Small droplet formation in a pneumatic drop-on-demand generator: Experiments and analysis

A pneumatic droplet generator is described in which a gas pulse of alternating negative and positive pressure is applied on liquid in a nozzle, driving a periodic motion of the free liquid surface. An analytic model of incompressible liquid motion in the nozzle demonstrates that the motion of the surface is out of phase with the exciting pressure oscillation. A jet of liquid issues far enough out of the nozzle that its tip becomes unstable, detaches and forms a droplet. Experiments showed the oscillation of the liquid surface prior to droplet ejection and the time lag between the pressure oscillation and droplet ejection. The model predicts that maximum liquid velocity is attained at an intermediate value of viscosity, and experiments confirmed that the largest liquid motion was achieved with this intermediate value, which eventually leads to droplet formation. By varying the duration of the pressure pulse droplets smaller than the nozzle diameter could be produced.