Long-term stability of droplet production by microchannel (step) emulsification in microfluidic silicon chips with large number of terraced microchannels

A long-term production stability of oil-in-water emulsions by microchannel (step) emulsification was investigated using two cross flow silicon chips consisting of 540 or 1,850 microchannels fabricated on 10 parallel terraces. Each terrace was 9.54 mm long and consisted of 54 channels with a depth of 5 µm and a width of 18 µm (chip 1) or 185 channels with a depth of 4 µm and a width of 8 µm (chip 2). The dispersed phase was a mixture of 2 wt% polycaprolactone (PCL) and 1 wt% poly(d,l-lactic) acid (PLA) dissolved in dichloromethane and the continuous phase was 2 wt% poly(vinyl alcohol). After solvent evaporation, the droplets were transformed into highly uniform composite polymer particles with an average diameter of 8.8 or 4.9 µm. The percentages of active channels, droplet sizes and droplet generation frequencies at individual terraces were investigated in both chips to reveal any flow maldistribution. After 6 h of production in chip 1, overall 95% of the channels produced droplets and the percentage of active channels on each terrace was at least 91%. The percentage of active channels decreased by decreasing the size of the channels. The mean droplet size varied negligibly across individual terraces over 7 h. Slightly higher droplet generation frequencies and smaller percentages of active channels were observed at central terraces. The droplet generation frequencies at the channels located close to each other were similar, indicating that droplet formation dynamics was coupled. The droplet size was not affected by the dispersed phase flow rate.