silicon and n-hexadecane

We report on the fabrication of silicon nanostructured superhydrophobic and superoleophobic surfaces also called "superomniphobic" surfaces. For this purpose, silicon interfaces with different surface morphologies, single or double scale structuration, were investigated. These structured surfaces were chemically treated with perfluorodecyltrichlorosilane (PFTS), a low surface energy molecule. The morphology of the resulting surfaces was characterized using scanning electron microscopy (SEM). Their wetting properties: static contact angle (CA) and contact angle hysteresis (CAH) were investigated using liquids of various surface tensions. Despite that we found that all the different morphologies display a superhydrophobic character (CA>150° for water) and superoleophobic behavior (CA ≈ 140° for hexadecane), values of hysteresis are strongly dependent on the liquid surface tension and surface morphology. The best surface described in this study was composed of a dual scale texturation i.e. silicon micropillars covered by silicon nanowires. Indeed, this surface displayed high static contact angles and low hysteresis for all tested liquids.

Topics: Alkanes; Fluorocarbons; Hydrophobic and Hydrophilic Interactions; Microscopy, Electron, Scanning; Nanostructures; Silicon; Surface Tension; Water; Wettability

Whether driven by external mechanical stresses (shear flow) or induced by membrane-active peptides and/or proteins, the collective growth of tubules in membranous fluids has seldom been reported. The pearling destabilization of these membranous tubules which requires an activation of the shape distortion, often induced by optical tweezers, membrane-active biomolecules or an electrical field, has also rarely been observed under mild experimental conditions. Here we report such events of collective tubulation and pearling destabilization in sessile drops of a didodecyl-dimethylammonium bromide (DDAB) vesicular solution that are confined by a surrounding oil medium. Based on the wetting dynamics and the features of the tubulation process, we show that the growth of the tubules here relies on a mechanism of "pinning-induced pulling" from the retracting drop, rather than the classical hydrodynamic fingering instability. We show that the whole tubulation process is driven by a strong coupling between the bulk properties of the ternary (DAAB/water/oil) system and the dynamics of wetting. Finally, we discuss the pearling destabilization of these tubules under vanishing static interface tension and quite mild tensile force arising from their pulling. We show that under those mild conditions, shape disturbances readily grow, either as pearling waves moving toward the drop-reservoir or as Rayleigh-type peristaltic modulations. Besides revealing singular non-Rayleigh pearling modes, this work also brings new insights into the flow dynamics in membranous tubules anchored to an infinite reservoir.

Topics: Alkanes; Chemical Phenomena; Kinetics; Mechanical Phenomena; Microscopy, Video; Models, Chemical; Oils; Quaternary Ammonium Compounds; Rheology; Silicon; Solutions; Squalene; Surface Tension; Tensile Strength; Viscosity; Wetting Agents

To further improve the coverage of organic monolayers on hydrogen-terminated silicon (H-Si) surfaces with respect to the hitherto best agents (1-alkynes), it was hypothesized that enynes (H-C≡C-HC═CH-R) would be even better reagents for dense monolayer formation. To investigate whether the increased delocalization of β-carbon radicals by the enyne functionality indeed lowers the activation barrier, the kinetics of monolayer formation by hexadec-3-en-1-yne and 1-hexadecyne on H-Si(111) were followed by studying partially incomplete monolayers. Ellipsometry and static contact angle measurements indeed showed a faster increase of layer thickness and hydrophobicity for the hexadec-3-en-1-yne-derived monolayers. This more rapid monolayer formation was supported by IRRAS and XPS measurements that for the enyne show a faster increase of the CH2 stretching bands and the amount of carbon at the surface (C/Si ratio), respectively. Monolayer formation at room temperature yielded plateau values for hexadec-3-en-1-yne and 1-hexadecyne after 8 and 16 h, respectively. Additional experiments were performed for 16 h at 80° to ensure full completion of the layers, which allows comparison of the quality of both layers. Ellipsometry thicknesses (2.0 nm) and contact angles (111-112°) indicated a high quality of both layers. XPS, in combination with DFT calculations, revealed terminal attachment of hexadec-3-en-1-yne to the H-Si surface, leading to dienyl monolayers. Moreover, analysis of the Si2p region showed no surface oxidation. Quantitative XPS measurements, obtained via rotating Si samples, showed a higher surface coverage for C16 dienyl layers than for C16 alkenyl layers (63% vs 59%). The dense packing of the layers was confirmed by IRRAS and NEXAFS results. Molecular mechanics simulations were undertaken to understand the differences in reactivity and surface coverage. Alkenyl layers show more favorable packing energies for surface coverages up to 50-55%. At higher coverages, this packing energy rises quickly, and there the dienyl packing becomes more favorable. When the binding energies are included the difference becomes more pronounced, and dense packing of dienyl layers becomes more favorable by 2-3 kcal/mol. These combined data show that enynes provide the highest-quality organic monolayers reported on H-Si up to now.

Topics: Alkanes; Alkynes; Hydrogen; Molecular Structure; Silicon; Surface Properties

Self-assembled monolayers of fatty acids were formed on stainless steel by room-temperature solution deposition. The acids are covalently bound to the surface as carboxylate in a bidentate manner. To explore the effect of saturation in the carbon backbone on friction in sliding tribology, we study the response of saturated stearic acid (SA) and unsaturated linoleic acid (LA) as self-assembled monolayers using lateral force microscopy and nanotribometry and when the molecules are dispersed in hexadecane, using pin-on-disc tribometry. Over a very wide range (10 MPa-2.5 GPa) of contact pressures it is consistently demonstrated that the unsaturated linoleic acid molecules yield friction which is significantly lower than that of the saturated stearic acid. It is argued, using density functional theory predictions and XPS of slid track, that when the molecular backbone of unsaturated fatty acids are tilted and pressed strongly by a probe, in tribological contact, the high charge density of the double bond region of the backbone allows coupling with the steel substrate. The interaction yields a low friction carboxylate soap film on the substrate. The saturated fatty acid does not show this effect.

Topics: Alkanes; Fatty Acids; Friction; Linoleic Acid; Lubrication; Microscopy, Atomic Force; Silicon; Spectrometry, X-Ray Emission; Spectroscopy, Fourier Transform Infrared; Stainless Steel; Stearic Acids