Self-assembled filament layers in drying sessile droplets: from morphology to electrical conductivity

TL;DR

This study uses numerical simulations to explore how evaporation regimes and filament properties influence deposit morphology and electrical conductivity in drying droplets, offering insights for optimizing printed electronic devices.

Contribution

It reveals how evaporation regimes and filament characteristics affect deposition patterns and conductivity, providing new guidelines for controlling microstructure in printed electronics.

Findings

Diffusion-limited evaporation causes the coffee-ring effect, leading to non-uniform deposits.

Reaction-limited evaporation suppresses edge accumulation, resulting in more uniform, centered conductive networks.

Tuning evaporation regimes and filament properties can lower percolation thresholds and enhance conductivity.

Abstract

Controlling the deposition of filaments, such as nanowires and nanotubes, from evaporating droplets is critical for the performance of emerging technologies like flexible sensors and printed electronics. The final deposit morphology strongly governs functional properties, such as electrical conductivity, yet remains challenging to control. In this work, we numerically investigate how filament length, stiffness, and concentration affect deposition patterns during the drying process. We compare reaction-limited and diffusion-limited evaporation regimes, demonstrating that their distinct velocity fields and flow magnitudes fundamentally alter filament arrangement. While diffusion-limited evaporation drives the ``coffee-ring effect", compromising network uniformity, reaction-limited evaporation suppresses edge accumulation, promoting centered conductive deposits. We map out the spatial variation of filament alignment - tangential at the contact line, radial in the intermediate region, and random near the center. Longer filaments tend to favour more tangential alignment overall and suppress edge accumulation. We find that by tuning the evaporation regime, filament deposition can lead to significantly lower percolation thresholds and significantly higher conductivity exponents. These results quantify the link between evaporation kinetics and microstructure, providing guidelines for optimizing conductive network formation in printed electronics.