Wettability is the determining factor in many applications ranging from coatings to enhanced oil recovery and medical implants to pesticides. Due to a wide range of applications, the measurement of wettability, i.e., the contact angle measurement can be done with several different methods.
Sessile drop is the most used contact angle measurement method. It is done with the optical tensiometer which ranges from manual instruments to completely automated systems. In the sessile drop method, the droplet, typically water, is placed on the solid sample and the image of the drop is taken by a high-resolution camera. The contact angle is then automatically determined by the software.
Sessile drop measurement gives a so-called static contact angle for the surface. Sessile drop measurements are most often done with water. If the water contact angle is lower than 90 degrees the surface is said to be hydrophilic and if the contact angle is higher than 90 the surface is hydrophobic. Hydrophilicity and hydrophobicity are important in many applications. Water contact angle measurement is often used for quality control purposes as it offers a quick and non-destructive way to check the surface chemistry. Sessile drop is also often utilized for surface free energy calculations.
Typical applications: Surface free energy calculations, Quality control, surface treatment optimization
Advancing and receding contact angles with the needle method
The so-called needle method offers a way to measure advancing and receding contact angles. In this method, the needle is brought close to the surface. The size of the drop, most typically water, is gradually increased by pumping more liquid into the drop. As the baseline of the drop starts to move the advancing contact angle is measured. After that, the size of the drop is gradually decreased by sucking the liquid back to the dispenser. As the baseline starts to withdraw the receding contact angle is being measured. The measurement of advancing and receding contact angles is always recommended but the measurement is especially utilized to study the wetting of smart surfaces.
Typical applications: Superhydrophobic surfaces
The tilting method is another method for the measurement of advancing and receding contact angle. In this method, the drop is placed on the surface after which the substrate is tilted until the drop starts to move. The tilting can be done either by tilting the sample stage or by tilting the whole instrument. As the droplet starts to move the advancing contact angle is measured at the front of the drop and receding from the back of the drop. The method also allows you to determine the so-called roll-off angle which is the angle at which the droplet starts to move.
Typical applications: Smart surfaces, measurement of roll of angle
Captive bubble method
In the captive bubble method, the static or advancing and receding contact angles are measured so that the sample is immersed in liquid. The liquid is most typically water. The contact angle measurements are then performed by using an air bubble or some less dense liquid than water such as oil. Captive bubble measurement allows the measurements to be done for solid-liquid-liquid systems which are especially meaningful in oil reservoir wettability studies. It also makes it possible to measure samples that need to be immersed in water for proper measurements, such as contact lenses. In the case of a contact lens, an air bubble is used to determine the hydrophilicity of the lens surface.
Typical application: Contact lens, superhydrophilic surfaces, enhanced oil recovery
Wilhelmy plate is a measurement method that is often used for surface tension measurements but can be as well adapted to the measurement of the contact angle. The method is done with the force tensiometer. The sample is hung on the hook that is connected to a very sensitive balance. The sample is then immersed in liquid (most typically water). The force exerted on the sample is recorded and based on the surface tension of the liquid and the dimensions of the sample, the contact angle is calculated. The Wilhelmy plate method gives the advancing contact angle as the sample is being immersed and receding angle when the sample is pulled up. The important aspect to note here is that the contact angle is an average of the whole immersed surface area. Both sides of the sample need to then be identical. Due to the sensitivity of the method, it is often used to measure contact angle on fibers with diameters down to 7 um in size.
Typical applications: Fibers (down to 7 um)
The meniscus is a method that uses an optical tensiometer to measure the contact angle on thin objects such as fibers. The sample is immersed in a liquid which is typically water and then pulled up so that the meniscus of liquid is formed. The contact angle is then determined by looking at the angle that is formed in between the liquid meniscus and the sample.
Typical applications: Fibers (down to 200 um), other thin objects
The Washburn method is based on the capillary rise of the liquid into powders or other porous materials. The material is most typically pack to a stainless-steel holder that is hung on the hook of a force tensiometer. The holder with holes at the bottom is then immersed into a liquid and the mass uptake to the powder as a function of time is recorded. The Washburn equation is used for calculations. An important thing here to note is that to be able to determine the contact angle for the material, the measurement with completely wetting liquid needs to be performed first to determine the so-called material constant. The material constant is dependent on the packing of the material into the holder. This means that the packing method is one of the most critical parts of the measurement process.
Typical applications: Powders, porous materials
To read more about different contact angle measurement methods, please download the white paper through the link below.