Critical Marangoni Number for Disappearance of Striations during Spin-Coating

Area of Science:

• Materials Science
• Fluid Dynamics

Background:

• Spin-coating is a common technique for thin film deposition.
• Striations, or spoke-like thickness undulations, can form during spin-coating.
• The underlying mechanisms for striation formation and disappearance are not fully understood.

Purpose of the Study:

• To investigate the formation and disappearance of striations during spin-coating.
• To analyze the dynamics of striation wavelengths.
• To determine the critical Marangoni number for striation decay.

Main Methods:

• Spin-coating of an epoxy resin and diglyme solvent mixture.
• Optical measurement of spatiotemporal thickness variation.
• Short-time Fourier analysis for wavelength spectrum.
• Numerical simulation to predict transient Marangoni number.

Main Results:

• Striations were observed to appear and subsequently disappear.
• Wavelength components of striations decreased by three orders of magnitude.
• The Marangoni number was found to decrease below a critical value.
• Critical Marangoni numbers for striation disappearance were determined to be in the range of 90-160.

Conclusions:

• Striation disappearance is attributed to the Marangoni number falling below a subcritical value.
• The study provides quantitative insights into the solutal Marangoni-Bénard instability during spin-coating.
• The findings contribute to understanding and controlling thin film uniformity.