Top-View Method as a Robust Alternative for Contact Angle Measurement

Area of Science:

• Surface Science and Engineering
• Materials Characterization
• Fluid Dynamics

Background:

• Conventional side-view contact angle measurements face limitations on rough, deformable, or irregular surfaces.
• These limitations include compromised baseline visibility, meridian clarity, and drop symmetry, hindering accurate analysis.
• Alternative methods are needed for reliable contact angle determination in challenging scenarios.

Purpose of the Study:

• To validate the top-view method for contact angle measurement, relying on the spherical cap assumption.
• To assess its practicality as an alternative to traditional side-view techniques, particularly for difficult surfaces.
• To introduce and evaluate a drop projection index (DPI) for quantifying gravity effects in top-view imaging.

Main Methods:

• Simultaneous acquisition of top and side-view images of sessile drops.
• Precise determination of drop volume using drop mass measurements.
• Calculation of contact angles from drop volume and contact area, applying the spherical cap model.

Main Results:

• The top-view method provided statistically comparable contact angle values to side-view methods for small drops (1-10 μL).
• Contact angles for water and formamide on PMMA and Teflon surfaces were measured across wetting and non-wetting regimes.
• The drop projection index (DPI) was introduced and quantified gravity effects, showing distinct values for different surface wettabilities and liquids.

Conclusions:

• The top-view method, under the spherical cap assumption, is a practical and accurate alternative for contact angle measurement, especially for small drops.
• The study provides a foundation for enhancing the applicability of the top-view method across diverse material systems.
• The newly introduced DPI offers a valuable tool for understanding and correcting for gravity-induced distortions in top-view sessile drop analysis.