Thickness of Nano-Scale Poly(Dimethylsiloxane) Layers Determines the Motion of Sliding Water Drops.

Layers of nanometer thick polydimethylsiloxane (PDMS) are applied as hydrophobic coatings because of their environmentally-friendly and chemically-inert properties. In applications such as heat exchangers or fog harvesting, low water drop friction on surfaces is required. While the onset of motion (static friction) has been studied, our knowledge of dynamic friction needs to be improved. Which processes lead to energy dissipation and cause dynamic friction? This knowledge is important to minimize friction. Here, we measured the dynamic friction of drops on PDMS brushes of different thicknesses, covering the whole available velocity regime. The brush thickness L turned out to be a predictor for drop friction. 4-5 nm thick PDMS brush showed the lowest dynamic friction. A certain minimal thickness is necessary to form homogeneous surfaces and reduce the attractive Van der Waals interaction between water and the substrate. The increase in dynamic friction above L = 5 nm is also attributed to the increasing viscoelastic dissipation of the capillary ridge formed at the contact line. The height of the ridge is related to the brush thickness. Fluorescence correlation spectroscopy and atomic force measurements support this interpretation. Sum-frequency generation further indicates a maximum order at the PDMS-water interface at intermediate thickness. This article is protected by copyright. All rights reserved.

Slide electrification of drops at low velocities.

Slide electrification of drops is mostly investigated on tilted plate setups. Hence, the drop charging at low sliding velocity remains unclear. We overcome the limitations by developing an electro drop friction force instrument (eDoFFI). Using eDoFFI, we investigate slide electrification at the onset of drop sliding and at low sliding velocities ≤ 1 cm s-1. The novelty of eDoFFI is the simultaneous measurements of the drop discharging current and the friction force acting on the drop. The eDoFFI tool facilitates control on drop length and width using differently shaped rings. Hereby, slide electrification experiments with the defined drop length-to-width ratios >1 and <1 are realized. We find that width of the drop is the main geometrical parameter which determines drop discharging current and charge separation. We combine Kawasaki-Furmidge friction force equation with our finding on drop discharging current. This combination facilitates the direct measurement of surface charge density (σ) deposited behind the drop. We calculate σ ≈ 45 µC m-2 on Trichloro(1H,1H,2H,2H-perfluorooctyl)silane (PFOTS) and ≈20 µC m-2 on Trichloro(octyl)silane (OTS) coated glass surfaces. We find that the charge separation by moving drops is independent of sliding velocity ≤ 1 cm s-1. The reverse sliding of drop along the same scanline facilitates calculation of the surface neutralization time constant. The eDoFFI links two scientific communities: one which focuses on the friction forces and one which focuses on the slide electrification of drops.

Molecular transport and water condensation inside mesopores with wettability step gradients.

The wettabilities of nanoscale porous surfaces play important roles in the context of molecular and fluid transport or oil-water separation. The wettability pattern along a nanopore strongly influences fluid distribution throughout the membrane. Mesoporous silica thin films with gradually adjusted wettabilities were fabricated via cocondensation. With consecutive mesoporous layer depositions, double-layer mesoporous silica films with asymmetric or so-called Janus wettability patterns were generated. The effects of these wetting gradients on mass transport, water imbibition, and water vapor condensation were investigated with ellipsometry, cyclic voltammetry (CV), drop friction force instrument (DoFFIs), fluorescence microscopy and interferometry. By increasing the film thickness of the hydrophobic mesoporous silica top layer deposited on a hydrophilic mesoporous silica layer up to 205 nm, molecular transport through both the layers was prevented. However, water was observed to condense onto the bottom layer, and transport occurred for thinner top layers.

Kinetic drop friction.

Liquid drops sliding on tilted surfaces is an everyday phenomenon and is important for many industrial applications. Still, it is impossible to predict the drop's sliding velocity. To make a step forward in quantitative understanding, we measured the velocity [Formula: see text], contact width [Formula: see text], contact length [Formula: see text], advancing [Formula: see text], and receding contact angle [Formula: see text] of liquid drops sliding down inclined flat surfaces made of different materials. We find the friction force acting on sliding drops of polar and non-polar liquids with viscosities ([Formula: see text]) ranging from 10-3 to 1 [Formula: see text] can empirically be described by [Formula: see text] for a velocity range up to 0.7 ms-1. The dimensionless friction coefficient [Formula: see text] defined here varies from 20 to 200. It is a material parameter, specific for a liquid/surface combination. While static wetting is fully described by [Formula: see text] and [Formula: see text], for dynamic wetting the friction coefficient is additionally necessary.

When crystals flow.

Semicrystalline polymers are solids that are supposed to flow only above their melting temperature. By using confinement within nanoscopic cylindrical pores, we show that a semicrystalline polymer can flow at temperatures below the melting point with a viscosity intermediate to the melt and crystal states. During this process, the capillary force is strong and drags the polymer chains in the pores without melting the crystal. The unexpected enhancement in flow, while preserving the polymer crystallites, is of importance in the design of polymer processing conditions applicable at low temperatures, e.g., cold drawn polymers such as polytetrafluoroethylene, self-healing, and in nanoconfined donor/acceptor polymers used in organic electronics.

Fast-release kinetics of a pH-responsive polymer detected by dynamic contact angles.

Polymers conjugated with active agents have applications in biomedicine, anticorrosion, and smart agriculture. When the active agent is used as a drug, corrosion inhibitor, or pesticide, it can be released upon a specific stimulus. The efficiency and the sustainability of active agents are determined by the released kinetics. In this work, we study the fast-release kinetics of 8-hydroxyquinoline (8HQ) from a pH-responsive, random copolymer of methyl methacrylate and 8-quinolinyl-sulfide-ethyl acrylate [P(MMA-co-HQSEA)] by hydrolysis of the ß-thiopropionate groups. We used contact angle measurements of sliding drops as an elegant way to characterize the release kinetics. Based on the results gained from 1H nuclear magnetic resonance measurement, fluorescent intensity measurement, and velocity-dependent contact angle measurement, we found that both the hydrolysis rate and polymer conformation affect the release kinetics of 8HQ from a P(MMA-co-HQSEA) film. Polymer chains collapse and further suppress the release from the inner layer in acidic conditions, while polymer chains in a stretched condition further facilitate the release from the inner layer. As a result, the cumulative release rate of 8HQ is higher in the basic condition than in the acidic condition.

Free Standing Dry and Stable Nanoporous Polymer Films Made through Mechanical Deformation.

A new straight forward approach to create nanoporous polymer membranes with well defined average pore diameters is presented. The method is based on fast mechanical deformation of highly entangled polymer films at high temperatures and a subsequent quench far below the glass transition temperature Tg . The process is first designed generally by simulation and then verified for the example of polystyrene films. The methodology does not need any chemical processing, supporting substrate, or self assembly process and is solely based on polymer inherent entanglement effects. Pore diameters are of the order of ten polymer reptation tube diameters. The resulting membranes are stable over months at ambient conditions and display remarkable elastic properties.

Understanding the evolution of lithium dendrites at Li6.25Al0.25La3Zr2O12 grain boundaries via operando microscopy techniques.

The growth of lithium dendrites in inorganic solid electrolytes is an essential drawback that hinders the development of reliable all-solid-state lithium metal batteries. Generally, ex situ post mortem measurements of battery components show the presence of lithium dendrites at the grain boundaries of the solid electrolyte. However, the role of grain boundaries in the nucleation and dendritic growth of metallic lithium is not yet fully understood. Here, to shed light on these crucial aspects, we report the use of operando Kelvin probe force microscopy measurements to map locally time-dependent electric potential changes in the Li6.25Al0.25La3Zr2O12 garnet-type solid electrolyte. We find that the Galvani potential drops at grain boundaries near the lithium metal electrode during plating as a response to the preferential accumulation of electrons. Time-resolved electrostatic force microscopy measurements and quantitative analyses of lithium metal formed at the grain boundaries under electron beam irradiation support this finding. Based on these results, we propose a mechanistic model to explain the preferential growth of lithium dendrites at grain boundaries and their penetration in inorganic solid electrolytes.

Deep Learning to Analyze Sliding Drops.

State-of-the-art contact angle measurements usually involve image analysis of sessile drops. The drops are symmetric and images can be taken at high resolution. The analysis of videos of drops sliding down a tilted plate is hampered due to the low resolution of the cutout area where the drop is visible. The challenge is to analyze all video images automatically, while the drops are not symmetric anymore and contact angles change while sliding down the tilted plate. To increase the accuracy of contact angles, we present a 4-segment super-resolution optimized-fitting (4S-SROF) method. We developed a deep learning-based super-resolution model with an upscale ratio of 3; i.e., the trained model is able to enlarge drop images 9 times accurately (PSNR = 36.39). In addition, a systematic experiment using synthetic images was conducted to determine the best parameters for polynomial fitting of contact angles. Our method improved the accuracy by 21% for contact angles lower than 90° and by 33% for contact angles higher than 90°.

Scanning Drop Friction Force Microscopy.

Wetting imperfections are omnipresent on surfaces. They cause contact angle hysteresis and determine the wetting dynamics. Still, existing techniques (e.g., contact angle goniometry) are not sufficient to localize inhomogeneities and image wetting variations. We overcome these limitations through scanning drop friction force microscopy (sDoFFI). In sDoFFI, a 15 µL drop of Milli-Q water is raster-scanned over a surface. The friction force (lateral adhesion force) acting on the moving contact line is plotted against the drop position. Using sDoFFI, we obtained 2D wetting maps of the samples having sizes in the order of several square centimeters. We mapped areas with distinct wetting properties such as those present on a natural surface (e.g., a rose petal), a technically relevant superhydrophobic surface (e.g., Glaco paint), and an in-house prepared model of inhomogeneous surfaces featuring defined areas with low and high contact angle hysteresis. sDoFFI detects features that are smaller than 0.5 mm in size. Furthermore, we quantified the sliding behavior of drops across the boundary separating areas with different contact angles on the model sample. The sliding of a drop across this transition line follows a characteristic stick-slip motion. We use the variation in force signals, advancing and receding contact line velocities, and advancing and receding contact angles to identify zones of stick and slip. When scanning the drop from low to high contact angle hysteresis, the drop undergoes a stick-slip-stick-slip motion at the interline. Sliding from high to low contact angle hysteresis is characterized by the slip-stick-slip motion. The sDoFFI is a new tool for 2D characterization of wetting properties, which is applicable to laboratory-based samples but also characterizes biological and commercial surfaces.

Acidic pH Promotes Refolding and Macroscopic Assembly of Amyloid ß (16-22) Peptides at the Air-Water Interface.

Assembly by amyloid-beta (Aß) peptides is vital for various neurodegenerative diseases. The process can be accelerated by hydrophobic interfaces such as the cell membrane interface and the air-water interface. Elucidating the assembly mechanism for Aß peptides at hydrophobic interface requires knowledge of the microscopic structure of interfacial peptides. Here we combine scanning force microscopy, sum-frequency generation spectroscopy, and metadynamics simulations to probe the structure of the central fragment of Aß peptides at the air-water interface. We find that the structure of interfacial peptides depends on pH: at neutral pH, the peptides adopt a less folded, bending motif by forming intra-hydrogen bonds; at acidic pH, the peptides refold into extended ß-strand fibril conformation, which further promotes their macroscopic assembly. The conformational transition of interfacial peptides is driven by the reduced hydrogen bonds, both with water and within peptides, resulting from the protonation of acidic glutamic acid side chains.

Adaptation and Recovery of a Styrene-Acrylic Acid Copolymer Surface to Water.

Drops sliding down an adaptive surface lead to changes of the dynamic contact angles. Two adaptation processes play a role: 1) the adaptation of the surface upon bringing it into contact to the drop (wetting) and 2) the adaptation of the surface after the drop passed (dewetting). In order to study both processes, the authors investigate samples made from random styrene (PS)/acrylic acid (PAA) copolymers, which are exposed to water. Sum-frequency generation spectroscopy and tilted-plate measurements indicate that during wetting, the PS segments displace from the interface, while PAA segments are enriched. This structural adaptation of the PS/PAA random copolymer to water remains after dewetting. Annealing the adapted polymer induces reorientation of the PS segments to the surface.

Clathrate Adhesion Induced by Quasi-Liquid Layer.

The adhesive force of clathrates to surfaces is a century-old problem of pipeline blockage for the energy industry. Here, we provide new physical insight into the origin of this force by accounting for the existence of a quasi-liquid layer (QLL) on clathrate surfaces. To gain this insight, we measure the adhesive force between a tetrahydrofuran clathrate and a solid sphere. We detect a strong adhesion, which originates from a capillary bridge that is formed from a nanometer-thick QLL on the clathrate surface. The curvature of this capillary bridge is nanoscaled, causes a large negative Laplace pressure, and leads to a strong capillary attraction. The microscopic capillary bridge expands and consolidates over time. This dynamic behavior explains the time-dependent increase of measured capillary forces. The adhesive force decreases greatly upon increasing the roughness and the hydrophobicity of the sphere, which founds the fundamental basics for reducing clathrate adhesion by using surface coating.

Adaptation of a Styrene-Acrylic Acid Copolymer Surface to Water.

Solid surfaces, in particular polymer surfaces, are able to adapt upon contact with a liquid. Adaptation results in an increase in contact angle hysteresis and influences the mobility of sliding drops on surfaces. To study adaptation and its kinetics, we synthesized a random copolymer composed of styrene and 11-25 mol% acrylic acid (PS/PAA). We measured the dynamic advancing (θA) and receding (θR) contact angles of water drops sliding down a tilted plate coated with this polymer. We measured θA ≈ 87° for velocities of the contact line <20 µm/s. At higher velocities, θA gradually increased to ∼98°. This value is similar to θA of a pure polystyrene (PS) film, which we studied for comparison. We associate the gradual increase in θA to the adaptation process to water: The presence of water leads to swelling and/or an enrichment of acid groups at the water/polymer interface. By applying the latest adaptation theory (Butt et al. Langmuir 2018, 34, 11292), we estimated the time constant of this adaptation process to be ≪1 s. For sliding water drops, θR is ∼10° lower compared to the reference PS surface for all tested velocities. Thus, at the receding side of a sliding drop, the surface is already enriched by acid groups. For a water drop with a width of 5 mm, the increase in contact angle hysteresis corresponds to an increase in capillary force in the range of 45-60 µN, depending on sliding velocity.

Electronic effects of nano-confinement in functional organic and inorganic materials for optoelectronics.

When various optically and/or electronically active materials, such as conjugated polymers, perovskites, metals, and metal oxides, are confined at the nanoscale, they can exhibit unique nano-confined behavior that significantly differs from the behavior observed at the macroscale. Although controlled nano-confinement of functional materials can allow modulation of their electronic properties without the aid of any synthetic methodologies or additional chemical treatments, limited assembly approaches for nano-confinement and insufficient analytical tools for electronic characterization remain critical challenges in the development of novel optoelectronic materials and the investigation of their modulated properties. This review describes how the nano-confined features of organic and inorganic materials are related to the control and improvement of their optoelectronic properties. In particular, we focus on various assembly approaches for effective nano-confinement as well as methods for nano-electronic characterization. Then, we briefly present challenges and perspectives on the direction of nano-confinement in terms of the preparation of optoelectronic materials with desired functionalities. Furthermore, we believe that this review can provide a basis for developing and designing next-generation optoelectronics through nano-confinement.

Adaptive Wetting of Polydimethylsiloxane.

To better understand the wetting of cross-linked polydimethylsiloxane (PDMS), we measured advancing and receding contact angles of sessile water drops on cross-linked PDMS as a function of contact line velocity (up to 100 µm/s). Three types of samples were investigated: pristine PDMS, PDMS where oligomers were removed by toluene treatment, and PDMS with an enriched concentration of oligomers. Depending on the velocity of advancing contact lines and the contact time with water, different modes of wetting were observed: one with a relatively low contact angle hysteresis (Δθ ≈ 10°) and one with a larger hysteresis. We attribute the low hysteresis state, called the lubricated state, to the enrichment of free oligomers at the water-PDMS interface. The enrichment of oligomers is induced by drop contact. The kinetics of the transition to the lubricated state can be described by adaptation theory. PDMS adapts to the presence of water by an enrichment of free oligomers at the interface and a correlated reduction in interfacial tension.

Vitamin C Loaded Polyethylene: Synthesis and Properties of Precise Polyethylene with Vitamin C Defects via Acyclic Diene Metathesis Polycondensation.

A polyethylene-like polymer with an in-chain vitamin C group was synthesized by olefin metathesis polymerization. Here, we describe both the synthesis and a comprehensive physical characterization. Because of the olefin metathesis synthesis, the vitamin C groups are equidistantly arranged in the polyethylene (PE) main chain. Their separation was adjusted to 20 CH2 units. After hydrogenation, a semicrystalline polymer is obtained that is soluble in polar solvents. Because of its size and steric effect, the vitamin C acts as a chain defect, which is expelled from the crystal lattice, yielding a lamellar crystal with a homogeneous thickness corresponding to the interdefect distance. The physical properties were examined by various methods including differential scanning calorimetry, X-ray scattering, and transmission electron microscopy. We show that vitamin C retains its radical scavenger properties despite being incorporated into a polyethylene chain. Furthermore, we demonstrate that it is degrading in alkaline conditions. To complete its suitability as a biocompatible material, cytotoxicity and cell uptake experiments were performed. We show that the polymer is nontoxic and that it is taken up in nanoparticular form via endocytosis processes into the cytoplasm of cells.

Hygroscopic properties of NaCl nanoparticles on the surface: a scanning force microscopy study.

We investigated the hygroscopic growth of sodium chloride (NaCl) nanoparticles with curvature related diameters ranging from 10 nm to 200 nm, at different relative humidities using scanning force microscopy. Hygroscopic aerosol nanoparticles play a vital role in the Earth's climate and human health. We report that 10 nm NaCl nanoparticles adsorbed on silicon surfaces have a higher deliquescence relative humidity than larger NaCl nanoparticles (size > 30 nm). This finding is consistent with the observations for airborne nanoparticles by hygroscopicity tandem differential mobility analyzer. Therefore, the presence of silicon surfaces plays no significant role in the deliquescence relative humidity. Moreover, the study of individual airborne particles by means of scanning force microscopy revealed that the ability of water uptake, i.e. growth factor, of NaCl particles differs by as large as 40% at the same relative humidity. This finding indicates that the individual nature of NaCl particles influences the growth factor.

Premelting-Induced Agglomeration of Hydrates: Theoretical Analysis and Modeling.

Resolving the long-standing problem of hydrate plugging in oil and gas pipelines has driven an intense quest for mechanisms behind the plug formation. However, existing theories of hydrate agglomeration have critical shortcomings, for example, they cannot describe nanometer-range capillary forces at hydrate surfaces that were recently observed by experiments. Here, we present a new model for hydrate agglomeration which includes premelting of hydrate surfaces. We treat the premelting layer on hydrate surfaces such as a thin liquid film on a substrate and propose a soft-sphere model of hydrate interactions. The new model describes the premelting-induced capillary force between a hydrate surface and a pipe wall or another hydrate. The calculated adhesive force between a hydrate sphere (R = 300 µm) and a solid surface varies from 0.3 mN on a hydrophilic surface (contact angle, θ = 0°) to 0.008 mN on a superhydrophobic surface (θ = 160°). The initial contact area is 4 orders of magnitude smaller than the cross-sectional area of the hydrate sphere and can expand with increasing contact time because of the consolidation of hydrate particles on the solid surface. Our model agrees with the available experimental results and can serve as a conceptual guidance for developing a chemical-free environmentally friendly method for prevention of hydrate plugs via surface coating of pipe surfaces.

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.