Reliable measurement of slip using colloid probe atomic force microscopy

Recent research has shown that Newtonian liquids can slip at solid surfaces in confined geometries, which contradicts the classical no-slip boundary condition in which the liquid is stationary at the solid surface. The study of liquid boundary conditions that provides a fundamental understanding of the physics of liquid flow in confined geometries, such as in porous media, and also could benefit various commercial applications, such as micro and nanofluidic applications. The aim of our work was to build a reliable experimental and theoretical framework to investigate liquids slip on solid surfaces by colloid probe atomic force microscopy (AFM). Colloid probe AFM provides an accurate way to study slip at a solid surface by measuring the hydrodynamic drainage force between a colloid probe and a solid substrate as the two surfaces approach to contact. In our studies, we have investigated the slip of a one-component viscous liquid (di-n-octylphthalate) on bare silicon substrates and hydrophobised silicon substrates. In order to obtain reliable slip results, we solved experimental problems in previously published experiments and improved the theoretical modeling which affects the reliability and accuracy of the measured slip lengths. In the new improved experimental protocol we used a closed loop scanner to produce a constant driving velocity, minimised the virtual deflection due to top-scan AFM by removing a constant slope in the force curve, and clarified the true compliance and zero separation in the force curve. The need for tight control over experimental conditions in slip measurements was highlighted, such as extremely careful surface cleaning, the use of a one-component liquid, continuous monitoring of the liquid temperature, and repeat measurements in different locations of the substrate. By performing slip measurements in symmetric and asymmetric systems, a new method was developed to self-assess the accuracy and reproducibility of the slip force measurements. A new mathematical algorithm was built to predict the hydrodynamic drainage force independently of experimental data. This new mathematical algorithm reduced the noise greatly in the theoretical forces over that in the previous treatments; it was demonstrated by blind test that this new calculation method provides reproducible and reliable slip length values and spring constant values with the uncertainty within 3%. The new mathematical algorithm can be easily applied to simulate slip lengths and hydrodynamic forces in different experimental conditions, such as the presence of nanoparticle contamination on the substrate surface and the flattening of the colloid probe, which were both demonstrated to affect the measured slip lengths. The exact variable drag force on soft cantilevers was calculated for the first time and applied to fit the experimental force. This calculation revealed that the dependence of slip on the driving velocity and the cantilever shape found in literature could be a spurious effect due to the assumption that the drag force on the cantilever is constant during force measurements. In our studies, it was also shown that the measured slip length actually decreases with increasing shear rate, rather than being a constant value as commonly assumed. A new shear dependent model for slip fitted well experimental hydrodynamic forces for all separations down to a few nanometres. A possible molecular explanation was proposed for the mechanism of shear rate dependent slip in our experiments

Reliable measurement of slip using colloid probe atomic force microscopy

Recent research has shown that Newtonian liquids can slip at solid surfaces in confined geometries, which contradicts the classical no-slip boundary condition in which the liquid is stationary at the solid surface. The study of liquid boundary conditions that provides a fundamental understanding of the physics of liquid flow in confined geometries, such as in porous media, and also could benefit various commercial applications, such as micro and nanofluidic applications. The aim of our work was to build a reliable experimental and theoretical framework to investigate liquids slip on solid surfaces by colloid probe atomic force microscopy (AFM). Colloid probe AFM provides an accurate way to study slip at a solid surface by measuring the hydrodynamic drainage force between a colloid probe and a solid substrate as the two surfaces approach to contact. In our studies, we have investigated the slip of a one-component viscous liquid (di-n-octylphthalate) on bare silicon substrates and hydrophobised silicon substrates. In order to obtain reliable slip results, we solved experimental problems in previously published experiments and improved the theoretical modeling which affects the reliability and accuracy of the measured slip lengths. In the new improved experimental protocol we used a closed loop scanner to produce a constant driving velocity, minimised the virtual deflection due to top-scan AFM by removing a constant slope in the force curve, and clarified the true compliance and zero separation in the force curve. The need for tight control over experimental conditions in slip measurements was highlighted, such as extremely careful surface cleaning, the use of a one-component liquid, continuous monitoring of the liquid temperature, and repeat measurements in different locations of the substrate. By performing slip measurements in symmetric and asymmetric systems, a new method was developed to self-assess the accuracy and reproducibility of the slip force measurements. A new mathematical algorithm was built to predict the hydrodynamic drainage force independently of experimental data. This new mathematical algorithm reduced the noise greatly in the theoretical forces over that in the previous treatments; it was demonstrated by blind test that this new calculation method provides reproducible and reliable slip length values and spring constant values with the uncertainty within 3%. The new mathematical algorithm can be easily applied to simulate slip lengths and hydrodynamic forces in different experimental conditions, such as the presence of nanoparticle contamination on the substrate surface and the flattening of the colloid probe, which were both demonstrated to affect the measured slip lengths. The exact variable drag force on soft cantilevers was calculated for the first time and applied to fit the experimental force. This calculation revealed that the dependence of slip on the driving velocity and the cantilever shape found in literature could be a spurious effect due to the assumption that the drag force on the cantilever is constant during force measurements. In our studies, it was also shown that the measured slip length actually decreases with increasing shear rate, rather than being a constant value as commonly assumed. A new shear dependent model for slip fitted well experimental hydrodynamic forces for all separations down to a few nanometres. A possible molecular explanation was proposed for the mechanism of shear rate dependent slip in our experiments

Triple condensate halo from water droplets impacting on cold surfaces

Understanding the dynamics in the deposition of water droplets onto solid surfaces is of importance from both fundamental and practical viewpoints. While the deposition of a water droplet onto a heated surface is extensively studied, the characteristics of depositing a droplet onto a cold surface and the phenomena leading to such behavior remain elusive. Here we report the formation of a triple condensate halo observed during the deposition of a water droplet onto a cold surface, due to the interplay between droplet impact dynamics and vapor diffusion. Two subsequent condensation stages occur during the droplet spreading and cooling processes, engendering this unique condensate halo with three distinctive bands. We further proposed a scaling model to interpret the size of each band, and the model is validated by the experiments of droplets with different impact velocity and varying substrate temperature. Our experimental and theoretical investigation of the droplet impact dynamics and the associated condensation unravels the mass and heat transfer among droplet, vapor and substrate, offer a new sight for designing of heat exchange devices

Electron Bunch Train Excited Higher-Order Modes in a Superconducting RF Cavity

Higher-order mode (HOM) based intra-cavity beam diagnostics has been proved effectively and conveniently in superconducting radio-frequency (SRF) accelerators. Our recent research shows that the beam harmonics in the bunch train excited HOM spectrum, which have much higher signal-to-noise ratio than the intrinsic HOM peaks, may also be useful for beam diagnostics. In this paper, we will present our study on bunch train excited HOMs, including the theoretic model and recent experiments carried out based on the DC-SRF photoinjector and SRF linac at Peking University.Comment: Supported by National Natural Science Foundation of China (11275014

Excellent performance of Pt-C/TiO2 for methanol oxidation:contribution of mesopores and partially coated carbon

Partial deposition of carbon onto mesoporous TiO2 (C/TiO2) were prepared as supporting substrate for Pt catalyst development. Carbon deposition is achieved by in-situ carbonization of furfuryl alcohol. The hybrid catalysts were characterized by XRD, Raman, SEM and TEM and exhibited outstanding catalytic activity and stability in methanol oxidation reaction. The heterogeneous carbon coated on mesoporous TiO2 fibers provided excellent electrical conductivity and strong interfacial interaction between TiO2 support and Pt metal nanoparticles. Methanol oxidation reaction results showed that the activity of Pt-C/TiO2 is 3.0 and 1.5 times higher than that of Pt-TiO2 and Pt-C, respectively. In addition, the Pt-C/TiO2 exhibited a 6.7 times enhanced stability compared with Pt-C after 2000 cycles. The synergistic effect of C/TiO2 is responsible for the enhanced activity of Pt-C/TiO2, and its excellent durability could be ascribed to the strong interfacial interaction between Pt nanoparticles and C/TiO2 support

Mechanical properties of Ropaque hollow nanoparticles

The elastic properties and strength upon compression of commercial Ropaque polystyrene hollow particles were investigated by atomic force microscopy (AFM). These particles are commonly used in paints as opacifying agents, as their internal air void effectively scatters light. A sharp AFM tip was used to apply a point load to the particle surface, and increased to probe both the elastic and plastic deformation of the shell, and then further until the shell broke. For small deformations, the deformation increased linearly with applied force. The Young’s modulus was calculated by accounting for the effect of the rigid substrate, and compare the modulus obtained from the Reissner and Hertz models. The minimum stress needed to destroy the integrity of the shell was extracted and found to be smaller than or close to that of silica hollow particles with different shell thickness tested in the literature.Australian Research Council and DuluxGroup Australia through Linkage gran

Self-Lubricating Polytetrafluoroethylene/Polyimide Blends Reinforced with Zinc Oxide Nanoparticles

ZnO nanoparticle reinforced polytetrafluoroethylene/polyimide (PTFE/PI) nanocomposites were prepared and their corresponding tribological and mechanical properties were studied in this work. The influences of ZnO loading, sliding load, and velocity on the tribological properties of ZnO/PTFE/PI nanocomposites were systematically investigated. Results reveal that nanocomposites reinforced with 3 wt% ZnO exhibit the optimal tribological and mechanical properties. Specifically, the wear loss decreased by 20% after incorporating 3 wt% ZnO compared to unfilled PTFE/PI. Meanwhile, the impact strength, tensile strength, and elongation-at-break of 3 wt% ZnO/PTFE/PI nanocomposite are enhanced by 85, 5, and 10% compared to pure PTFE/PI blend. Microstructure investigation reveals that ZnO nanoparticles facilitate the formation of continuous, uniform, and smooth transfer film and thus reduce the adhesive wear of PTFE/PI

Factorized Inverse Path Tracing for Efficient and Accurate Material-Lighting Estimation

Inverse path tracing has recently been applied to joint material and lighting estimation, given geometry and multi-view HDR observations of an indoor scene. However, it has two major limitations: path tracing is expensive to compute, and ambiguities exist between reflection and emission. Our Factorized Inverse Path Tracing (FIPT) addresses these challenges by using a factored light transport formulation and finds emitters driven by rendering errors. Our algorithm enables accurate material and lighting optimization faster than previous work, and is more effective at resolving ambiguities. The exhaustive experiments on synthetic scenes show that our method (1) outperforms state-of-the-art indoor inverse rendering and relighting methods particularly in the presence of complex illumination effects; (2) speeds up inverse path tracing optimization to less than an hour. We further demonstrate robustness to noisy inputs through material and lighting estimates that allow plausible relighting in a real scene. The source code is available at: https://github.com/lwwu2/fiptComment: Updated experiment results; modified real-world section

Bioinspired cellulose-integrated MXene-based hydrogels for multifunctional sensing and electromagnetic interference shielding

Bioinspired hydrogels are complex materials with distinctive properties comparable to biological tissues. Their exceptional sensitivity to various external stimuli leads to substantial application potential in wearable smart devices. However, these multifaceted hydrogels are often challenging to be combined with pattern customization, stimulus responsiveness, self-healing, and biocompatibility. Herein, inspired by mussel secretions, a printable, self-healing, and biocompatible MXene-based composite hydrogel was designed and prepared by incorporating Ti3C2Tx MXene nanosheets into the hydrogel framework through the chelation of calcium ions (Ca2+) with polyacrylic acid and cellulose nanofibers at alkaline conditions. The biocompatible conductive hydrogel exhibited sensitivity (gauge factor of 2.16), self-healing (within 1 s), recognition, and adhesion, distinguishing it as an ideal candidate for wearable multifunctional sensors toward strain sensing, vocal sensing, signature detection, and Morse code transmission. Additionally, the multifunctional hydrogel manifested efficient electromagnetic interference shielding properties (reaching more than 30 dB at a thickness of 2.0 mm), protecting electronics and humans from electromagnetic radiation and pollution. Therefore, the presented work represents a versatile strategy for developing environmentally friendly conductive hydrogels, demonstrating the perspectives of intelligent hydrogels for multifunctional applications