Microfluidic devices have been used in various applications including automated analysis systems, biological applications such as DNA sequencing, antigen-antibody reactions, protein studies, chemical applications, single cell studies, etc. These devices are controlled by an electric field and thus unlike continuous devices , digital microfluidic devices are free of mechanically moving parts.
Microfluidics
At low capillary number (Ca < 1), the interfacial force dominates and thus spherical droplets are formed.
Digital microfluidics (DMF) and Electrowetting on di- electric (EWOD)electric (EWOD)
EWOD devices have advantages similar to those of continuous microfluidic devices, such as low reagent and sample requirements, compact size, portability, low cost, mass production, and easy integration with analytical devices. But the main advantage of EWOD chips over other microfluidic devices is flexibility in operations.
Theory of electrowetting
- Contact angle change
- Droplet transport
- Dispensing droplets form reservoir
- Splitting droplets in smaller droplets
- Merging and mixing of two droplets
The Young-Lippmann equation clearly shows that the change in the contact angle of a droplet with a solid surface is directly proportional to the square of the applied voltage and inversely proportional to the thickness of the dielectric layer. The cut-off electrode helps to pinch off a larger reagent droplet from the reservoir, and the transport electrode moves that droplet forward.
Applications of EWOD
Following a detailed discussion in the first chapter on the electrowetting effect, how it is achieved on dielectric layers at low voltages and its applications, this chapter discusses the motivation behind the presented work. A brief description of the previous work done in numerical analysis of electrowetting, fabrication of EWOD devices, and work on drop routing algorithms is given.
Motivatioin: Alkaline phosphatase and Osteoporosis
This incidence of osteoporosis in women may be due to low calcium intake, vitamin D deficiency, early menopause and no access to screening and diagnostic facilities [27]. The drops of reagents and blood plasma can be mixed and analyzed to know the levels of BALP in the patient.
Digital Microfluidics
These operations include droplet creation from the reservoir, transport of this droplet over the electrode array, mixing and droplet separation in the EWOD device. These applications are important to make EWOD devices suitable for micro total analysis (µTAS) systems. The device they fabricated was sealed. channel equipment. Moon et al studied silicon dioxide, parylene and barium strontium titanate (BST) as dielectric material [3]. They experimentally proved that the dots can move at voltages up to 15 V. These devices can be used for multiple applications just by changing the sequence of applied voltage.
Yu et al discussed the types of electrode addressing modes and their advantages and disadvantages [26]. The work showed that the algorithm is able to handle multiple responses at the same time. Pop et al proposed a routing algorithm that implements responses on chip based on current execution scenario with operation execution time variability [31]. Cahill et al used the level set method to simulate Young-Lippmann equation to show droplet manipulation using COMSOL Multiphysics [9].
Xiong et al demonstrated that the EWOD chip can be used as a high-performance droplet spreader [25]. To verify the functionality and accuracy of the proposed device, Xiong et al performed numerical analysis in COMSOL Multiphysics. These interfaces can be linked to include the effect of one parameter on another.
Objectives
- Setting up COMSOL Multiphysics model
- Simulation objectives
- Setting up COMSOL Multiphysics module
- Simulation of objectives
Here in this section, 2D simulations of Young-Lippmann equation, transport of droplets over a series of electrodes and mixing of two droplets with different concentrations of analytes are performed. To set up a COMSOL Multiphysics model for 2D simulation of Lippmann-Young equation, electrostatics and laminar two-phase flow, moving mesh modules are used. Laminar two-phase flow, moving mesh interface is defined only in water and air regions.
To model the 2D transport of a droplet in the direction of applied voltage, a conservative two-phase laminar flow, level set method is used. By default, the eǫls value is tpf.hmax/2 where tpf.hmax is the dimension of the largest mesh element in the level set domain. From the list, the electrostatics and two-phase laminar flow modules, placed in the level, are selected.
In the final stage, the initialization is performed only for the level setting method and both the electrostatics and the level setting modules are solved in a time-dependent solver. To solve the governing partial differential equations of electrostatics and laminar two-phase flow, the level set method (tpf), variables such as the contact angle of the liquid droplet with the solid wall, volume forces acting on the liquid droplet and the dielectric constant of the region where the level set method is applied must be specified. To study this switching of voltage between electrodes, electrostatics and level adjustment method modules are used.
Merging and mixing two droplets
Setting up COMSOL Multiphysics model
Simulation of objectives
These parameters help us understand the effects of different parameters on the electrowetting of a liquid droplet, its transport across an electrode array, and the mixing efficiency. Design and Fabrication of EWOD Chip and Switching Circuit When we talk about EWOD devices as a system, it is intended to include the fabricated electrode array chip and an electronic switching circuit that controls the sequence of voltage from an external source. This section discusses the design and fabrication of the EWOD chip and electronic switching circuit.
Fabrication of the EWOD device
Design of the EWOD device
Fabrication process
Glass is allowed to cool and the same spin coating procedure is followed once more. When two rounds of spin coating are finished, ITO glass is baked at 110°C for 90 sec. Power of UV Writer is set to 80 When this exposure is done, ITO glass is baked again at 110°C for 90 sec.
After the etching is perfectly done, the patterned ITO glass is washed with deionized water, acetone and again with DI water. To isolate the patterned ITO electrodes from liquid droplets, the EWOD device is coated with dielectric and hydrophobic layers. Also to study the properties of different materials as an option for the dielectric layer, pattern-free ITO glass is coated with SU8 photoresist and polydimethylsiloxane (PDMS)[23].
To ensure smooth and frictionless transportation of droplets over an array of electrodes, the top plate of the fabricated device is coated with Teflon solution.
Electrode addressing
- Microcontroller
- Switching circuit
- Relays
- Metal oxide field effect transistor (MOSFET)
- Connections
- PCB design
The voltage required to perform basic operations such as transport, mixing, fusing is in the order of tens of thousands of volts. Therefore, it is necessary to have a switching circuit which will connect high voltage source to electrodes, but with some control. On the other hand, solid state relays, which are electronic switching circuits, operate without moving parts.
They are switched ON and OFF when a small external voltage is applied across the control terminals. If this voltage is greater than the threshold voltage of MOSFET, it allows current to flow through it. Gate of MOSFET receives signal from Arduino, source is grounded and drain is connected to high voltage source.
To drive one electrode in direct addressing mode, we need one MOSFET and one resistor. Connections of the PCB and layout of the PCB designed in Eagle are shown in Fig. (4.7) and Fig. (4.8) respectively.
Designed experiments
Once the contact angle change is observed, these points will be placed on electrodes with 0V applied to them. The actuation voltage will be kept as low as possible and gradually increased to indicate the threshold voltage for droplet transport. After achieving droplet movement, this device will be used to efficiently mix two liquid droplets.
Once mixing is achieved, the device can be used to mix blood plasma with BCIP disodium salt and nitro blue tetrazolium. This chapter demonstrates the results obtained after numerical analysis in COMSOL Multiphysics and experiments performed on fabricated EWOD device. The first part of this chapter discusses the simulation results for Young-Lippmann equation, droplet transport and on-chip mixing.
Second part in this chapter is about results obtained after conducting experiments on fabricated EWOD device.
Numerical Analysis in COMSOL Multiphysics
- Modelling Young-Lippmann equation
- Reversibility of Lippmann effect
- Demonstrating dependency of voltage required for electrowetting on dielectric thicknesson dielectric thickness
- Moving droplet to adjacent electrode
- Demonstrating dependency of velocity of droplet on voltage applied (Brochards model)(Brochards model)
- Realizing droplet transport over an array of electrodes
- Demonstrating merging of two droplet
- Visualizing concentration field after mixing
- Demonstrating dependency of mixing quality on voltage applied
With the help of a boundary probe, the contact angle of the droplet with the solid surface is calculated at each step. As mentioned earlier, a liquid droplet changes its contact angle when a voltage is applied to it. When the voltage source is turned off, the drop bounces back and tries to regain its original shape with a contact angle of 120.
The contact angle change in a liquid droplet is inversely proportional to the thickness of the dielectric layer. Therefore, the voltage requirement for achieving certain change in contact angle is different for different values of dielectric thickness. The limit probe is used to record the value of contact angle at each voltage.
From these values it is evident that the lower the dielectric layer, the lower the required voltage. Fig(5.4), fig (5.5) and fig (5.6) demonstrate the contact angles at different voltages when d=0.1um, 1um and 10um respectively. It is necessary to move the droplet over a set of electrodes. Due to computational time, the point is not moved over the electrode array. One of the critical parameters that can change the quality of the mixture is the level of voltage applied to the electrodes.
Experimental Results
These bubbles are only partially formed where Teflon AF is dispensed, which can be seen in fig(5.16). To find the best suitable option as dielectric, SU8 and PDMS are coated on simple ITO coated glass substrate. So to increase the thickness of this SU8 film, two rounds of spin coating were done.
But this film was also destroyed due to electrolysis of drops of KCl, PBS and DI water which can be seen in fig(5.17). Figure 5.17: a) SU8 film destroyed after application of 5V b) Comparatively thicker SU8 film destroyed after application of 5V. Even after the voltage is gradually increased to 40V, PDMS film is stable as seen in fig (5.18). Theoretically, the required voltage to change contact angle of droplet by 10 is 200V. This is very high tension when biological applications are involved.
In order to achieve the electrowetting effect at a lower voltage, the PDMS must be spin coated at a very high speed.
Conclusion
Creating, Transporting, Cutting and Aggregating Liquid Droplets by Electrowetting-Based Actuation for Digital Microfluidic Circuits”. An Acoustically Actuated Microliter Flow Chamber on a Chip for Cell-Cell and Cell-Surface Interaction Studies”.
