ELECTROSTATIC DROPLET GENERATION - MECHANISM OF POLYMER DROPLET FORMATION

The mechanism of alginate droplet formation and experimental parameter s for producing very small polymer microbeads (less than 100 mum dia.) using an electrostatic droplet generator studied showed that the micr obead size was a function of needle diameter, charge arrangement (elec trode geometry and spacing) and strength of electric field. Perfectly spherical and uniform polymer beads, 170 mum dia., for example, were o btained at a potential difference of 6 kV with a 26-gauge needle and a n electrode distance of 2.5 cm. Increasing the electric field, and thu s the surface charge in the vicinity of the needle, by increasing the applied potential, resulted in needle oscillation, giving a bimodal be ad 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 ana lysis/video system revealed distinct stages. After a voltage was appli ed, the liquid meniscus at the needle tip was distorted from a spheric al shape into an inverted cone-like shape. Consequently, alginate solu tion flowed into this cone at an increasing rate causing formation of a neck-like filament. When this filament broke away, producing small d roplets, the meniscus relaxed back to a spherical shape until flow of the polymer caused the process to start again. A large-scale multineed le device with a processing capacity of 0.7 L/h was also designed and produced uniform 400 +/- 150 mum microbeads.