Evaporative Morphology Tuning of Conducting Polymer Films Under Controlled Vacuum Conditions

The evaporation of drops on solid surfaces is a ubiquitous natural phenomenon, and their dynamics play a pivotal role in many biological, environmental, and industrial processes. However, the complexity of the underlying mechanisms has largely confined previous studies to liquid drop evaporation under atmospheric conditions. In this study, the first comprehensive investigation of the evaporation dynamics of conducting polymer-containing drops under controlled vacuum environments is presented. Utilizing high-speed imaging of a drop within a vacuum chamber, it is observed that the evaporation of a sessile drop under vacuum conditions unfolds through four distinct stages: Constant Contact Radius (CCR), Constant Contact Angle (CCA), Increasing Contact Angle (ICA), and Stick and Slip (S&S) modes. The detailed analysis of the force balance reveals that the depinning dynamics of the contact line, significantly driven by vacuum-induced forces, are the primary factor distinguishing these evaporation modes. A modified diffusion-limited model specifically tailored for vacuum conditions is further developed, which closely aligns with the experimental data on volume reduction over time. Importantly, the study demonstrates that by carefully adjusting the vacuum level, it is possible to precisely manipulate the final film morphology, with uniform deposition achieved at an optimal pressure of 20 kPa. This research introduces a novel approach for controlling drop evaporation dynamics in vacuum, with potential applications in advanced manufacturing processes.