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Characterization of the dynamic flow response in microfluidic devices
Elgack, Mohammed Elmahdi
Elgack, Mohammed Elmahdi
Date
2024-05
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Thesis
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Description
A Master of Science thesis in Mechanical Engineering by Mohammed Elmahdi Elgack entitled, “Characterization of the dynamic flow response in microfluidic devices”, submitted in May 2024. Thesis advisor is Dr. Mohamed Abdelgawad. Soft copy is available (Thesis, Completion Certificate, Approval Signatures, and AUS Archives Consent Form).
Abstract
Microfluidics, which pertains to liquid manipulation on the microscale, have become an essential tool in many chemical and biological applications during the past two decades. Proper functioning of microfluidic devices requires precise control of the flow inside microchannels which is a challenging task given the unconventional fluid flow phenomena that arise on the microscale. For example, the flow rate-pressure relation in microfluidic devices made of soft material like polydimethylsiloxane (PDMS), is significantly changed by the wall compliance. Moreover, the small volume changes due to liquid compressibility in the case of syringe pump-driven flows can be the same order of magnitude as the liquid volume in narrow sections of the device. This will result in unexpectedly long times to initiate liquid motion, which is known as the “bottleneck effect”. Therefore, the underlying effects of such common situations must be carefully considered. This compressible fluid-structure interaction problem was investigated here with different system parameters that govern the transient times in microchannels. Numerical simulation and experiments were conducted to characterize both compliance and bottlenecking effects in microfluidic devices of different elasticities filled with different liquids. A numerical simulations-based model was created to predict the compliance of thick PDMS microchannels as a direct function of the channel dimensions, and it gave a good fit for data outside of the study range with an error within 4%. This model predicted the dynamic response of microchannels accurately as long as the pressure drop was not high. A less accurate (10% error) pressure-dependent model was created to capture the increase in microchannel compliance due to the non-linear behavior of PDMS dominance under high pressures. Experiments were used to characterize the effect of syringe volume, microchannel resistance, and liquid type on the flow dynamic response caused by the bottleneck effect. When the bottleneck effect was present, the elasticity of the PDMS channels, controlled through the monomer to curing agent mixing ratio, did not have a noticeable influence on the system dynamic response. The models and characterization presented here allow for predicting the dynamic behavior of PDMS microchannels using simple hydraulic-circuit analysis and enable the proper design of transient microfluidic applications.