Optical Contact Angle Measuring
OCA & Contour Analysis Systems
The optical contact angle measuring, and contour analysis systems of the OCA series are high precision optical measuring devices for the measurement of interfacial parameters and phenomena. In the following you will learn about the possibilities that the optical analysis of drop contours offers and about our OCA models with their extensive range of accessories and intuitive software control.
OCA & Drop Contour Analysis
The optical analysis of drops that hang from a dosing needle or are placed on a solid surface facilitates the determination of different surface and interfacial parameters. The contact angle that a liquid drop establishes on a solid surface characterises the solid’s wetting behaviour with said liquid.
Having measured the contact angles of multiple test liquids the surface energy of the solid can be determined and the latter can be used to calculate the work of adhesion for different liquids.
The reliable and experimentally robust measurement of the contact angle aids on the development of surface coatings, composite materials, paints and varnishes or cleaning agents. In short: the measurement of contact angle helps in all situations where solids and liquids meet and advantage is to be gained by the control of wetting and adhesion properties.
Contact Angle Measurement
An equilibrium of vectorial forces dictates the contact angle ΘC at the three phase contact line of a deposited drop. The surface energy of the solid σS acts along the solid surface. The solid-liquid interfacial energy σSL acts in the opposite direction and the surface tension σL of the liquid acts tangential to the drop surface. This can be described by a simple scalar equation, the Young equation:
The drop is viewed in profile during the contact angle measurement. The image processing software recognises and records the drop contour, as well as the base line at the solid-liquid interface and fits a mathematical function to the drop shape.
Optical Determination of Surface & Interfacial Tension
When no other factor is in play a drop of liquid tends to form a sphere, due to its surface tension. The typical drop shape materialises because the drop is elongated due to gravity. The Young-Laplace evaluation of pendant drops recognises this fact: The characteristic shape of the drop profile yields the surface tension σL of a liquid.
In the case where a pendant drop is surrounded by a second liquid, rather than air, the interfacial tension between the two liquids can be deduced from the drop shape. For optical analysis the outer liquid has to be transparent. Depending on the relative densities, the inner liquid can be dosed either as a pendant drop or upwards, via a bent needle.
Surface Energy of Solids
To determine the surface energy of a solid one measures the contact angles of test liquids whose surface tensions including their dispersive and polar parts are known. These dispersive and polar parts are used to calculate the interfacial tension σSL between the solid and a liquid based on a suitable model.
An often applied model is the one of Owens, Wendt, Rabel and Kaelble (OWRK model) which considers the geometric mean of the dispersive and polar parts of the liquid’s surface tension σL and of the solid’s surface energy σS:
Substituting this expression in the Young equation, the polar and the dispersive part of the solid’s surface energy can be determined from the regression line in a suitable plot.
The linear regression requires contact angle measurements with at least two different test liquids. However, as a regression line based on just two points contains no information on the accuracy of the result, contact angle measurements with at least three test liquids are recommended for the determination of the surface energy of solids.
A well quoted example of large contact angles can be found in nature: when water droplets come into contact with a lotus leaf they roll off without wetting the surface. During ‘roll off’ the drops take dirt particles with them, resulting in the self-cleaning of the leaf. Mimicking this “Lotus effect” is a popular research and product development topic in many technical fields. Self-cleaning facades, ceramics and other surfaces are regarded as highly desirable. In this context the ability to measure and record contact angles, of course, plays an essential role.
The OCA Models
The optical contact angle measuring and contour analysis systems of the OCA series combine high resolution optics, exact liquid dosing and precise sample positioning into powerful and reliable measuring systems.
The modular approach to all hardware components allows for a multitude of configurations, ranging from manually operated basic devices to fully automated high-performance measuring systems. All OCA models are based on a common design philosophy and are built with a sturdy aluminium metal frame. Moreover, they all feature a LED lighting with manually and software-controlled intensity. Due to an automatic temperature drift compensation a stable and homogeneous illumination of the sample is guaranteed at all times.
For a compact overview of the OCA models download the product brochure or use our convenient online comparison of the different models.
Measurements conforming to standards
Measurements that comply with various national and international standards are possible due to the high precision and extensive range of accessories of the OCA models. These standards are among others:
- ASTM C813
- ASTM D724
- ASTM D2578
- ASTM D5725
- ASTM D5946
- ASTM D7334
- ASTM D7490
- ASTM D7541
- DIN 55660
- DIN EN 15802
- ISO 8296
- ISO 15989
- ISO 27448
The high temperature measuring system OCA 25-HTV 1800 is capable of measuring contact angles at high temperatures of up to 1800 °C and under vacuum down to 10-5 mbar or under inert gas atmosphere, respectively.
The OCA 25-PMC 750 with its high pressure measuring chamber provides the opportunity to measure interfacial tensions and contact angles at pressures of up to 750 bar and under temperature conditions ranging from -30 °C to 200 °C
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