Thermo-mechanical analysis of fluids using microfluidic cantilever in dynamic resonance mode
Creators: Abraham, Rosmi
Publisher: name: University of Alberta Library
Year: 2023

<p>This thesis investigates the thermo-mechanical behavior of picograms of fluids using a suspended microchannel resonator called microfluidic cantilever operating in dynamic mode by exploring the interfacial effects of liquids on the resonance frequency. Microfluidic cantilever is a promising candidate in micro electromechanical systems (MEMS) which can provide high degree of accuracy in mass sensing, stress sensing, thermo-mechanical sensing of rare fluid samples. While characterizing fluid samples, the existing analytical model based on Euler-Bernoulli beam equation to calculate the frequency of the microfluidic cantilever does not consider the effects of surface tension and interfacial tension provided by the liquid inside the microfluidic cantilever. Hence, there exists a discrepancy between the experimental values and conventional model values. For a microfluidic cantilever with curvature, the surface stress and interfacial stress provided by the liquid inside the channel can effectively change the stiffness of the cantilever and can alter the frequency of the cantilever. An analytical model based on modified Euler Bernoulli beam equation that incorporates Young-Laplace equation to consider the effects of surface stress and the interfacial tension of fluids was proposed in this thesis. This modified model was validated by comparing the experimental values of frequencies obtained for the microfluidic cantilever filled with different alkanes with the analytical values obtained from the conventional model and the proposed modified model.</p>

<p>The proposed analytical model was used to measure the thermal properties of liquids quantitatively using a microfluidic cantilever irradiated with a diode laser. The thermal stress generated by the volumetric expansion of the liquids upon photothermal heating influenced the stiffness of the cantilever and altered the resonance frequency of the microfluidic cantilever. This effect of thermal stress on the resonance frequency for different liquids was studied using the modified Euler Bernoulli beam equation. In addition to that, thermal time constant of the system was observed and the relation with the density and the heat capacity of the liquids was established.  To overcome the shortcomings in using a bi-material cantilever, for the thermal characterization of liquids, such as fabrication of complicated structures and the drift in the static mode, we used a silicon nitride microfluidic cantilever filled with liquids operating in dynamic mode and analyzed the shift in the resonance frequency upon laser heating to measure the thermal properties of liquids.</p>

<p>As an application of the measurement of thermal properties of liquids using microfluidic cantilever, an investigation was carried out to understand the mechanical property evolution of a thermo-sensitive polymer Poly N-isopropylacrylamide (PNIPAM) as a function of temperature.<br>
A microfluidic cantilever was used with an external heater attached to it and was operated in dynamic mode to overcome the challenges of drift and mechanical losses. Here, the thermo-mechanical property changes of PNIPAM at lower critical solution temperature (LCST) were studied by analyzing the dynamic mechanical outputs of the cantilever such as shift in frequency and quality factor.  As the temperature was varied, there was conformational change for the PNIPAM aqueous solution filled inside the cantilever at LCST along with the mechanical property changes. The modulus changes of the PNIPAM at LCST altered the stiffness of the cantilever and resulted in the shift of the frequency.</p>

<p>Furthermore, a solution with optimum concentration of Kaolinite and PNIPAM was taken through the same thermal cycle inside the microfluidic cantilever and the coil to globule transition of PNIPAM adsorbed on Kaolinite at LCST resulted in the sedimentation of Kaolinite on the walls of the cantilever. This in turn, increased the stiffness of the cantilever changing the resonance frequency of the cantilever. The flocculation performance of the PNIPAM and Kaolinite in the microfluidic channel was compared with the macro scale flocculation experiments. </p>

<p>This study which uses the mechanical response of the cantilever to characterize the thermo- mechanical properties of fluids by incorporating the interfacial effects can be used to improve the precision of sensing of the portable, high thorough put devices for the in situ real-time detections of various biomolecular and chemical interactions in rare samples.</p>