Droplet Formation and Deformation in a Two-Stage Flow-Focusing Microchannel

While droplet deformation analysis can shed light on droplet composition, the bulk of previous microchannel-based droplet deformation work has focused on unidirectional deformation, limiting the insights that can be gained. In contrast, sequential bidirectional droplet deformation in microchannels remains underexplored despite its potential to provide richer insights into droplet behavior and characterization. Understanding droplet deformation is particularly important for advancing label-free droplet analysis, which underpins applications in diagnostics, drug screening, and material synthesis. In this work, we report a novel application of a flow-focusing microchannel to investigate sequential bidirectional droplet deformation. Both experiments and computational modeling confirm the formation of localized high-strain regions driven by the interaction between the main and sheath flows. Using high-speed imaging, we capture the sequential bidirectional deformation as the droplets traverse these regions. Importantly, we establish a relationship between nondimensional parameters and droplet deformation, providing a framework for characterizing droplet rheology. This approach highlights the potential for developing high-throughput methods to estimate rheological properties on a single-droplet basis. Such capabilities have the potential to open new avenues for more efficient and precise droplet-based assays in biological and chemical research.