Manipulation of droplets and cells in microfluidic platforms

Manipulation of liquid droplets and cells is crucial for several key processes such as heat transfer, energy generation, and clinical diagnostics. In particular, encapsulation of single cells in droplets is becoming a powerful tool to map cellular heterogeneity in diseased and healthy tissues, even at the single-cell level. This PhD thesis is focused on the development of engineered smart surfaces and microfluidic Platforms for the manipulation of droplets and cells. The work has two main goals: on the one hand, manipulation of water droplets by light patterns projected on engineered surfaces, and on the other hand, encapsulation of cells within droplets in microfluidic devices designed for a large variety of essays, primarily single-cell sequencing. Manipulation of droplets on engineered surfaces. Lithium Niobate (LN) is a functional Material characterized by photoactivated properties such as the photovoltaic effect: when illuminated (typically by a laser) ,charge accumulation occurs at the two faces of the material. This leads to the formation of virtual reconfigurable electrodes, which can be exploited to manipulate neutral and charged fluids through dielectrophoretic and electrophoretic effects. As happens for any solid surface, the motion of droplets on LN is hard to control over large distances because of droplet pinning on surface defects. Taking inspiration from the natural world, a special coating, called liquid-infused surface (LIS), is realized on the surface of the material. A liquid surface is smooth down to the molecular scale and is a defect-free surface; therefore, it enables low-friction droplet motion, avoiding uncontrolled pinning on surface defects. An optofluidic platform based on the photovoltaic effect of Lithium Niobate is realized: droplets are actuated, merged, and even split on illuminated LN, tuning a few experimental parameters such as droplet volume, the illumination pattern, and light intensity. Remarkably, controlled merging and splitting, typically performed in closed devices, is achieved by optowetting on an engineered surface. Encapsulation of cells in microfluidic droplets. The second focus is on the manipulation of cells within microfluidic channels. The main limitations of the use of cells in microfluidics are their sedimentation inside the storage container and, for some applications, the necessity to work in a highly diluted regime. Consequently, a cell mixing device (CMD) is developed to limit cell sedimentation. It is characterized from a biological point of view, monitoring cell viability under different experimental conditions, and studying the injection of cells into a microfluidic device. It is discovered to play an important role in limiting cell sedimentation, and cell viability is not affected. To increase the number of droplets with one cell encapsulated, inertial microfluidic and Dean flows are exploited: it is possible to create stable trains, where cells are aligned and separated one from each other. Then, the encapsulation of single cells aligned via inertial microfluidics in droplets is investigated. This last step becomes crucial to understand whether it is possible to increase the concentration of cells with respect to the case of single-cell experiments, obtaining a larger number of droplets with one single cell inside. As a consequence, the integration of the inertial microfluidics part with the single-cell encapsulation part on the same microfluidic platform is finally discussed.