Inkjet printing of precisely defined features using contact-angle hysteresis

Motivated by the process of inkjet printing of electronics, we study experimentally and theoretically the processes limiting the printing of sharply defined, equilibrium corners. Using a non-volatile ionic liquid, we inkjet print squares with rounded corners on a substrate of roughened, display-grade glass. We show experimentally that with increasing roughness, corner radius decreases, allowing more precisely defined features to be printed. To interpret these results in terms of contact-angle hysteresis (difference between the advancing and retreating contact angles 0(A) and 0(R)), we implement the following model with the Surface Evolver program. With drop volume fixed, we minimize drop surface energy subject to a prescribed contact line. We identify 0(A) and 0(R) as the minimum and maximum contact angles around the drop perimeter. We find that with decreasing corner fidelity, contact-angle hysteresis also decreases. We are thus able to infer 0(R) from the corner radius of printed features. We conclude that increasing contact-angle hysteresis allows the printing of more precisely defined features. (C) 2013 Elsevier Inc. All rights reserved.