Micro Elastofluidics for Tuneable Droplet Merging

ABSTRACT Droplet microfluidics enables precise handling of discrete fluid volumes at the microscale, with broad applications in biomedicine, food and beverage, and material synthesis. Droplet merging is an essential droplet manipulation capability that allows controlled reagent delivery, sample mixing, biochemical reactions, and single‐cell analysis. Among the various active and passive droplet merging techniques, micropillar‐assisted merging provides better control over merged droplet size and number, yet still requires laborious flow tuning and redesign for varying merging scenarios. To overcome these limitations, we developed a flexible, stretchable microfluidic platform for tuneable droplet merging. The merging element consists of three parallel channels, separated by two micropillar arrays. Three parallel channels can be laterally stretched to tune channel dimensions and hydraulic resistances, resulting in a change in the associated pressure distribution that determines droplet merging. We first characterised droplet generation and merging in rigid devices. Next, we conducted a theoretical and numerical analysis on the effects of stretching on hydraulic resistance and pressure distribution in the merging element. Subsequently, we fabricated the stretchable microfluidic devices and tested droplet generation and merging under various device elongation strains. We found that device stretching allows for selective droplet merging of varying numbers and tuneable mixing efficiency. This approach demonstrates a new route for controlling droplet deceleration, spacing, and merging without modifying flow conditions or device redesign. This stretchable microfluidic platform introduces a reconfigurable, efficient approach for droplet merging, with potential applications in droplet‐based chemical reactions, biological assays and single‐cell analysis.