syringe/peristaltic pump) and the microfluidic device. After filling, the trap is interconnected between a pump ( e.g. If water is used, hydrophilic materials or materials that can be rendered hydrophilic, for instance by O 2 plasma treatment, can be used. The trapping and shield tubes can be made of various materials, with the condition that the contact angle of the solid–fluid interface is small. To finalize the fabrication of the trap, the connection tubes are coupled at both ends of the shield tube. Afterwards, the trapping tube is pushed inside the spiral, where it is kept in place, in the middle of the shield tube. copper) is inserted into the shield tube as the spacer. The fabrication of the trap starts by cutting the tubes to the desired length. The maximum volume of air which can be trapped is defined by the dimensions of the trapping tube and can be tailored to specific needs (desired flow rate and autonomy of the system). Bubble trap design and fabrication The bubble trap is made of three tubes of different diameters and lengths: the connection, the trapping and the shield tubes ( Fig. Consequently, the presented concept can be used to trap air bubbles but may also be used to create a microfluidic oscillator. An additional feature of the presented trapping concept is the generation of an oscillation of the trapped air volume. The trap can easily be integrated into existing microfluidic flow systems without any modifications. Further, the bubble trap functions with various fluids, such as water, 100% ethanol or cell culture medium. The fabrication of the bubble trap is inexpensive and straightforward. The concept takes advantage of the interaction of surface tension and hydrodynamic forces acting on the air bubbles. In this technical innovation, we present a new air/gas bubble trapping concept that does not require a complex setup and can be used for various fluids. 12 All of these concepts either require advanced setups that are expensive or are complicated to fabricate, or only work with aqueous solutions. They are based on membranes, 1–3 systems using vacuum, 4–6 specific channel geometries with or without hydrophobic coatings 7–10 or the use of ultrasound 11 or pressurized fluid. To avoid and minimize the unintended introduction of bubbles into microfluidic systems, a number of bubble trap concepts have been reported. They can abruptly increase the internal pressure in a microfluidic system, which may lead to important shear force variations, change the compliance of the system or even block small channels. Introduction Air bubbles are a major issue in microfluidic systems as they dramatically and unexpectedly modify the intrinsic physical properties of the microfluidic environment. These oscillations may be exploited as a basis for fluidic oscillators in future microfluidic systems. Furthermore, the natural oscillations of trapped air bubbles created in this system are explained and quantified in terms of bubble displacement over time and oscillation frequency. We describe the general working principle and derive a simple theoretical model to explain the trapping. The trap is made of tubes of different sizes and can easily be integrated into any microfluidic setup. The design is simple, easy to fabricate and straightforward to use. The bubble trap is based on the combined interaction of surface tension and hydrodynamic forces. A new approach to trap air bubbles before they enter microfluidic systems is presented.
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