Pore-scale immiscible interfacial transport facilitates low-cost droplet generation

We devised an economical method for droplet generation utilizing a "Y"-shaped paper strip. This approach employs passive capillary action, distinguishing it from traditional microfluidic droplet generators that require external pumping. To investigate the phenomenon of oil droplet generation in a water-wicking medium, we performed multiple experiments by changing the grade of paper (grades 1 and 4) and the inclination of the paper strip. Capillary pressure in the grade 1 paper surpasses that in the grade 4 paper at elevated liquid saturation levels. The microscopic droplets are produced within the pores and transported to the outlet by two primary mechanisms. These mechanisms entail the merging and elongation of oil droplets driven by the capillary action of water. The gravitational force markedly reduces the droplet size with considerable uniformity. We identified three regimes of temporal droplet generation based on the pattern of saturation-dependent capillary pressure. The type of paper used strongly influences the droplet size, with the smaller pore size of the grade 1 paper enabling the formation of smaller droplets. Finally, we established the temporal stability of the droplets, which is relevant for cellular research. To sum up, the results of this work provide a low-cost method for generating micro-sized droplets suitable for the chemical and biological investigation of micro- or nanoscale analytes.