Drop volume modulation via modulated contact angle in inkject systems

To date, there are limited options in the ability to adjust feature sizes when inkjet printing. The reduction of nozzle size has been the primary method by which smaller features are created. Prior attempts to reduce feature sizes have all come with some restriction or another, as they are only accessible on certain systems, are difficult to manufacture, tune, cause clogging, etc. Few attempts to create droplets of smaller radii than that of the nozzle from which they are produced have been successful. Additionally, many fluids cannot be jetted whatsoever. Existing methods pertain largely to piezoelectric inkjet printing and time scale manipulation of negative pressure pulse versus fluid properties, rendering them inapplicable to thermal inkjet technologies as these are unable to apply a negative pressure pulse. In this work, a simple method for drop volume control in inkjet systems is proposed in which the stable drop volume can be reduced by an order of magnitude with a constant nozzle radius by adjusting the back pressure (pressure applied in the opposite direction of the ejection in a nozzle) in the reservoir that supplies the print head. However, this technique carries with it a surprising benefit, the increase in printability in regions previously unavailable. To evaluate this effect across many printing platforms, simulations of the effects of back pressure were performed. Computational fluid dynamics in its many forms (phase field, level set, etc.) has been historically found to be extremely accurate for predicting inkjet phenomena [1-16]. Using only the two most prominent forms of inkjet, thermal and piezo, to represent all inkjet phenomena is justified as most inkjets simply are the application of a brief pressure pulse at the back of a nozzle. These two inkjet varieties are no different, the only difference is its point of origin. In these, the back pressure is regulated and the fluid meniscus is allowed to come to rest and then a positive pressure impulse is applied creating a droplet. From this, droplet sizes, stability, and velocities are recorded. These results are then corroborated by rigorous analytical modeling. The results of this modelling can be used to generate mappings of printability in regards to contact angle; this allows for the design of both dynamic printing waveforms and static application techniques. The general contribution of this work, is not in the novelty of its results or the methods by which they are understood, as they have all been developed in some fashion previously, but rather the new combinations and resulting conclusions. Prior works in this area have all explored some facet of the implicit manipulation of meniscus shapes prior to fluid ejection, but by viewing this phenomenon from a more explicit viewpoint the result is more broadly applicable and understandable.