When and how self-cleaning of superhydrophobic surfaces works

Despite the enormous interest in superhydrophobicity for self-cleaning, a clear picture of contaminant removal is missing, in particular, on a single-particle level. Here, we monitor the removal of individual contaminant particles on the micrometer scale by confocal microscopy. We correlate this space- and time-resolved information with measurements of the friction force. The balance of capillary and adhesion force between the drop and the contamination on the substrate determines the friction force of drops during self-cleaning. These friction forces are in the range of micro-Newtons. We show that hydrophilic and hydrophobic particles hardly influence superhydrophobicity provided that the particle size exceeds the pore size or the thickness of the contamination falls below the height of the protrusions. These detailed insights into self-cleaning allow the rational design of superhydrophobic surfaces that resist contamination as demonstrated by outdoor environmental (>200 days) and industrial standardized contamination experiments.

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Mapping micrometer-scale wetting properties of superhydrophobic surfaces

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1916772116 ◽

2019 ◽

Vol 116 (50) ◽

pp. 25008-25012 ◽

Cited By ~ 4

Author(s):

Dan Daniel ◽

Chee Leng Lay ◽

Anqi Sng ◽

Coryl Jing Jun Lee ◽

Darren Chi Jin Neo ◽

...

Keyword(s):

Contact Angles ◽

Superhydrophobic Surfaces ◽

Atomic Force Microscope Cantilever ◽

Time Resolved ◽

Friction Forces ◽

Wetting Properties ◽

Angle Measurements ◽

Atomic Force ◽

Contact Angle Measurements ◽

Micrometer Scale

There is a huge interest in developing superrepellent surfaces for antifouling and heat-transfer applications. To characterize the wetting properties of such surfaces, the most common approach is to place a millimetric-sized droplet and measure its contact angles. The adhesion and friction forces can then be inferred indirectly using Furmidge’s relation. While easy to implement, contact angle measurements are semiquantitative and cannot resolve wetting variations on a surface. Here, we attach a micrometric-sized droplet to an atomic force microscope cantilever to directly measure adhesion and friction forces with nanonewton force resolutions. We spatially map the micrometer-scale wetting properties of superhydrophobic surfaces and observe the time-resolved pinning–depinning dynamics as the droplet detaches from or moves across the surface.

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Picosecond time-resolved photon antibunching measures nanoscale exciton motion and the true number of chromophores

Nature Communications ◽

10.1038/s41467-021-21474-z ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Gordon J. Hedley ◽

Tim Schröder ◽

Florian Steiner ◽

Theresa Eder ◽

Felix J. Hofmann ◽

...

Keyword(s):

Rational Design ◽

Three Dimensional ◽

Dna Origami ◽

Fluorescent Nanoparticles ◽

Time Resolved ◽

Exciton Diffusion ◽

Polymer Aggregates ◽

True Number ◽

Particle Level ◽

Photon Antibunching

AbstractThe particle-like nature of light becomes evident in the photon statistics of fluorescence from single quantum systems as photon antibunching. In multichromophoric systems, exciton diffusion and subsequent annihilation occurs. These processes also yield photon antibunching but cannot be interpreted reliably. Here we develop picosecond time-resolved antibunching to identify and decode such processes. We use this method to measure the true number of chromophores on well-defined multichromophoric DNA-origami structures, and precisely determine the distance-dependent rates of annihilation between excitons. Further, this allows us to measure exciton diffusion in mesoscopic H- and J-type conjugated-polymer aggregates. We distinguish between one-dimensional intra-chain and three-dimensional inter-chain exciton diffusion at different times after excitation and determine the disorder-dependent diffusion lengths. Our method provides a powerful lens through which excitons can be studied at the single-particle level, enabling the rational design of improved excitonic probes such as ultra-bright fluorescent nanoparticles and materials for optoelectronic devices.

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Spatiotemporal imaging of anisotropic charge transfer in photocatalyst particles

10.21203/rs.3.rs-251007/v1 ◽

2021 ◽

Author(s):

Can Li ◽

Ruotian Chen ◽

Zefeng Ren ◽

Yu Liang ◽

Thomas Dittrich ◽

...

Keyword(s):

Charge Transfer ◽

Rational Design ◽

Catalytic Surface ◽

Time Resolved ◽

Transfer Processes ◽

Ballistic Electron ◽

Photogenerated Electrons ◽

Particle Level ◽

Efficient Charge ◽

Charge Transfer Dynamics

Abstract Water-splitting reactions using photocatalyst particles are promising routes for solar fuel production1-4. Photoinduced charge transfer from a photocatalyst to catalytic surface sites is key in ensuring photocatalytic efficiency5; however, it is challenging to understand this process, which spans a wide spatiotemporal range from nanometers to micrometers and from femtoseconds to seconds6-8. Although the steady-state charge distribution on single photocatalyst particles has been mapped using microscopic techniques9-11 and the averaged charge transfer dynamics in photocatalyst aggregations have been revealed via time-resolved spectroscopy12,13, spatiotemporally evolving charge transfer processes in single photocatalyst particles cannot be tracked, and the mechanism of charge transfer is unknown. Here, we report spatiotemporally resolved surface photovoltage measurements on Cu2O photocatalyst particles to map complete charge transfer processes throughout the femtosecond to second time scale at the single-particle level. We found that photogenerated electrons are transferred to the catalytic surface ballistically on a sub-picosecond timescale and are retained at this location for the duration, whereas photogenerated holes are transferred to a spatially separated surface and stabilized via selective trapping on a microsecond timescale. We demonstrate that these ballistic electron transfer and anisotropic trapping regimes, which challenge the classical perception of the drift–diffusion model, contribute to efficient charge separation in photocatalysis and improve the photocatalytic performance. We anticipate our findings to demonstrate the universality of other photoelectronic devices and facilitate the rational design of photocatalysts.

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The Adsorption Behaviors of Citric Acid on Abrasive Particles in Cu CMP Slurry

MRS Proceedings ◽

10.1557/proc-867-w7.5 ◽

2005 ◽

Vol 867 ◽

Cited By ~ 1

Author(s):

Young-Jae Kang ◽

Yi-Koan Hong ◽

Jae-Hoon Song ◽

In-Kwon Kim ◽

Jin-Goo Park

Keyword(s):

Citric Acid ◽

Friction Force ◽

Adhesion Force ◽

Removal Rate ◽

Friction Forces ◽

Abrasive Particles ◽

Ph Values ◽

Adsorption Behaviors ◽

Cu Cmp ◽

Citrate Ions

AbstractThe interaction between Cu surface and abrasive particles in slurry solution was characterized. The adsorption behavior of the citrate ions was dependent on the pH of the slurry and the concentration of the citric acid. The adsorption of citrate ions generated a highly negative charge on the alumina surface and shifted isoelectric point (IEP) to lower pH values. The Cu removal rate of alumina slurry was higher than that of colloidal silica based slurry in the investigated pH ranges. Although lower friction forces of Cu were observed in alumina based slurry of pH 4, 6 and 8, a higher friction force was observed at pH 2. This high friction force was attributed to the positive zeta potential and greater adhesion force of particle. It indicates that the magnitudes of particle adhesions on Cu surfaces in slurries can be directly related to the frictional behavior during CMP process.

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The Friction Forces Between Si Tip and Multilayer Graphene

Volume 9: Micro- and Nano-Systems Engineering and Packaging, Parts A and B ◽

10.1115/imece2012-87131 ◽

2012 ◽

Author(s):

Yan Zhang ◽

Yingying Wang ◽

Yunfei Chen ◽

Yujuan Wang

Keyword(s):

Friction Force ◽

Graphene Layer ◽

Adhesion Force ◽

Normal Load ◽

Multilayer Graphene ◽

Friction Forces ◽

Layer Number ◽

Si Substrates ◽

Atomic Force ◽

Natural Oxide

Mechanical peeling method is used to prepare multilayer graphene on silicon wafer with natural oxide, and the layer number of graphene is determined through atomic force microsopy (AFM) topography image and optical image. The friction force between Silicon tip and multilayer graphene and SiO2/Si substrates is measured with AFM. It is found that the friction force is reduced with the increase of the graphene layer number and approaches the value between the Si tip and graphite. Through comparing the tip sliding on graphene with different layers, the deformation of graphene is believed to be the main reason causing the decrease of the friction force with the layer number. When the normal load is much larger than the adhesion force, friction force increases with normal load linearly. However, while normal load closes to the adhesion force, friction force is independent of the normal load.

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Real-time observation of tetrapyrrole binding to an engineered bacterial phytochrome

Communications Chemistry ◽

10.1038/s42004-020-00437-3 ◽

2021 ◽

Vol 4 (1) ◽

Author(s):

Yusaku Hontani ◽

Mikhail Baloban ◽

Francisco Velazquez Escobar ◽

Swetta A. Jansen ◽

Daria M. Shcherbakova ◽

...

Keyword(s):

Real Time ◽

Rational Design ◽

Near Infrared ◽

Fluorescent Proteins ◽

Covalent Bond ◽

Binding Pocket ◽

Tissue Imaging ◽

Covalent Attachment ◽

Time Resolved

AbstractNear-infrared fluorescent proteins (NIR FPs) engineered from bacterial phytochromes are widely used for structural and functional deep-tissue imaging in vivo. To fluoresce, NIR FPs covalently bind a chromophore, such as biliverdin IXa tetrapyrrole. The efficiency of biliverdin binding directly affects the fluorescence properties, rendering understanding of its molecular mechanism of major importance. miRFP proteins constitute a family of bright monomeric NIR FPs that comprise a Per-ARNT-Sim (PAS) and cGMP-specific phosphodiesterases - Adenylyl cyclases - FhlA (GAF) domain. Here, we structurally analyze biliverdin binding to miRFPs in real time using time-resolved stimulated Raman spectroscopy and quantum mechanics/molecular mechanics (QM/MM) calculations. Biliverdin undergoes isomerization, localization to its binding pocket, and pyrrolenine nitrogen protonation in <1 min, followed by hydrogen bond rearrangement in ~2 min. The covalent attachment to a cysteine in the GAF domain was detected in 4.3 min and 19 min in miRFP670 and its C20A mutant, respectively. In miRFP670, a second C–S covalent bond formation to a cysteine in the PAS domain occurred in 14 min, providing a rigid tetrapyrrole structure with high brightness. Our findings provide insights for the rational design of NIR FPs and a novel method to assess cofactor binding to light-sensitive proteins.

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Influence of Nanoscale Textured Surfaces and Subsurface Defects on Friction Behaviors by Molecular Dynamics Simulation

Nanomaterials ◽

10.3390/nano9111617 ◽

2019 ◽

Vol 9 (11) ◽

pp. 1617 ◽

Cited By ~ 1

Author(s):

Ruiting Tong ◽

Zefen Quan ◽

Yangdong Zhao ◽

Bin Han ◽

Geng Liu

Keyword(s):

Molecular Dynamics ◽

Molecular Dynamics Simulation ◽

Friction Force ◽

Copper Substrate ◽

Dynamics Simulation ◽

Textured Surfaces ◽

Friction Forces ◽

Subsurface Structures ◽

Texture Orientation ◽

Subsurface Defects

In nanomaterials, the surface or the subsurface structures influence the friction behaviors greatly. In this work, nanoscale friction behaviors between a rigid cylinder tip and a single crystal copper substrate are studied by molecular dynamics simulation. Nanoscale textured surfaces are modeled on the surface of the substrate to represent the surface structures, and the spacings between textures are seen as defects on the surface. Nano-defects are prepared at the subsurface of the substrate. The effects of depth, orientation, width and shape of textured surfaces on the average friction forces are investigated, and the influence of subsurface defects in the substrate is also studied. Compared with the smooth surface, textured surfaces can improve friction behaviors effectively. The textured surfaces with a greater depth or smaller width lead to lower friction forces. The surface with 45° texture orientation produces the lowest average friction force among all the orientations. The influence of the shape is slight, and the v-shape shows a lower average friction force. Besides, the subsurface defects in the substrate make the sliding process unstable and the influence of subsurface defects on friction forces is sensitive to their positions.

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