Nano-scaled roughness effect on air bubble-hydrophilic surface adhesive strength

Trapping and manipulation of bubbles with holographic optical tweezers

A methodology to manipulate bubbles and measure adhesion forces is presented and validated. Holographic optical tweezers are employed to establish a circular array of high intensity points to effectively trap a gas bubble within a liquid medium. This approach includes an efficient calibration protocol based on a theoretical framework for the calculation of optical forces using a ray tracing algorithm, which allows enhancing the versatility of optical manipulation to micro-objects with a lower refractive index than the surrounding medium. As an initial application, the adhesion force between two stable bubbles at different sizes is measured, finding a minimum when they have the same diameter.

Nanoscale-roughness influence on pull-off adhesion force in liquid and air

The pull-off adhesion force was measured by atomic force microscopy in sphere-plate geometry in water where a capillarylike behavior develops due to nanobubbles and was compared to the corresponding capillary adhesion in air. The sphere and the plate were coated with gold, and the pull-off adhesion force was measured as a function of the evolving surface roughness of the plate, and the retraction velocity of the interacting surfaces. In absolute magnitude, the pull-off force in air is larger than that in liquid by an order of magnitude or more, but in both cases, the pull-off force follows a monotonic decrease with increasing roughness. However, the relative decrement of the adhesion force in water was approximately 300%, and significantly higher than that in air for the same change of the rms roughness in the range ∼7-14 nm. Finally, the adhesion force in water shows a relatively complex dependence on the retraction velocity of the interacting surfaces as the roughness increases due to possible deformation of the nanobubbles and the bridges they form between the surfaces.

Electrochemically Grown Ultrathin Platinum Nanosheet Electrodes with Ultralow Loadings for Energy-Saving and Industrial-Level Hydrogen Evolution

Nanostructured catalyst-integrated electrodes with remarkably reduced catalyst loadings, high catalyst utilization and facile fabrication are urgently needed to enable cost-effective, green hydrogen production via proton exchange membrane electrolyzer cells (PEMECs). Herein, benefitting from a thin seeding layer, bottom-up grown ultrathin Pt nanosheets (Pt-NSs) were first deposited on thin Ti substrates for PEMECs via a fast, template- and surfactant-free electrochemical growth process at room temperature, showing highly uniform Pt surface coverage with ultralow loadings and vertically well-aligned nanosheet morphologies. Combined with an anode-only Nafion 117 catalyst-coated membrane (CCM), the Pt-NS electrode with an ultralow loading of 0.015 mgPt cm-2 demonstrates superior cell performance to the commercial CCM (3.0 mgPt cm-2), achieving 99.5% catalyst savings and more than 237-fold higher catalyst utilization. The remarkable performance with high catalyst utilization is mainly due to the vertically well-aligned ultrathin nanosheets with good surface coverage exposing abundant active sites for the electrochemical reaction. Overall, this study not only paves a new way for optimizing the catalyst uniformity and surface coverage with ultralow loadings but also provides new insights into nanostructured electrode design and facile fabrication for highly efficient and low-cost PEMECs and other energy storage/conversion devices.

Single bubble probe atomic force microscope and impinging-jet technique unravel the interfacial interactions controlled by long chain fatty acid in anaerobic digestion

Anaerobic digestion of lipid-rich wastewater generally suffers from foaming induced by long chain fatty acid (LCFA). However, a systematic understanding of LCFA inhibition, especially the physical inhibition on interfacial interaction still remains unclear. Here, we combined bubble probe atomic force microscope and impinging-jet technique to unravel the interfacial interactions controlled by long chain fatty acids in anaerobic digestion. We showed that LCFA had a significant inhibition on methane production in anaerobic reactors for the inhibition of the conversion of VFAs to methane. By measuring the LCFA influence on methanogenic archaea Methanosarcina acetivorans C2A, the results demonstrated that methanogenesis was limited for substrates utilization but not metabolic pathways. The impinging-jet technique results indicated that LCFA enhanced bubble separation from anaerobic granules and reduced the bubble-bubble coalescence probability. In addition, the bubble probe atomic force microscope (AFM) revealed that LCFA enhanced the adhesion force between bubbles by enhancing electrical double layer (EDL) repulsion and decreasing hydrophobic interactions. Overall, these results complement framework of LCFA inhibition in anerobic digestion and provide a nanomechanical insight into the fundamental interfacial interactions related to bubbles in anaerobic reactors.

Generation and stabilization of CO2 nanobubbles using surfactants for extraction of polyphenols from Camellia oleifera shells

Camellia oleifera shells are abundant in polyphenolic compounds. Green extraction methods of polyphenolic compounds are essential to ensure product quality, efficiency, process cost, environment, and safety. This study investigated the effect of Tween 80 and Rhamnolipid surfactants on the production and utilization of stabilized carbon dioxide nanobubbles (CO₂ -NBs). The results confirmed the presence of the CO₂ -NBs in ultra-pure water with a concentration of 8.45 ± 1.05 × 10⁸  ml^-1 , among which the stable CO₂ -NBs possessed a mean size of 40-90 nm and a negative zeta potential (-41.6 ± 1.3 mV). Further, the efficiency of CO₂ -NBs combined with ultrasonication (CO₂ -NBs-Rh-UAE) was evaluated to extract polyphenols from Camellia oleifera shells (waste). The CO₂ -NBs treatment with ultrasonication showed the highest total phenolic content (TPC) and total flavonoid content (TFC) (36.75 ± 0.22 mg GAE/g DW and 24.06 ± 0.22 mg RE/g DW, respectively). Overall, this study demonstrated an innovative approach for producing, stabilizing, and utilizing biosurfactant stabilized CO₂ -NBs to extract polyphenolic compounds from the waste agricultural products. These findings highlighted the potential application of biosurfactant-stabilized CO₂ -NBs.

Droplet Spreading and Adhesion on Spherical Surfaces

Adhesion of a liquid droplet to a solid surface is a result of solid surface interactions with surrounding fluids, affected by its wettability and morphology. Unfortunately, the direct measurements of adhesion forces are rarely reported in the scientific literature, especially for solids with curvatures. In this study, by using a high-sensitivity microelectronic mechanical balance which vertically deposits and then pulls liquid droplets, the spreading and adhesion forces for water and ethylene glycol droplets on spherical surfaces of polyethylene terephthalate (PET) with radii of curvature from 2 to 8 mm were recorded. Results show that the surface curvature does not affect the advancing and most-stable contact angles but affects the extent of spreading and maximum adhesion forces. The solid surface curvature affects both surface tension and Laplace pressure forces at the spreading point, whereas it mainly affects the Laplace pressure force at the maximum adhesion point.