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TA : Pelin Kübra İşgör |
Topics covered:
Microfluidic systems
Droplet generation devices
Fabrication of microfluidic devices
Introduction to Elveflow pressure pump and Microqubic 3D digital microscope
Droplet generation at varying sizes
Droplet detection using via video/image processing
Experiment details:
DROPLET GENERATION EXPERIMENT
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Continuous microfluidics is based on continuous liquid flow manipulation. These systems are usually used for simple and well-defined functions such as chemical separation and biochemical applications. Since the surface property of the entire system affects fluid flow at any location in the system, these systems are not suitable for integration and scalability. Discrete microfluidics compartmentalizes and manipulates small volumes of liquid with two immiscible phases. Microdroplets are suitable for very small amount of liquid handling. Since droplet is isolated from its surrounding, any material inside droplet (reagent, cell, protein etc.) is preserved throughout the system. Droplet loading, mixing, sorting, merging, break-up enables high-throughput chemical and biological experimentation due to kHz level droplet generation. Both continuous and discrete microfluidics operate in microchannels. However, digital microfluidics is manipulation of small liquid volumes on open structures using electrowetting method. Electrowetting is changing surface properties of a material by applying electric field. On independently addressed electrodes, a small volume of liquid is moved from one electrode to another. These systems enable merging of different material loaded droplets. Electrowetting on-dielectric (EWOD) is a common method in digital microfluidics.
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Paper-based microfluidic systems are considered Microfluidics 2.0. This method is inexpensive and easy-to-use. Changing hydrophobicity of the paper at different zones using a printer and wax, channels are fabricated [5]. Printing reagents and other materials to the test zones makes microfluidic paper-based analytical devices (µPADs) cheaper point-of-care diagnostic devices. Being user-friendly and cheap, paper-based microfluidic systems are developed for disease diagnostics in third world countries [6].
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Two immiscible fluids are driven from two separate channels and meet at a junction that is determined by the specific geometry of the channels. In 2001, Thorsen et al. published an article titled "Dynamic pattern formation in a Vesicle-Generating Microfluidic Device" [16]. For the first time, they accomplished droplet generation with two immiscible fluids using a T-junction. Both water and oil were continuously driven to the microchannel. The water obstructs the main channel at the junction, while oil flows through the channel. At this moment, high shear forces occur. The flow is not linear and static due to interactions between the boundary of two liquids. This instability arises from the competition between surface tension and shear forces. The competition generates droplets. The size and speed of droplets are finely tuned by adjusting water and oil flow rates or pressures.
Fig. x Droplet generation strategies: (ii) T-Junction in a planar chip format ; (iii) flow focusing in a planar chip
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How do we use Navier-Stokes eqn for microfluidics?
Hardware
The experimental system comprised of Elveflow pressure pump, Microqubic 3D digital microscope and microfluidic flow focusing device.
Figure X : Set-up photo with insets
Fabrication of microfluidic Fabrication of microfluidic devices
For fabrication of microchannel molds, a negative photoresist, SU-8 2050, can be used by applying photolithography techniques. During this process, a transparency mask can be used. SU-8 2050 can be spin coated on Si wafer. At the end of the process, SU8 developer was used for getting the microchannel structure. Process flow is given in Figure 3.1. Soft lithography is a method that refers to replicating mold structure using polymeric, "soft", materials by stamping. For fabrication of microchannels, PDMS (polydimethylsiloxane) is used as soft material.
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Hardware
The experimental system comprised of Elveflow pressure pump, Microqubic 3D digital microscope and microfluidic flow focusing device.
Figure X : Set-up photo with insets