Dynamics of drying colloidal suspensions, measured by optical coherence tomography

Area of Science:

• Materials Science
• Physical Chemistry
• Fluid Dynamics

Background:

• Colloidal suspensions are widely used in coatings, forming solid films upon drying.
• Observing the dynamic processes during film formation is challenging.
• Understanding these dynamics is crucial for controlling coating properties.

Purpose of the Study:

• To demonstrate the utility of optical coherence tomography (OCT) for non-contact, in-situ monitoring of drying colloidal suspensions.
• To characterize the evolution of particle concentration and structure during film formation.
• To investigate the factors influencing the induction time and final film properties.

Main Methods:

• Utilized a scanning Michelson interferometer with a broadband laser for OCT imaging.
• Achieved high-resolution (10 μm transverse, 1.8 μm depth) cross-sectional imaging of colloidal silica drying in Hele-Shaw cells.
• Calibrated OCT images to quantify particle concentration profiles over time and space.
• Combined OCT with optical microscopy and particle tracking for comprehensive analysis.

Main Results:

• Successfully profiled the dynamics within a drying colloidal suspension with high spatial and temporal resolution.
• Quantified the variation in colloidal particle concentration during the drying process.
• Observed the development of the drying front and determined the induction time for solid film formation.
• Identified density-driven circulation near the drying front as a factor delaying film formation.

Conclusions:

• OCT is a powerful tool for studying the complex dynamics of drying colloidal suspensions.
• The findings provide insights into transport properties, such as collective diffusion dependence on particle concentration.
• Understanding the interplay between diffusion, evaporation, and fluid motion is key to controlling coating formation and properties.