Statistical contact angle analyses; “slow moving” drops on a horizontal silicon-oxide surface

Robust Contact Angle Determination for Needle-in-Drop Type Measurements

One of the main approaches for contact angle determination using sessile drops with a missing apex (e.g., because of the presence of the needle tip) is the polynomial drop-profile fitting method. The major disadvantage of this fitting procedure is that the derived contact angle is highly sensitive to the polynomial order and the number of pixels involved in the actual fit. In the present work, an easily implementable method is introduced to effectively tackle these drawbacks. Instead of fitting the drop-profile itself, the polynomial fitting is applied to the difference between the drop profile and the circumcircle, independently for both sides of the drop. The derivative value of this difference at the contact point is used to correct the slope obtained analytically from the circumcircle. It is shown that this approach allows the robust determination of the contact angle with high (≤0.6°) accuracy in a straightforward manner, and the results are not affected by the actual contact angle, drop volume, or the resolution of the captured image. Validation of this new approach is also given in the contact angle range of 20°-150° by comparing the results to the values calculated by the Young-Laplace fit.

Silane Deposition via Gas-Phase Evaporation and High-Resolution Surface Characterization of the Ultrathin Siloxane Coatings

Siloxane coatings for surfaces are essential in many scientific and industrial applications. We describe a straightforward gas-phase evaporation technique in inert atmosphere and introduce a practical and reliable silanization protocol adaptable to different silane types. The primary aim of depositing ultrathin siloxane films on surfaces is to enable a reproducible and homogenous surface functionalization without agglomeration effects during the layer formation. To realize high-quality and large-area coatings, it is fundamental to understand the reaction conditions of the silanes, the process of the siloxane layer formation, and the possible influence of the substrate morphology. We used three typical silane types to exemplify the potential and versatility of our process: aminopropyltriethoxysilane, glycidoxypropyltrimethoxysilane, and 1 H,1 H,2 H,2 H-perfluorooctyl-trichlorosilane. The ultrathin siloxane layers, which are generally difficult to characterize, were precisely investigated with high-resolution surface-characterization methods to verify our concept in terms of reproducibility and coating quality. Our results show that this gas-phase evaporation protocol is easily adaptable to all three, widely used silane types also enabling a large-area upscale.

Advancing and receding contact angle investigations for highly sticky and slippery aluminum surfaces fabricated from nanostructured anodic oxide

The fabrication of sticky and slippery superhydrophobic aluminum was achieved by anodizing in pyrophosphoric acid and modification with self-assembled monolayers (SAMs). In addition, the corresponding sliding behaviors of a water droplet were investigated by contact angle measurements and direct observations. For the formation of anodic alumina nanofibers, 4N aluminum plates were anodized in a concentrated pyrophosphoric acid solution at 25–75 V. The morphology of the anodic oxide successively changed to barrier oxide, porous oxide, nanofibers, bundle structures with many nanofibers, and then weak nanofibers during anodizing. The anodized specimens were immersed in a fluorinated phosphonic acid/ethanol solution to form SAMs on the surface of the anodic oxide. The contact angle hysteresis drastically changed with anodizing time: it increased with the formation of porous oxide, decreased for the nanofibers and bundle structures, and then increased once again for the weak nanofibers. Correspondingly, the adhesion interaction between the water droplet and the aluminum surface also drastically changed to show sticky, slippery, and sticky behaviors with anodizing time. More sticky and slippery aluminum surfaces can be obtained by anodizing at higher voltages. The slippery behavior was further improved through two distinct anodizing processes with the formation of ordered alumina nanofibers. A superhydrophobic aluminum surface with coexisting sticky and slippery properties was fabricated by the selective anodizing method.

Highly sticky and slippery superhydrophobic aluminum surfaces were successfully fabricated by pyrophosphoric acid anodizing.