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February – 2012 Volume – 1, Issue – 1 Article #04
IJASETR Research Paper ISSN: 1839-7239
Design and Simulation of Electrokinetic Valve by COMSOL Multiphysics Dr. Srinivasa Rao Karumuri* Associate Professor, Department of Electronics & Instrumentation, Lakireddy Bali Reddy College of Engineering (Autonomous), Mylavaram-521230, A.P, India *Corresponding author’s e-mail:
[email protected]
Abstract In this paper, we have design a model presents an example of pressure-driven flow and electrophoresis in a micro channel system. Researchers often use a device similar to the one in this model as an electro kinetic sample injector in biochips to obtain well-defined sample volumes of dissociated acids and salts and to transport these volumes. The model presents a study of a pinched injection cross valve during the focusing, injection, and separation stages. Focusing is obtained through pressure-driven flow of the sample and buffer solution, which confines the sample in the focusing channel. When the system reaches steady state, the pressure-driven flow is turned off and an electric field is applied along the channels. This field drives the dissociated sample ions in the focusing zone at right angles to the focusing channel and through the injection channel. A clean separation of the sample ions is important, so the model examines the effect on ion separation of different configurations of the electric field. This specific case does not account for electro osmosis because the channel surfaces are subjected to a treatment that minimizes the extension of the electric double layer. Keywords: Sensors, Actuators, MEMS, Electrokinetic Valve, COMSOL Multiphysics Citation: Karumuri SR (2012), Design and Simulation of Electro Kinetic Valve by COMSOL Multiphysics. IJASETR 1(1): Article #04.
Received: 20-01-2012
Accepted: 05-02-2012
Copyright: @ 2012 Karumuri S.R. This is an open access article distributed under the terms of the Creative Common Attribution 3.0 License.
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1. INTRODUCTION “Micromechatronic is the synergistic integration of micro electromechanical systems, electronic technologies and precision mechatronics with high added value”. Micro-Electro-MechanicalSystems (MEMS) is the integration of mechanical elements, sensors, actuators & electronics on a Common silicon substrate through micro fabrication technology [1-3]. They are fabricated using integrated circuit(IC) batch processing techniques and can range in size from micrometers to millimeters. These systems can sense control and actuate on the micro scale and function individually or in arrays to generate effects on the micro scale. It can be difficult for one to imagine the size of MEMS device. The general size of MEMS is on the order of microns .The main characteristic of MEMS is their small size. Due to their size, MEMS cannot be seen with the unaided eye. An optical microscope is usually required for one to be able to see them. MEMS revolutionize silicon based micro electronics with micro machining technology, making possible the realization of complete System-on-a-chip [4]. MEMS is an enabling technology allowing the development of smart products, augmenting the computational ability of microelectronics with the perception and control capabilities of micro sensors and micro actuators and expanding the space of possible designs and applications. Microelectronic integrated circuits can be thought of as the “brains” of a system and MEMS augments this decision making capability with eyes and arms to allow Microsystems to sense and control the environment [5-7]. Sensors gather information from the environment through measuring mechanical, thermal, biological, chemical, optical and magnetic phenomena. The electronics then process the information derived from the sensors and through some decision making capability direct the actuators to respond by moving, positioning, regulating, pumping and filtering, there by controlling the environment for some desired outcome or purpose [8-10]. Because MEMS devices are manufactured using batch fabrication techniques similar to those used for integrated circuits unprecedented levels of functionality, reliability and sophistication can be placed on a small silicon chip at a relatively low cost. In this paper, we have design, modeling and simulation of electrokinetic valve by COMSOL.
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2. GEOMETRY OF ELECTROKINETIC VALVE During the past decade, significant advance has been achieved in the area of the micro total analysis systems (lTAS) or lab-on-a-chip (LOC) devices showing great promise for performing a variety of chemical and biochemical analyses. An integrated LOC device can incorporate many of the necessary components and functions of a typical room-sized laboratory into a small chip that performs a specific biological or chemical analysis, including sample treatment, transport, reaction, and detection. Lab-on-a-chip devices are not simply the smaller versions of the conventional instruments; miniaturization raises many new challenges, and LOC devices often involve new physical phenomena and new processes that are dominated by the forces important at micro scale. Controlled transport of liquids and chemical and biological samples is one of crucial issues in these LOC devices. Electrokinetic methods, including electroosmosis, electrophoresis, and dielectrophoresis, are playing important roles in microfluidic devices. Electroosmosis and electrophoresis refer to the motion of the liquid and particles/cells, respectively, in an applied electrical field, and are based on the interaction of electrostatic charge at the liquid-solid interfaces with the externally applied electrical field. Dielectrophoresis is the motion of dielectric particles or cells in a non-uniform electrical field, and is caused by the asymmetric polarization of the particles/cells. These phenomena have been extensively used for pumping, mixing, gradient generation, separation, and sorting on LOC platforms. Electroosmosis is widely used as a pumping method due to its significant advantages over the conventional pressure-driven flow, such as pluglike velocity profile, ease to control and switch flow, and no mechanical moving parts. Generally, electro kinetically driven flows in microfluidic devices are laminar because of the slow velocity and small characteristic length scale and thus small Reynolds number [11]. Consequently, mixing in such a laminar flow of multiple parallel streams occurs only by diffusion, which is problematic for situations requiring rapid mixing of different solutions in micro channels. Some electrokinetic based devices have been developed to perform mixing enhancement, such as T-shaped microchannel mixers, which employ electroosmotic flow (EOF) to pump liquids from two horizontal channels to the T-intersection and mix liquids in the vertical channel while the liquid flows to downstream. T-shaped mixers have been applied in various LOC devices, for example, to dilute sample in a buffer solution and to generate concentration gradients The method was further extended with a non-uniform distribution of zeta potential along the channel wall to generate
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micro flow circulation to enhance the mixing between two solutions. In addition, creative channel and voltage arrangements were utilized to offer the benefits in mixing enhancement These electrokinetic based mixing methods highly rely on the complex channel design, surface modification and voltage control. To regulate the flow in the micochannels, typically membrane pumps and valves are used with different actuation methods including pizoelelctric, electrostatic, and thermopneumatic actuation in silicon, glass, some plastic devices and pneumatic actuation in PDMS devices All these mechanisms require complicated fabrication techniques to introduce moving parts into the microfluidic systems. Electroosmotic flow (EOF) involves simple channel design and fabrication, and does not require mechanical valves. However, EOF-based flow regulating can be more complex because it requires multiple power supplies and timed switching of voltages among these power supplies In searching for potential solutions to the abovementioned challenges, we look into a new type of electrokinetic flow called induced-charge electroosmosis (ICEO) . The most notable feature of ICEO is the micro flow circulation generated near a highly polarizable conducting object in an external electric field. This is because the induced non-uniform charges at the conductor–liquid interface and the resulting non-uniform EOF. Thus, it can be predicted that, by introducing conducting surfaces in a microchannel, irregular flow field with micro vortex can be obtained, which can be used to enhance the species mixing. Additionally, the flow circulations in the microchannel may provide a potential way to gate the flow in the microchannel. The applications of ICEO for mixing enhancement using microelectrode arrays and streaming flow pumping using asymmetric conducting bodies were theoretically predicted These applications of ICEO require fabrication of complex microelectrode arrays and no flow regulating effect was studied. In this article, we suggest a new microchannel design with a pair of triangle-shaped conducting hurdles, forming a converging–diverging section. A correction method is proposed to numerically estimate the induced zeta potential on the conducting surface. A two-dimensional numerical model is used to obtain the electric field, the flow field and the concentration field. The induced-charge electrokinetic flow behaviors in the channel are investigated. The purpose of this study is to propose new methods for rapid electrokinetic mixing and for the unique flow regulating with simple fabrication and easy operation.Figure 1 shows a 2D cross section of the geometry in the xz-plane and points out the different channels and boundaries. The horizontal channel serves as the focusing channel, while
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the vertical channel is the injection channel. The actual model is in 3D with rectangular pipes whose corners are rounded. For geometry dimensions refer to Table 1 below. Table 1: Geometry dimensions
Dimensions(micrometer) HORIZONTAL CHANNEL X 340 Y 20 Z 20
VERTICAL CHANNEL CROSSING AREA
Dimensions(micrometer) HORIZONTAL CHANNEL X -100 Y 0 Z 0 rounding(micrometer) Radius 4 Direction1 In
VERTICAL CHANNEL CROSSING AREA
20 20 340
28 20 28
0 0 -200
-4 0 -4
4 In
4 out
Figure 1: The focusing stage involves pressure-driven flow of both the sample and the buffering solution. The device applies an electric field over the focusing channel.
The device operation and hence the modeling procedure takes place in two stages: focusing and injection. In the focusing stage, the device injects a buffering solution through pressure-driven 45
convection into the vertical channels from the top and bottom. At the same time, it forces the sample solution through the horizontal focusing channel (see Figure 1). The buffering solution neutralizes the acids contained in the sample except for a very thin region confined to the crossing between the horizontal and vertical channels. This means that the dissociated ions are only in a needle-shaped region in the focusing zone. Next, in the injection stage the device turns off the convective flow and then applies a vertical field to migrate the sample from the focusing channel to the injection point at the lower end of the vertical channel. The sample ions are negatively charged and migrate in opposite direction to the electric field. This model studies two different configurations (See Table 2) for the applied electric field. In the first configuration (Injection stage, Mode A) electric field is only applied in the vertical direction. In the second configuration (Injection stage, Mode B) the electric field is applied in both the horizontal and vertical directions (Figure 2).The horizontal field focuses the sample during the initial part of the injection stage in order to obtain a well-separated sample.
Figure 2: During the injection stage, the device turns off convective flow and applies an electric field. The horizontal field avoids the broadening of the sample, while the vertical field injects the sample into the vertical channel in the direction opposite to the electric field.
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Table 2: Configurations for electric fields INLET
MODE A
MODE B
Sample inlet
Electric insulation
Electric potential=-1V
Outlet
Electric insulation
Electric potential=0V
Upper buffer inlet
Electric potential=-3.2V
Electric potential=-3.2V
Lower buffer inlet
Electric potential=0V
Electric potential=0V
The model assumes that the charged sample concentration is very low compared to other ions dissolved in the solution. This implies that the sample concentration does not influence the solution’s conductivity and that you can neglect the concentration gradients of the charge-carrying species, which are present in a much higher concentration than the sample ions. Such an electrolyte is known as a supporting electrolyte.
3. RESULTS and DISCUSSIONS In COMSOL Multiphysics you define the model with the following physics interfaces: •The Laminar Flow interface solves the fluid flow in the channels governed by Stokes equations. •The Electric Currents interface solves the equation for current balance. •The Transport of Diluted Species interface solves the Nernst-Planck equation. 3.1 COMPUTING ELECTROKINETIC VALVE MODEL The operation of the actual device proceeds in two stages, the focusing stage and the injection stage. This model simulates two settings of the injection stage so in total it works in three phases. The first phase defines the domain settings and boundary conditions for the focusing phase. Then the model solves the interfaces sequentially with a nonlinear solver in the following sequence: 1. Laminar Flow interface 2. Electric Currents interface 3. Transport of Diluted Species interface Each step uses the solution from the previous one. The model stores the last solution for use as the initial value for the consequent modeling. In the second phase you change the domain settings and 47
boundary conditions to handle the injection stage Mode A. In a real device you would turn off the convective flow; in the model you simulate this by setting the velocity parameters of the Electrokinetic Flow interface to zero. Thus it uses no information from the Laminar Flow interface. Solving the second phase starts from the stored solution of the first phase, and the model solves the Electric Currents interface with a nonlinear solver. Then you select a time-dependent solver and solve the Transport of Diluted Species interface. This solution is the result for the injection stage Mode A. This example analyzes the focusing stage and two configurations for the injection stages. Recall that the first injection-stage configuration (Mode A) applies the electric field only over the injection channel while the inlet and outlet boundaries of the focusing channel are insulated; the second injection-stage configuration (Mode B) applies the electric field over both channels. Figure 3 shows the steady-state concentration distribution during the focusing stage along with the distribution at the beginning of the injection stage. Note that the vertical flows from the upper and lower injection channels focus the concentration on a very narrow region near the crossing area of the channels. Further away from the crossing area, however, the concentration spreads again more equally over the channel.
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Figure 3: The steady-state concentration distribution during the focusing stage and prior to the injection stage.
Figure 4 and Figure 5 compare the concentration distribution for the two configurations at two times, specifically 0.06 s and 0.12 s after the beginning of the injection stage. The figures on the left show that for Mode A the concentration boundary is practically stationary in the horizontal direction. Consequently, the vertical electric field can continuously draw ions from the focusing channel, which results in poor separation and a poorly defined sample volume of the substance. For Mode B the situation is very different. The horizontal electric field draws the concentration boundary to the left, and the channels separate rapidly. Consequently, this scheme draws a welldefined sample volume of the substance into the injection channel.
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Figure 4: The concentration distribution at a time 0.06 s after starting the injection stage for the Mode A configuration (left) and Mode B configuration (right).
Figure 5: The concentration distribution at a time 0.12 s after starting the injection stage for the Mode A configuration (left) and Mode B configuration (right).
4. CONCLUSION In the transportation of different types of acids, salts in large volume the problem of dissociation of ions in the flow is solved by using electrokinetic valve, designed using COMSOLE MULTI PHYSICS software. It is also possible to observe the difference between the two configurations if you look at the concentration along a line through the middle of the injection channel, examining it at several times after the start of the injection stage (Figure 3). The maximum concentration moves down the injection channel with time. The peaks are higher in the upper axis corresponding to Mode A, but they are much wider than for Mode B. A considerable amount of concentration appears at the left of the peak, and the sample remains attached to the focusing area—resulting in an unwanted distortion of the sample package. The narrow peaks of Mode B, on
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the other hand, form nice bell curves throughout the downward transport in the injection channel, resulting in a well-defined sample package. The model presents a study of a pinched injection cross valve during the focusing, injection, and separation stages. This electrokinetic valve is most commonly used in bio-chips to obtain well defined sample volumes. Acknowledgements Dr.K.Srinivasa Rao would like to thank IISc, Bangalore, for providing research facilities under NPMASS Scheme. The author also would like to thank the director and The Management of Lakireddy Bali Reddy College of engineering for constant encouragement for the development of MEMS Technology.
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