Control of monodisperse droplet size in microfluidics: A combined experimental and numerical investigation

Monodisperse droplets, characterized by their uniform size distribution and well-controlled physicochemical properties, have found extensive applications in drug delivery, bioengineering, and food science. However, experimentally tuning droplet size typically requires multiple rounds of photomask design and microfluidic chip fabrication, which is costly and inefficient. To address this, we developed a numerical model for droplet generation in microfluidics based on the Volume of Fluid method in Fluent. The model was experimentally validated, achieving a maximum relative error below 5.43%. We systematically investigated the droplet formation mechanism, with a particular focus on the influence of structural and operational parameters on the control of droplet size. Three flow-focusing chips with dispersed phase inlet widths of 20, 40, and 70 μm enabled controlled modulation of droplet diameters ranging from 13.6 to 108.55 μm. Pressure variations at the geometric center of the flow-focusing structure inlet region revealed four distinct stages of droplet formation: lagging, filling, necking, and detachment. The associated pressure curves exhibited periodic oscillations, serving as a reliable indicator of monodispersity and generation stability. Structural parameters significantly affected both droplet size and frequency. The chip with a 20 μm inlet generated droplets at frequencies ranging from 625 to 7695 Hz, markedly higher than those of the 70 μm inlet (66.67-715 Hz). This study demonstrates the capability of high-precision, low-cost control of droplet size via numerical simulation. The findings provide theoretical insights for the design of microfluidic chips and practical guidance for applications in drug formulation, cell encapsulation, and microcapsule synthesis.