ELECTROSTATIC DROPLET GENERATION - MECHANISM OF POLYMER DROPLET FORMATION

The mechanism of alginate droplet formation and experimental parameters for producing very small polymer microbeads (less than 100 mum dia.) using an electrostatic droplet generator studied showed that the microbead size was a function of needle diameter, charge arrangement (electrode geometry and spacing) and strength of electric field. Perfectly spherical and uniform polymer beads, 170 mum dia., for example, were obtained at a potential difference of 6 kV with a 26-gauge needle and an electrode distance of 2.5 cm. Increasing the electric field, and thus the surface charge in the vicinity of the needle, by increasing the applied potential, resulted in needle oscillation, giving a bimodal bead size distribution with a large fraction (30-40%) of microbeads with a mean diameter of 50 mum. The process of alginate droplet formation under the influence of electrostatic forces assessed with an image analysis/video system revealed distinct stages. After a voltage was applied, the liquid meniscus at the needle tip was distorted from a spherical shape into an inverted cone-like shape. Consequently, alginate solution flowed into this cone at an increasing rate causing formation of a neck-like filament. When this filament broke away, producing small droplets, the meniscus relaxed back to a spherical shape until flow of the polymer caused the process to start again. A large-scale multineedle device with a processing capacity of 0.7 L/h was also designed and produced uniform 400 +/- 150 mum microbeads.