Bubble Entrapment and Stability in Complex Passageways

Abstract

The presence of bubbles in the complex passageways of inkjet print heads can cause a variety of ill-effects from changes in local wave dampening to droplet misfire and ejection asymmetry leading to weak, intermittent, chronically missing jets or defects leading to crosstalk. In this study we present macroscale observations of capillarity-driven flow phenomena in simulated rectangular microchannels. The high resolution video results are gathered following brief experiments conducted in a drop tower which enables the use of large length scale transparent channels with representative flows that are readily captured at 60 fps. Conduit geometries simulating unit tortuous passageways in print head stacks are studied identifying geometries that inadvertently generate bubbles during in-fill or re-fill. The flow in each channel is interrupted by a ‘feature,’ the geometry of which is systematically varied to identify critical shapes and flow rates. The mechanisms for such bubble generation and entrapment depend strongly on the feature geometry, where if local capillary flow rates exceed global flow rates the bubble entrapment does not occur. Scaling laws and numerical computations are employed to predict the critical flow rate conditions under which such entrapment may be avoided.