Dimensionless Constant
Fluid dynamics ratios relate the viscous forces to the square root of the product of surface tension and inertial forces. The Ohnesorge number characterizes the droplet formation behavior in inkjet printing and liquid spraying. It helps predict whether a specific ink will form a single stable drop or break into unwanted fragments.
Lower values suggest that surface tension dominates, while higher values indicate that viscosity is the primary dampening force.
Inkjet Application
Droplet stability in a piezo printhead depends on the physical properties of the fluid staying within a specific window of the stability ratio, which is the inverse of the Ohnesorge number. If the value is too low, the fluid filament is too long and likely to snap into satellites. If it is too high, the energy required to eject the drop becomes excessive.
Most industrial textile inks are formulated to land in a stable range between 0.1 and 1.0.
Physical Variable
Calculating the value requires measurements of the liquid density, the nozzle diameter, the surface tension and the dynamic viscosity. Because the Ohnesorge number is independent of the velocity of the jet, it functions as a pure description of the fluid and the geometry of the orifice. Changes in temperature at the printhead will shift the viscosity and surface tension, thereby altering the number.
Formulaic consistency is required to maintain the same jetting quality across different batches of ink.
Outcome Prediction
The result provides a reliable way to compare different ink chemistries on the same hardware. A stable drop formation leads to fewer print defects.
