Measuring contact angle hysteresis can give a lot of additional information on your surface. The measurement methods can be divided into optical (needle and tilting methods) and force (Wilhelmy method) depending on the measurement principle used. All the methods are measuring advancing and receding contact angles which are used to calculate contact angle hysteresis. All of these methods have their pros and cons which makes some more suitable for certain types of substrates and applications.
Contact angle hysteresis by needle method
Needle method is the first one of the optical methods. Here, a needle is brought close to the substrate surface and the liquid is dispensed slowly to increase the volume of the drop. As the contact line between the substrate and the drop is increasing, the advancing contact angle is measured. When the liquid is sucked slowly off the substrate and the contact line is withdrawing, the receding contact angle is measured. The method is used especially with superhydrophobic samples, as this is the only practical method for such a high contact angle substrate.
Contact angle hysteresis by tilting method
Tilting method is the second optical method. The droplet is placed on the substrate to be measured and the tilting of the sample is started. The advancing and receding angles are measured at the point at which the droplet starts to move on the substrate. At the same point, the roll-off angle can be defined. The method allows for the mapping of the substrate but is often criticized as the size of the drop can have an impact on the measured values. A large enough and same drop volume should be used to allow comparison between different substrates.
Contact angle hysteresis by Wilhelmy method
Wilhelmy method is based on the force measurement. The object to be measured, a rectangular plate or round rod, is immersed into a liquid with the known surface tension. As the plate or rod is immersed, the advancing contact angle is measured and when the plate is emerged, the receding angle is obtained. The dimensions of the plate or the diameter of the rod needs to be known and the measured contact angles are averaged over the whole area immersed into the liquid. Thus the method is best suited for objects, like thin fiber, which measurement is difficult with other methods.
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