What is the value of water contact angle on silicon?

Silicon is a widely applied material and the wetting of silicon surface is an important phenomenon. However, contradictions in the literature appear considering the value of the water contact angle (WCA). The purpose of this study is to present a holistic experimental and theoretical approach to the WCA determination. To do this, we checked the chemical composition of the silicon (1,0,0) surface by using the X-ray photoelectron spectroscopy (XPS) method, and next this surface was purified using different cleaning methods. As it was proved that airborne hydrocarbons change a solid wetting properties the WCA values were measured in hydrocarbons atmosphere. Next, molecular dynamics (MD) simulations were performed to determine the mechanism of wetting in this atmosphere and to propose the force field parameters for silica wetting simulation. It is concluded that the best method of surface cleaning is the solvent-reinforced de Gennes method, and the WCA value of silicon covered by SiO2 layer is equal to 20.7° (at room temperature). MD simulation results show that the mechanism of pure silicon wetting is similar to that reported for graphene, and the mechanism of silicon covered by SiO2 layer wetting is similar to this observed recently for a MOF

Revisiting wetting, freezing, and evaporation mechanisms of water on copper

Wetting of metal surfaces plays an important role in fuel cells, corrosion science, and heat-transfer devices. It has been recently stipulated that Cu surface is hydrophobic. In order to address this issue we use high purity (1 1 1) Cu prepared without oxygen, and resistant to oxidation. Using the modern Fringe Projection Phase-Shifting method of surface roughness determination, together with a new cell allowing the vacuum and thermal desorption of samples, we define the relation between the copper surface roughness and water contact angle (WCA). Next by a simple extrapolation, we determine the WCA for the perfectly smooth copper surface (WCA = 34°). Additionally, the kinetics of airborne hydrocarbons adsorption on copper was measured. It is shown for the first time that the presence of surface hydrocarbons strongly affects not only WCA, but also water droplet evaporation and the temperature of water droplet freezing. The different behavior and features of the surfaces were observed once the atmosphere of the experiment was changed from argon to air. The evaporation results are well described by the theoretical framework proposed by Semenov, and the freezing process by the dynamic growth angle model

Electrophoretic deposition of layer-by-layer unsheathed carbon nanotubes - A step towards steerable surface roughness and wettability

It is well known that carbon nanotube (CNT) oxidation (usually with concentrated HNO3) is a major step before the electrophoretic deposition (EPD). However, the recent discovery of the “onion effect” proves that multiwalled carbon nanotubes are not only oxidized, but a simultaneous unsheathing process occurs. We present the first report concerning the influence of unsheathing on the properties of the thus-formed CNT surface layer. In our study we examine how the process of gradual oxidation/unsheathing of a series of multiwalled carbon nanotubes (MWCNTs) influences the morphology of the surface formed via EPD. Taking a series of well-characterized and gradually oxidized/unsheathing Nanocyl™ MWCNTs and performing EPD on a carbon fiber surface, we analyzed the morphology and wettability of the CNT surfaces. Our results show that the water contact angle could be gradually changed in a wide range (125–163°) and the major property determining its value was the diameter of aggregates formed before the deposition process in the solvent. Based on the obtained results we determined the parameters having a crucial influence on the morphology of created layers. Our results shed new light on the deposition mechanism and enable the preparation of surfaces with steerable roughness and wettability

Water nanodroplet on a hydrocarbon “Carpet”—The mechanism of water contact angle stabilization by airborne contaminations on Graphene, Au, and PTFE surfaces

Wetting is very common phenomenon, and it is well documented that the wettability of a solid depends on the surface density of adsorbed airborne hydrocarbons. This “hydrocarbon hypothesis” has been experimentally confirmed for different surfaces, for example, graphene, TiO2, and SiO2; however, there are no scientific reports describing the influence of airborne contaminants on the water contact angle (WCA) value measured on the polytetrafluoroethylene (PTFE) surface. Using experimental data showing the influence of airborne hydrocarbons on the wettability of graphene, gold and PTFE by water, together with Molecular Dynamics simulation results we prove that the relation between the WCA and the surface concentration of hydrocarbons (n-decane, n-tridecane, and n-tetracosane) is more complex than has been assumed up until now. We show, in contrast to commonly approved opinion, that adsorbed hydrocarbons can increase (graphene, Au) or decrease (PTFE) the WCA of a nanodroplet sitting on a surface. Using classical thermodynamics, a simple theoretical approach is developed. It is based on two adsorbed hydrocarbon states, namely, “carpet” and “dimple”. In the “carpet” state a uniform layer of alkane molecules covers the entire substrate. In contrast, in the “dimple” state, the preadsorbed layer of alkane molecules covers only the open surface. Simple thermodynamic balance between the two states explains observed experimental and simulation results, forming a good starting point for future studies

The Origin of Amphipathic Nature of Short and Thin Pristine Carbon Nanotubes—Fully Recyclable 1D Water-in-Oil Emulsion Stabilizers

Short and thin pristine carbon nanotubes (CNTs) emerge as 1D emulsion stabilizers capable of replacing aquatoxic low‐molecular surfactants. However, inconsistencies in understanding of water–solid interfaces for realistic CNTs hamper their individualization‐driven functionalities, processability in benign media, and compatibility with a broad‐scale of matrices. Pristine CNT processing based on water and inexpensive n‐alkanes within a low energy regime would constitute an important step toward greener technologies. Therefore, structural CNT components are quantitatively assessed, placing various CNTs on the scale from hydrophobicity to hydrophilicity. This structural interweave can lead to amphipathicity enabling the formation of water‐in‐oil emulsions. Combining experiments with theoretical studies, CNTs and CNT emulsions are comprehensively characterized establishing descriptors of the emulsifying behavior of pristine and purified CNTs. They emerge as having hydrophilic open‐ends, small number of oxygen–functionalized/vacancy surface areas, and hydrophobic sidewalls and full caps. The interplay of these regions allows short and thin CNTs to be utilized as fully recyclable 1D surfactants stabilizing water/oil emulsions which, as demonstrated, can be applied as paints for flexible conductive coatings. It is also shown how the amphipathic strength depends on CNT size, the pristine‐to‐oxidized/vacancy domains and the oil‐to‐water ratios

Are nanohedgehogs thirsty? Toward new superhydrophobic and anti-icing carbon nanohorn-polymer hybrid surfaces

85% of the world’s polymers production is made up of thermoplastics, and among them, one can find: polyethylene (PE), polypropylene (PP) and polyethylene terephthalate (PET). In this study, these three polymers were applied for the first time as supports for superhydrophobic and anti-icing surfaces created using single walled carbon nanohorns (SWCNH). A facile method of thermal “feathering” was used and the obtained materials were deeply characterized using thermal (thermogravimetry, differential scanning calorimetry) and microscopic (scanning electron microscopy, atomic force microscopy, confocal microscopy) methods as well as tribological and nanoindentation tests. We show that, depending on the polymer, the process of thermal “bulk” or “surface feathering” occurs. The results supported by Hansen Solubility Parameters calculation combined with Molecular Dynamics Simulations allow to explain the stability of surfaces as well as the wetting behavior revealing that all new materials are superhydrophobic. Moreover, some of them exhibit ice-phobic properties, while droplet freezing process is well described by the dynamic growth angle model. Finally, by discussing the mechanical stability, superhydrophobic and anti-icing properties two optimal surfaces (PP + SWCNH 5 min, PE + SWCNH 10 min) are chosen. Both of them possess intermediate values of Young’s modulus and show intermediate values of static contact angle hysteresis. Since the PE-based solids show enhanced tribological performance, tests, PE + SWCNH 10 min sample emerges as the most perspective in practical hydrophobic and anti-icing applications