Integrating Surface Acoustic Waves and Microfluidic Flows to Control Nanoparticles and Liquid Crystals

Microfluidic technologies have enabled precise manipulation of small volumes of fluids reducing sample consumption, enhancing reaction control, increasing throughput, and facilitating automation. Important challenges to be addressed include precise particle manipulation, separation of complex samples, and integration of functionalities. Achieving high-resolution manipulation, particularly for small-scale particles, demands advanced techniques and precise flow control challenging when dealing with multiple particle types or heterogeneous populations. Thus, efficient separation, sorting, and characterization of such samples is critical. In parallel, microfluidic solutions can integrate optical components with fluidic systems, enabling on-chip optical analysis, sensing and manipulation that leverage the interaction of light with fluids. Optofluidics technologies faces challenges in the alignment of optical components, integration of light sources and detectors, and the optimization of fluidic and optical interfaces. Here, liquid crystals offer benefits, enhancing their functionality and performance being highly tunable and reconfigurable structured materials.The integration of acoustic waves into microfluidic devices have been applied to address challenges in mixing, atomizing, droplet manipulation, and particle manipulation for Lab-on-a-chip applications with an emphasis on actuation of micro-objects, such as droplets, bacteria, red blood cells, cancer cells, exosomes, and extracellular vesicles. This technology offers a label-free, contactless filtration approach. However, the acoustofluidic filtration of nanoparticles has proven to be challenging and no reliable approach has put forth to separate nanoparticles from microparticles and smaller molecules in solution. This dissertation explores the implementation of acoustic waves in microfluidic devices to engineer solutions in two emerging field in microfluidics: I) acoustofluidic nanoparticle purification; and II) optofluidic applications with liquid crystals. From the interaction of waves with fluids emerge two distinct mechanisms that are commonly at odds: the acoustic radiation force and the acoustic streaming. In this work, a microfluidic platform that combines both mechanism driving the separation of 150-300 nm particles from micron-size particles and small molecules in solution is presented. This strategy shows differential separation of particles yielding 83% particle recovery and 75% reduction in concentration of solutes. The acoustic focusing and separation in microfluidic channels separates both polystyrene particles and extracellular vesicles from cell culture. Supporting numerical simulations identify asymmetries in the force fields acting on the particles arising from changes in wave amplitude as the mechanism driving particle displacement. Taken together, the findings presented here show a novel method of acoustofluidic purification towards continuous, high-throughput, and highly automatable point-of-care purification systems. In addition, this work characterizes the effects of microfluidic flows and acoustic fields on the molecular orientation and optical response of nematic liquid crystals. After introducing acoustic waves in confinement, previously unknown structures are identified which are rationalized in terms of a state diagram as a function of the strengths of the flow and the acoustic field. The new structures are interpreted by relying on calculations with a free energy functional expressed in terms of the tensorial order parameter, using continuum theory simulations in the Landau-de Gennes framework. Taken together, the findings presented here offer promise for the development of new systems based on combinations of sound, flow, and confinement.