Electronic Thesis and Dissertation Repository

Thesis Format



Master of Engineering Science


Mechanical and Materials Engineering


Boutilier, Michael


Micro-scale flow sensors present several advantages over traditional flow sensing
methods, including minimal flow disruption, high spatial resolution, and low unit
cost. Many existing micro-scale thermal and piezo flow sensors struggle with temperature drift and require complicated fabrication processes. This thesis details
the development of a 60 μm by 60 μm by 50 μm drag-based capacitive flow sensor
constructed from vertically aligned carbon nanotube forests. The construction
of a thermal chemical vapour deposition system for sensor synthesis is also de-
tailed. Manual manipulation of the sensor with an atomic force microscope probe
was found to produce a full scale signal of ∼30 fF, measured by an AD7746 integrated circuit. A flow experiment producing wall shear stresses from 1 to 95 Pa
did not produce a measurable output. With more development, such a sensor
would provide a low-cost, highly configurable device for all manner of flow sensing

Summary for Lay Audience

Flow sensors measure the speed and direction of fluid movement and have a variety of important applications, such as meteorological measurements, industrial
processes, and in biomedical devices. The advent of the microchip in the late
20th century heralded an era of miniaturization. Micro-scale flow sensors offer
several advantages over traditional devices. Firstly, their small size means that
they do not disturb the fluid flow they seek to measure. Second, their compatibility with widely available manufacturing methods used for microchip production
makes them cheap to produce in large numbers. Finally, their small size means
they provide flow measurements at a very precise location. Unfortunately, current
micro-scale flow sensors suffer from temperature drift, and require complicated
manufacturing processes, hampering widespread adoption.
This work outlines the development of a micro-scale flow sensor 60 μm by 60 μm
by 50 μm in size. The flow sensor is constructed from forests of vertically aligned
carbon nanotubes grown in a custom, purpose-built chemical vapour deposition
system. Discrete forests form the structural components of the sensor. When fluid
flows over the sensor, the drag force bends the forests over, closing the gap between
them. This movement can be measured by a change in electric charge stored in
adjacent structures. The signal is measured with an off-the-shelf integrated circuit
used primarily in the automotive industry. The sensor was tested by physically
poking the carbon nanotube forests with the probe on an atomic force microscope,
and found to produce a discernible signal of ∼30 fF. A flow experiment that
followed using a blower connected to a 16 mm by 16 mm square channel showed
no signal. It is posited that a poor understanding of the stiffness of the sensor
structure led to a flow experiment with insufficient flow rates to properly actuate
the sensor.
While the produced flow sensor is not ready for real-world applications, this project fulfilled its objective as a first-generation proof of concept. Sensitivity of the device may be increased in subsequent work by refining the sensor geometry, and better
understanding the material properties. The resulting sensor will be simpler and
cheaper than existing micro-scale flow sensing options.

Creative Commons License

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