Graduation Year

2022

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Chemical Engineering

Major Professor

Venkat R. Bhethanabotla, Ph.D.

Committee Member

John N. Kuhn, Ph.D.

Committee Member

Lawrence A. Stern, Ph.D.

Committee Member

Robert Frisina, Ph.D.

Committee Member

Humberto Rodriguez Gutierrez, Ph.D.

Keywords

Acoustic radiation force, Acoustic streaming, Acoustothermal heating, Multiple timescale, Second-order fluids

Abstract

Acoustofluidics is a well-proven tweezer serving various applications such as particle manipulation, cell trapping and separations, fluid mixing, and bio-sensing. In this thesis, we explored several aspects of acoustofluidics such as acoustic streaming, acoustothermal heating and acoustophoretic manipulations of micron sized objects, in the context of biosensing applications.

In Chapter 2, inner acoustic streaming for second-order fluids has been studied analytically by employing asymptotic expansions for thin Stokes layer and low acoustic Mach number. In addition, a multiple-time-scale approach has been adopted to separate the primary oscillatory flow and the steady acoustic streaming. The study considers two sample cases: (i) motionless boundary and (ii) vibrating boundary, and compares the characteristics associated with their streaming. It is observed that both the primary oscillatory flow and acoustic streaming flow fields are suppressed in second-order fluids, due to the extra stress components present in the fluids. The study considers both compressible and incompressible Stokes layer, to bring out the acoustic streaming characteristics associated with fluid compressibility. For the compressible Stokes layer, stronger acoustic streaming flow results for the motionless boundary, leveraging the deeper interaction between the primary oscillatory pressure field and the steady streaming. In the case of a vibrating boundary, the primary oscillatory pressure field is independent of the Stokes layer compressibility and hence the acoustic streaming flow remains unaltered. The extra stresses in second-order fluids reduce the acoustic body force density and the maximum reduction has been observed for the vibrating boundary. In order to understand Lagrangian streaming, Stokes drift has also been calculated and compared for all the scenarios. The theoretical analysis and fundamental insights derived from this study have potential for applications in diverse fields such as particle manipulation, biosensing, cell sorting, and removal of loosely bound material such as non-specifically bound proteins.

Chapter 3 investigates the acoustic streaming phenomena in a standing surface acoustic wave (SAW) driven microfluidic channel filled with second-order fluids. We have developed a multiple-timescale-based theoretical model where a perturbation technique was adopted to separate the fast and slow timescales associated with the oscillatory flow field (i.e., acoustic field) and the mean flow field (acoustic streaming), respectively. The governing equations have been expressed in non-dimensional form to effectively show the dependence of the acoustic streaming fields on Reynolds number (Re), Deborah number (De), and the ratio of material constants related to normal stress coefficients (b). Contrary to our intuition, we observed that with increasing Deborah number (which is a measure of the extra stress present in the second-order fluids), acoustic streaming first increases, and thereafter, a further increase in Deborah number leads to a gradual suppression of streaming. Our study also reveals that the acoustic field and the acoustic radiation force show negligible dependence on the fluid rheology. For the following ranges: , , and , the maximum variation of the acoustic streaming is observed to be ~ 161.3% whereas the variation in the acoustic field stays within just 0.15%. This significant finding can help design efficient acoustofluidic systems that can manipulate acoustic streaming without affecting the acoustic radiation forces, as strong acoustic streaming can impair the acoustofluidic devices.

In Chapter 4, we present a theoretical model based on multiple scale perturbation approach to solve the fluid flow and heat transfer equations for SAW-driven acoustothermal heating of a Newtonian fluid in a microchannel. The first order fields are oscillatory with the same frequency as that of the SAW, whereas the second order components are time-averaged to account for the mean flow and temperature fields. We find that the temperature rise depends solely on the acoustic energy density and its conversion into internal energy via pressure work on the fluid, and hydrodynamic transportation of heat. For a fixed aspect ratio, an increase in system size essentially increases the conversion of acoustic energy into internal energy, leading to an increase in temperature rise. On the other hand, an increase in SAW frequency for a given system size causes the acoustic energy density to increase, and thereby increases the temperature rise. Temperature rise is found to increase linearly with SAW power, in agreement with experimental results reported in literature. The quantitative model for the temperature field presented in this work will find applications in designing biosensors, microreactors, and in other SAW driven controllable digital microfluidic heating applications.

In Chapter 5, we have studied numerically the acoustothermal heating phenomena in a standing surface acoustic wave driven open polydimethylsiloxane (PDMS) microchannel, where influences of both shear and bulk viscosities on the temperature rise have been analyzed. We have used a perturbation-based multiple timescale approach to distinguish the inherent slow and fast dynamics associated with acoustofluidic systems. Here, for the first time, we have developed a pressure acoustic model for the thermoviscous fluid that can be applied for a large fluid domain at a much-reduced computational cost and successfully validated that with the solutions of the full model comprised of Navier-Stokes and energy equations. Our study shows that both the shear and bulk viscosities play important roles in converting acoustic energy into internal energy via viscous damping, resulting in the temperature rise. We found that at zero shear and bulk viscosities, the acoustic wave remains undamped while traveling inside the fluid and hence, no acoustothermal heating is observed. We also show that for open microchannel, there are significant reflections of the acoustic waves occurring at the fluid top surface due to impedance mismatch, which helps in confining the acoustic energy inside fluid and causes enhanced acoustothermal heating.

In Chapter 6, we studied standing SAW driven acoustothermal heating in sessile droplets both via theoretically and experimentally. Our experimental results show droplet temperature rise similar to that of a first order step response and the associated time constant was observed to decrease at higher power. Our theoretical study employed Fourier transform to effectively calculate the acoustic fields, which otherwise was extremely difficult to calculate via 3D simulations. Our results show presence of two eyes where significant fluctuations of acoustic fields were observed. The acoustothermal temperature rise has been observed to vary linearly with input SAW power and our theoretically estimated temperature rise shows very good agreement with the experimental results.

In Chapter 7, we study autonomous motion of hybrid PEDOT/Pt and Au/Pt micromotors in presence of surface acoustic waves. The catalytic decomposition of H2O2 fuel in presence of Pt generates oxygen bubbles which provide propulsion thrust for the micromotor motions. We have identified that the Bjerknes force arising due to the acoustic-bubble interaction causes an addition thrust, leading to a higher speed of the micromotors. Accordingly, a theoretical model has been developed to estimate the micromotor speed which can explain the motion in presence and absence of SAW. Moreover, the dependence of SDS and Triton X-100 surfactant concentrations on the micromotor movement has also been shown. Our theoretical model agrees well with the associated experimental results.

In Chapter 8, we have analyzed swimming motions of Au/Ni bimetallic microwires under acoustic and magnetic fields. While the presence of nickel in the microwire makes it responsive to the externally applied magnetic field, both gold and nickel respond to acoustic actuation. Our study shows that magnetic power can provide excellent directional control while both acoustic and magnetic powers generate large propulsive thrust providing excellent swimming speed. We have also presented theoretical modeling and explained the swimming mechanism for such systems. The results of this study will play a pivotal role in designing efficient micro-/nanorobotic systems for healthcare solutions.

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