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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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Microfluidics is the key science and technology that enables fluid manipulation and control in channels that have dimensions on the order of micrometers. The change in fluid physics in microscale ensures novel use of microfluidic systems. Although the first microfluidic device was a gas chromatograph invented in 1975, these systems have not used for biological or chemical applications since 1990s [4]. Microfluidic device fabrication technology was derived from microelectromechanical system (MEMS) technology.
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Fig.1. A microfluidic chemostat [ref] During the last decades, microfluidic systems became a widespread research area and gave birth to easy-to-use and miniaturized platforms that can be applied to biological and chemical analysis. Microfluidics can be divided into three groups as continuous microfluidics, discrete (multi-phase flow, droplet-based, segmented flow) microfluidics and digital microfluidics.Fig.2. Comparison between continuous and segmented flow microfluidics: microfluidic droplets elegantly address several issues of continuous flow, such as Taylor dispersion of reagents due to parabolic flow: the enlargement of the dotted area illustrates this spreading effect; cross-contamination: the single continuous phase allows diffusion between different fluid portions—in this case A and B eventually combine and become C (radial diffusion is omitted for the sake of simplicity); and reagent adsorption on channel walls (illustrated as green channel edges) leading to reagent loss and cross-contamination. [ref]
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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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In the literature, there are two main passive droplet formation generators, T-junction (;
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) and flow-focusing device (;
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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.
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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.