Fabrication and Application of Halloysite Nanotube-Embedded Photocatalytic Nanofibers with Antibacterial Properties.

With decreasing indoor air quality, increased time spent at indoors, and especially with the COVID-19 pandemic, the development of new materials for bacteria and viruses has become even more important. Less material consumption due to the electrospinning process, the easy availability/affordability of the halloysite nanotube (HNT), and the antibacterial effect of both TiO2 and ZnO nanoparticles make the study even more interesting. HNTs have attracted research attention in recent years due to their low cost, high mechanical strength, natural and environmentally friendly structure, and non-toxicity to human health and ecosystem. In this study, HNT-embedded composite nanofiber filters were fabricated as filter materials using the electrospinning method. Photocatalysts (TiO2 and ZnO) were incorporated into the composite nanofibers by the electrospraying method. The results showed that the combination of both HNT/TiO2 and HNT/ZnO additives was successfully integrated into the filter structure. The effect of embedding the HNT and spraying photocatalysts enables the fabrication of composite filters with lower pressure drop, high filtration efficiency, improved mechanical properties, and high antibacterial properties against Escherichia coli, making the nanofibers suitable and promising for face masks and air filter materials.

Effect of contact angle hysteresis on the removal of the sporelings of the green alga Ulva from the fouling-release coatings synthesized from polyolefin polymers.

Wettability is one of the surface characteristics that is controlled by the chemical composition and roughness of a surface. A number of investigations have explored the relationship between water contact angle and surface free energy of polymeric coatings with the settlement (attachment) and adhesion strength of various marine organisms. However, the relationship between the contact angle hysteresis and fouling-release property is generally overlooked. In the present work, coatings were prepared by using commercial hydrophobic homopolymer and copolymer polyolefins, which have nearly the same surface free energy. The effects of contact angle hysteresis, wetting hysteresis, and surface free energy on the fouling-release properties for sporelings of the green alga Ulva from substrates were then examined quantitatively under a defined shear stress in a water channel. The ease of removal of sporelings under shear stress from the polymer surfaces was in the order of PP>HDPE>PPPE>EVA-12 and strongly and positively correlated with contact angle and wetting hysteresis; i.e., the higher the hysteresis, the greater the removal.

Range of applicability of the Wenzel and Cassie-Baxter equations for superhydrophobic surfaces.

The Wenzel and Cassie-Baxter equations depending on the extent of liquid/solid interfacial contact area were generally used to estimate water contact angles on superhydrophobic surfaces. In this study, a simple method is proposed on the criterion to use the Wenzel and Cassie-Baxter equations to evaluate the contact angle results on superhydrophobic surfaces. In this method, the difference between the theoretical (geometric) and experimental contact angle-dependent Wenzel roughness parameter, Delta r(w), and Cassie-Baxter solid/liquid contact area fraction, Delta f(s)(CB) were determined, and the validity of these equations was evaluated. We used the data of eight recent publications where the water drop sits on square and cylindrical pillar structured superhydrophobic model surfaces. We evaluated the contact angle results of 166 patterned samples with our method. We also found that the effect of contact angle error margins was low to vary these parameters. In general, the use of the Wenzel equation was found to be wrong for most of the samples (74% of the samples for cylindrical and 58% for square pillar patterned surfaces), and the deviations from the theory were also high for the remaining (26% for cylindrical and 42% for square) samples, and it is concluded that the Wenzel equation cannot be used for superhydrophobic surfaces other than a few exceptions, especially for cylindrical patterns. For the Cassie-Baxter equation, two situations are possible: for positive Delta f(s)(CB), there is only a partial contact of the drop with the top solid surface, and, for negative Delta f(s)(CB), the penetration of the drop in between the pillars is possible, and thus the liquid drop is in contact with the lateral sides of the pillars. We found that 65% of the samples containing cylindrical pillars (52-77% with error margins) and 44% of the samples containing square pillars (38-50% with error margins) resulted in negative Delta f(s)(CB)(red) values. In addition, large deviations of experimental water contact angle results, theta r(e) from the theoretical theta r(CB) were also determined for most of the samples, indicating that the Cassie-Baxter equation should be applied to superhydrophobic surfaces with caution.