Investigations of interfacial dynamics of droplet formation via a particle-free flow visualization method

In this paper, we investigate the interfacial dynamics of droplet formation in a flow-focusing microchannel using a particle-free liquid crystal (LC) polarization method. Driven by flow-induced shear, the anisotropic LC molecules reorient to generate distinct optical fringe patterns that map the localized flow field. Quantitatively, our results reveal that the fringe density within the liquid filament increases exponentially as the neck thins, providing a direct visual signature of the rapidly intensifying shear stress prior to pinch-off. Furthermore, we demonstrate that the LC droplet diameter exhibits an exponential decrease relative to the capillary number (Ca), quantitatively scaling as D = αCa^-0.315. The implementation of an AC electric field in the microfluidic environment enabled precise tuning of the droplet size between 20 and 80 µm. Specifically, the electric field causes a rapid initial decrease in droplet diameter at 500 Hz, followed by gradual shrinkage as the frequency increases to 5000 Hz. The LC-based visualization framework established in this study can be extended to probe the rheological behavior of complex fluids in constrained geometries, while the electro-hydrodynamic control mechanism lays the foundation for next-generation optofluidic actuators and responsive soft functional materials.