The contact angle of a drop on a surface strongly depends on the chemistry of the surface and on the test liquid: If both are polar (e.g. a water droplet on a surface with many OH-groups), there is a good wetting and the drop spreads out on the surface. This is then a hydrophilic surface with a contact angle θ typically smaller than 45°. If there are only few polar groups, the contact angle of water will be higher. Above a contact angle of 90° a surface is called hydrophobic. On superhydrophobic surfaces (θ >160°), the water drop balls up to a spherical shape enclosing air underneath. This effect is also called lotus effect and is due to a combination of a hydrophobic surface and a rough topography.
Figure 1: Equipment in the RMS Foundation for measuring the contact angle.
The biological performance of an implant is generally controlled by surface properties. For example a more hydrophilic dental implant leads to a faster osseointegration [1]. This property can be controlled by measuring the contact angle: a smaller contact angle was found on a clean implant compared to an implant which was stored for several weeks on a bench (figure 2).
Figure 2: Contact angle of a clean (θ=44°) and of a «dirty» implant (θ=73°, after several weeks on a bench).
Contact angle measurements can be used for the characterisations of materials too, as shown in a research project on poly(HEMA-co-MMA) mixed polymers (figure 3). With increasing MMA-fraction of the co-polymer, the surface changes from hydrophilic to hydrophobic.
In general, a surface can be characterised quickly by measuring the contact angle. But this is only one aspect of the surface analysis and it is ideally accompanied by XPS (X-ray photoelectron spectroscopy for the surface chemistry) and/or SEM (scanning electron microscopy → topography).
1. F. Schwarz et al. Journal of biomedical materials research – Part B Applied biomaterials 88, 544-557 (2009).
Figure 3: Contact angle of poly(HEMA-co-MMA) (poly(2-hydroxyethylmethacrylate-co-methylmethacrylate)) co-polymers.
Our equipment for measuring the contact angle:
Surftens universal from OEG GmbH (Frankfurt, Germany)
Options:
- Static measurement of the contact angle:
Measurement of the left and right contact angle of a static drop on a surface in its equilibrium. - Dynamic measurement of the contact angle: The volume of the drop is continually increased and later decreased and the advancing and receding contact angles are measured on this drop (average of several angles/drop). Higher reliability for rough or inhomogeneous surfaces.
- Acquisition of movies with the possibility to analyze each image separately.
Preset values:
- Flat and homogeneous surfaces particularly for static measurements.
- Dimension: up to 20x20x6 cm, in certain circumstances even larger.
Normative references:
ASTM D7334 and DIN 55660-2
