Bubbles nucleating on superhydrophobic micropillar arrays under flow

Solutal Marangoni force controls lateral motion of electrolytic gas bubbles

Electrochemical gas-evolving reactions have been widely used for industrial energy conversion and storage processes. Gas bubbles form frequently at the electrode surface due to a small gas solubility, thereby reducing the effective reaction area and increasing the over-potential and ohmic resistance. However, the growth and motion mechanisms for tiny gas bubbles on the electrode remains elusive. Combining molecular dynamics (MD) and fluid dynamics simulations (CFD), we show that there exists a lateral solutal Marangoni force originating from an asymmetric distribution of dissolved gas near the bubble. Both MD and CFD simulations deliver a similar magnitude of the Marangoni force of ∼0.01 nN acting on the bubble. We demonstrate that this force may lead to lateral bubble oscillations and analyze the phenomenon of dynamic self-pinning of bubbles at the electrode boundary.

Generating Stable Nitrogen Bubble Layers on Poly(methyl methacrylate) Films by Photolysis of 2-Azidoanthracene

2-Azidoanthracene (2N3-AN) can act as a photochemical source of N2 gas when dissolved in an optically transparent polymer such as poly(methyl methacrylate) (PMMA). Irradiation at 365 or 405 nm of a 150 μm-thick polymer film submerged in water causes the rapid appearance of a surface layer of bubbles. The rapid appearance of surface bubbles cannot be explained by normal diffusion of N2 through the polymer and likely results from internal gas pressure buildup during the reaction. For an azide concentration of 0.1 M and a light intensity of 140 mW/cm2, the yield of gas bubbles is calculated to be approximately 40%. The dynamics of bubble growth depend on the surface morphology, light intensity, and 2N3-AN concentration. A combination of nanoscale surface roughness, high azide concentration, and high light intensity is required to attain the threshold N2 gas density necessary for rapid, high-yield bubble formation. The N2 bubbles adhered to the PMMA surface and survived for days under water. The ability to generate stable gas bubbles "on demand" using light permits the demonstration of photoinduced flotation and patterned bubble arrays.

Nanobubbles explain the large slip observed on lubricant-infused surfaces

Lubricant-infused surfaces hold promise to reduce the huge frictional drag that slows down the flow of fluids at microscales. We show that infused Teflon wrinkled surfaces induce an effective slip length 50 times larger than expected based on the presence of the lubricant alone. This effect is particularly striking as it occurs even when the infused lubricant’s viscosity is several times higher than that of the flowing liquid. Crucially, the slip length increases with increasing air content in the water but is much higher than expected even in degassed and plain Milli-Q water. Imaging directly the immersed interface using a mapping technique based on atomic force microscopy meniscus force measurements reveals that the mechanism responsible for this huge slip is the nucleation of surface nanobubbles. Using a numerical model and the height and distribution of these surface nanobubbles, we can quantitatively explain the large fluid slip observed in these surfaces.

Why are lubricant-infused surfaces so effective at reducing drag in microfluidic flow? Here, authors reveal that infused nanostructured Teflon wrinkles induce large interfacial slip due to the spontaneous nucleation of surface nanobubbles, a mechanism likely to occur on most rough infused surfaces.

Fabrication of a waterborne, superhydrophobic, self-cleaning, highly transparent and stable surface

Superhydrophobic surfaces have received tremendous attention worldwide. However, the synthesis of a superhydrophobic surface possessing two paradoxical characteristic properties - stability and transparency, is a vital aspect that has been addressed in this paper. The surface was fabricated by an environmentally friendly process, which used distilled water for the dissolution of SiO2 nanoparticles in the presence of surfactants, instead of organic solvents. Moreover, the surface was transparent and had self-cleaning properties and stability. The optimal balance of roughness and multi-porous structure imparted excellent transparency to this surface. Importantly, both the conformal coating and the SiO2 nanoparticles embedded in the half solidified conformal coating contributed to the excellent stability, thus overcoming the paradox. The surface could withstand a temperature of 150 °C for 24 h and also different temperature regimes between 0-200 °C for 2 h. In addition, this surface could resist repeated scratches and abrasion as well as strong acids and alkali. The surface achieved its self-cleaning ability due to the introduction of surfactants containing the F element. This simple but novel strategy and surface have the advantages of high safety, low cost and environmental-friendliness.