Mechanisms of coffee-ring suppression in Picoliter silver nanoparticle ink droplets: Implications for printed electronics

The coffee-ring effect (CRE) remains a major obstacle to achieving uniform functional deposits in high-resolution printed electronics, particularly for picoliter-scale droplets where evaporation dynamics are complex and poorly understood. This study demonstrates that precise control of substrate temperature is a highly effective strategy for suppressing the CRE in inkjet-printed picoliter droplets of a commercial silver nanoparticle ink. We identify a distinct morphological transition: uniform deposits form at low temperatures (20-40 °C), pronounced coffee-rings develop at intermediate temperatures (50-70 °C), and central accumulation emerges at high temperatures (>90 °C). Through a combination of experimental analysis, scaling arguments, and numerical simulations, we systematically rule out Marangoni flows and viscous dissipation as the primary suppression mechanisms at low temperatures. Instead, we show that the extended drying time at low substrate temperatures drastically reduces the Péclet number, shifting the dominant transport mechanism from outward capillary advection to inward particle diffusion. This diffusion-driven homogenization ensures uniform particle redistribution prior to immobilization. Our findings provide a robust, practical, and readily applicable thermal strategy for eliminating capillary-driven inhomogeneities, paving the way toward the reliable fabrication of high-resolution printed electronic devices with superior morphological and functional uniformity.