A pneumatic droplet generator is described, which consists of a T-junction with a nozzle fit into one opening, the second opening is connected to a gas cylinder through a solenoid valve and the third connected to a length of steel tubing. The droplet generator is filled with liquid. Opening the valve for a preset time creates a pulse of alternating negative and positive pressure in the gas above the surface of the liquid. A jet of liquid issues far enough out of the nozzle that its tip becomes unstable, detaches and forms a droplet. Experiments were conducted using water/glycerin mixtures and molten metals including tin, zinc and zinc alloy. Droplet formation was photographed and the pressure variation inside the droplet generator recorded. The effect of various experimental parameters such as nozzle size, pressure pulse width, secondary gas flow pressure, liquid viscosity and temperature on droplet formation were investigated.;An analytic model of incompressible liquid motion in the nozzle is used to explain the behavior of water/glycerin solutions inside the nozzle and droplet formation. The model demonstrates that the motion of the surface is out of phase with the exciting pressure oscillation. 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. Similarly, with molten metals, a simple analytical method was used to estimate the diameter of droplets. The size of tin droplets measured from experiments was in good agreement with that obtained from the model.
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