A Microfluidic Platform for Five-Order-of-Magnitude Concentration Control With Stable Droplet Size.

Generating monodisperse droplets with tunable concentrations is essential for a wide range of microfluidic applications. We present a platform that overcomes the trade-off between concentration range and droplet size stability using a passive, water-head-driven system with asymmetric fluidic resistances. This design effectively decouples concentration tuning from droplet generation within the tested regime by stabilizing the junction pressure to ensure a steady total aqueous flow rate. We demonstrate theoretically and experimentally that this results in monodisperse droplets of nearly constant size across the programmed concentration range. Concurrently, by adjusting a single passive pressure inlet, the system achieves continuous control of the concentration ratio over a five-order-of-magnitude dynamic range. This robust, simple method eliminates complex pumps, offering a powerful tool for high-throughput screening and combinatorial studies.