After overcoming the viscous resistance, the drop must also overcome the liquid-air surface tension barrier at the nozzle for ejection. For this, the drop must have sufficient surface energy. This leads to a minimum velocity required for drop ejection, which has been formulated in terms of 𝑅𝑒 and 𝑂ℎ numbers to give the following constraint for drop ejection: 𝑅𝑒𝑂ℎ ≥ 2