Generation Characteristics of Highly Uniform Nonspherical Droplets of Soybean Oil Using Microchannel Array Devices

This paper investigated the generation characteristics of nonspherical oil-in-water (O/W) droplets consisting of food-grade components using microchannel (MC) array devices that have many rectangular MCs with shallow wells. The well height was designed to be twice the MC height. Two hydrophilic MC array devices made of surface-oxidized single-crystal silicon with equivalent MC diameters of 3.2 and 8.4 μm were used. Refined soybean oil was used as the to-be-dispersed phase, and a Milli-Q water solution of 1.0 wt% polyoxyethylene (20) sorbitan monolaurate (Tween20) was used as the continuous phase. Highly uniform discoid droplets with diameters of 9.0 and 21.5 μm, heights of 4.6 and 9.8 μm, and coefficients of variation of less than 4% were generated by simply forcing a to-be-dispersed phase via rectangular MCs into a well filled with a continuous phase. The to-be-dispersed phase pressures necessary for droplet generation were less than 8 kPa. The detailed generation process of the discoid droplets was analyzed using movie clips taken by a high-speed camera. Key phenomena during the detachment process were considered to be rapid flow of the to-be-dispersed phase into the well and instantaneous pinch-off of the neck. The effect of the to-be-dispersed phase velocity inside a rectangular MC (U _d,MC) on the resultant droplet diameter and the droplet-generation rate was also analyzed. Size-controlled discoid droplets were stably generated via the rectangular MC below the critical U _d,MC, and the droplet-generation rate became maximum at the critical U _d,MC.