Instability and droplet formation in evaporating thin films of a binary solution

Area of Science:

• Physical Chemistry
• Materials Science
• Fluid Dynamics

Background:

• Bilayer thin films composed of immiscible solutions on a water surface are subject to complex phenomena during solvent evaporation.
• Phase separation within these layers influences their stability and morphology.
• Understanding these instabilities is crucial for controlling thin film properties.

Purpose of the Study:

• To investigate the instability phenomenon in a bilayer thin film system during solvent evaporation.
• To analyze the morphological changes and droplet formation in the top layer.
• To validate experimental observations with theoretical energetic calculations.

Main Methods:

• Experimental observation of a bilayer thin film (hexane/hexadecylamine and amyl acetate/cellulose solutions) during solvent evaporation.
• Monitoring changes in layer thickness, surface tension, and viscosity over time.
• Theoretical calculations based on energetic arguments to model droplet pattern formation.

Main Results:

• The top layer (hexane/hexadecylamine) decomposes into droplets as its thickness decreases rapidly due to high hexane evaporation rate.
• Droplet formation occurs when the top layer thickness reaches a few nanometers, while the bottom layer (amyl acetate/cellulose) remains significantly thicker (approx. 100 nm).
• Experimentally determined droplet pattern parameters (diameter, height, interdroplet distance, density) show good agreement with theoretical predictions.

Conclusions:

• The differing evaporation rates of solvents in bilayer thin films are the primary drivers of instability and droplet formation.
• Energetic arguments provide a reliable framework for predicting the geometrical characteristics of the resulting droplet patterns.
• This study offers insights into controlling thin film morphology through solvent selection and evaporation dynamics.