Abstract
Superhydrophobic surfaces have attracted considerable attention because of their unique water-repellency and their wide range of applications. The conventional method to characterize the surface wetting properties of surfaces, including superhydrophobic surfaces, relies on measuring static and dynamic contact angles, and sliding angles of water drops. However, because of the inhomogeneities inherently present on surfaces (smooth and textured), such optical methods can result in relatively large variability in sliding angle measurements. In this work, by using a force-based technique with ±1 μN sensitivity, the friction force between water drops and various surfaces is measured. The friction force can then be used to accurately predict the sliding angle of water drops of various sizes with improved consistency. We also show that the measured friction force can be used to determine the critical drop size below which a water drop is not expected to slide even at a tilt angle of 90°. The proposed technique to characterize the wetting properties of surfaces has a higher accuracy (between 15% and 65%, depending on the surface) compared to optical methods.
Highlights
Lotus leaf surfaces have inspired scientists to gain a better understanding of the basic science and mechanisms behind their water-repellant properties and to fabricate biomimetic superhydrophobic (SH) surfaces.[1]
A preload corresponding to the weight of the drop is applied and the drop is allowed to equilibrate for 20 s, which mimics the interaction of the water drop on the surface under its own weight
We again note the improved accuracy of the force-based measurement compared to the conventional technique as shown by the smaller standard deviations obtained for the predicted sliding angle (PSA) measurements
Summary
Lotus leaf surfaces have inspired scientists to gain a better understanding of the basic science and mechanisms behind their water-repellant properties and to fabricate biomimetic superhydrophobic (SH) surfaces.[1]. Several research groups have explored force-based techniques to characterize the wetting properties of surfaces.[22−25] In a forcebased measurement, a water drop is sheared against a surface over a predetermined distance and velocity, while the friction force is monitored. Shi et al.[32] proposed a new technique to accurately characterize the wetting properties of surfaces They utilized a capillary sensor (with ±0.7−2 μN sensitivity) attached to a water drop to measure the friction force between the drop and solid surfaces. A force-based approach is proposed to predict the SA of water drops on surfaces with greater accuracy compared to optical-based techniques.
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