Phase diagram of pinch-off behaviors during drop-on-demand inkjetting of alginate solutions

Viscoelastic polymer solutions have been extensively utilized in inkjet printing for a variety of biomedical applications. The pinch-off of viscoelastic jets is a key step toward the generation of droplets in inkjet printing. This complex process is governed by the interplay of four stresses, including inertial stress, capillary stress, viscous stress, and elastic stress. Depending on polymer solution properties and process conditions, four types of pinch-off phenomenon were observed during inkjetting of viscoelastic alginate solutions. In this study, material properties of alginate solutions with different concentrations have been characterized, and three dimensionless numbers (Ohnesorge number Oh, Deborah number De, and Weber number We) have been proposed to analyze different pinch-off behaviors. The phase diagram in terms of these three dimensionless numbers has been constructed to classify the regimes for different pinch-off types during inkjetting of viscoelastic alginate solutions. It is found that (1) at low De and Oh, the capillary stress is mainly balanced by the inertial stress, resulting in front pinching. (2) At medium De and low Oh, with the increase of We, the pinch-off type may change from front pinching to hybrid pinching to exit pinching. (3) At low Oh and high De, the capillary stress is mainly balanced by the elastic stress, resulting in exit pinching. (4) At high Oh and De, the viscoelastic effect is dominant. With the increase of We, middle pinching turns to be exit pinching due to the increase in the initial ligament diameter near the forming droplet.