Acoustic manipulation in two-phase systems

Droplet-microfluidic platforms enable precise processing, manipulation, and analysis of biological samples with improved throughput, cost-efficiency and reduced response times compared to conventional bulk methods. At the microscale, shifts in the relative significance of physical parameters lead to distinct emergent properties, facilitating the creation of physiologically relevant microenvironments with spatiotemporal control over chemical (e.g., growth factors) and mechanical (e.g., hydrogel scaffolds) cues. These capabilities make droplet-microfluidic systems highly suitable for biology and biomedical research, and for applications such as disease diagnostics, cell therapeutics, and regenerative medicine. A fundamental requirement for such platforms is the ability to manipulate the spatial positioning and movement of biological entities for sample enrichment, separation, localization, and patterning. Acoustic forces offer a label-free and non-invasive approach to achieve this control, making them particularly attractive in microfluidic-based devices. This thesis focuses on studying the manipulation of microparticles and cells within droplets and microgels using bulk acoustic wave, aiming to develop on-chip processes that could ultimately be integrated into various cell-related research and applications. In droplet microfluidics, understanding the effects of different fluidic environments on the acoustic manipulation of intra-droplet particles is essential, particularly given the presence of two immiscible phases. Due to biocompatibility constraints, the selection of solutions for the droplet phase is limited. Therefore, the surrounding oil phase was modified in Paper I. Results of Paper I reveal that acoustic field uniformity and strength within the droplet are optimized when the speed of sound of the aqueous droplet and the surrounding oil are closely matched. It was concluded that hydrocarbon oils are more compatible with intra-droplet acoustic focusing, as their acoustic properties more closely approximate those of water. In contrast, fluorinated oils, conventionally favored in droplet microfluidics, were found to be less suitable due to greater disparities in their acoustic properties relative to water. In a subsequent study, the environmental risks associated with fluorinated oils, classified as per- and polyfluoroalkyl substances (PFAS), were assessed. The findings indicate that viable oil/surfactant alternatives to conventional fluorinated solutions exist for specific droplet microfluidic applications, such as the mineral oil/PGPR system for bacterial culture. In Paper III, it was demonstrated that intra-droplet acoustic focusing remains effective when the droplet phase consists of more viscous hydrogel formulations. Precise spatial control of bioparticles within continuously generated hydrogel droplets was successfully achieved, followed by on-chip crosslinking to immobilize particle positions without significantly compromising cell viability. Finally, the acoustic assembly of three-dimensional polystyrene bead clusters within hydrogel matrices was demonstrated, serving as a proof-of-concept for point focusing in microgel constructs via ultrasonic techniques.