courtesy of DataPhysics Instruments
Surface free energy (SFE) is one of the most critical surface parameters in industrial manufacturing processes such as painting,
coating, printing, and adhesive bonding. The success of these processes strongly depends on proper wetting of the
substrate surface — and wetting behavior is directly governed by the surface free energy of the solid. A widely used method
for determining solid SFE is the Owens–Wendt–Rabel–Kaelble (OWRK) method, which divides the total surface energy into
two components: Dispersive (non-polar) and Polar.
To calculate these components, contact angles of at least two test liquids
must be measured on the substrate surface. However, the selection of suitable test liquids is not arbitrary — it has a direct
and significant impact on the reliability, stability, and accuracy of the calculated surface energy values. For robust OWRK
analysis, DataPhysics Instruments recommends the use of diiodomethane, thiodiglycol, and ethylene glycol. The following
application note explains the selection criteria and demonstrates how to choose the test liquids.
Theory
Based on an appropriate model, the surface energy (SFE) of a solid, including its dispersive and polar parts, can be determined by contact angle (CA) measurements with at least two different
test liquids, whose surface tensions including their dispersive and polar parts are known. The OWRK model is the most commonly used method.
Requirements for Test Liquids
The selection of suitable test liquids is essential for reliable surface energy determination. Test liquids must fulfill both practical handling requirements and defined physicochemical criteria
to ensure accurate OWRK evaluation.
Handling and safety
For optical contact angle measurements very small liquid volumes are required, typically in the microlitre range.
Test liquids should:
-Be safe and easy to handle
-Exhibit good chemical stability (no rapid degradation due to light, oxygen, or moisture)
-Not react with or alter the substrate surface
-Have low vapour pressure to prevent evaporation during measurement
Stable and non-reactive liquids help ensure reproducible and reliable contact angle data.
Physicochemical criteria
For surface energy determination according to OWRK, the following properties are required:
√ Known total surface tension
√ Known dispersive and polar parts of the surface tension
√ High purity and stable composition
Although literature values are often available, verification of surface tension by pendant drop or tensiometric measurement is recommended to
exclude contamination. In addition, liquids with relatively high surface tension are preferred. They form measurable contact angles rather
than spreading completely, improving regression stability and evaluation accuracy.
• Number of test liquids
As described above the surface energy of a solid can be calculated from the measured contact angles and the known surface tension
components of at least two liquids. Although two test liquids are mathematically sufficient to define a regression line, a two-point evaluation does
not provide information about the reliability of the measurement. Any unnoticed error in one contact angle measurement may directly lead to an
incorrect surface energy result.
For this reason, DataPhysics Instrument recommends using three different test liquids. Additional data points improve the stability of the
regression, enable the identification of potential outliers, and increase the overall accuracy of the evaluation. Furthermore, the selected liquids
should have significantly different polar-to-dispersive ratios to ensure well-separated data points and a robust surface energy determination.
Recommended Test Liquids Set
DataPhysics Instrument recommends diiodomethane, thiodiglycol, and ethylene glycol as the preferred test liquids for contact angle
measurements and subsequent surface energy determination according to the OWRK method.
These substances present manageable handling risks, particularly since only very small volumes are required for optical contact
angle measurements (e.g., with the OCA systems). Ethylene glycol is chemically stable, while thiodiglycol and diiodomethane, although slightly
sensitive to light, remain sufficiently stable for reliable measurements when stored properly. Storage in light-protected containers is recommended.
In addition, their relatively low vapour pressures prevent rapid evaporation during measurement. All three recommended liquids exhibit
surface tensions above 47 mN/m, minimizing unwanted spreading and enabling stable, measurable contact angles on most solid surfaces. Most
importantly, they provide excellent contrast in their dispersive and polar surface tension components:
• Diiodomethane: purely dispersive with 0% of polar part
• Thiodiglycol: significant polar contribution with 27% of polar part
• Ethylene glycol: higher polar contribution with 35% of polar part
This wide distribution ensures well spaced data points in the OWRK plot, resulting in a robust regression line and reliable determination of the
solid’s surface energy.
Caution when using water:
Water is a commonly used and readily available test liquid. It is non-toxic, chemically stable, and has a high surface
tension (72.8 mN/m) with approximately 70% polar contribution, which in principle makes it suitable for surface
energy analysis. In particular, water is very useful for quality control and routine wettability checks.
However, due to its strong polarity, such as its high polar component, hydrogen bonding capability, and sensitivity
to contamination and surface heterogeneity, water can sometimes produce outlier measurements. Therefore,
it should not be used as one of only two test liquids in scientific surface energy analysis.
If used, it is recommended as:
• A third liquid (allowing exclusion in case of outliers), or preferably
• A fourth liquid in combination with diiodomethane, thiodiglycol, and ethylene glycol.
This approach ensures reliable regression while maintaining control over potential measurement deviations.
Summary
For reliable OWRK surface energy analysis, DataPhysics Instruments recommends using diiodomethane, thiodiglycol, and ethylene glycol.
This combination provides a broad range of polar and dispersive parts, ensuring stable regression and accurate results. Water is very useful for quality control
and routine wettability checks. It may be included as an additional test liquid; however, it should not be used as one of only two liquids for scientific surface energy evaluation.