Many-body Dissipative Particle Dynamics Research Articles

In Situ Liquid Phase TEM of Nanoparticle Formation and Diffusion in a Phase-Separated Medium.

Colloidal nanoparticles are synthesized in a complex reaction mixture that has an inhomogeneous chemical environment induced by local phase separation of the medium. Nanoparticle syntheses based on micelles, emulsions, flow of different fluids, injection of ionic precursors in organic solvents, and mixing the metal organic phase of precursors with an aqueous phase of reducing agents are well established. However, the formation mechanism of nanoparticles in the phase-separated medium is not well understood because of the complexity originating from the presence of phase boundaries as well as nonuniform chemical species, concentrations, and viscosity in different phases. Herein, we investigate the formation mechanism and diffusion of silver nanoparticles in a phase-separated medium by using liquid phase transmission electron microscopy and many-body dissipative particle dynamics simulations. A quantitative analysis of the individual growth trajectories reveals that a large portion of silver nanoparticles nucleate and grow rapidly at the phase boundaries, where metal ion precursors and reducing agents from the two separated phases react to form monomers. The results suggest that the motion of the silver nanoparticles at the interfaces is highly affected by the interaction with polymers and exhibits superdiffusive dynamics because of the polymer relaxation.

Coalescence-induced jumping of droplets on superhydrophobic substrates with a beam structure

Coalescence-induced jumping motion of droplets on superhydrophobic surfaces has potential applications for anti-icing, self-cleaning and anti-fogging, etc. In the present study, the coalescence-induced jumping of mesoscale water droplets on substrate without and with beam is investigated by using many-body dissipative particle dynamics (MDPD) simulations. For the cases of jumping on flat substrate without beam, the jumping velocity scaled by the inertial-capillary velocity (vic) is nearly constant at 0.105 for droplet radius varying from 116.04 nm to 290.10 nm. For coalescence-induced jumping on substrate with a beam, the jumping velocity can be enhanced to 0.7vic, and the jumping is accompanied by the merging process, which is different from that of the droplets merging on substrate without beam. The stages of merging on substrate with a beam can also be divided into 4 stages : I. expansion of liquid bridge; II. detaching from substrate; III. jumping away from beam; IV. moving freely in air. The conversion efficiency of jumping kinetic energy increases with the beam height in the range from zero to the radius of droplet. Hydrophobic and hydrophilic beams lead to coalesced droplets standing on the beam or wrapping the beam. Our work can provide new insights in the droplets coalescence dynamics, and help design water resource collection system and microfluidics devices.

Transverse effect on liquid viscosity: A many-body dissipative particle dynamics simulation study

Fluid viscosity plays an important role in multiphase flows, and the many-body dissipative particle dynamics (MDPD) method is an efficient means of simulating such flows at the mesoscopic scale. As the viscosity of the standard MDPD (S-MDPD) fluid cannot be efficiently adjusted, a transverse MDPD (T-MDPD) scheme is newly proposed to tune the viscosity of an MDPD fluid over a large range. With a lateral friction coefficient added to the S-MDPD form, the viscosity of the T-MDPD fluid is higher than that of an S-MDPD fluid, and the viscosity is about five times larger than that of an S-MDPD fluid with a friction coefficient of 40.5. In a T-MDPD fluid, the viscosity is much more sensitive to the new transverse friction coefficient, as it increases about five times higher when this coefficient increases from 4.5 to 40.5, while the viscosity only increases two times higher with the same variation of the original coefficient. By increasing the repulsive coefficient, the liquid particle number density, or the cutoff radius, the viscosity of the T-MDPD fluid is enhanced as well. Based on this extension of the MDPD scheme, a quantitative expression for the variation of the viscosity in the current T-MDPD fluid is derived. In future simulations of multiphase flows using an MDPD scheme, the transverse effect can be extended to effectively tune the viscosity, and this empirical expression will be useful to predict the viscosity of the T-MDPD fluid.

Dynamics of droplet impact on a ring surface

We investigate the dynamics of droplet impacts on a ring-decorated solid surface, which is reported to reduce the integral of contact area over contact time by up to 80%. By using many-body dissipative particle dynamics (MDPD), a particle-based simulation method, we measure the temporal evolution of the shape and the impact force of two specific types of phenomena, overrun and ejection. The numerical model is first validated with experimental data on a plain surface from literature. Then, it is used to extract the impacting force of the ring and substrate separately, showing the ring does not provide the majority of vertical force to redirect the horizontal spreading. The impacting pressure in different concentric rings is also present as a function of time, showing pressure waves traveling from ring to center. The effect of the ring's height and radius on the impacting force is also discussed. To the best of our knowledge, this is the first MDPD study on droplets impacting on a solid surface with a validated force analysis.

Pore Size Dependence of Permeability in Bicontinuous Nanoporous Media.

In this work, we employ many-body dissipative particle dynamics (mDPD) simulations to investigate the fluid flow process through bicontinuous nanoporous media, which are representative models for a broad class of nanoporous materials. The mDPD formulation includes attractive and repulsive interactions describing accurately fluid-fluid and fluid-solid interactions. As a mesoscale simulation method, mDPD can bridge the length and time scale gap between continuum and atomistic simulations. The bicontinuous nanoporous models are constructed considering a defined morphology, the porosity level, and varying pore sizes in the range from 3.41 to 13.63 nm. All models have a 0.65 porosity level and the same topology. The models provide a stochastic description of the morphology and pore size distribution and allow for a direct investigation of the dependence of permeability on the average pore size. The stationary nanoporous models are filled with fluid particles, and flow is induced by the action of confining pistons. Simulation results, obtained by imposing different pressure differences on the surfaces of the nanoporous media, indicate a linear pressure drop within the nanoporous model. Regardless of the complexities and different scales of the porous media considered, the steady-state fluid flow through the nanoporous models is proportional to the pressure gradient applied, in agreement with Darcy's law. The calculated pore size dependence of permeability is well described by the Hagen-Poiseuille law, considering a single shape correction factor that accounts for the flow resistance due to the complex nanoporous morphology. This work highlights the effect of the average pore size of a complex stochastic bicontinuous nanoporous medium on fluid properties. The results indicate rather a relatively simple dependence of permeability on the average pore size. The novel method we employ to generate the stochastic bicontinuous nanoporous structure allows the control of different geometric features that can be explored in future studies.

Influence of hydrophobicity and roughness on the wetting and flow resistance of water droplets on solid surface: A many-body dissipative particle dynamics study

Hydrophobicity and roughness are crucial properties for determining rock wetting and flow resistance. The wetting and flow behavior of water droplets on smooth and rough solid surfaces are investigated by performing a many-body dissipative particle dynamics simulation. The results show that, in terms of the roughness, a solid surface can be divided into three regions. In Region I, as inherent hydrophobicity increases, roughness can enhance the apparent hydrophilicity of a hydrophilic surface. In Region II, the increased hydrophobicity improves the apparent hydrophobicity of a more hydrophobic surface. In Region III, when the hydrophobicity is beyond the superhydrophobic boundary, the enhancement effect of the rough surface on the apparent hydrophobicity is weakened or even disappears. With an increase in the roughness, the boundaries among these three regions and their ranges show nonmonotonic changes, making the contact mode (Wenzel state/Cassie and Baxter state), contact angle hysteresis and drag reduction effect change nonmonotonically.

Imbibition dynamics in an open-channel capillary with holes

Spontaneous capillary flow is generally described by Washburn’s equation. However, the modification must be made for imbibition dynamics in an open channel. Moreover, the presence of the holes in the capillary for flow division or bifurcation complicates the outcome of the imbibition process. In this work, capillary imbibition into a two-rail channel with/without holes is investigated by many-body dissipative particle dynamics (MDPD). The influences of channel height (h), channel width (w), hole size (wh), and surface wettability (θY) on the imbibition rate are examined. For two-rail channel without holes, the simulation results agree reasonably with Washburn’s equation with the correction 1-(h/w)/cosθY. The penetration rate grows with reducing the channel height or enlarging the channel width. However, in the presence of holes, the wicking dynamics is slowed down and four types of outcomes are observed: “covering”, “separation”, “pass/stop”, and “stoppage”. These states depend on the size and (symmetric or asymmetric) position of the rectangular hole. Finally, the phase diagrams which vary with θY, wh/h, and w/h are constructed. The domains of those states agree reasonably with the equations acquired simply from the analyses of surface free energy.

A many-body dissipative particle dynamics with energy conservation study of droplets icing on microstructure surfaces

Droplets icing has important applications in real life. The icing process of droplets on microstructure is explored based on the MDPDE method in this study. Firstly, the correctness of the heat transfer model was verified by one-dimensional heat conduction simulation, and then the feasibility of the phase change model was verified by investigating the icing process of droplets. The influence of cold surface temperature, droplet volume and contact angle on freezing time of droplets was discussed, and it was found that the temperature of cold surfaces had a greater effect on freezing. We finally explored the influence of different microstructure surfaces on the icing of droplets, and results showed that the presence of microstructures greatly enhanced the anti-icing effect of the surface. In our research, the contact angle is a relatively large factor affecting the icing of droplets. In addition, it was discovered that the droplet had the strongest ability to delay freezing on the surface of triangle microstructures with a contact angle of 157.1°.

A review of many-body dissipative particle dynamics (MDPD): Theoretical models and its applications

Many-body dissipative particle dynamics (MDPD) is a novel coarse-grained numerical method that originated from dissipative particle dynamics. In the MDPD system, a density-dependent repulsive interaction and an attractive term are introduced into a conservative force, enabling the formation of vapor–liquid coexistence. In the last two decades, the MDPD is becoming a powerful tool to study various interfacial problems at mesoscale due to its Lagrangian and adaptive features. In the present paper, we review the developments in the theoretical models and applications for the MDPD. First, the MDPD theoretical backgrounds of single- and multi-component system are introduced. Then, the parameter analysis and mapping protocols in the MDPD are discussed. Furthermore, recent applications based on the MDPD, including droplet and microbubble dynamics, evolution of liquid bridges, capillary wetting, polymer solutions, and phase change, are revisited with some comments. Finally, we summarize several unsolved issues in the MDPD and outline its future developments.

Influence of thermal fluctuations on nanoscale free-surface flows: A many-body dissipative particle dynamics study

Free-surface fluid dynamics at the nanoscale is shown to strongly depend on thermal fluctuations. These fluctuations can be predicted by particle-based models numerically and analyzed by stochastic partial differential equations theoretically. In this work, we employ one of the particle methods, known as many-body dissipative particle dynamics (mDPD), to investigate the influence of thermal fluctuations on nanoscale free-surface flows. After enough independent simulations are performed for convincing ensemble-averaged results, mDPD not only shows good consistency with theories derived from the Landau–Lifshitz Navier–Stokes equations, but also indicates high efficiency in contrast with traditional molecular dynamics (MD). These results confirm that mDPD has capabilities of capturing the physics of fluctuating hydrodynamics in nanoscale free-surface flows, especially the complicated three-dimensional behaviors that cannot be predicted by theoretical models, both accurately and efficiently.

A numerical study of droplet impact on solid spheres: The effect of surface wettability, sphere size, and initial impact velocity

The present work numerically studies the dynamics of a droplet impact on solid spheres with different sizes and surface wettability. Different initial impact velocities are also set to the droplet to consider the velocity effect. A particle-based numerical method, many-body dissipative particle dynamics (MDPD), is employed to perform the simulations. The simulation results show that the post-impact droplet morphology highly depends on the impact velocity, size and surface wettability of solid spheres. At lower impact velocities, the droplet can sit on the top of the solid surfaces and keep a semi-sphere shape; while at higher impact velocities, for smaller solid spheres, the droplet can fully wrap the spheres and even drip off. For impact on larger spheres, on the hydrophilic surfaces, the final stable liquid film thickness is highly velocity-dependent, while on the hydrophobic surfaces, the droplet always sits on top of the sphere with the same liquid thickness.

Flow reduction of hydrocarbon liquid in silica nanochannel: Insight from many-body dissipative particle dynamics simulations

A modified many-body dissipative particle dynamics (mDPD) model recently developed for realistic mesoscale multiphase flow simulations is rigorously parameterized, calibrated, and applied for elucidating the flow mechanisms of hydrocarbon liquids (i.e., heptane in this work) in amorphous silica cylindrical nanochannels with inner diameters ranging from 4.5 to 22.5 nm. The simulation results suggest the presence of a strong threshold of pressure gradient under which heptane cannot be driven to flow. The threshold for the 4.5 nm diameter pore is 10 to 100 times as high as for the 9–22.5 nm diameter pore, highlighting a remarkable nanoconfinement effect. Fluid viscosity is found to exhibit a shear-thinning phenomenon with intensity to weaken with increasing channel diameter — a phenomenon not observed in nanochannel flow of liquid water and gas in literature. Most remarkably, the radial profiles of average longitudinal flow velocity fitted by the modified Hagen-Poiseuille equation showed a negative slip length (−2.5% to −0.5% relative to the diameter) and a reduction of apparent permeability by 16% to 23%. This finding suggests silica nanochannels tend to deter hydrocarbon flow, a phenomenon that is opposed to the flow enhancement reported in most of the prior nanochannel flow studies in literature.

Interfacial behavior of phospholipid monolayers revealed by mesoscopic simulation

A mesoscopic model with molecular resolution is presented for dipalmitoyl phosphatidylcholine (DPPC) and palmitoyl oleoyl phosphatidylcholine (POPC) monolayer simulations at the air-water interface using many-body dissipative particle dynamics (MDPD). The parameterization scheme is rigorously based on reproducing the physical properties of water and alkane and the interfacial property of the phospholipid monolayer by comparison with experimental results. Using much less computing cost, these MDPD simulations yield a similar surface pressure-area isotherm as well as similar pressure-related morphologies as all-atom simulations and experiments. Moreover, the compressibility modulus, order parameter of lipid tails, and thickness of the phospholipid monolayer are quantitatively in line with the all-atom simulations and experiments. This model also captures the sensitive changes in the pressure-area isotherms of mixed DPPC/POPC monolayers with altered mixing ratios, indicating that the model is promising for applications with complex natural phospholipid monolayers. These results demonstrate a significant improvement of quantitative phospholipid monolayer simulations over previous coarse-grained models.

Study on a mesoscopic model of droplets freezing considering the recalescence process

There are many practical applications of droplets freezing, and in many cases, it is necessary to prevent the droplets freezing to reduce the loss caused by freezing. Based on the many-body dissipative particle dynamics with energy conservation method, this research proposes an icing model that considers the recalescence process and initial ice mass fraction of droplets for the first time, which obtains a complete simulation of the two-phase four-stage freezing process of droplets. The accuracy and applicability of this model are verified by studying the single-phase Stefan problem, the recalescence process of droplet, and whether the initial ice mass fraction is considered for freezing. Then, the freezing process of droplets under four surface temperatures and five types of surface wettability was studied, and it was found that the temperature of droplets in recalescence stage would jump from nucleation temperature to equilibrium temperature, and almost unaffected by external factors. Change of the temperature distribution with dimensionless height H* before recalescence is only affected by the surface temperature and nucleation temperature. At the end of droplets recalescence, the initial ice mass fraction has little relationship with volume. As the contact angle, surface temperature, and droplet volume increase, temperature changes in the pre-cooling and solidification stages of droplets will slow down, and the solidification time will increase. Additionally, the temperature of the solid wall surface has almost no effect on the final ice shape, and the final ice tip phenomenon is more obvious on the surface with a larger contact angle.

Spontaneous spreading of nanodroplets on partially wetting surfaces with continuous grooves: Synergy of imbibition and capillary condensation

The wetting behavior of nanodroplets deposited on hydrophilic textured surfaces with discontinuous and continuous grooves was investigated by many-body dissipative particle dynamics. For surfaces with discontinuous grooves, the apparent contact angle (θa) can be described by the Wenzel model with an effective area ratio. However, for surfaces with continuous grooves, θa of nanodroplets depends on roughness topology, and spontaneous spreading of nanodroplets can take place even for partially wetting surfaces (θY > 0). That is, the partial-to-total wetting transition can occur on hydrophilic surfaces as surface textures change from discontinuous to continuous grooves. The dynamics of the spreading can be characterized by the apparent radius and groove radius (Rg). While the former is time-invariant at early stage, the latter follows the power law Rg ~ t1/4. This spreading of nanodroplets along continuous grooves can be explained from the synergy of spontaneous imbibition into continuous grooves, arrest by sharp edges, and capillary condensation for resuming the flow.

Simulations for Pulsating Breakups of a Nano Taylor Cone

In this paper, a Taylor cone model in the nanoscale is configured using the many-body dissipative particle dynamics method. The sharpening process of the Taylor cone and the breakup process at different electric field intensities and different charge concentrations are systematically investigated. Under a strong electric field, the hemispherical droplet is sharpened over time and evolves into a conic one. Then the conic cusp emits a thin jet. Finally, the cone shrinks into a semi-sphere after jet breaking. These deformation processes occur several times until no charges are emitted from the conic cusp. It is found that the electric field force is responsible for jet emitting, while the Coulombic force causes a jet breakup. With the rising of the intensity of the electric field, the breakup times also increase. However, the breakup times decrease with the rising of the charge concentration. It indicates that a conductive liquid with low electrical conductivities and subjected to a strong electric field is more prone to undergo pulsating breakups.

Thermally assisted mobility of nanodroplets on surfaces with weak defects

HypothesisThermal activation plays an essential role in contact line dynamics on nanorough surfaces. However, the relation between the aforementioned concept and the sliding motion of nanodroplets remains unclear. As a result, thermally assisted motion of nanodroplets on nanorough surfaces is investigated in this work. ExperimentsSteady slide and random motion of nanodroplets on surfaces with weak defects are investigated by Many-body Dissipative Particle Dynamics. The surface roughness is characterized by the slip length acquired from the velocity profile associated with the flowing film. FindingsThe slip length is found to decline with increasing the defect density. The linear relationship between the sliding velocity and driving force gives the mobility and reveals the absence of contact line pinning. On the basis of the Navier condition, a simple relation is derived and states that the mobility is proportional to the slip length and the reciprocal of the product of viscosity and contact area. Our simulation results agree excellently with the theoretical prediction. In the absence of external forces, a two-dimensional Brownian motion of nanodroplets is observed and its mean square displacement decreases with increasing the defect density. The diffusivity is proportional to the mobility, consistent with the Einstein relation. This consequence suggests that thermal fluctuations are able to overcome contact line pinning caused by weak defects.

Many-body dissipative particle dynamics study of the local slippage over superhydrophobic surfaces

The gas–liquid interface (GLI) over superhydrophobic surfaces (SHSs), where the flow slips, is the key to reduce frictional drag in underwater applications. Many-body dissipative particle dynamics simulations are used to explore the slip behavior of a shear flow over a rectangular grooved SHS, and a flat GLI is obtained by tuning the contact angle of the GLI. Due to the slip, the normal profiles of the local velocity, which are perpendicular to the GLI, are curved and shifted away from the linear form near the GLI. Then, a polynomial function is proposed to fit the velocity profile to extract the local shear rate and calculate the slip length. Based on this fitting method, a hybrid slip boundary condition is derived for both longitudinal and transverse flows. That is, the shear stress and slip length are finite near the groove edge, and the stress is nearly zero and the slip length is infinite in the center region of the GLI. This new hybrid slip boundary condition not only explains the inconsistent slip conditions reported in the literature under different groove length scales, but also unifies the existing exclusive slip assumptions.

Molecular View of the Distortion and Pinning Force of a Receding Contact Line: Impact of the Nanocavity Geometry.

We present a molecular view using many-body dissipative particle dynamics simulations to unravel the pinning phenomenon of a liquid film receding over a solid substrate with a nanocavity. We find that the pinning force and distortion of the pinned contact line vary across different nanocavity shapes. We show that the mechanism of a caterpillar motion, which has previously been proposed for advancing precursor films, persists in a partially pinned receding contact line. Our results also demonstrate a localized clamping effect, which is originated from the variation of the dynamic contact angle along the pinned contact line. The simulation results suggest that the clamping effect can be controlled by the geometry of the nanocavity and hydrophilicity of the underlying substrate.

Spontaneous motion of droplet in tubes: The effects of surface wettability and segment length

This work studies the spontaneous droplet flow in tubes with different wettability gradient and number of segments by using a particle-based numerical method, many-body dissipative particle dynamics (MDPD). The study aims to understand how the different contact angle sets (gradient contact angle distribution along with different segments of a tube) and the number of segments could affect the droplet flow velocity and time to reaching a stable state. Simulation results show that even with the same wettability gap, a contact angle set with higher values of contact angle along the tube segments can drive the droplet to flow faster in the tubes; moreover, with the same contact angle set and tube length, the tubes with fewer segments can allow a faster flow of droplet, thus a shorter time to reaching the stable state. The numerical findings in this work can provide a new idea and direction for the design of droplet-based microfluidic systems.