Stability of line shapes in inkjet printing at low substrate speeds

Abstract

Line formation control plays a critical role in inkjet printing stability for its high relevance to industrial processes. The present study describes experiments for depositing droplets of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solutions using high-speed imaging technology. The line printing dynamics and ink drops coalescence were analyzed. Two stability criteria are proposed for the stability of the printed line at low substrate speeds, including the continuity criterion derived from the volume conservation and the bulging instability determined by the ratio of the transported flow rate and the applied flow rate. Stable printed lines are bound by the two stability criteria and equipment limitation, represented in a parameter space bound by the normalized drop spacing, p*, and the normalized substrate speed, U T*. We then discuss the changes of the normalized critical substrate speed between the stable and bulging regions U T 1∗ and between the stable and discontinuous regions U T 2∗ under a fixed injection frequency. Finally, the reasons for the formation of different printed line shapes are demonstrated by observing the coalescence processes of an impacting droplet and a sessile printed line. The relationship between the location of the liquid bridge and the drop spacing is discussed for determining the line shapes.