A Method for Calculating the Critical Velocity of Microdroplets Produced by Circular Nozzles

Piezoelectric print-heads (PPHs) are used with a variety of fluid materials with specific functions; thus, matching the drive waveform of the PPH and the physical characteristics of the fluid can greatly improve deposition accuracy and quality. In this study, a distribution equation for the critical point for producing droplets defined in a dimensionless plane composed of Weber and Reynolds numbers was proposed. Computational fluid dynamics (CFD) was used to calculate the critical point distribution of the droplets generated by various materials in the dimensionless plane. Simulation results showed that the proposed distribution equation could describe the varying laws of the critical points of different materials. Process parameters in the distribution equation were identified using a fitting method based on CFD simulation results. The formula for calculating the critical characteristic velocity and dimensionless coordinates of the generated droplets was proposed based on the identified process parameters. The critical characteristic velocity and dimensionless coordinates of water, ethanol, aniline, and glycol were calculated using the proposed method. The average relative error between the calculated results and those from CFD is less than 5%. The drive waveforms of PPH are designed using the critical characteristic velocity of four different materials. Their droplet generation processes were recorded through a droplet watch system. The experimental results indicate that the flight velocity of the droplets generated at the critical characteristic velocity is close to zero, which indicates that the method proposed in this study is accurate.