Purpose and strength of the technique
- Contact angle between a liquid and a solid
- Free surface energy from contact angles of several test fluids for all current models
- Static contact angle, advancing and receding angle
- Measuring the surface tension and the liquid-liquid interfacial tension by the pendant drop method
- Simple and rapid method to evaluate the wettability of surfaces
- Investigation of the polarity of surfaces
- Contact angle: 1-180°, detection limit ~0,1°
- Surface tension: 0,01 - 1000 mN/m, detection limit ~0,01 mN/m
- Sample support: manual motion in x-y-z direction
- Max. Sample Size: 300 x 300 x 50 mm (B x T x H)
Lateral resolution: ~3 mm
- Characterization of surface pretreatments
- Investigation of the adhesion and stability of bonds and coatings
- Testing the wettability of plastic, glass, ceramics, paper, wood or metal
- Quality control for wafer and microelectronics
- Examination of surface cleanliness
The contact angle (also edge or wetting angle) is defined as the angle which a liquid drop forms on a surface of a solid to that surface. It is a dimension of the wettability of the surface. The size of the contact angle between liquid and solid depends on the interaction between the substances at the interface. The weaker this interaction, the greater is the contact angle.
The contact angle
Certain properties of the surface of a solid, surface energy for example, can be determined from the contact angle. In the special case of using water as liquid at low contact angles (about 0°) the surface is called to be hydrophilic (= water loving), at angles of 90° to be hydrophobic (= water repellent) and at even greater angles as superhydrophobic. The latter is also called lotus effect at very high angles (160°) and corresponds to an extremely low wettability.
The contact angle can be changed by surface treatments.
The contact angle measurement
Contact angle measurements provide information about the wettability of a surface or about the surface tension (from the Young's equation). These variables are often helpful in evaluating the condition of a surface, for example, after pre-treatment or cleaning steps.
Beside the direct measurement of contact angles test inks are often used. A coloured liquid ("ink") with a defined surface tension is applied on the surface to be tested by brush. If the surface is wetted by the ink (means that the brush stroke persists for more than 3 seconds without contraction), the surface tension of the surface under test is equal to or greater than that of the test ink. On the other hand, if the brush stroke contracts within 3 seconds, the surface tension of the tested surface is smaller than the surface tension of the test ink.
The surface can be roughly characterized by this way. Using a contact angle measuring device, much more accurate measurements are possible.
The contact angle is important in all areas where the quality of the contact has to be checked or evaluated between liquid and solid substances, for example coating / painting, cleaning, printing, hydrophobic or hydrophilic coating, bonding, dispersing.
Carrying out the measurement
For this purpose, a drop of test fluid is deposited on the surface to be tested. The lying drops are recorded using a camera. Special software measures the contour of the droplet and the base line of the substrate from the image. The figure shows the determined contour in green and the base line in red.
In a further step the contact angle or the surface tension can be calculated using mathematical models (for example by the conic, polynomial or Young-Laplace method).
Application of contact angle measurements
Printing, conglutination or coating of polymers requires optimal pre-treatment steps for hydrophobic plastic surfaces.
The different methods such as corona process, flame treatment or plasma treatment and chemical action of ozone or fluorine increase the surface free energy and thus ensure improved wetting, initial adhesion and long-term stability.
The effectiveness of such treatments can be quantified by contact angle measurements.
In addition, parameters can be acquired which allow predicting the surface wettability of liquids with known surface tension out of systematic measurements.
Complementary Analytical Methods
Contact angle measurement can prove wetting differences between different surfaces in a comparatively simple manner. However, in order to obtain specific information on the underlying chemical differences of the surfaces further complementary measurement methods are available:
- Infrared spectroscopy (ATR-IR)
- Photoelectron spectroscopy (XPS/ESCA)
- Time of flight secondary ion mass spectroscopy (TOF-SIMS)
With these methods it is possible to discover which lubricant could be removed by a cleaning method.