Dissipative-particle-dynamics model for two-phase flows

Single-Site Local-Density Potentials for the Mesoscopic Representation of Water Based on the SAFT-VR Mie Equation of State

In this article, we present three mesoscopic models for water. All three models make use of local density-dependent interaction potentials, as employed within the Pagonabarraga-Frenkel framework [Pagonabarraga, I.; Frenkel, D. J. Chem. Phys. 2001, 115, 5015-5026]. The forms of these three interaction potentials are based on the free energy function of the SAFT-VR Mie equation of state (EoS) [Lafitte, T. J. Chem. Phys. 2013, 139, 154504]. Two of these models represent the water-water interaction as a spherically symmetric Mie interaction with temperature-dependent parameters, while the third model works with a temperature-independent Mie potential and explicitly models the effect of hydrogen bonding using an association term. All three models provide good predictions of the vapor-liquid equilibrium of water over a wide temperature range. They also give accurate predictions of the isothermal compressibility for both sub- and supercritical conditions. To model the interfacial tension of the vapor-liquid interface with our mesoscale simulations, we added a square-gradient term to our potential energy function. We show that the addition of this term has a minimal effect on the bulk properties of water. However, by parametrizing the coefficient of this term as a function of temperature, all three models again provide excellent predictions of water's interfacial tension over a wide temperature range. Of the three models, our preference is for the model that includes an association term, as this model can operate successfully over a wider range of conditions.

Development of a Meshless Kernel-Based Scheme for Particle-Field Brownian Dynamics Simulations

We develop a meshless discretization scheme for particle-field Brownian dynamics simulations. The density is assigned on the particle level using a weighting kernel with finite support. The system's free energy density is derived from an equation of state (EoS) and includes a square gradient term. The numerical stability of the scheme is evaluated in terms of reproducing the thermodynamics (equilibrium density and compressibility) and dynamics (diffusion coefficient) of homogeneous samples. Using a reduced description to simplify our analysis, we find that numerical stability depends strictly on reduced reference compressibility, kernel range, time step in relation to the friction factor, and reduced external pressure, the latter being relevant under isobaric conditions. Appropriate parametrization yields precise thermodynamics, further improved through a simple renormalization protocol. The dynamics can be restored exactly through a trivial manipulation of the time step and friction coefficient. A semiempirical formula for the upper bound on the time step is derived, which takes into account variations in compressibility, friction factor, and kernel range. We test the scheme on realistic mesoscopic models of fluids, involving both simple (Helfand) and more sophisticated (Sanchez-Lacombe) equations of state.

Local density dependent potentials for an underlying van der Waals equation of state: A simulation and density functional theory analysis

There is an ever increasing use of local density dependent potentials in the mesoscale modeling of complex fluids. Questions remain, though, about the dependence of the thermodynamic and structural properties of such systems on the cutoff distance used to calculate these local densities. These questions are particularly acute when it comes to the stability and structure of the vapor/liquid interface. In this article, we consider local density dependent potentials derived from an underlying van der Waals equation of state. We use simulation and density functional theory to examine how the bulk thermodynamic and interfacial properties vary with the cutoff distance, rc, used to calculate the local densities. We show quantitatively how the simulation results for bulk thermodynamic properties and vapor-liquid equilibrium approach the van der Waals limit as rc increases and demonstrate a scaling law for the radial distribution function in the large rc limit. We show that the vapor-liquid interface is stable with a well-defined surface tension and that the interfacial density profile is oscillatory, except for temperatures close to critical. Finally, we show that in the large rc limit, the interfacial tension is proportional to rc and, therefore, unlike the bulk thermodynamic properties, does not approach a constant value as rc increases. We believe that these results give new insights into the properties of local density dependent potentials, in particular their unusual interfacial behavior, which is relevant for modeling complex fluids in soft matter.

Insights into Waterflooding in Hydrocarbon-Bearing Nanochannels of Varying Cross Sections from Mesoscopic Multiphase Flow Simulations

Waterflooding is one of the geotechniques used to recover fuel sources from nanoporous geological formations. The scientific understanding of the process that involves the multiphase flow of nanoconfined fluids, however, has lagged, mainly due to the complex nanopore geometries and chemical compositions. To enable the benchmarked flow of nanoconfined fluids, architected geomaterials, such as synthesized mesoporous silica with tunable pore shapes and surface chemical properties, are used for designing and conducting experiments and simulations. This work uses a modified many-body dissipative particle dynamics (mDPD) model with accurately calibrated parameters to perform parametric flow simulations for studying the influences of waterflooding-driven power, pore shape, surface roughness, and surface wettability on the multiphase flow in heptane-saturated silica nanochannels. Remarkably, up to an 80% reduction in the effective permeability is found for water-driven heptane flow in a baseline 4.5-nm-wide slit channel when compared with the Hagen-Poiseuille equation. In the 4.5-nm-wide channels with architected surface roughness, the flow rate is found to be either higher or lower than the baseline case, depending on the shape and size of cross sections. High wettability of the solid surface by water is essential for achieving a high recovery of heptane, regardless of surface roughness. When the solid surface is less wetting or nonwetting to water, the existence of an optimal waterflooding-driven power is found to allow for the highest possible recovery. A detailed analysis of the evolution of the transient water-heptane interface in those nanochannels is presented to elucidate the underlying mechanisms that impact or dictate the multiphase flow behaviors.

Flow Reduction in Pore Networks of Packed Silica Nanoparticles: Insights from Mesoscopic Fluid Models

A modified many-body dissipative particle dynamics (mDPD) model is rigorously calibrated to achieve realistic fluid-fluid/solid interphase properties and applied for mesoscale flow simulations to elucidate the transport mechanisms of heptane liquid and water, respectively, through pore networks formed by packed silica nanoparticles with a uniform diameter of 30 nm. Two million CPU core hours were used to complete the simulation studies. Results show reduction of permeability by 54-64% in heptane flow and by 88-91% in water flow, respectively, compared to the Kozeny-Carman equation. In these nanopores, a large portion of the fluids are in the near-wall regions and thus not mobile due to the confinement effect, resulting in reduced hydraulic conductivity. Moreover, intense oscillations in the calculated flow velocities also indicate the confinement effect that contests the external driven force to flow. The generic form of Darcy's law is considered valid for flow through homogeneous nanopore networks, while permeability depends collectively on pore size and surface wettability. This fluid-permeability dependency is unique to flow in nanopores. In addition, potential dependence of permeability on pore connectivity is observed when the porosity remains the same in different core specimens.

Dissipative particle dynamics simulations in colloid and Interface science: a review

Dissipative particle dynamics (DPD) is one of the most efficient mesoscale coarse-grained methodologies for modeling soft matter systems. Here, we comprehensively review the progress in theoretical formulations, parametrization strategies, and applications of DPD over the last two decades. DPD bridges the gap between the microscopic atomistic and macroscopic continuum length and time scales. Numerous efforts have been performed to improve the computational efficiency and to develop advanced versions and modifications of the original DPD framework. The progress in the parametrization techniques that can reproduce the engineering properties of experimental systems attracted a lot of interest from the industrial community longing to use DPD to characterize, help design and optimize the practical products. While there are still areas for improvements, DPD has been efficiently applied to numerous colloidal and interfacial phenomena involving phase separations, self-assembly, and transport in polymeric, surfactant, nanoparticle, and biomolecules systems.

Size dependence of static polymer droplet behavior from many-body dissipative particle dynamics simulation

We used molecular simulation to study the static behavior of polymer droplets in vacuum and on solid surfaces, namely the size of the droplet and the contact angle, respectively. The effects of the polymer chain length and the total number of particles were calculated by the many-body dissipative particle dynamics method. For the spherical droplet containing the same number of particles, we show that its radius depends on the polymer chain length. The radius of the droplet is also proportional to one-third power of the total number of particles for all given chain lengths. For the hemispherical droplet, the contact angle increases with the number of particles in the droplet, and this effect is relatively strong, especially for longer polymer chains. The effect of wettability of the solid surface was also investigated by using polymerphobic (low-affinity) and polymerphilic (high-affinity) surfaces. As the chain length increases, the contact angle on the low-affinity surface decreases, while that on the hydrophilic surface increases. The simulation reveals that there is a critical affinity for the monomer on the solid surface; above and below which the wettability increases and decreases as the molecular length increases, respectively.

Hydrodynamic Interactions and Entanglements of Polymer Solutions in Many-Body Dissipative Particle Dynamics

Using many-body dissipative particle dynamics (MDPD), polymer solutions with concentrations spanning dilute and semidilute regimes are modeled. The parameterization of MDPD interactions for systems with liquid⁻vapor coexistence is established by mapping to the mean-field Flory⁻Huggins theory. The characterization of static and dynamic properties of polymer chains is focused on the effects of hydrodynamic interactions and entanglements. The coil⁻globule transition of polymer chains in dilute solutions is probed by varying solvent quality and measuring the radius of gyration and end-to-end distance. Both static and dynamic scaling relations for polymer chains in poor, theta, and good solvents are in good agreement with the Zimm theory with hydrodynamic interactions considered. Semidilute solutions with polymer volume fractions up to 0.7 exhibit the screening of excluded volume interactions and subsequent shrinking of polymer coils. Furthermore, entanglements become dominant in the semidilute solutions, which inhibit diffusion and relaxation of chains. Quantitative analysis of topology violation confirms that entanglements are correctly captured in the MDPD simulations.

Numerical Simulations of the Digital Microfluidic Manipulation of Single Microparticles

Single-cell analysis techniques have been developed as a valuable bioanalytical tool for elucidating cellular heterogeneity at genomic, proteomic, and cellular levels. Cell manipulation is an indispensable process for single-cell analysis. Digital microfluidics (DMF) is an important platform for conducting cell manipulation and single-cell analysis in a high-throughput fashion. However, the manipulation of single cells in DMF has not been quantitatively studied so far. In this article, we investigate the interaction of a single microparticle with a liquid droplet on a flat substrate using numerical simulations. The droplet is driven by capillary force generated from the wettability gradient of the substrate. Considering the Brownian motion of microparticles, we utilize many-body dissipative particle dynamics (MDPD), an off-lattice mesoscopic simulation technique, in this numerical study. The manipulation processes (including pickup, transport, and drop-off) of a single microparticle with a liquid droplet are simulated. Parametric studies are conducted to investigate the effects on the manipulation processes from the droplet size, wettability gradient, wetting properties of the microparticle, and particle-substrate friction coefficients. The numerical results show that the pickup, transport, and drop-off processes can be precisely controlled by these parameters. On the basis of the numerical results, a trap-free delivery of a hydrophobic microparticle to a destination on the substrate is demonstrated in the numerical simulations. The numerical results not only provide a fundamental understanding of interactions among the microparticle, the droplet, and the substrate but also demonstrate a new technique for the trap-free immobilization of single hydrophobic microparticles in the DMF design. Finally, our numerical method also provides a powerful design and optimization tool for the manipulation of microparticles in DMF systems.

Dewetting of a stratified two-component liquid film on a solid substrate

Dissipative particle dynamics simulations for the dewetting of a stratified thin film, composed of two immiscible fluids of different viscosity, reveal a nontrivial dewetting kinetics depending on the location of the more viscous liquid with respect to the solid substrate. Our simulations show that when the layer of higher viscosity is in contact with the substrate, the energy loss through viscous dissipation is concentrated near the contact line. The kinetics of dewetting is then inversely proportional to the viscosity of the lower layer eta_{A} (with respect to eta_{B} for the upper layer). However, when the liquid of higher viscosity is on the upper layer, a new dynamics appears with a dewetting velocity V approximately eta_{B};{-0.44}eta_{A};{-0.56}. The difference between these two scenarios lies in the different routes through which the interfacial energy is converted into heat by viscous dissipation in the rim.

Simulations of liquid nanocylinder breakup with dissipative particle dynamics

In this work, we use a dissipative-particle-dynamics-based model for two-phase flows to simulate the breakup of liquid nanocylinders. Rayleigh's criterion for capillary breakup of inviscid liquid cylinders is shown to apply for the cases considered, in agreement with prior molecular dynamics (MD) simulations. Also, as shown previously through MD simulations, satellite drops are not observed, because of the dominant role played by thermal fluctuations which lead to a symmetric breakup of the neck joining the two main drops. The parameters varied in this study are the domain size, cylinder radius, thermal length scale, viscosity, and surface tension. The breakup time does not show the same scaling dependence as in capillary breakup of liquid cylinders at the macroscale. The time variation of the radius at the point of breakup agrees with prior theoretical predictions from expressions derived with the assumption that thermal fluctuations lead to breakup.

Simulation model of concentrated colloidal nanoparticulate flows

This paper presents a simulation model of concentrated colloidal nanoparticulate flows to investigate self-organization of the nanoparticles and rheology of the colloid. The motion of solid nanoparticles is treated by an off-lattice Newtonian dynamics. The flow of solvent is treated by an on-lattice fluctuating Navier-Stokes equation. A fictitious domain method is employed to couple the motion of nanoparticles with the flow of solvent. The surface of nanoparticles is expressed by discontinuous solid-liquid boundary to calculate accurately contact interaction and Derjaguin-Landau-Verwey-Overbeek interaction between the nanoparticles. At the same time, the surface is expressed by continuous solid-liquid boundary to calculate efficiently hydrodynamic interaction between the nanoparticles and the solvent. Unlike other simulation models that focus on the hydrodynamic interaction, the present model includes all crucial interactions, such as contact force and torque, van der Waals force, electrostatic force, hydrodynamic force, and torque including thermal fluctuation of the solvent that causes translational and rotational Brownian motions of the nanoparticles. Especially the present model contains the frictional force that plays a significant role on nanoparticles in contact with one another. A fascinating novelty of the present model is that computational cost is constant regardless of the concentration of nanoparticles. The capability of the present simulation model is demonstrated by two-dimensional simulations of concentrated colloidal nanoparticles in simple shear flows between flat plates. The self-organization of concentrated colloidal nanoparticles and the viscosity of colloid are investigated in a wide range of Péclet numbers.