ramé-hart Technical Document

Technical Bulletin

Tips for Liquids for Two-Liquid Tool

Nov 2025

When using the 2-Liquid Surface Energy Tool, ramé-hart's DROPimage Advanced software uses one of two methods.

The Wu method (harmonic mean) uses two pure liquids denoting their dispersive and non-dispersive values. This method is recommended for determining the surface energy of polymer solids.

The Owens-Wendt-Rabe-Kaelble (OWRK) method (geometric mean) is also used to determine the two components of total free surface energy of a solid surface, the polar component, and the dispersive component. Measuring the contact angle of two liquids is required to solve a system of linear equations with two variables.

When the 2-Liquid Surface Energy Tool is used, it's necessary to use two high purity liquids. The recommended combination is pure deionized water and diiodomethane (AKA methylene iodide). Here are the reasons this pair is the recommended combination:

1. Water is the "Polar Standard." It has a dominant polar component of 51.0 mN/m and a moderate dispersive component 21.8 mN/m while diiodomethane is the "Dispersive Standard." It acts as a proxy for a purely dispersive liquid, with a very high dispersive component of 50.8 mN/m and a polar component of essentially zero nN/m. Water and Diiodomethane provide the necessary mathematical contrast to stabilize the calculation. Other combinations would not provide the necessary contrast for reliable results.

2. Water and diiodomethane are well-characterized liquids and can be obtained at a high purity, leading to more consistent and accurate results.

3. To use Young’s Equation (the basis of all contact angle math), the liquid must form a measurable angle on the surface (greater than 0°). The probe liquid must have a higher surface tension than the solid being tested. Water, with a surface tension of 72.8 mN/m, is a high surface tension liquid and diiodomethane with a surface tension of 50.8 mN/m is also relatively high. Since both test liquids have a surface energy over 45 mN/m, there is no problem with testing solids with high surface energies.

4. Water is inert and while diiodomethane is a heavy liquid, it is chemically distinct from solvents which will immediately begin to dissolve, swell, or etch the surface. This alters the surface topography and chemistry during the measurement, rendering the contact angle data invalid.

5. Water and Diiodomethane have been studied for decades. Their dispersive and polar splits are universally agreed upon in surface science literature (Ström, Fowkes, etc.). If you use lesser known liquids, you may find their total surface tension in a handbook or measure it yourself, but finding reliable data on exactly how much of that tension is polar vs. dispersive is difficult. If these input parameters are wrong, the Surface Energy calculated by DROPimage will be wrong.

6. Other liquids may introduce factors that can confound the OWRK analysis, such as reacting with the solid, absorption, or complete spreading.

In short, using other combinations aside from water and diiodomethane usually results in data that is mathematically unstable (high standard deviation) or physically invalid (due to wetting or swelling) and will fail to produce accurate results.

Using random liquids, or liquids that are too similar chemically, leads to invalid data for three specific reasons:

A. The Mathematical "Ill-Conditioned" Matrix

If you choose two liquids that are mathematically similar (e.g., Water and Glycerol, or Water and Formamide), both have high polar components.

The Visualization: Imagine drawing two lines on a graph to find where they cross (the solution). If the lines are perpendicular (like Water vs. Diiodomethane), the crossing point is exact. If the lines are almost parallel (like Water vs. Glycerol), the crossing point is blurry.
The Consequence: A tiny error in your contact angle measurement will result in a massive error in the final surface energy calculation. The software may produce a result, but the standard deviation will be unacceptable.

B. Surface Tension Hierarchy (Young's Equation Limits)

For a valid contact angle to form, the surface tension of the liquid generally needs to be higher than the surface energy of the solid.

If you use a random liquid with low surface tension (like Ethanol or Hexane), the liquid will likely spread completely and the contact angle will be zero.
A contact angle of zero cannot be used in the Owens-Wendt calculation because it implies complete wetting, making it impossible to calculate specific surface energy components.

C. Chemical Incompatibility (The "Observer Effect")

Random liquids introduce variables that the OWRK method assumes do not exist. The method assumes the liquid sits on top of the solid without altering it.

Solvation: If you use a solvent (like Acetone or Toluene) on a polymer surface, the liquid may begin to dissolve or swell the surface immediately.
Reaction: The liquid might chemically react with the substrate.
Absorption: The liquid might absorb into the matrix of the solid.

Any of these interactions invalidate the contact angle geometry, meaning the input data into DROPimage is flawed, rendering the output meaningless.

Conclusion: For optimal results, pure deionized water and diiodomethane are the recommended liquids for use with the 2-Liquid Surface Energy Tool found in ramé-hart's DROPimage software whether you're using the harmonic or geometric mean method.

If you require any additional technical assistance, please contact us with your serial number and the nature of your inquiry.