Help from a Hindrance: Using Astigmatism in Round Capillaries To Study Contact Angles and Wetting Layers

Area of Science:

• Soft-matter science
• Interfacial science
• Optical physics

Background:

• Round glass capillaries are standard tools in soft-matter science.
• Astigmatism introduced by capillaries often hinders accurate analysis.
• Refraction effects in capillaries are typically considered a hindrance.

Purpose of the Study:

• To demonstrate how capillary refraction can be utilized to measure interfacial properties.
• To investigate the measurement of contact angles and thin films on capillary walls.
• To explore applications under demanding conditions and detect previously invisible phenomena.

Main Methods:

• Analysis of optical cusps formed by refraction at the fluid-meniscus interface.
• Utilizing the relationship between meniscus dimensions and contact angle.
• Investigating conditions for total internal reflection to detect thin films.

Main Results:

• Direct measurement of contact angles and capillary inner diameter at the meniscus.
• Detection of submicron thin films on capillary walls via total internal reflection.
• Measurement of CO2-water/brine contact angles up to 200°C and 600 bar, showing temperature effects on glass wettability.
• Observation of a tenuous gas hydrate layer using refraction, invisible by other methods.

Conclusions:

• Capillary refraction offers a powerful, non-intrusive method for characterizing interfacial properties.
• The technique enables measurements under extreme conditions (high T/P, small volumes).
• Refraction-based optical cusps reveal subtle interfacial phenomena like thin film formation and temperature-dependent wettability.