Abstract
Predicting and controlling a droplet’s behavior on surfaces is very complex due to several factors affecting its nature. These factors play a crucial role in colloidal material deposition and related solution-based manufacturing methods such as printing. A better understanding of the processes governing the droplet in the picoliter regime is needed to help develop novel thin-film manufacturing methods and improve the current ones. This study introduces the substrate temperature as a method to control the droplet’s behavior during inkjet printing, especially the coffee-ring phenomena, at an unprecedented temperature range (25–250 °C). To explain the particular behavior of the droplet, this research associates the creation of specific coffee-ring micro/nanostructures at elevated temperatures with the Leidenfrost effect that is responsible for creating a vapor pocket under the drying drop. Herein, we combine experimental data and numerical methods to explain the drying dynamic of the picoliter-size droplet on the substrate at elevated temperatures. The achieved results indicate that the coffee-ring effect is correlated with the heat-transfer changes caused by the Leidenfrost effect and can be controlled and used to produce micro/nanostructured thin films without additional processing steps.
Highlights
When droplets, containing solid materials or dissolved nonevaporating compounds, dry on a surface, they leave patterns and structures which could be utilized in various applications.[1]For decades, this phenomenon has been studied to better understand the patterning schemes for coating applications.[2−4]Recent experimental and theoretical advances have revealed the potential of drop-evaporated patterns in biomedicine and nanotechnology
We focus on the empirical and numerical explanation of drop behavior and various patterns created on the heated substrates by combining the coffee-ring and the Leidenfrost effects to explain the droplet drying process
The 3D images in Figure 2a−d show the influence of the substrate temperature on the creation of the PEDOT:PSS structures affected by the coffee-ring effect
Summary
When droplets, containing solid materials or dissolved nonevaporating compounds, dry on a surface, they leave patterns and structures which could be utilized in various applications.[1]. For the inks used in this research, the calculated values of the Weber and Reynolds numbers are relatively small, meaning that the spreading of the drop upon impact is negligible because the deformation of the drop is governed by the viscous stress, surface tension, and inertial forces, imposing an almost ideal spherical shape.[51] It leads us to the assumption that the width of the structure is related to the formation of the vapor pocket and heat-transfer changes occurring during the drying process. Shen et al provide information about the influence of humidity on coffee-ring formation, further research is needed to understand the impact of environmental factors better.[27] the revealed correlation between the coffee-ring and the Leidenfrost effects in the wide spectrum of substrate temperatures offers the possibility to control the drop behavior for the creation of desired micro/nanostructures
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