Kinetic Aspects of Hysteresis

According to its definition, contact angle hysteresis should be concerned only with thermodynamic equilibrium situations and not with nonequilibrium kinetic events. However, because of experimental limitations, such as the time required for measurements, certain kinetic effects are almost unavoidable. Theoretically, such effects should fall under the heading of kinetic contact angles but are often reflected in equilibrium measurements. 

Because the measurement of a contact angle must involve some movement of the wetting line, it is possible, or even probable, that the act of spreading of the hquid will displace certain surface equilibria that will not be reestablished over the time frame of the experiment. For example, the displacement of a second fluid may result in the estabhshment of a nonequilibrium situation in terms of the adsorption of the various components at the solid-liquid, solid-fluid 2, and liquid-fluid 2 interfaces. Time will be required for adsorption equilibrium to be attained, and it may not be attained during the time of the contact angle measurement if the transport and adsorption-desorption phenomena involved are slow. The kinetic effect may be especially significant for solutions containing surfactants, polymers, or other dissolved adsorbates. 

A second potential kinetic effect may result from bulk interactions between the surface and the spreading liquid. For example, if the liquid can penetrate the surface (e.g., if the liquid can be absorbed as opposed to adsorbed), the rate of penetration may be so slow that the measured contact angle will not reflect the true equilibrium situation. Likewise, if the liquid swells the surface, the wetting line may lie on a ridge of swollen surface rather than on a flat surface, resulting in an error in 6 (Fig. 17.6). 

FIGURE 17.6. If the liquid used for measuring the contact angle is absorbed into the solid surface (i.e., it swells) the resulting contact angle will be smaller than the true value. 

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