Effects of surface heterogeneities on wetting and contact line dynamics as observed with the captive bubble technique

Understanding the effects of surface heterogeneities on wetting characteristics and contact line dynamics is significant for a number of applications. In this work, the advancing and receding contact angles on well-defined heterogeneous surfaces were measured using the captive bubble technique. Furthermore, contact line distortions were observed to clarify the origin of the changes in the contact angle. For heterogeneous surfaces with different hydrophilic area fractions but the same hydrophilic defect diameter, the defects had no restrictive effect on the retreating behavior of the contact line, resulting in a nearly constant advancing contact angle. Conversely, during receding contact angle measurements, the presence of a few hydrophilic defects caused three-phase contact line pinning. Increasing the number of hydrophilic defects led to more corrugations of the three-phase contact line, resulting in a decrease in the receding contact angle. For heterogeneous surfaces with the same hydrophilic area fraction but different hydrophilic defect diameters, the advancing contact angles were also nearly constant because in the hydrophilic region, no energy barrier prevented contact line movement. However, the greater energy barriers caused by larger hydrophilic defects led to a slight decrease in the receding contact angles. A comparison of the measured and calculated values for both the advancing and receding contact angles showed that the experimental contact angle results were consistent with the contact angles calculated using the line-fraction Cassie equation. These insights into the wetting behavior of heterogeneous surfaces under liquids are expected to lead to advanced practical applications of heterogeneous surfaces.