water in oil emulsions). The 3D simulation domain was large enough (256(3) ~ 10(7) grid points) to obtain good statistics with droplet numbers ranging from a few thousand at early times to a few hundred near equilibrium. Increased surfactant contents slowed down the coalescence rate between droplets due to the Gibbs-Marangoni effect, and the coalescence was driven by a quasi-turbulent velocity field. The kinetic energy decayed at a relatively slow rate at early times, due to conversion of interfacial energy to kinetic energy in the flow during coalescence. Phenomenological, coupled differential equations for the mean droplet diameter D(t) and the number density n(d)(t) were obtained from the simulation data and from film draining theories. Local (in time) power law exponents for the growth of the mean diameter (and for the concomitant decrease of n(d)) were established in terms of the instantaneous values of the kinetic energy, coalescence probability, Gibbs elasticity, and interfacial area. The model studies indicated that true power laws for the growth of the droplet size and decrease of the number of droplets with time may not be justified, since the exponents derived using the phenomenological model were time dependent. In contrast to earlier simulation results for symmetric blends with surfactant, we found no evidence for stretched logarithmic scaling of the form D ~ [ln (ct)](α) for the morphology length, or exponential scalings associated with arrested growth, on the basis of the phenomenological model.

Particle transport in a turbulent boundary layer: Non-local closures for particle dispersion tensors accounting for particle-wall interactions

Continuum equations derived from a probability density function kinetic equation contain dispersion tensors that describe the interaction between inertial particles and the underlying turbulent flow in which they are transported. These tensors require closure treatment and recent work has shown that traditional closure approximations perform poorly when applied to the case of particle dispersion in turbulent boundary layers. The dispersion tensors are intrinsically non-local, being sensitive to both the strong inhomogeneity of wall-bounded turbulence and the influence of particle-wall collisions. A new strategy for constructing non-local closure models is presented to account for such influences. An important feature of the approach is that it utilizes exactly the same input parameters required for the traditional closures. Differences between the two approaches are therefore a reflection of the improved closure strategy, rather than a consequence of improved or additional input data. Predictions from bot...

Direct numerical simulation of surfactant-stabilized emulsions

A 3D lattice Boltzmann model for two-phase flow with amphiphilic surfactant was used to investigate the evolution of emulsion morphology and shear stress in starting shear flow. The interfacial contributions were analyzed for low and high volume fractions and varying surfactant activity. A transient viscoelastic contribution to the emulsion rheology under constant strain rate conditions was attributed to the interfacial stress. For droplet volume fractions below 0.3 and an average capillary number of about 0.25, highly elliptical droplets formed. Consistent with affine deformation models, gradual elongation of the droplets increased the shear stress at early times and reduced it at later times. Lower interfacial tension with increased surfactant activity counterbalanced the effect of increased interfacial area, and the net shear stress did not change significantly. For higher volume fractions, co-continuous phases with a complex topology were formed. The surfactant decreased the interfacial shear stress due mainly to advection of surfactant to higher curvature areas. Our results are in qualitative agreement with experimental data for polymer blends in terms of transient interfacial stresses and limited enhancement of the emulsion viscosity at larger volume fractions where the phases are co-continuous.

Reduction of the effective shear viscosity in polymer solutions due to crossflow migration in microchannels: Effective viscosity models based on DPD simulations

ABSTRACT Molecular dynamics simulations (dissipative particle dynamics–DPD) were developed and used to quantify wall-normal migration of polymer chains in microchannel Poseuille flow. Crossflow migration due to viscous interaction with the walls results in lowered polymer concentration near the channel walls. A larger fraction of the total flow volume becomes depleted of polymer when the channel width h decreases into the submicron range, significantly reducing the effective viscosity. The effective viscosity was quantified in terms of channel width and Weissenberg number Wi, for 5% polymer volume fraction in water. Algebraic models for the depletion width δ(Wi, h) and effective viscosity μe(δ/h, Wi) were developed, based on the hydrodynamic theory of Ma and Graham and our simulation results. The depletion width model can be applied to longer polymer chains after a retuning of the polymer persistence length and the corresponding potential/thermal energy ratio. GRAPHICAL ABSTRACT

A DPD study of asphaltene aggregation: The role of inhibitor and asphaltene structure in diffusion-limited aggregation

ABSTRACT The kinetic effects of DBSA (dodecyl benzene sulfonic acid) and a linear amphihile on asphaltene aggregation was investigated, using dissipative particle dynamics molecular simulations. The simulation results indicated that without inhibitor, diffusion-limited asphaltene aggregation can be initiated by a kinetic/diffusive capture process between polar side chain groups rather than by interaction between polyaromatic rings. The most likely reason for this is that the side chains have higher diffusive mobility than the more massive aromatic ring structures. The DBSA acidic head groups adhered to the asphaltene side chain polar groups (the basic functional groups), resulting in lowered mobility of the side chain/DBSA complexes, thereby suppressing asphaltene aggregation initiation. A more mobile amphiphilic inhibitor without the aromatic ring gave a higher asphaltene aggregation rate. Adsorption of asphaltenes on a solid surface was suppressed with DBSA, due to an adsorbed mono-layer of DBSA that occupied a significant fraction of the surface area. GRAPHICAL ABSTRACT

Multiphase Flow Research with a Surfactant Lattice Boltzmann Model

Surfactant-dependent rheologies of emulsions and surfactant-controlled droplet size distributions in turbulent flows were studied with 3D lattice Boltzmann simulations. We review the surfactant related aspects of the model, where the amphiphilic surfactant is treated as a continuous density and vector field. The free energy formulation of the model allows for thermodynamic interpretations of the model results. GRAPHICAL ABSTRACT

Effects of polymer adsorption on the effective viscosity in microchannel flows: phenomenological slip layer model from molecular simulations

Abstract Molecular simulations (Dissipative Particle Dynamics - DPD) were used to quantify the effect of polymer adsorption on the effective shear viscosity of a semi-dilute polymer solution in microchannel Poseuille flow. It is well known that polymer depletion layers develop adjacent to solid walls due to hydrodynamic forces, causing an apparent wall slip and reduced effective viscosity (increased total flow rate). We found that depletion layers also developed in the presence of hydrodynamically rough adsorbed layers on the wall. Polymer-polymer (steric) repulsion between flowing and adsorbed polymer expanded the depletion layer compared to no-adsorption cases, and the effective viscosity was reduced further. Desorption occurred for higher shear rates, reducing the repulsion effect and shrinking the depletion layers. A phenomenological algebraic model for the depletion layer thickness, including a shear modified adsorption isotherm, was developed based on the simulation data. The depletion layer model can be used together with the effective viscosity model we developed earlier. Graphical Abstract

Development of electrochemical DPD molecular simulations for oil/water partitioning of organic acids at varying pH

ABSTRACT Electrostatic DPD simulations were developed to model the partitioning of organic acid from the lipid phase to the aqueous phase with increasing pH. The ionization-kinetic mechanisms were accounted for by implementing proton transfer between acid and base via double well Morse potentials, as reported by Lee et al. The DPD model was extended with hydration of the ionized acid A− at the oil/water interface. Thermodynamics predict a rapid increase of acid concentration in the water above a certain value in pH, corresponding to high ionization fraction of acid in water. This transition was regenerated by a careful tuning of the DPD interaction parameters. However, relatively high acid and ion concentrations are needed to provide a sufficient number of molecules in a limited simulation volume. Consequently, the simulations operate at very high and very low pH. High pH was obtained by adding a base providing OH− that accept protons from the acid at the interface. Likewise, low pH was obtained by adding H+, either forming hydronium or recombining with the ionized acid A− at the interface. In conclusion, the current simulation model contains the essential kinetic ionization mechanisms in partitioning. It seems reasonable to use the simulation model to study molecular transport over the interface in the presence of surfactant layers (surfactant indigenous to the oil, or added to the water). GRAPHICAL ABSTRACT