Mohammad Robiul Hossan

Modeling and simulation of dielectrophoretic particle–particle interactions and assembly

Electric field induced particle-particle interactions and assembly are of great interest due to their useful applications in micro devices. The behavior of particles becomes more complex if multiple particles interact with each other at the same time. In this paper, we present a numerical study of two dimensional DC dielectrophoresis based particle-particle interactions and assembly for multiple particles using a hybrid immersed interface-immersed boundary method. The immersed interface method is employed to capture the physics of electrostatics in a fluid media with suspended particles. Particle interaction based dielectrophoretic forces are obtained using Maxwells stress tensor without any boundary or volume integration. This electrostatic force distribution mimics the actual physics of the immersed particles in a fluid media. The corresponding particle response and hydrodynamic interactions are captured through the immersed boundary method by solving the transient Navier-Stokes equations. The interaction and assembly of multiple electrically similar and dissimilar particles are studied for various initial positions and orientations. Numerical results show that in a fluid media, similar particles form a chain parallel to the applied electric field, whereas dissimilar particles form a chain perpendicular to the applied electric field. Irrespective of initial position and orientation, particles first align themselves parallel or perpendicular to the electric field depending on the similarity or dissimilarity of particles. The acceleration and deceleration of particles are also observed and analyzed at different phases of the assembly process. This comprehensive study can be used to explain the multiple particle interaction and assembly phenomena observed in experiments.

Hybrid immersed interface-immersed boundary methods for AC dielectrophoresis

Abstract Dielectrophoresis, a nonlinear electrokinetic transport mechanism, has become popular in many engineering applications including manipulation, characterization and actuation of biomaterials, particles and biological cells. In this paper, we present a hybrid immersed interface–immersed boundary method to study AC dielectrophoresis where an algorithm is developed to solve the complex Poisson equation using a real variable formulation. An immersed interface method is employed to obtain the AC electric field in a fluid media with suspended particles and an immersed boundary method is used for the fluid equations and particle transport. The convergence of the proposed algorithm as well as validation of the hybrid scheme with experimental results is presented. In this paper, the Maxwell stress tensor is used to calculate the dielectrophoretic force acting on particles by considering the physical effect of particles in the computational domain. Thus, this study eliminates the approximations used in point dipole methods for calculating dielectrophoretic force. A comparative study between Maxwell stress tensor and point dipole methods for computing dielectrophoretic forces are presented. The hybrid method is used to investigate the physics of dielectrophoresis in microfluidic devices using an AC electric field. The numerical results show that with proper design and appropriate selection of applied potential and frequency, global electric field minima can be obtained to facilitate multiple particle trapping by exploiting the mechanism of negative dielectrophoresis. Our numerical results also show that electrically neutral particles form a chain parallel to the applied electric field irrespective of their initial orientation when an AC electric field is applied. This proposed hybrid numerical scheme will help to better understand dielectrophoresis and to design and optimize microfluidic devices.

A new fabrication technique to form complex polymethylmethacrylate microchannel for bioseparation

Recent studies show that reduction in cross-sectional area can be used to improve the concentration factor in microscale bioseparations. Due to simplicity in fabrication process, a step reduction in cross-sectional area is generally implemented in microchip to increase the concentration factor. But the sudden change in cross-sectional area can introduce significant band dispersion and distortion. This paper reports a new fabrication technique to form a gradual reduction in cross-sectional area in polymethylmethacrylate (PMMA) microchannel for both anionic and cationic isotachophoresis (ITP). The fabrication technique is based on hot embossing and surface modification assisted bonding method. Both one-dimensional and two-dimensional gradual reduction in cross-sectional area microchannels were formed on PMMA with high fidelity using proposed techniques. ITP experiments were conducted to separate and preconcentrate fluorescent proteins in these microchips. Thousand fold and ten thousand fold increase in concentrations were obtained when 10 × and 100 × gradual reduction in cross-sectional area microchannels were used for ITP.

Review: Electric field driven pumping in microfluidic device

Pumping of fluids with precise control is one of the key components in a microfluidic device. The electric field has been used as one of the most popular and efficient nonmechanical pumping mechanism to transport fluids in microchannels from the very early stage of microfluidic technology development. This review presents fundamental physics and theories of the different microscale phenomena that arise due to the application of an electric field in fluids, which can be applied for pumping of fluids in microdevices. Specific mechanisms considered in this report are electroosmosis, AC electroosmosis, AC electrothermal, induced charge electroosmosis, traveling wave dielectrophoresis, and liquid dielectrophoresis. Each phenomenon is discussed systematically with theoretical rigor and role of relevant key parameters are identified for pumping in microdevices. We specifically discussed the electric field driven body force term for each phenomenon using generalized Maxwell stress tensor as well as simplified effective dipole moment based method. Both experimental and theoretical works by several researchers are highlighted in this article for each electric field driven pumping mechanism. The detailed understanding of these phenomena and relevant key parameters are critical for better utilization, modulation, and selection of appropriate phenomenon for efficient pumping in a specific microfluidic application.

Bipolar Janus particle assembly in microdevice

In recent years, there are significant interests in the manipulation of bipolar Janus particles. In this article, we investigate the transient behavior of the electro‐orientation process and particle–particle interaction of ellipsoidal bipolar Janus particles in the presence and absence of a DC electric field. The bipolar particle dynamics is modeled with a body force term in the fluid flow equations based on the Maxwell stress tensor. This force is due to presence of bipolar surface charges on the particles as well as their interactions with an imposed field. An interface resolved numerical scheme that consider the finite size of the particle is adopted for computation of the electric and flow fields. Our numerical results show that in the absence of an electric field, particles can undergo self‐orientation to reach an equilibrium position. The time taken to reach a stable orientation depends on the initial configuration and inter‐particle separation distance. Bipolar particles experience forces only on their polar ends, a phenomena that is difficult to capture with noninterface resolved methods. When bipolar particles are exposed to an external electric field, they rotate to align along the external electric field direction. Depending upon the initial configuration, particles orient via clockwise or counter clockwise rotations to form head to tail chains. The time required to form particle assembly strongly depends on particle size and bipolar charge density. The present numerical algorithm can be applied to a wider class of dual‐faced Janus particles.

Strength Evaluation and Failure Prediction of Short Carbon Fiber Reinforced Nylon Spur Gears by Finite Element Modeling

In this paper, short carbon fiber reinforced nylon spur gear pairs, and steel and unreinforced nylon spur gear pairs have been selected for study and comparison. A 3D finite element model was developed to simulate the multi-axial stress–strain behaviors of the gear tooth. Failure prediction has been conducted based on the different failure criteria, including Tsai-Wu criterion. The tooth roots, where has stress concentration and the potential for failure, have been carefully investigated. The modeling results show that the short carbon fiber reinforced nylon gear fabricated by properly controlled injection molding processes can provide higher strength and better performance.

Numerical Investigation of DC Dielectrophoretic Particle Transport

In this paper, we investigate the mechanism of two dimensional DC dielectrophoresis (DEP) using a hybrid immersed interface-immersed boundary method where both electric and hydrodynamic forces are obtained with interface-resolved approach instead of point-particle method. Immersed interface method is employed to predict DC electric field in a fluid media with suspended particles while immersed boundary method is used to study particle transport in a fluid media. The Maxwell stress tensor approach is adopted to obtain dielectrophoretic force. This hybrid numerical scheme demonstrates the underlying physics of positive and negative dielectrophoresis, and explains their contribution in particle assembly with consideration of size, initial configurations and electrical properties of particles as well as fluid media. The results show that the positive DEP provides accelerating motion while negative DEP provides decelerating motion depending on the electrode configurations and initial particle positions. The results also show that the local nonuniformity in electric field induced by the suspended particles guides the particles to form stable chain. Both positive and negative DEP can contribute in the process of particle assembly formation based on the properties of particles and fluid media. This hybrid immersed interface-immersed boundary scheme could be an efficient numerical tool for understanding fundamental mechanism of dielectrophoresis as well as designing and optimization of DEP based microfluidic devices.Copyright

Reynolds Number Dependence of Laminar Loss Coefficients for a Rectangular-Cross Section Square-Cornered Tee Junction

Energy losses in junctions are often found by using an assumed loss coefficient for the particular geometry. While this coefficient remains nearly constant under turbulent conditions, this is not true for laminar flow. The loss coefficient through a dividing junction tends to decrease as a function of Reynolds number converging to some value once fully turbulent flow occurs. Research has been done to catalog losses for various geometries under turbulent flow; yet, there has not been the same detailed research for the laminar conditions. This is mostly due to many applications at the macro scale where turbulent flow is prevalent. Flow in microchannels and nanochannels is mostly laminar. Applications at this small scale have created a demand for the study of these loss coefficients under the laminar flow conditions.This paper focuses on simulations done of flow through a square-cornered tee-junction with a rectangular cross sectional area of 6 by 7 millimeters. The fluid consisted of a glycerin and water mixture that was 30 percent water and 70 percent glycerin by volume. Measurements were made of an actual mixture’s density and viscosity and these parameters were used in the simulations. The mixture was chosen to give sufficient pressure drop to measure in validation experiments. Using these simulations, a detailed account of the energy losses in the junction was observed as a function of Reynolds number. The Reynolds number range in this study was 1–100. Higher Reynolds numbers were simulated where signs of a transition to turbulence were observed.The stagnation loss coefficient, which includes kinetic energy and pressure changes through the junction, was found to be inversely proportional to Reynolds number. Initial experimental verification has been performed, in which the experimental stagnation loss coefficient followed the same trend as the simulations. Additional experimental validation is underway.Copyright

ANALYTICAL SOLUTION FOR TEMPERATURE DISTRIBUTION IN MICROWAVE HEATING OF RECTANGULAR OBJECTS

Microwave heating is extremely popular as a household method for warming up foodstuffs quickly. However the industrial application of this convenient, pollution free heating technique is very limited because of its non-uniform temperature distribution with hot and cold spots. The temperature distribution in microwave heating depends on the electromagnetic frequency, processing time, size and shape, and dielectric properties of the object being heated. In this paper, we present a close form analytic solution for temperature distribution in a three dimensional rectangular block under microwave processing. With the knowledge of electric field distribution from Maxwell’s equation, a three dimensional, unsteady, non-homogenous energy equation containing a microwave source term is solved by the integral transform technique. The effects of various parameters such as sample thickness, electromagnetic frequencies, dielectric properties, and processing time are studied for a salmon fillet. The results indicate that the proper integration of incident frequencies, dielectric properties, sample thickness and processing time could provide homogenous temperature distribution in a salmon fillet under microwave heating.Copyright

Modeling and simulation of electric field guided cell deformation

Application of electric field has become one of the most promising actuation tools in bio-microfluidics for cell deformation, poration and manipulation. However, electric field guided cell deformation process is still not well understood due to its complex multiphysics nature. In this paper, we present an electric field induced cell deformation study using hybrid immersed interface-immersed boundary method where both electric and hydrodynamic forces are evaluated with interface-resolved approach. Immersed interface method is employed to evaluate electric field in fluids with submerged cells while immersed boundary method is used to study hydrodynamics with flexible immersed boundaries. Electric field induced force is calculated using Maxwell’s stress tensor approach. The results show that the deformation process depends on the electrical properties of fluid as well as cells, the direction of applied electric field and nature of electric field induced forces. When electrical conductivity of cell is less than that of fluid medium, cell experiences compressive force and deformation rate is faster. The response of fluids to the deformation process is also depend on the relative electrical properties of cell and fluid medium. This study provides critical insight of the transient cell deformation mechanism and better understanding which can help in designing experimental studies and exploring new applications of electric field guided cell deformation.Application of electric field has become one of the most promising actuation tools in bio-microfluidics for cell deformation, poration and manipulation. However, electric field guided cell deformation process is still not well understood due to its complex multiphysics nature. In this paper, we present an electric field induced cell deformation study using hybrid immersed interface-immersed boundary method where both electric and hydrodynamic forces are evaluated with interface-resolved approach. Immersed interface method is employed to evaluate electric field in fluids with submerged cells while immersed boundary method is used to study hydrodynamics with flexible immersed boundaries. Electric field induced force is calculated using Maxwell’s stress tensor approach. The results show that the deformation process depends on the electrical properties of fluid as well as cells, the direction of applied electric field and nature of electric field induced forces. When electrical conductivity of cell is less th...