Goniometry is a technique that is used to investigate the interactions between a solid surface and a liquid. When a small volume of liquid is placed onto a solid surface it usually forms a discrete drop on the surface. The contact angle is geometrically defined as the angle on the liquid side of the tangential line drawn through the three phase boundary where a liquid, gas and solid intersect, or two immiscible liquids and solid intersect. A high contact angle is indicative of a repellant action between the surface and liquid. Where attractive forces occur between the substrate and liquid low contact angles are observed. Water is commonly of interest when examining contact angles, since it is the most abundant liquid at room temperature. Solids which exhibit a high (>90 ) contact angle at a water / air interface are termed hydrophobic, while those exhibiting and angle less than 90° are known as hydrophilic.
The CAM 200 consists of a camera, lens, light source and PC interface, illustrated in the top left figure. It collects images of the fluid surface interaction and stores them digitally. These images are then processed to provide the contact angle at each side of the drop, surface tension and drop volume as shown in the top right figure.
- The camera has a resolution of 512 x 480 pixels and runs at a maximum frame rate of 25 fps.
- Illumination is provided by a pulsed LED source. This provides a monochromatic light source, improving the image sharpness and decreasing sample heating.
- The lens has a zoom configuration allowing different sizes of drops to be measured while retaining optimal resolution.
- A tilt stage enables dynamic contact angle measurements to be made.
- A precision syringe allows precise control over the drop volume.
- The software enables simple, accurate calculation of the contact angle, surface tension and volume.
The Main Unique Aspects Of The Machine
- The software is capable of capturing a large number of images at defined time periods, this is very useful for looking at the variation of contact angle with time.
- The tilt stage enables dynamic analysis of contact angle. This give quantitative data on both advancing and receding contact angles which can determine the contact angle hysteresis. The hysteresis is related to the homogeneity of the sample surface. Roughness and surface heterogeneity contribute to the hysteresis.
Examples Of Work Undertaken At The Institute
A number of projects have utilised this technique for assessment of a wide variety of surfaces. For example we have studied contact angles on surface modified titanium, on soluble calcium phosphate based glasses (figures) and on polymer systems degraded by a number of different biological agents. The figures below show that varying the composition of phosphate glasses can have a significant effect on the contact angle. Both the hydrophilic (figure below, left) and hydrophobic (figure below, right) surfaces are from the same system. Certain surfaces undergo changes in surface properties when exposed to water. The figure below, centre shows the rapid change in the contact angle of a phosphate glass exposed to water.