Confined Liquid Transport Engineering for Compact and High-Performance Microfluidic Transparency-Switching Glass

Microfluidic transparency-switching glass (MTSG) systems based on liquid-filled cavities offer a promising approach for dynamic control of optical transmission, but their practical implementation is limited by high liquid consumption, slow drainage, and bulky device architectures. Here, we investigate fluid transport in confined microfluidic cavities to establish design principles for efficient transparency switching to a desired level, effectively serving as a built-in curtain. By systematically varying cavity thickness (0-300 µm) and liquid viscosity (1-20 mPa·s), we show that reducing the cavity gap to 100 μm and employing a low-viscosity refractive index matching liquid (RIML), such as methyl benzoate, enables stable liquid curtain formation while reducing liquid consumption. Low-viscosity RIMLs also promote more effective drainage, resulting in enhanced opacity (T = 16.26 ± 0.20%) in the drained state. Dynamic display patterns were achieved, enabling switchable decorative and functional features within the smart-glass assembly. Furthermore, beam-engineering-based theoretical calculations, validated experimentally, were used to determine the maximum viable dimensions of large-area glass panels. These findings provide practical guidelines for the development of compact, energy-efficient, and scalable MTSG systems.