Hysteresis of the contact angle

The term contact angle hysteresis describes the phenomenon that, in the case of surfaces that cannot be completely wetted, the contact angle (advancing angle) that occurs during wetting is greater than the contact angle during dewetting (receding angle).

In general, hysteresis describes the behavior of a system in which an output variable depends on an input variable and also on the previous state of the output variable. The output variable has different values when the input variable increases than when it decreases.

How does contact angle hysteresis occur?

Hysteresis occurs on almost all surfaces that cannot be completely wetted (contact angle > 0°), although it is usually more pronounced on chemically and/or topographically inhomogeneous surfaces. In the course of wetting, for example, depressions in rough surfaces can be filled with liquid, which increases wettability during the retraction movement. In addition, the degree of hysteresis is associated with the adhesion force between the liquid and the solid surface.

For which questions is the hysteresis of the contact angle of interest?

First of all, wetting and dewetting processes often occur together in the formation of solid-liquid phase boundaries, so that both processes and their differences should be considered for a complete characterization of wettability.

Hysteresis is particularly revealing when assessing hydrophobic, anti-adhesive and/or self-cleaning surfaces. The common belief that a high initial contact angle characterizes the water-repellent properties of such materials is not always true; the adhesion of the liquid can even be surprisingly strong (rose petal effect). The complementary measurement of the behavior during dewetting and the quantification of hysteresis can therefore be used to analyze hydrophobic materials more comprehensively.

How is the hysteresis of the contact angle determined?

As the hysteresis describes the different behavior during wetting and dewetting, contact angle measurement methods are used that depict both processes. The difference between the contact angle values obtained is then calculated.

Optical measurement with contactlessly pressure-dosed drops: The Liquid Needle is used to measure the recently advanced contact angle (RACA) on a drop that has spread with low dynamics and has not retracted. Complementarily, the Stood-up Drop method measures the recently receded contact angle (RRCA) immediately after the formation of a self-contracting drop.
Optical measurement with a needle dosing unit (needle-in): The drop is enlarged by adding volume using a dosing needle to measure the advancing contact angle (ACA) and reduced by aspiration for the receding contact angle (RCA).
Optical measurement combined with a tilting device: The drop slides over the tilted surface. In the direction of movement, the advancing angle is measured at the front and the receding angle at the rear.
Meniscus method (optical method for fibers): The contact angle on the meniscus of an immersing fiber is measured in the course of immersion (advancing angle) and withdrawal (receding angle).
Mechanical method according to Wilhelmy with a tensiometer: The wetting force (capillary force) on a sample or fiber is measured during immersion and withdrawal (by lifting and lowering of the liquid).