ABSTRACT
Most fluids eject much more reliably if negative pressure with respect to the atmosphere is maintained inside the fluid reservoir. This negative pressure suppresses the tendency of the fluid to leak out of the ejection aperture hole and wet the surface of the orifice plate, producing an unpredictably variable fluid layer through which the drop ejector will have to eject its fluid jet. At best this can result in a variable ejection threshold as a function of time and ejection rate. Another negative effect is that the drop ejection direction can be deflected by tens of degrees from its desired perpendicular orientation, if this layer is asymmetric around the ejection aperture by amounts as little as microns. At worst a very thick fluid buildup can totally suppress ejection. An additional failure mode, if the fluid is being used as a volatile carrier for particulates, is that the fluid built up on the outside can evaporate and solidify, thus jamming the aperture.
