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Thesis Format

Monograph

Degree

Master of Engineering Science

Program

Chemical and Biochemical Engineering

Supervisor

Boutilier, Michael S. H.

Abstract

Miniature fluid flow sensors are being integrated in autonomous air and water vehicles, microfluidic devices, medical equipment, and intelligent systems. This work presents a wall shear stress sensor that has a 50 µm by 60 µm footprint, 200 µm height, sensitivity of 0.05–1 fF/Pa and range up to ±8 Pa. The sensor consists of two carbon nanotube pillars and produces a capacitance change in response to deformation in flow. Sensor elasticity and touch sensitivity were quantified by atomic force microscopy. Sensing element deformation was confirmed via optical microscopy. Capacitance response was established by calibration in channel flow. Sensor operating range widens when the thinner pillar was downstream since greater deflection did not cause pillar contact. Some sensors responded differently to flow due to pillar twisting or a dominant Bernoulli effect. Potential topics for future work include liquid flow sensing, external circuitry integration, and fabrication tuning.

Summary for Lay Audience

Miniature flow sensors are being integrated into autonomous air and water vehi- cles, microfluidic devices, medical equipment, and intelligent systems. As the sizes of these devices continue to decrease, flow sensors must also shrink accordingly. This work presents a novel miniature flow sensor that has a length of 60 µm, a width of 50 µm, and an average height of 200 µm. The sensor is made of two carbon nanotube (CNT) pillars, a thin pillar that acts as a sensing element and a thick pillar that serves as a reference element. CNT sensors are advantageous as CNT fabrication methods readily scale to the order of tens of microns. With this design, we seek to further efforts to miniaturize fluid flow sensors and enable smaller devices.

When the sensor is exposed to flow, the thin pillar will bend like a thin beam that is fixed at one end. When it bends, the free end of the thin pillar moves closer to the thick pillar, and since the carbon nanotube pillars are conductive, the capacitance of the sensor changes. This capacitance change can be measured with off-the-shelf electronics. Atomic force microscopy was used to measure the CNT pillar elasticity and sensitivity to touch as these parameters were needed to model the capacitance response to wall shear stress. The deflection of this thin pillar was confirmed via observation under an optical microscope and the capacitance response to wall shear stress was established by calibration in a flow channel.

In the flow channel, the sensitivity of the sensor was found to be 0.13 fF/Pa when the thin pillar was positioned downstream of the thick pillar. When the thin pillar was upstream of the thick pillar, the capacitance response reached a maximum of 0.1 fF at a wall shear stress of 2 Pa due to the pillars making contact with each other. However, some sensors responded differently to flow, usually caused by either twisting of the thin pillar or a dominant Bernoulli effect. Future work could exploit these diverse sensing mechanisms to tune sensitivity or operating range.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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