Many-body Dissipative Particle Dynamics Simulations Research Articles

Self-Assembly Mechanisms of Triblock Janus Particles.

We present a detailed model to study the nucleation of triblock Janus particles from solution. The Janus particles are modeled as cross-linked polystyrene spheres whose poles are patched with sticky alkyl groups and their middle band is covered with negative charges. To mimic the experimental conditions, solvent, counterions, and a substrate, on which the crystallization takes place, are included in the model. A many-body dissipative particle dynamics simulation technique is employed to include hydrodynamic and many-body interactions. Metadynamics simulations are performed to explore the pathways for nucleation of Kagome and hexagonal lattices. In agreement with experiment, we found that nucleation of the Kagome lattice from solution follows a two-step mechanism. The connection of colloidal particles through their patches initially generates a disordered liquid network. Subsequently, orientational rearrangements in the liquid precursors lead to the formation of ordered nuclei. Biasing the potential energy of the largest crystal, a critical nucleus appears in the simulation box, whose further growth crystallizes the whole solution. The location of the phase transition point and its shift with patch width are in very good agreement with experiment. The structure of the crystallized phase depends on the patch width; in the limit of very narrow patches strings are stable aggregates, intermediate patches stabilize the Kagome lattice, and wide patches nucleate the hexagonal phase. The scaling behavior of the calculated barrier heights confirms a first-order liquid-Kagome phase transition.

Many-body dissipative particle dynamics (MDPD) simulation of a pseudoplastic yield-stress fluid with surface tension in some flow processes

The dynamics of Newtonian and pseudoplastic yield-stress fluid drops impacting on solid surfaces are studied numerically by the many body dissipative particle dynamics (MDPD) method. The pseudoplastic yield stress fluid is modelled by two groups of different MDPD particles, in which the parameters are tuned so that the overall fluid behaves in a similar manner to the Herschel–Bulkley pseudoplastic material in simple shear flow. The droplet characteristics are measured by its central drop diameter and its height, and an effective unyielded region is observed in the drop impacting process. The MDPD simulation results compare well with previous drop impact experiments and simplified theoretical studies on the slump test, serving to demonstrate the capability of the MDPD method to explore yield-stress fluid drop’s flow processes.

Rheology of polymers in many-body dissipative particle dynamics simulations: Schmidt number effect

The Schmidt number effect on the rheology of finitely extensible nonlinear elastic chains (FENE) in many-body dissipative particle dynamics (MDPD) is investigated in this work. We find that the Schmidt number, ranging from (101) to (103), has limited influence on the polymer properties, such as its radius of gyration (Rg), diffusion coefficient (D) and relaxation time (τ). The simulation results follow Zimm model's predictions well. The hydrodynamic interaction strength parameter h* demonstrates that the full hydrodynamic interaction can be simulated for Schmidt number from (100-106) in MDPD. Next, the rheology of FENE polymers is studied using Lees-Edward boundary condition in shear flow. The shear-thinning and normal stress difference are measured and analysed with MDPD; meanwhile, the volume fraction, solvent quality and chain length are varied to explore their effects on the extent of the Newtonian region. Finally, the non-Newtonian droplet is firstly simulated in MDPD. Its maximum spreading diameter is measured for both Newtonian and non-Newtonian droplet with Weber number (We) ranging from 4.37 to 109.2 on hydrophilic, moderate and hydrophobic surfaces, respectively. The fluid shear-thinning property increases at high shear rate and is further enhanced on more hydrophobic surface, from the maximum spreading diameter results. The non-Newtonian (FENE) droplet can be now well simulated in MDPD and this provides additional insight to further research concerning polymer-solvent-surface interactions, which is crucial in various applications.

Sliding Dynamic Behavior of a Nanobubble on a Surface

The sliding dynamics of a nanobubble adhered to surfaces is an intriguing and yet to be fully understood aspect. In this work, the dynamics of stable surface nanobubbles in a fluid subject to body force is explored by many-body dissipative particle dynamics simulations at low Reynold numbers Re ∼ O(0.1). It is found that in addition to surface roughness, the nanobubble motion depends significantly on the bubble shape which relates to the bubble angle (θb) associated with surface wettability. The mobility grows substantially with decreasing θb and the surface nanobubble can rise faster than a freely rising nanobubble. Moreover, owing to the decrease of both the frontal area and drag coefficient, the mobility of a flat bubble on a smooth surface can be many times greater than that of a spherical bubble. The influence of surface roughness depends on the surface wettability. The sliding motion is impeded by surface roughness because of capillary pinning. However, it is interesting to find that for θb less tha...

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.

A mesoscopic simulation of static and dynamic wetting using many-body dissipative particle dynamics

A many-body dissipative particle dynamics simulation is applied here to pave the way for investigating the behavior of mesoscale droplets after impact on horizontal solid substrates. First, hydrophobic and hydrophilic substrates are simulated through tuning the solid–liquid interfacial interaction parameters of an innovative conservative force model. The static contact angles are calculated on homogeneous and several patterned surfaces and compared with the predicted values by the Cassie’s law in order to verify the model. The results properly evaluate the amount of increase in surface superhydrophobicity as a result of surface patterning. Then drop impact phenomenon is studied by calculating the spreading factor and dimensionless height versus dimensionless time and the comparisons made between the results and the experimental values for three different static contact angles. The results show the capability of the procedure in calculating the amount of maximum spreading factor, which is a significant concept in ink-jet printing and coating process.

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.

Application of many-body dissipative particle dynamics to determine liquid characteristics

Purpose – The purpose of this paper is to find out the applicability of the many-body dissipative particle dynamics (MDPD) method for various real fluids by specifically focusing on the effects of the MDPD parameters on the MDPD fluid properties. Design/methodology/approach – In this study, the MDPD method based on van der Waals (vdw) equation of state is employed. The simulations are conducted by using LAMMPS with some modifications of the original package to include the many-body features in the simulation. The simulations are investigated in a three-dimensional Cartesian box solution domain in which MDPD particles are distributed. In order to evaluate the MDPD liquid characteristics for a stationary liquid film, self-diffusivity, viscosity, Schmidt number (Sc) and surface tension, are estimated for different MDPD parameters. The parameters are carefully selected based on previous studies. A set of single-droplet simulations is also performed to analyze the droplet characteristics and its behavior on a solid-wall. Besides, the relationship between the characteristic length in the DPD simulations and scaling parameters for the stationary liquid-film case is discussed by employing the Ohnesorge number. Findings – The results show that the liquid properties in the MDPD simulations can be widely ranged by varying the MDPD parameters. The values are highly influenced by the many-body feature in the conservative force which is not included in the original DPD method. It is also found that the wetting ability of the MDPD fluid on solid walls can be easily controlled by changing a many-body parameter. The characteristic length between the MDPD reduced unit and real unit is related for the stationary liquid-film case by employing the Ohnesorge number. Originality/value – The present parametric study shows that the liquid properties in the MDPD method can vary by carefully controlling the MDPD parameters, which demonstrates the high-potential applicability of the method for various real fluids. This will contribute to research areas in multi-phase transport phenomena at nano and sub-micron scales in, for example, fuel cells, batteries and other engineering devices involving porous media.

A many-body dissipative particle dynamics study of fluid–fluid spontaneous capillary displacement

The fluid–fluid spontaneous capillary displacement is a model related to many important natural processes and industrial applications, such as groundwater remediation and oil recovery. To study the displacement process in microscale where experimental observations are hard to perform, computational simulations are an adequate approach. In the present work, we report a many-body dissipative particle dynamics (MDPD) simulation study on the fluid–fluid spontaneous capillary displacement. System parameters such as the wetting property of both fluids, the miscibility between them, and the capillary radius, can be adjusted independently and separately in the simulation, thus their influences on the spontaneous capillary displacement process can be observed individually. Based on our previous study of MDPD simulations on fluid imbibition, we are able to monitor the motion of the fluid–fluid interface in this spontaneous capillary displacement simulation accurately. Combined with the new method for three-phase contact angle calculation proposed in the present study, the displacement curve obtained is in good accordance with the theoretical description even in the very beginning of the spontaneous capillary displacement motion. Our findings provide microscopic information about the fluid–fluid spontaneous displacement process, and lay a basis for further studies on more complex systems.

Local pressure components and surface tension of spherical interfaces. Thermodynamic versus mechanical definitions. I. A mesoscale modeling of droplets

We report mesoscale simulations of spherical drops to investigate the surface tension and mechanical properties. The Monte Carlo simulations are performed with the multibody potential commonly used in the many-body dissipative particle dynamics simulations. We establish here the calculation of the local normal and transverse components of the pressure tensor via the perturbation volume within the thermodynamic route. The different profiles of these components are compared to those calculated using the mechanical approach. To complete the mesoscale modeling of drops, we investigate the curvature dependence of the surface tension in order to calculate the Tolman's length, which is found to be negative.

Many-body dissipative particle dynamics simulation of liquid/vapor and liquid/solid interactions

The combination of short-range repulsive and long-range attractive forces in many-body dissipative particle dynamics (MDPD) is examined at a vapor/liquid and liquid/solid interface. Based on the radial distribution of the virial pressure in a drop at equilibrium, a systematic study is carried out to characterize the sensitivity of the surface tension coefficient with respect to the inter-particle interaction parameters. For the first time, the approximately cubic dependence of the surface tension coefficient on the bulk density of the fluid is evidenced. In capillary flow, MDPD solutions are shown to satisfy the condition on the wavelength of an axial disturbance leading to the pinch-off of a cylindrical liquid thread; correctly, no pinch-off occurs below the cutoff wavelength. Moreover, in an example that illustrates the cascade of fluid dynamics behaviors from potential to inertial-viscous to stochastic flow, the dynamics of the jet radius is consistent with the power law predictions of asymptotic analysis. To model interaction with a solid wall, MDPD is augmented by a set of bell-shaped weight functions; hydrophilic and hydrophobic behaviors, including the occurrence of slip in the latter, are reproduced using a modification in the weight function that avoids particle clustering. The dynamics of droplets entering an inverted Y-shaped fracture junction is shown to be correctly captured in simulations parametrized by the Bond number, confirming the flexibility of MDPD in modeling interface-dominated flows.

A Many-Body Dissipative Particle Dynamics Study of Spontaneous Capillary Imbibition and Drainage

The spontaneous capillary imbibition and drainage processes are studied using many-body dissipative particle dynamics (MDPD) simulations. By adjusting the solid-liquid interaction parameter, different wetting behavior between the fluid and the capillary wall, corresponding to the static contact angle ranging from 0 degrees to 180 degrees, can be controllably simulated. For wetting fluids, the spontaneous capillary imbibition (SCI) is evident in MDPD simulations. It is found that, whereas the corrected Lucas-Washburn equation (taking into account the dynamic contact angle and the fluid inertia) can well describe the SCI simulation result for the completely wetting fluid, it deviates, to a notable degree, from the results of partly wetting fluids. In particular, this corrected equation cannot be used to describe the spontaneous capillary drainage (SCD) processes. To solve this problem, we derive an improved form of the Lucas-Washburn equation, in which the slip effects of fluid particles at the capillary wall are treated. Such an improved equation turns out to be capable of describing all the simulation results of both the SCI and the SCD. These findings provide new insights into the SCI and SCD processes and improve the mathematical base.

Many-body dissipative particle dynamics simulation of wetting phenomena

With the development of the simulation of particle dynamics, the traditional dissipative particle dynamics (DPD) method can not satisfy the needs of research in static or dynamic wetting phenomena. However, the Many-body DPD approach extends the ability of the traditional method to simulate the interface between solid and liquid or some other situation. In this paper, we propose a Many-body DPD program to simulate the solid-liquid interface and get satisfactory results.