DEVELOPMENT OF A REPEATABLE SINGLE-DROPLET GENERATOR USING A STEP MOTOR AND PWM CONTROL OF A TWO-WAY SOLENOID VALVE

A device capable of repeatedly generating a single droplet was developed. A piston pushes the liquid into a cylinder, creating droplets that are emitted from the nozzle outlet. A one-dimensional linear stage connected to a stepper motor was used for piston displacement. To repeat the generation of single droplets, a method that consists of generating droplets, supplying liquid into the cylinder, and blocking the inflow of external air through the nozzle outlet during the process of supplying liquid into the cylinder was developed. To prevent the inflow of external air into the cylinder through the nozzle outlet, a two-way solenoid valve was installed in the supply passage between the cylinder and the reservoir holding the liquid supply. The sudden opening of the two-way solenoid valve is prevented by PWM (pulse width modulation) control of the current supplied to the solenoid, and the air inflow into the cylinder through the nozzle outlet can be successfully blocked by this method. PWM control of the driving current in the solenoid enabled the repeated generation of single droplets. A single-droplet-generation experiment was repeatedly conducted while varying the nozzle diameter and the hydraulic head difference h between the nozzle outlet and the liquid reservoir. The ratio of the droplet diameter to the nozzle diameter was 5.45 with a nozzle diameter of 0.23 mm and convergence to 2.1 with an increase in the nozzle diameter to 1.51 mm. When repeated measurements were taken under fixed experimental conditions, the droplet diameter deviation % (=stdev/average*100) showed a distribution of 1 ~ 6%.