Dissipative particle dynamics simulation of flow through periodic arrays of circular micropillar

Flow through arrays of micropillar embedded inside microfluidic chip systems is important for various microfluidic devices. It is critical to accurately predict the mass flow rate through pillar arrays based on the pillar design. This work presents a dissipative particle dynamics （DPD） model to simulate a problem of flow across periodic arrays of circular micropillar and investigates the permeability of two types of micropillar arrays. The flow fields including horizontal and vertical velocity fields, the number density field, and the streamline of the flow are analyzed. The predicted solid volumes by the presented DPD simulation of both types of arrays are quite close to the actual counterparts. These quantitative agreements show usefulness and effectiveness of the DPD model in simulating arrays of micropillar. By comparing two types of micropillar arrangement patterns, we find that the arrangement pattern of micropillar does not have significant influence on the permeability of the array.

A new model for dissipative particle dynamics boundary condition of walls with different wettabilities

The implementation of solid-fluid boundary condition has been a major challenge for dissipative particle dynamics(DPD)method.Current implementations of boundary conditions usually try to approach a uniform density distribution and a velocity profile close to analytical solution.The density oscillations and slip velocity are intentionally eliminated,and different wall properties disappear in the same analytical solution.This paper develops a new wall model that combines image and frozen particles and a new strategy to emphasize different wall properties especially wettabilities.The strategy first studies the realistic wall-fluid system by molecular dynamics(MD)simulation depending on physical parameters.Then,a DPD simulation is used to match the density and velocity profiles with the new wall model.The obtained DPD parameters can simulate the systems with the same wall and fluid materials.With this method,a simulation of the Poiseuille flow of liquid argon with copper walls is presented.Other walls with super-hydrophilic,hydrophilic,and hydrophobic wettabilities are also simulated.The limitations of the analytical solution and the effect of the wall-fluid interaction are discussed.The results show that the method suggested in this paper can simulate the mesoscale behavior of the microchannel flow related to realistic systems.

A note on hydrodynamics from dissipative particle dynamics

We calculate current correlation functions （CCFs） of dissipative particle dy- namics （DPD） and compare them with results of molecular dynamics （MD） and solutions of linearized hydrodynamic equations. In particular, we consider three versions of DPD, the empirical/classical DPD, coarse-grained （CG） DPD with radial-direction interactions only and full （radial, transversal, and rotational） interactions between particles. To fa- cilitate quantitative discussions, we consider specifically a star-polymer melt system at a moderate density. For bonded molecules, it is straightforward to define the CG variables and to further derive CG force fields for DPD within the framework of the Mori-Zwanzig formalism. For both transversal and longitudinal current correlation functions （TCCFs and LCCFs）, we observe that results of MD, DPD, and hydrodynamic solutions agree with each other at the continuum limit. Below the continuum limit to certain length scales, results of MD deviate significantly from hydrodynamic solutions, whereas results of both empirical and CG DPD resemble those of MD. This indicates that the DPD method with Markovian force laws possibly has a larger applicability than the continuum description of a Newtonian fluid. This is worth being explored further to represent gen- eralized hydrodynamics.

Effect of shear on the symmetric diblock copolymer/nanorod mixture:A dissipative particle dynamics study

The phase behaviours of a lamellar diblock copolymer/nanorod composite under steady shear are investigated using dissipative particle dynamics. We consider a wide range of nanorod concentrations, where the nanorods each have a preferential affinity to one of the blocks. Our results suggest that shear not only aligns the orientations of the diblock eopolymer templates and nanorods towards flow direction, but also regulates the distribution of the nanorods within the polymer matrix. Meanwhile, the shear-induced reorientation and morphology transitions of the systems also significantly depend on the nanorod concentration. At certain nanorod concentrations, the competitions between shearinduced polymer thinning and nanorods dispersion behaviours determine the phase behaviours of the composites. For high nanorod concentrations, no morphology transition is observed, but reorientation is present, in which the sheared nanorods are arranged into hexagonal packing arrays. Additionally, the orientation behaviour of nanorods is determined directly by the applied shear, also interfered with by the shear-stretched copolymer molecules.

Many-body dissipative particle dynamics with energy conservation:temperature-dependent long-term attractive interaction

Heat and mass transfer during the process of liquid droplet dynamic behaviors has attracted much attention in decades.At mesoscopic scale,numerical simulations of liquid droplets motion,such as impacting,sliding,and coalescence,have been widely studied by using the particle-based method named many-body dissipative particle dynamics(MDPD).However,the detailed information on heat transfer needs further description.This paper develops a modified MDPD with energy conservation(MDPDE)by introducing a temperature-dependent long-term attractive interaction.By fitting or deriving the expressions of the strength of the attractive force,the exponent of the weight function in the dissipative force,and the mesoscopic heat friction coefficient about temperature,we calculate the viscosity,self-diffusivity,thermal conductivity,and surface tension,and obtain the Schmidt number Sc,the Prandtl number P r,and the Ohnesorge number Oh for 273 K to 373 K.The simulation data of MDPDE coincide well with the experimental data of water,indicating that our model can be used to simulate the dynamic behaviors of liquid water.Furthermore,we compare the equilibrium contact angle of droplets wetting on solid surfaces with that calculated from three interfacial tensions by MDPDE simulations.The coincident results not only stand for the validation of Young’s equation at mesoscale,but manifest the reliability of our MDPDE model and applicability to the cases with free surfaces.Our model can be extended to study the multiphase flow withcomplex heat and mass transfer.

Polymer translocation through nanopore under external electric field:dissipative particle dynamics study

The DNA sequencing technology has achieved a leapfrog development in recent years. As a new generation of the DNA sequencing technology, nanopore sequenc- ing has shown a broad application prospect and attracted vast research interests since it was proposed. In the present study, the dynamics of the electric-driven translocation of a homopolymer through a nanopore is investigated by the dissipative particle dynam- ics （DPD）, in which the homopolymer is modeled as a worm-like chain （WLC）. The DPD simulations show that the polymer chain undergoes conformation changes during the translocation process. The different structures of the polymer in the translocation process, i.e., single-file, double folded, and partially folded, and the induced current block- ades are analyzed. It is found that the current blockades have different magnitudes due to the polymer molecules traversing the pore with different folding conformations. The nanoscale vortices caused by the concentration polarization layers （CPLs） in the vicinity of the sheet are also studied. The results indicate that the translocation of the polymer has the effect of eliminating the vortices in the polyelectrolyte solution. These findings are expected to provide the theoretical guide for improving the nanopore sequencing tech- nique.

Discussions on the correspondence of dissipative particle dynamics and Langevin dynamics at small scales

We investigate the behavior of dissipative particle dynamics （DPD） within different scaling regimes by numerical simulations. The paper extends earlier analytical findings of Ripoll, M., Ernst, M. H., and Espafiol, P. （Large scale and mesoscopic hy- drodynamics for dissipative particle dynamics. Journal of Chemical Physics, 115（15）, 7271-7281 （2001）） by evaluation of numerical data for the particle and collective scaling regimes and the four different subregimes. DPD simulations are performed for a range of dynamic overlapping parameters. Based on analyses of the current auto-correlation functions （CACFs）, we demonstrate that within the particle regime at scales smaller than its force cut-off radius, DPD follows Langevin dynamics. For the collective regime, we show that the small-scale behavior of DPD differs from Langevin dynamics. For the wavenumber-dependent effective shear viscosity, universal scaling regimes are observed in the microscopic and mesoscopic wavenumber ranges over the considered range of dynamic overlapping parameters.

Temperature dependence of microscopic properties in diblock copolymer films:A dissipative particle dynamics simulation

Temperature dependence of microscopic properties in diblock copolymer films has been investigated by dissipative particle dynamics simulations. Results show the relation between mean-square bond length （MSBL） and system temperature can be described as a quadratic curve. The root-mean-square radius of gyration （RMSGR） and end-end distance （RMSED） increase gradually as the temperature rises and composition fraction changes from 0.1 to 0.5, in which the effect of the former is primary. Especially, the relation between RMSGR and temperature is nearly linear in the confinement-introduced direction. Density distribution of each component in the films can be controlled and adjusted effectively by its interaction with other components and boundaries. Moreover, the changes of system temperature and composition fraction can both affect the density distributions to a certain extent.

Pickering emulsion transport in skeletal muscle tissue:A dissipative particle dynamics simulation approach

Lymph node targeting is a commonly used strategy for particulate vaccines,particularly for Pickering emulsions.However,extensive research on the internal delivery mechanisms of these emulsions,especially the complex intercellular interactions of deformable Pickering emulsions,has been surprisingly sparse.This gap in knowledge holds significant potential for enhancing vaccine efficacy.This study aims to address this by summarizing the process of lymph-node-targeting transport and introducing a dissipative particle dynamics simulation method to evaluate the dynamic processes within cell tissue.The transport of Pickering emulsions in skeletal muscle tissue is specifically investigated as a case study.Various factors impacting the transport process are explored,including local cellular tissue environmental factors and the properties of the Pickering emulsion itself.The simulation results primarily demonstrate that an increase in radial repulsive interaction between emulsion particles can decrease the transport efficiency.Additionally,larger intercellular gaps also diminish the transport efficiency of emulsion droplet particles due to the increased motion complexity within the intricate transport space compared to a single channel.This study sheds light on the nuanced interplay between engineered and biological systems influencing the transport dynamics of Pickering emulsions.Such insights hold valuable potential for optimizing transport processes in practical biomedical applications such as drug delivery.Importantly,the desired transport efficiency varies depending on the specific application.For instance,while a more rapid transport might be crucial for lymph-node-targeted drug delivery,certain applications requiring a slower release of active components could benefit from the reduced transport efficiency observed with increased particle repulsion or larger intercellular gaps.

Statics,Dynamics and Linear Viscoelasticity from Dissipative Particle Dynamics Simulation of Entangled Linear Polymer Melts

Dissipative particle dynamics(DPD)with bond uncrossability shows a great potential in studying entangled polymers,however relatively little is known of applicability range of entangled DPD model to be use as a model for ideal chains and properly describe the full dynamics of entangled melts.Therefore,we perform a comprehensive study on structure,dynamics and linear viscoelasticity of a typical DPD entangled model system,semiflexible linear polymer melt.These polymers obey Flory's ideality hypothesis in chain dimensions,but their local structure exhibits nonideal behavior due to weak correlated hole effect.Both monomer motion and viscoelasticity relaxation reproduce the full pictures as predicted by reptation theory.The stronger chain length dependent diffusion coefficient and relaxation time as well as dynamic moduli are in close agreement with predictions of modern tube model that accounts for additional relaxation mechanisms besides chain reptation.However,an anomalous sub-diffusive center of mass motion is observed both before and after the intermediate reptation regime and the cross-correlation between chains is not negligible even these polymers obey stress-optical law,indicating limitations of the reptation theory.Hence semiflexible linear entangled DPD model can correctly describe statics and dynamics of entangled polymer melts.

Dissipative Particle Dynamics Study on Aggregation of MPEG- PAE-PLA Block Polymer Micelles Loading Doxorubicine

To guide the molecular design of the pH-sensitive triblock amphiphilic polymer MPEG-PAE-PLA and the for- mula design of its doxorubicine （DOX）-loaded micelles, dissipative particle dynamics （DPD） simulations are em- ployed to investigate the aggregation behaviors of the DOX-loaded micelles. The simulation results showed that the aggregate morphologies of micelles and DOX distribution are influenced by degree of polymerization of blocks, and the proposed structure of polymer is MPEG44-PAE3-PLA4. With different contents of polymer or DOX, differ- ent aggregate morphologies of the micelles, like microsphere, spindle/column, reticulation or lamella are observed. To prepare the micro-spherical DOX-loaded micelles, the polymer content is proposed as 10%--15%, and the DOX content less than 10%.

Dissipative Particle Dynamics Simulation of Onion Phase in Star-block Copolymer

A dissipative particle dynamics simulation technique was used to investigate the effect of molecular architecture of star-block copolymer on the patterned structure in a nanodroplet. With increasing the ratio of solvophilic to block length to solvophobic block length（RwT）, solvophobic sphere, ordered hexagonal phase, onion phase, perforated onion phase and flocculent phase are formed, respectively. Since onion phase has potential application in controlled drug release, it has received wide attention experimentally and theoretically. Our simulation indicates onion phase forms at a certain RH/T（close to but less than 1）. A star-block copolymer molecule has two conformations in onion phase： either fully located in a shell or shared by two neighboring shells. Central structure affects onion＇s final shape. The molecular number of the copolymer in each shell is a quadratic function of the shell＇s radius. The arm number of star-block copolymer has little influence on onion＇s structure, but slightly affects the solvent content. Additionally, we studied the influence of arm length on onion＇s structure.

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 solidliquid interface and get satisfactory results.

Stable and accurate schemes for smoothed dissipative particle dynamics

Smoothed dissipative particle dynamics （SDPD） is a mesoscopic particle method that allows to select the level of resolution at which a fluid is simulated. The numerical integration of its equations of motion still suffers from the lack of numerical schemes satisfying all the desired properties such as energy conservation and stability. Similarities between SDPD and dissipative particle dynamics with energy （DPDE） con- servation, which is another coarse-grained model, enable adaptation of recent numerical schemes developed for DPDE to the SDPD setting. In this article, a Metropolis step in the integration of the fluctuation/dissipation part of SDPD is introduced to improve its stability.

Impingement and mixing dynamics of micro-droplets on a solid surface

The hydrodynamics and mixing during the nonaxisymmetry impingement of a micro-droplet and a sessile droplet of the same fluid are investigated by many-body dissipative particle dynamics(MDPD) simulation.In this work,the range of the impingement angle(θ_(i)) between the impinging droplet and the sessile droplet is 0°-60° and the contact angle is set as 45° or 124°.The droplets impingement and mixing behavior is analyzed based on the droplet internal flow field,the concentration distribution and the time scale of the decay of the kinetic energy of the impinging droplet.The dimensionless total mixing time(τ_(m)) is calculated by a modified mixing function.With the Weber number(We) ranging from 5.65 to22.7 and the Ohnesorge number(Oh) ranging from 0.136 to 0.214,we find rm hardly changes with We and Oh.Whereas,θ_(i)and surface wettability are found to have a significant effect on τ_(m).We find that θ_(i)has no clear effect on τ_(m)on a hydrophobic surface,while on the hydrophilic surface,τ_(m)increase with the θ_(i).Thus,reducing the impinging angle is a valid method to shorten the τ_(m).

Numerical simulation of pore-scale flow in chemical flooding process

Chemical flooding is one of the effective technologies to increase oil recovery of petroleum reservoirs after water flooding.Above the scale of representative elementary volume(REV), phenomenological modeling and numerical simulations of chemical flooding have been reported in literatures,but the studies alike are rarely conducted at the pore-scale,at which the effects of physicochemical hydrodynamics are hardly resolved either by experimental observations or by traditional continuum-based simulations.In this paper,dissipative particle dynamics(DPD),one of mesoscopic fluid particle methods,is introduced to simulate the pore-scale flow in chemical flooding processes.The theoretical background,mathematical formulation and numerical approach of DPD are presented.The plane Poiseuille flow is used to illustrate the accuracy of the DPD simulation,and then the processes of polymer flooding through an oil-wet throat and a water-wet throat are studies, respectively.The selected parameters of those simulations are given in details.These preliminary results show the potential of this novel method for modeling the physicochemical hydrodynamics at the pore scale in the area of chemical enhanced oil recovery.

Mesoscale modeling of microgel mechanics and kinetics through the swelling transition

The mechanics and swelling kinetics of polymeric microgels are simulated using a mesoscale computational model based on dissipative particle dynamics. Microgels are represented by a random elastic network submerged in an explicit viscous solvent. The model is used to probe the effect of different solvent conditions on the bulk modulus of the microgels. Comparison of the simulation results through the volume phase transition reveals favorable agreement with Flory-Rehner＇s theory for polymeric gels. The model is also used to examine the microgel swelling kinetics, and is found to be in good agreement with Tanaka＇s theory for spherical gels. The simulations show that, during the swelling process, the microgel maintains a nearly homogeneous structure, whereas deswelling is characterized by the formation of chain bundles and network coarsening.

Mesoscale Simulation for Polymer Migration in Confined Uniform Shear Flow

The structure and dynamics of confined single polymer chain in a dilute solution, either in equilibrium or at different shear rates in the uniform shear flow fields, were investigated by means of dissipative particle dynamics simulations. The no-slip boundary condition without density fluctuation near the wall was taken into account to mimic the environment of a nanochannel. The dependences of the radius of gyration, especially in three different di- rections, and the density profile of the chain mass center on the strength of the confinement and the Weissenberg number（Wn） was studied. The effect of the interaction between polymer and solvent on the density profile was also investigated in the cases of moderate and strong Wn. In the high shear flow, the polymer migrates to the center of the channel with increasing Wn. There is only one density profile peak in the channel center in the uniform shear flow, which is in agreement with the results of the experiments and theory.

Everything you always wanted to know about SDPD~★(~★but were afraid to ask)

An overview of the smoothed dissipative particle dynamics （SDPD） method is presented in a format that tries to quickly answer questions that often arise among users and newcomers. It is hoped that the status of SDPD is clarified as a mesoscopic particle model and its potentials and limitations are highlighted, as compared with other methods.

Temperature Effect on the Conformation Transition of Ultra-high Molecular Weight Polyethylene/Polypropylene Blends Undergoing Continuous Volume Extensional Flow:A Mesoscopic Simulation

Due to the multiformity and complexity of chain conformation under external flow and the challenge of systematically investigating the transient conformation and dynamic evolution process of polymer chains at the molecular level by means of present experimental techniques,a universal description of both chain conformation and dynamics with respect to continuous volume extensional flow(CVEF)is still absent.Taking into account the temperature effect,we performed dissipative particle dynamics(DPD)simulations with the particles corresponding to the repeat units of polymers over a wide temperature range and analyzed the correlation with the conformational properties of ultra-high molecular weight polyethylene/polypropylene(UHMWPE/PP)blend in response to the CVEF.With time evolution,the polymer chains become highly oriented parallel to the flow direction instead of the initial random coiling and self-aggregation.It is found that a high temperature is necessary for more substantial compactness to take place than low temperature.The low-k plateau and low-k peak in structure factor S(k)curves suggest a low degree of conformational diversity and a high degree of chain stretching.It is also concluded that the intra-molecular C-C bond interaction is the main driving force for the dynamics process of the chain conformations undergoing CVEF,where the motion of the alkyl chains is seriously restricted owing to the increase in bond interaction potential,resulting in a reduction of the difference in diffusion rates among alkyl chains.