Pinning Effects of Wettability Contrast on Pendant Drops on Chemically Patterned Surfaces

The morphology and dynamics of the pendant drops attached to chemically patterned surfaces (pattern-pinned pendant drops) with different hydrophilic/hydrophobic contrasts were investigated experimentally and numerically. During the experiments, the evolution of the contact angle and the maximum drop volume were found to be different from those of traditional pendant drops, whose contact line is pinned on the edge of the tips (tip-pinned pendant drops), and the deviation is related to both the pattern radius and the wettability contrast. Then, a hypothesis was proposed to illustrate the behavior of the contact line after it reached the pattern boundary, based on the premise that the pattern boundary possessed a certain width or fuzziness. It was concluded that the special phenomena in this case were due to the movement of the contact line, and the maximum contact radius was presented as a key parameter for the pattern-pinned drops, which is directly related to the stability and the maximum volume of the drops. Furthermore, through a simulation study on pattern-pinned pendant drops, the vibration performance of the meniscus was revealed as a superposition of two vibration behaviors including a low-frequency vibration due to the inertia effects and a high-frequency vibration due to the surface tension gradient within the boundary region. In addition, the hypothesis proposed above was also verified. Finally, a forecasting model to predict the maximum contact radius for the pattern-pinned pendant drops was built for different liquids and pattern wettabilities. This allows us to effectively design and optimize chemically patterned surfaces to achieve a desired pinning function or a pendant drop with desired properties.