Hydrophobicity in clearwing butterflies and moths: impact of scale 2 micro and nanostructure, and trade-off with optical transparency

In opaque butterflies and moths, scales ensure vital functions like camouflage, thermoregulation, and hydrophobicity. Wing transparency in some species - achieved via modified or absent scales - raises the question of whether hydrophobicity can be maintained and of it dependence on scale microstructural (scale presence, morphology, insertion angle, and coloration) and nanostructural (ridge spacing and width) features. To address these questions, we assessed hydrophobicity in 23 clearwing species differing in scale micro and nanofeatures by measuring static contact angle (CA) of water droplets in the opaque and transparent patches of the same individuals at different stages of evaporation. We related these measures to wing structures (macro, micro, and nano) and compared them to predictions from Cassie-Baxter and Wenzel models. We found that overall, transparency is costly for hydrophobicity and this cost depends on scale microstructural features: transparent patches are less hydrophobic and lose more hydrophobicity with water evaporation than opaque patches. This loss is attenuated for higher scale densities, coloured scales (for erect scales), and when combining two types of scales (piliform and lamellar). Nude membranes show lowest hydrophobicity. Best models are Cassie-Baxter models that include scale microstructures for erect scales, and scale micro and nanostructures for flat scales. All findings are consistent with the physics of hydrophobicity, especially on multiscale roughness. Finally, wing hydrophobicity negatively relates to optical transparency. Moreover, tropical species have more hydrophobic transparent patches but similarly hydrophobic opaque patches compared to temperate species. Overall, diverse microstructures are likely functional compromises between multiple requirements.

Observation of contact angle hysteresis due to inhomogeneous electric fields

Static contact angle hysteresis determines droplet stickiness on surfaces, and is widely attributed to surface roughness and chemical contamination. In the latter case, chemical defects create free-energy barriers that prevent the contact line motion. Electrowetting studies have demonstrated the similar ability of electric fields to alter the surface free-energy landscape. Yet, the increase of apparent static contact angle hysteresis by electric fields remains unseen. Here, we report the observation of electrowetting hysteresis on micro-striped electrodes. Unlike most experiments with stripes, the droplet spreading on the substrate is experimentally found to be isotropic, which allows deriving a simple theoretical model of the contact angle hysteresis depending the applied voltage. This electrowetting hysteresis enables the continuous and dynamic control of contact angle hysteresis, not only for fundamental studies but also to manufacture sticky-on-demand surfaces for sample collection.

Transparent and Hard Siloxane Based Hybrid UV-Curable Coating Materials with Amphiphobic Properties

In this study, highly transparent siloxane-based hybrid UV-curable coating materials were prepared using (acryloxypropyl)methylsiloxane monomer (APMS), a thiol-ene monomer, with benzoin ethyl ether. For the thiol-ene monomer, either pentaerythritol tetrakis(3-mercaptopropionate) (PETTMP) or trimethylolpropane tris(3-mercaptopropionate) (TMPTMP) was used. The siloxane-based hybrid coating materials were highly transparent and hard (pencil hardness of 6–7H). The materials were also amphiphobic, with a water static contact angle of 92–100° and an oil contact angle of 46–63°, when prepared with a high siloxane-monomer-to-PETTMP/TMPTMP ratio. In general, both hybrid coating materials exhibited improved oleophobicity, high hardness, and surface smoothness with increasing siloxane content, although the TMPTMP-based hybrid coating films exhibited slightly higher oleophobicity (lower hydrophobicity) and a smoother surface than the PETTMP-based hybrid coating films.

An effective polymer nanocomposite based on tetraethoxysilane (TEOS) and SiO2-Al2O3 nanoparticles for super protection of damaged ancient Egyptian wall paintings

Purpose This contribution aims to introduce an effective low cost polymer-nanocomposite for possible application to achieve a super protection for highly damaged ancient Egyptian wall paintings. Design/methodology/approach SiO2 and Al2O3 nanoparticles were synthesized by the sol-gel method. Then, the polymer-nanocomposite was prepared by simple mixing and dispersing the nanoparticles into the tetraethoxysilane polymer solution, with the aid of an ultrasonic dismembrator. The application of the polymer-nanocomposite and other polymeric nanodispersions, on laboratory models, was performed by the brushing technique. Next, the materials stability was evaluated by means of digital optical microscope, colorimetry, FE-scanning electron microscope, measuring the static contact angle and water absorption rates. Findings The results were promising in creating a superhydrophobicity and the static contact angle (?S) measured for the polymer-nanocomposite reached 135o. An average of three measurements of the water absorption rate after polymer-nanocomposite treatment was 0.66 g/m2 s, compared to 2.60 g/m2 s for the control model (untreated). Further, an average of color difference (?E*) for the treated surface was 2.78, and after the accelerated thermal aging was 3.6. Observing the surface morphology, the polymer-nanocomposite enhanced the roughness of the treated surface and showed a high resistance to laboratory salt weathering. Practical implications Preparation of a polymer-nanocomposite by adding SiO2 and Al2O3 NPs to tetraethoxysilane polymer has been proposed. As a promising conservation material, the produced polymer-nanocomposite helped to form an efficient protective film. Originality/value This paper attains to develop an economic polymer-nanocomposite to maintain a high protection to damaged ancient Egyptian wall paintings and similar objects.

Superhydrophobic fabric mixed with polyester and cotton yarns modified by alkaline treatment and thermal aging

Superhydrophobic fabric composed of polyester and cotton single yarns was developed by alkali treatment and thermal aging. During the alkali treatment to make the nano-roughness of the polyester fibers, micro-roughness also increased due to differences in the thicknesses of the two yarns arising from the increased polyester surface roughness and swollen cotton. The superhydrophobicity, with a static contact angle of 155.8 ± 3.2° and shedding angle of 11.1 ± 0.8°, was achieved with 90% polyester/10% cotton fabric treated with 20% alkali concentration for 20 min under applied tension, then followed by 24 h thermal aging at 130℃. The tensile strength of the superhydrophobic polyester/cotton fabric (28.7 MPa) was higher than that of 100% polyester fabric (20.1 MPa). The breathability of the superhydrophobic polyester/cotton fabric was improved compared with 100% polyester fabric. In durability assessment, a static contact angle of ≥150° was shown for the tape tests. Five times of repeated adhesion with a clothing tape cleaner were conducted for the five samples each. Although washing and dry-cleaning decreased contact angles to as low as 137.7°, a static contact angle of 150° was achieved by additional thermal aging (130℃, 24 h). We developed a superhydrophobic fabric mixed with polyester and cotton yarns by exploiting differences in the characteristics of the two yarns induced by alkali treatment, which causes fabric surface roughness, and thermal aging without the use of any chemicals. Moreover, this superhydrophobic fabric has improved breathability.

Numerical Study of Formation of a Series of Bubbles from an Orifice by Applying Dynamic Contact Angle Model

The dynamic contact angle model is applied in the formation process of a series of bubbles from Period-I regime to Period-II regime by using the VOF method on a 2D axisymmetric domain. In the first process of the current research, the dynamic contact angle model is validated by comparing the numerical results to the experimental data. Good agreement in terms of bubble shape and bubble detachment time is observed from a lower flow rate Q = 150.8 cm3/min (Re = 54.77, Period-I regime) to a higher flow rate Q = 603.2 cm3/min (Re = 219.07, Period-III regime). The comparison between the dynamic contact angle model and the static contact angle model is also performed. It is observed that the static contact angle model can obtain similar results as the dynamic contact angle model only for smaller gas flow rates (Q ≤ 150.8 cm3/min and Re ≤ 54.77)). For higher gas flow rates, the static contact angle model cannot produce good results as the dynamic contact angle model and has larger relative errors in terms of bubble detachment time and bubble shape.

Direct observation of the attachment behavior of hydrophobic colloidal particles onto a bubble surface

This article addresses the attachment behavior of a single particle onto a bubble from a microscopic view, in which a hydrophobic particle abruptly “jumps into” a bubble to satisfy its static contact angle.