Using an electro-spraying microfluidic chip to produce uniform emulsions under a direct-current electric field

An electro-spraying microfluidic chip was integrated with a parallel electrode and flow-focusing device to successfully generate uniform emulsions with an electric field. This approach utilizes a high electric field driven by a direct-current voltage to form a stable Taylor cone in the flow-focusing position. The Taylor cone can then generate stable and uniform emulsions that are less than 5?μm in diameter. The emulsion size is controlled by the surfactant concentration, the ratio of the water and oil phase flow rates and the strength of the electric field. When the strength of the electric field increases at a high surfactant concentration and low ratio of flow rates, the Taylor angle decreases, which causes the emulsion size to decrease. In this study, the water emulsion diameter ranged from 1 to 98?μm, and the poly(lactic-co-glycolic acid) (PLGA) emulsion size ranged from 7 to 70?μm. The microfluidic chip developed in this work has the advantages of actively controlling the emulsion size and generating uniform emulsions (the relative standard deviation was less than 10%) and represents a new emulsion generation process.     

泰勒锥的形成及应用

带电液滴在电场中受力变形的现象得到了广泛的研究,而毛细管管口处的液滴在电场下则形成泰勒锥。本文综述了泰勒锥的研究历史、形成过程和在静电喷雾电离、静电纺丝以及在液滴滴定中的应用,使我们了解了泰勒锥的产生机理,从而做到对泰勒锥形成过程中各项参数的控制,为后续工作奠定基础。     

DPD enables mesoscopic MRI simulation of slow flow

We present a novel method to simulate magnetic resonance imaging (MRI) for the assessment of slow flow at Reynolds number \(Re \approx 0.02\). We couple Bloch equations with dissipative particle dynamics (DPD) to study the effect of flow dynamics at the mesoscopic level on acquired MR images. The Bloch equations are used to propagate the evolution of the magnetization of particles while their trajectories are being computed simultaneously based on DPD interaction forces. The magnetic resonance assessment of fluid velocities is performed using a phase-contrast MRI technique, implemented by a spin echo single-sided bipolar gradient sequence. The computational cost for simulating the fluid flow is successfully reduced by an efficient implementation of a vectorized isochromat algorithm. We demonstrate successful simulation of laminar flow, flow with diffusion effects, and flow around an obstacle. The method can be used to simulate convective and diffusive flow MRI experiments at the mesoscopic level.     

Electric Ejection of Viscous Inks From MEMS Capillary Array Head for Direct Drawing of Fine Patterns

Free-space microink streams are realized by an array of microcapillaries. Ink is ejected from capillaries accelerated by an electric field with low or no back pressure applied. The capillaries are 20 mum in diameter, whereas the microstreams are only a few micrometers thick due to the Taylor cone effect. This method enables one to draw very fine lines down to 1 mum without lithographical techniques. With a pulsed-field application, dotted lines can also be drawn. Direct drawing of both solid and dotted lines has successfully been demonstrated by applying dc and 500-Hz ac fields, respectively.     

The level-set method applied to droplet dynamics in the presence of an electric field

The present work will show the combination of the level-set method and the ghost-fluid method employed to simulate two-dimensional droplet dynamics in the presence of an electric field. The incompressible Navier-Stokes equation is used to model the fluid flow both inside and outside the droplet. Poisson equation with discontinuous coefficients for the electric potential is calculated using the ghost-fluid method. Then the electrically induced surface forces are incorporated into the hydrodynamic interfacial relations, which are handled in a sharp manner. The utility of the method is demonstrated in the calculation of droplet breakup and coalescence due to the electrically induced forces.     

Dissipative particle dynamics simulations of liquid nanojet breakup

In this work, we use a dissipative-particle-dynamics (DPD)-based two-phase model to study the breakup of liquid nanojets. We show that the breakup of nanojets is accelerated by the presence of thermal fluctuations. Satellite drops, which are undesirable from an application viewpoint, are not observed in our simulations. We find that the presence of a repulsive wall enclosing the DPD liquid is necessary to prevent clogging of nanojets. We are able to recover the time evolution of minimum jet radius as given by prior theoretical analysis. This study shows that DPD is able to capture the thermal induced breakup phenomena at the sub-micron level. The coarse-grained nature of DPD along with its flexibility to allow for the modeling of complex fluids in combination with the results from this study show that DPD is a useful tool for sub-micron fluid flow simulations.     

Numerical investigation of AC electrokinetic virus trapping inside high ionic strength media

A numerical investigation of the mechanism by which viral particles suspended in physiologically relevant (i.e., high ionic strength) media can be electrokinetically sampled on a surface is presented. Specifically, sampling of virus from a droplet is taking place by means of a high frequency non-uniform electric field, generated by energized planar quadrupolar microelectrodes deposited on an oxidized silicon chip. The numerical simulations are based on experimental conditions applied in our previous work with vesicular stomatitis virus. A 3D computer model is used to yield the spatial profiles of electric field intensity, temperature, and fluid velocity inside the droplet, as well as the force balance on the virus. The results suggest that rapid virus sampling can be achieved by the synergistic action of dielectrophoresis and electrothermal fluid flow. Specifically, electrothermal fluid flow can be used to transport the virus from the bulk of a sample to the surface, where dielectrophoretic forces, which become significant only at very small length scales away from the surface, can cause its stable capture.     

Simulation of Stokes flow over microelectrodes with least-squares meshfree method

Microelectrode structures in alternating current (AC) electrokinetics can generate high electric field strength to manipulate, characterize and separate particles in suspending medium. It has been widely used in biological, pharmaceutical and medical fields. In this paper, a least-squares meshfree method (LSMFM) based on the first-order velocity–pressure–vorticity formulation for the Stokes flow, electric potential–electric field strength expression for electric field and temperature–heat flux equations for heat transfer problem is presented to study two-dimensional electrothermally induced fluid flow over microelectrodes. Joule heat generated from electric field acts as heat source and gives rise to electric force and buoyancy force acting on the fluid. The discretization of all system of equations is completed by the least-squares method. The equal-order moving least-squares (MLS) approximation is used with Gaussian quadrature in the background cells constructed by the quadtree algorithm. A matrix-free element-by-element Jacobi preconditioned conjugate gradient method is applied to solve the resulting systems. Finally, an example of steady heat transfer problem with analytical solution is devised to analyze the error estimates of the LSMFM, and the examples of electric field of shielded microstrip line and Stokes flow over microelectrode are also solved to investigate the features of the LSMFM.     

An inverse scattering analysis without information of sources producing incident fields

An inverse scattering approach based on the field equivalence principle is developed for reconstructing the electrical parameters of a stratified medium using only electric field time‐domain data measured at two observation points in presence and absence of the medium. The magnetic field data at the observation points are calculated from the electric field data by solving two equivalent problems for the incident and scattered fields. By noting that the field of an equivalent problem for the total field in the interior region between two observation points is null in the exterior region, the functional of the electrical parameters is introduced. A genetic algorithm is applied for minimization of the functional to estimate the parameters. Numerical simulations demonstrate the effectiveness of the approach. © 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2013.     

Self Assembly of Polymer Structures Induced by Electric Field

A method has been developed for fabricating polymer microstructures based on electric field induced self assembly and pattern formation. A dielectric fluid placed in between two conductive plates experience a force in an applied electric field gradient across the plates, which can induce a diffusive surface instability and self construction of the fluid surface. This process is exploited for the fabrication of self assembled polymer microstructures as well as replicated patterns through the use of pre-patterned plates or electrodes. FEM simulation is used to decide the minimum wavelength and electric gradient distribution of polymer structures. A variety of structures in the micron and nanometer scales including bio-fluidic MEMS, polymer optoelectronic devices can be fabricated using this method.     

Two dimensional shape optimization using partial control and finite element method for compressible flows

The objective of this study is to determine the two dimensional shape of a body located in a compressible viscous flow, where the applied fluid force is minimized. The formulation to obtain the optimal shape is based on an optimal control theory. An optimal state is defined as a state, in which the performance function defined as the integration of the square sum of the applied fluid forces is minimized due to a reduction in the applied fluid forces. Compressible Navier–Stokes equations are treated as constraint equations. In other words, the body is considered to have a shape that minimizes the fluid forces under the constraint of the Navier–Stokes equations. The gradient of the performance function is computed using the adjoint variables. A weighted gradient method is used as the minimization algorithm. The volume of the body is assumed to be the same as that of the initial body. In the case of the algorithm used in this study, both the creation of a structured mesh around the surface of the body and the smoothing procedure are employed for the computation of gradient. In this study, a remeshing technique based on the structured mesh around the body changing its configuration in the iteration cycle is employed. For the correction to keep the volume constant, the surface coordinates are moved along the radial direction. For the discretization of both the state and adjoint equations, the efficient bubble function interpolation presented previously by the authors [18] is employed. The algorithm, which is known as the partial control algorithm, is applied to the numerical procedure to determine the movement of the coordinates. In the case of the gradient method, in order to avoid the convergence of the final shape to the local minimum shape, the new algorithm, which is called the partial control algorithm, is presented in this study. In numerical studies, the shape determination of a body in a uniform flow field is carried out in 2D domains. The initial shape of the body is assumed to be an elliptical cylinder. The shape is modified by minimizing the applied fluid forces. Finally, the desired shape of a body, whose performance function is reduced and converged to a constant value, is obtained. By carrying out a procedure that involves the use of the partial control algorithm, the desired shape of a body, whose performance function is reduced further, is obtained. Stable shape determination of a body in a compressible viscous flow is carried out by using the presented method. It is indicated that the optimal shape can be obtained by using the partial control algorithm.     

Dissipative particle dynamics simulation of circular and elliptical particles motion in 2D laminar shear flow

The dissipative particle dynamics (DPD) method is a relatively new computational method for modeling the dynamics of particles in laminar flows at the mesoscale. In this study, we use the DPD approach to model the motion of circular and elliptical particles in a 2D shear laminar flow. Three examples are considered: (i) evaluation of the drag force exerted on a circular particle moving in a stagnant fluid, (ii) rotation of an elliptical particle around its center in a shear flow, and (iii) motion of an ellipsoidal particle in a linear shear flow. For all cases, we found a good agreement with theoretical and finite element solutions available. These results show that the DPD method can effectively be applied to model motion of micro/nano-particles at the mesoscale. The method proposed can be used to predict the performances of intravascularly administered particles for drug delivery and biomedical imaging.     

Electroosmotic flow control in complex microgeometries

Numerical simulation results for pure electroosmotic and combined electroosmotic/pressure driven Stokes flows are presented in the cross-flow and Y-split junctions. The numerical algorithm is based on a mixed structured/unstructured spectral element formulation, which results in high-order accurate resolution of thin electric double layers with discretization flexibility for complex engineering geometries. The results for pure electroosmotic flows in cross-flow junctions under multiple electric fields show similarities between the electric and velocity fields. The combined electroosmotic/pressure driven flows are also simulated by regulating the flowrate in different branches of the cross-flow junctions. Flow control in the Stokes flow regime is shown to have linear dependence on the magnitude of the externally applied electric field, both for pure electroosmotic and combined flows. This linear behavior enables utilization of electroosmotic forces as nonmechanical means of flow control for microfluidic applications     

A distributed lagrange multiplier based computational method for the simulation of particulate-Stokes flow

In this paper we present a distributed lagrange multiplier (DLM) based Stokes flow algorithm for particulate flows. The entire fluid–particle domain is treated as a fluid. The “fluid” in the particle domain is ensured to move rigidly by adding a rigidity constraint. We modify the SIMPLER (Semi-Implicit Method for Pressure Linked Equations—Revised) algorithm for fluids (by Patankar [Numerical Heat Transfer and Fluid Flow, Taylor and Francis, London, 1980]), to account for the presence of the particle. The modification pertains to the presence of the rigidity constraint in the particle domain. We validate the algorithm with suitable test cases.     

1D wave propagation in saturated viscous geomaterials: Improvement and validation of a fractional step Taylor–Galerkin finite element algorithm

This paper tackles the numerical simulation of 1D wave propagation in saturated viscous porous media, and especially in soil-like geomaterials. For this purpose, an improved fractional step Taylor–Galerkin algorithm is first formulated and then validated on the basis of a new analytical solution.The algorithm, based on a stress–velocity–pressure formulation of the hydro-mechanical problem, combines an explicit Taylor–Galerkin method with a fractional time-stepping, while an accurate Runge–Kutta-type integrator is introduced to treat the viscosity of the porous skeleton. The overall algorithm results in an efficient stabilized scheme allowing for linear equal interpolation of field variables, even when the so-called “undrained incompressible limit” is approached.The accuracy and stability of the method are verified with reference to a 1D benchmark problem, concerning the propagation of P waves along a saturated viscoelastic soil stratum. For this problem, a frequency-domain analytical solution is derived, assuming incompressible interstitial fluid and soil grains. The assumption of Maxwell viscoelastic soil skeleton is analytically convenient to preserve the linearity of the problem, while the same rheology of a more realistic elasto-viscoplastic non-linear behaviour is maintained.The performance of the fractional step Taylor–Galerkin algorithm is explored simulating the dynamic response of the stratum to harmonic, impulsive and seismic input excitations. In particular, parametric analyses are performed to confirm the effectiveness of the method in reproducing fully undrained responses, as well as in dealing with weakly viscous materials.     

基于Intel MIC平台大规模耗散粒子动力学模拟的设计与优化

耗散粒子动力学(DPD)模拟是一种重要的研究流体动力学特性的计算模拟方法,基于Intel MIC平台设计实现了面向大规模耗散粒子动力学模拟,充分结合了DPD模拟本身的特性和MIC平台的特征。对DPD模拟中的近邻列表构建和短程作用力关键代码实现了向量化优化,在CPU和MIC协处理器之间采用任务计算负载平衡机制,支持MPI进程内线程数量负载平衡控制。分别在原型程序上和LAMMPS集成中做了性能对比分析,实验结果显示了引入相关优化技术的有效性,为进一步研究面向MIC众核平台的分子动力学相关工作奠定了基础。     

Physically based wall boundary condition for dissipative particle dynamics

In this paper, we present a novel wall boundary condition model, which stands just on the physical facts, for the dissipative particle dynamics (DPD) method. After the validation of this model by means of the common benchmarks such as the Couette and the Poiseuille flows, we study the effects of this model on the diffusion coefficient in a wide variety of different coarse-graining levels. The obtained results show that the proposed model preserves the thermodynamics of the system, also eliminates the spurious effects of the wall, and consequently is able to preserve the accurate structural characteristics of the working DPD fluid in the wall’s vicinity. We also study the fluid flow through a channel with the polymer-coated walls. A comprehensive investigation into the wall–solvent–polymer interactions is presented for the solvents with different qualities. Since the working DPD fluid’s structure is heterogeneous, the working fluid’s structure, its density variations and the force field that is experienced by the working DPD fluid particles near the wall cannot be predicted before the simulation. Hence, all of these data have to be determined systematically during the simulation. We show that the force field experienced by the particles near the wall depends substantially upon the solvent quality. We also show that the force fields experienced by the particles from different types (solvent/polymer) in the wall’s vicinity are significantly different from each other except in the athermal solvent.     

Parallel implementation of isothermal and isoenergetic Dissipative Particle Dynamics using Shardlow-like splitting algorithms

A parallel implementation of the Shardlow splitting algorithm (SSA) for Dissipative Particle Dynamics (DPD) simulations is presented. The isothermal and isoenergetic SSA implementations are compared to the DPD version of the velocity-Verlet integrator in terms of numerical stability and performance. The integrator stability is assessed by monitoring temperature, pressure and total energy for both the standard and ideal DPD fluid models. The SSA requires special consideration due to its recursive nature resulting in more inter-processor communication as compared to traditional DPD integrators. Nevertheless, this work demonstrates that the SSA exhibits stability over longer time steps that justify its regular use in parallel, multi-core applications. For the computer architecture used in this study, a factor of 10–100 speedup is achieved in the overall time-to-solution for isoenergetic DPD simulations and a 15–34 speedup is achieved for the isothermal DPD simulations.     

Distributed Constraint Force Approach for Coordination of Multiple Mobile Robots

A new approach to coordination of multiple mobile robots is presented in this paper. The approach relies on the notion of constraint forces which are used in the development of the dynamics of a system of constrained particles with inertia. A familiar class of dynamic, nonholonomic robots are considered. The goal is to design a distributed coordination control algorithm for each robot in the group to achieve, and maintain, a particular formation while ensuring navigation of the group. The theory of constraint forces is used to generate a stable control algorithm for each mobile robot that will achieve, and maintain, a given formation. The advantage of the proposed method is that the formation keeping forces (constraint forces) cancel only those applied forces which act against the constraints. Another feature of the proposed distributed control algorithm is that it allows to add/remove other mobile robots into/from the formation gracefully with simple modifications of the control input. Further, the algorithm is scalable. To corroborate the theoretical approach, simulation results on a group of six robots are shown and discussed.     

Migration of DNA molecules through entropic trap arrays: a dissipative particle dynamics study

Dissipative particle dynamics (DPD) simulations of worm-like chain bead-spring models are used to explore the electrophoresis migration of DNA molecules traveling through narrow constrictions. The DPD is a relatively new numerical approach that is able to fully incorporate hydrodynamic interactions. Two mechanisms are identified that cause the size-dependent trapping of DNA chains and thus mobility differences. First, small molecules are found to be trapped in the deep region due to higher Brownian mobility and crossing of electric field lines. Second longer chains have higher probability to form hernias at the entrance of the gap and can pass the entropic barrier more easily. Consequently, longer DNA molecules have higher mobility and travel faster than shorter chains. The present DPD simulations show good agreement with existing experimental data as well as published numerical data.