Dispensing of rheologically complex fluids at the dripping regime

• Effect of particles on dispensing dynamics is investigated at the dripping regime.
• Particles hardly change the dispensing characteristics of Newtonian fluids.
• Particles strongly affect the dispensing characteristics of elastic liquids.
• Mechanism of drop size determination and role of each force component are found.
• Both elasticity and inertia are important in dispensing of elastic liquids.

Dispensing characteristics of rheologically complex fluids are investigated experimentally at the dripping regime. Two mixtures of ethylene glycol and glycerin (50:50 and 31:69) were used as base Newtonian fluids. As the polymer solution, 1000 ppm of polyacrylamide of 5–6M g/mol was added to the 50:50 base Newtonian fluid. 2.5, 5 or 7.5 vol% of polystyrene particles of 2 μm in diameter were added to the liquids to prepare particle suspensions. The liquids were dispensed to the air by using a nozzle with the inner and outer diameters of 0.61 and 0.91 mm, respectively. It was found that the drop size is primarily determined by the balance of drop weight and surface tension force holding the drop, and hence drop size does not depend on volume flow rate. The drop diameter is approximately 10% smaller for elastic liquids due to the severe extension of the filament before the detachment from the nozzle. The result also shows that the dripping characteristics of the fluids without the polymer are mainly determined by the balance of the gravity force, surface tension force holding the liquid volume and pressure force. The addition of less than approximately 10% of 2 μm spherical particles hardly changes the dispensing characteristics of Newtonian fluids except at the final stage of detachment. For polymeric liquids, the transient extensional property drastically changes the dripping dynamics. The force balance shows that the strain hardening of the polyacrylamide solution under the extensional flow causes severe retardation of the filament breakup. The filament breakup of the polymeric suspension is further decelerated by the increase in zero shear viscosity as a result of the particle addition.