Ink Transfer in Ultrathin Flexographic Printing Using Nanoporous Stamps

Printing of ultrathin layers of liquid and colloidal inks is critical to manufacturing of low-cost electronics on non-conventional substrates such as polymer films and flexible glass. A recent invention from our research group, engineered nanoporous stamps made from polymer-coated carbon nanotube (CNT) forests, are highly porous (>90%) and can retain colloidal nanoparticle inks within their volume. Using these stamps, we have achieved printing of micron-scale features with highly uniform sub-100 nm thickness. In this work, we use high-speed imaging of the contact line motion during printing on transparent substrates to observe the dynamics of liquid spreading during contact, and the evolution of a capillary liquid bridge in the stamp-substrate gap. At high approach speeds, spreading speed increases with approach speed whereas at low approach speeds, flow from the porous medium defined by the fluid and stamp properties determine the spreading speed. After bridge rupture at a critical stamp-substrate gap, liquid respreads to fill the area defined by a precursor film matching the stamp geometry with high precision, and the respreading dynamics follow Tanner’s law. The transferred liquid volume decreases with retraction speed enabling speed based process control of layer thickness.