The Phenomenon of Contact Angle Hysteresis （接触角滞后的现象）.pdf

Application Note The Phenomenon of Contact Angle Hysteresis A great interest in surface energy is contact angle hysteresis. Contact Angle Hysteresis is defined as the difference between advancing and receding contact angles. This hysteresis occurs due to the wide range of “metastable” states which can be observed as the liquid meniscus scans the surface of a solid at the solid/liquid/vapor interface. Because there are free energy barriers which exist between these metastable states, a true “equilibrium” contact angle is impossible to measure in real time. For an “ideal” surface that is wet by a pure liquid, contact angle theory predicts one and only one thermodynamically stable contact angle. In the real world, however, the “ideal” surface is rarely found. To fully characterize any surface, therefore, it is important to measure both advancing and receding contact angles and report the difference as the contact angle hysteresis. Thermodynamic Hysteresis There are at least six known sources of contact angle hysteresis. These fall into one of two group classifications thermodynamic and kinetic hysteresis. The first and most common classification is thermodynamic or “true“ contact angle hysteresis (see Figure 1). To qualify as a “true” or classical thermodynamic hysteresis, both advancing and receding contact angles must be stable (i.e., reproducible) regardless of time (time independent) or number of immersion cycles. There are two sources of thermodynamic hysteresis - surface roughness and surface heterogeneity. These represent the two most common of all sources of hysteresis in real world surfaces.(Table 1 below is a summary of the two primary sources of true contact angle hysteresis. Figure 1 A typical stable reproducible two-cycle Wilhelmy plate hysteresis loop demonstrating true or thermodynamic hysteresis +49-6031-16223