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

Abstract

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 1850 microchannels fabricated on 10 parallel terraces. Each terrace was 9.54mm long and consisted of 54 channels with a depth of 5μm and a width of 18μm (chip 1) or 185 channels with a depth of 4μm and a width of 8μm (chip 2). The dispersed phase was a mixture of 2wt% polycaprolactone (PCL) and 1wt% poly(d,l-lactic) acid (PLA) dissolved in dichloromethane and the continuous phase was 2wt% poly(vinyl alcohol). After solvent evaporation, the droplets were transformed into highly uniform composite polymer particles with an average diameter of 8.8 or 4.9μm. 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 6h 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 7h. 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.