Wettability is a surface property of any material which is important to be studied to understand its affinity towards any liquid medium. Contact angle measurements are the way to understand wettability of the surface. The understanding of this property in tribological studies helps to design the surface of materials based on its properties. This also helps to improve the lubrication properties on the surface thereby increasing the efficiency of the materials in any desired applications. Contact angle is one of the important parameter that determines the wettability.
Fig-1 Surface characterization using contact angle measurements .
Contact angle is the angle formed by the liquid at the contacting solid surface. It is one the important parameter which is used to determine the surface free energy of a solid which is in contact with one or more liquids. It is also used to determine the intensity of phase contact between the solids and liquids which is applicable in cleaning, bonding, dispersing, painting etc.
Types of contact angles
Geometrically contact angle is the angle formed at the intersection of solid, liquid and gaseous phases. Based on these three phase boundaries it can be classified as static contact angle and dynamic contact angle .
- Static contact angle: Static contact angle is measured when the three boundary phases i.e. solid, liquid and gas phases are static. Static contact angles are the wettability measurements which is used to define the surface free energy of the substrate.
- Dynamic contact angle: The dynamic contact angles are measured when the three boundary phases are in motion. They are mainly measured when the droplet is advancing or receding the surface. The contact angle hysteresis is the difference between the advancing and the receding contact angles and these are caused due to the solution impurities absorbing on the surface.
Contact angle measurements
Based on the contact angle measured the wettability of the surface can be determined. If the contact angle on the surface is zero it resembles the complete wetting on the surface. If the contact angle is less then 90degrees that resembles the surface has good wetting property and finally if the contact is more than 90degrees it resembles the that the surface is not having the good wettability. In Fig-2 the contact angles with the surface wetting properties are shown.
Fig-2 Contact angle measurements with its surface wetting .
Contact angle Measuring methods
There are various methods of measuring the contact angle on the surface based on the applications and the measuring principles .
Fig-3 Contact angle measurement setup .
Drop shape analysis: In this method the contact angle is measured by a sessile drop which is formed at the three boundary phases. The sessile drop is basically the standard arrangement made using the drop shape analysis. To measure the contact angle the points of intersection are determined at the fitted drop shape and the baseline. Then the angles are measured respectively by using the determined points.
Wilhelmina plate method: This method is mainly used in measuring the surface tension, interfacial tension and the contact angles between the liquid and a solid. The contact angle is measured by immersing the solid sample in liquid medium with the known surface tension. Then it can be calculated using the wilhelmy equation shown below.
Θ = arc * cos ( F / L * σ )
Where Θ is contact angle, F is immersing force, L is wetted length and σ is the surface tension of the liquid.
Washburn method: This is one of the contact angle measuring technique along with the surface free energy measurement. This technique is used to measure the the contact angles and surface free energies of the porous substances such as bulk powders, papers etc. The increase in the weight the of the immersed powder filled tube is measured with respect to time.
Research on Contact angle measurements
The contact angle measurements are important for analysing the surface wetting which in turn helps in various applications. There are several researchers who have studied the contact angle measurements on various materials. Tianyi Zahoor et.al., studied the contact angle measurements for the biomaterials, in this study they have used the image capture method for measuring the wettability . Roshan et.al., have studied the review on contact angle measurements of shale wettability where they have summarised all the research works conducted over a period on this contact angle measurements . Dimitri Jansen et.al., have studied the contact angle measurements for static solvents. Based on this contact angles they have determined the wettability of the self-assembled mono layers on silicon dioxide .
 Janssen, D., De Palma, R., Verlaak, S., Heremans, P. and Dehaen, W., 2006. Static solvent contact angle measurements, surface free energy and wettability determination of various self-assembled monolayers on silicon dioxide. Thin Solid Films, 515(4), pp.1433-1438.
I am currently working as a Postgraduate Researcher at the University of Leeds. Previously I completed my master's under the prestigious Erasmus Mundus joint master's degree program (Master's in Tribology). I have also completed my bachelor's in Mechanical engineering from VTU, Belgaum, India. I am working as the social media manager for Tribnet and also I have my youtube channel Tribo Geek.