An apparatus for depositing droplets on a substrate is disclosed. The apparatus includes a support for the substrate, a droplet ejection assembly which includes a pumping chamber, a controller and a source of static pressure to maintain the total pressure in the pumping chamber above a threshold pressure level to avoid rectified diffusion type bubble growth in the pumping chamber. The droplet ejection assembly is positioned over the support for depositing the droplets on the substrate and includes, in addition to a pumping chamber, a displacement member and an orifice that ejects the droplets. The controller provides signals to the displacement member to eject drops.