Elucidating the role of water in collagen self-assembly by isotopically modulating collagen hydration.

Water is known to play an important role in collagen self-assembly, but it is still largely unclear how water-collagen interactions influence the assembly process and determine the fibril network properties. Here, we use the H[Formula: see text]O/D[Formula: see text]O isotope effect on the hydrogen-bond strength in water to investigate the role of hydration in collagen self-assembly. We dissolve collagen in H[Formula: see text]O and D[Formula: see text]O and compare the growth kinetics and the structure of the collagen assemblies formed in these water isotopomers. Surprisingly, collagen assembly occurs ten times faster in D[Formula: see text]O than in H[Formula: see text]O, and collagen in D[Formula: see text]O self-assembles into much thinner fibrils, that form a more inhomogeneous and softer network, with a fourfold reduction in elastic modulus when compared to H[Formula: see text]O. Combining spectroscopic measurements with atomistic simulations, we show that collagen in D[Formula: see text]O is less hydrated than in H[Formula: see text]O. This partial dehydration lowers the enthalpic penalty for water removal and reorganization at the collagen-water interface, increasing the self-assembly rate and the number of nucleation centers, leading to thinner fibrils and a softer network. Coarse-grained simulations show that the acceleration in the initial nucleation rate can be reproduced by the enhancement of electrostatic interactions. These results show that water acts as a mediator between collagen monomers, by modulating their interactions so as to optimize the assembly process and, thus, the final network properties. We believe that isotopically modulating the hydration of proteins can be a valuable method to investigate the role of water in protein structural dynamics and protein self-assembly.

Molecular Probing of the Microscopic Pressure at Contact Interfaces.

Obtaining insights into friction at the nanoscopic level and being able to translate these into macroscopic friction behavior in real-world systems is of paramount importance in many contexts, ranging from transportation to high-precision technology and seismology. Since friction is controlled by the local pressure at the contact it is important to be able to detect both the real contact area and the nanoscopic local pressure distribution simultaneously. In this paper, we present a method that uses planarizable molecular probes in combination with fluorescence microscopy to achieve this goal. These probes, inherently twisted in their ground states, undergo planarization under the influence of pressure, leading to bathochromic and hyperchromic shifts of their UV-vis absorption band. This allows us to map the local pressure in mechanical contact from fluorescence by exciting the emission in the long-wavelength region of the absorption band. We demonstrate a linear relationship between fluorescence intensity and (simulated) pressure at the submicron scale. This relationship enables us to experimentally depict the pressure distribution in multiasperity contacts. The method presented here offers a new way of bridging friction studies of the nanoscale model systems and practical situations for which surface roughness plays a crucial role.

Electroless Ionization Mass Spectrometry Using a Compact Electrokinetic Ionization Source.

We introduce a new ionization technique for compact, portable mass spectrometers. It consists of a syringe with sample liquid capped by a self-ionizing spray nozzle containing a microfabricated nozzle chip. Interaction of the sample liquid with the nozzle wall results in electrical charging without the need for electronics. Elaborate cleaning procedures are redundant when disposable syringes and mass-fabricated spray nozzles are used. This self-named electroless spray ionization (ELI) technique shows comparable performance to conventional ionization techniques. In contrast to commonly used electrospray ionization, ELI exhibits excellent ionization efficiency for low-conductive solutions such as water or acetonitrile. Due to its compact size and the absence of high-voltage electronics, it can also be readily integrated in other ionization sources. Besides reviewing the main properties of ELI, we showcase the technique's potential for two on-site, ambient mass spectroscopy applications: perfume fingerprinting and fast screening of fungicides on citrus fruits.

Droplet Size Distribution in Emulsions.

The droplet size in emulsions is known to affect the rheological properties and plays a crucial role in many applications of emulsions. Despite its importance, the underlying mechanisms governing droplet size in emulsification remain poorly understood. We investigate the average drop size and size distribution upon emulsification with a high-shear mixer for model oil-in-water emulsions stabilized by a surfactant. The size distribution is found to be a log-normal distribution resulting from the repetitive random breakup of drops. High-shear emulsification, the usual way of making emulsions, is therefore found to be very different from turbulent emulsification given by the Kolmogorov-Hinze theory, for which power-law distributions of the drop size are expected. In agreement with this, the mean droplet size does not follow a scaling with the Reynolds number of the emulsification flow but rather a capillary number scaling based on the viscosity of the continuous phase.

Quantitative Understanding of the Onset of Dense Granular Flows.

The question of when and how dense granular materials start to flow under stress, despite many industrial and geophysical applications, remains largely unresolved. We develop and test a simple equation for the onset of quasistatic flows of granular materials which is based on the frictional aging of the granular packing. The result is a nonmonotonic stress-strain relation which-akin to classical friction-is independent of the shear rate. This relation suffices to understand the quasistatic deformations of aging granular media, and its solid-to-liquid transition. Our results also elucidate the (flow) history dependence of the mechanical properties, and the sensitivity to the initial preparation of granular media.

Controlling Macroscopic Friction through Interfacial Siloxane Bonding.

Controlling macroscopic friction is crucial for numerous natural and industrial applications, ranging from forecasting earthquakes to miniaturizing semiconductor devices, but predicting and manipulating friction phenomena remains a challenge due to the unknown relationship between nanoscale and macroscopic friction. Here, we show experimentally that dry friction at multiasperity Si-on-Si interfaces is dominated by the formation of interfacial siloxane (Si─O─Si) bonds, the density of which can be precisely regulated by exposing plasma-cleaned silicon surfaces to dry nitrogen. Our results show how the bond density can be used to quantitatively understand and control the macroscopic friction. Our findings establish a unique connection between the molecular scale at which adhesion occurs, and the friction coefficient that is the key macroscopic parameter for industrial and natural tribology challenges.

Capillary Forces Lead to Pendant Crystals at the Liquid-Air Interface of Evaporating Salt Solutions.

We investigated the nucleation and growth processes of individual NaCl crystals from an evaporating salt solution that is supersaturated. We find that crystals nucleate at the liquid/vapor interface, resulting in distinct "pendant" crystals, which reach millimeter dimensions. The substantial size of the crystals induces deformation of the interface. This process and the evaporation rate, in turn, determine the final crystal shape, which features a deep central cavity. Our findings reveal that a delicate balance exists between gravity, buoyancy, and the surface tension of the liquid/vapor interface that allows the crystal to remain pendant. When the contact angle of the crystal with the meniscus reaches 90°, the crystal disconnects from the interface and falls into the solution. We quantitatively predict the critical mass at which this occurs.

Super-resolution Fluorescence Imaging of Recycled Polymer Blends via Hydrogen Bond-Assisted Adsorption of a Nile Red Derivative.

A key challenge in the recycling of multilayer plastic films of polyethylene and polyamide, as typically used for food packaging, is to assess and control the phase separation of the two types of polymers in the recycled material, the specifics of which determine the mechanical strength of the recycled material. However, visualizing the polyamide-in-polyethylene domains with conventional fluorescence methods or electron microscopy is challenging. We present a new approach that combines the point accumulation in nanoscale topography (PAINT) super-resolution method with a newly synthesized Nile Red probe (diOHNR) as the fluorescent label. The molecule was modified to undergo a hydrogen bond-assisted interaction with the polyamide phase in the blend due to its two additional hydroxyl groups but preserves the spectral properties of Nile Red. As a result, the localization density of the probe in the PAINT image is 13 times larger at the polyamide phase than at the polyethylene phase, enabling quantitative evaluation of the spatial polyamide/polyethylene distribution down to the nanoscale. The method achieved a spatial resolution of 18.8 nm, and we found that over half of the polyamide particles in a recycled sample were smaller than the optical diffraction limit. Being able to image the blends with nanoscopic resolution can help to optimize the composition and mechanical properties of recycled materials and thus contribute to an increased reuse of plastics.

Surface-Mediated Molecular Transport of a Lipophilic Fluorescent Probe in Polydisperse Oil-in-Water Emulsions.

Emulsions often act as carriers for water-insoluble solutes that are delivered to a specific target. The molecular transport of solutes in emulsions can be facilitated by surfactants and is often limited by diffusion through the continuous phase. We here investigate this transport on a molecular scale by using a lipophilic molecular rotor as a proxy for solutes. Using fluorescence lifetime microscopy we track the transport of these molecules from the continuous phase toward the dispersed phase in polydisperse oil-in-water emulsions. We show that this transport comprises two time scales, which vary significantly with droplet size and surfactant concentration, and, depending on the type of surfactant used, can be limited either by transport across the oil-water interface or by diffusion through the continuous phase. By studying the time-resolved fluorescence of the fluorophore, accompanied by molecular dynamics simulations, we demonstrate how the rate of transport observed on a macroscopic scale can be explained in terms of the local environment that the probe molecules are exposed to.

Is there a difference between surfactant-stabilised and Pickering emulsions?

What measurable physical properties allow one to distinguish surfactant-stabilised from Pickering emulsions? Whereas surfactants influence oil/water interfaces by lowering the oil/water interfacial tension, particles are assumed to have little effect on the interfacial tension. Here we perform interfacial tension (IFT) measurements on three different systems: (1) soybean oil and water with ethyl cellulose nanoparticles (ECNPs), (2) silicone oil and water with the globular protein bovine serum albumin (BSA), and (3) sodium dodecyl sulfate (SDS) solutions and air. The first two systems contain particles, while the third system contains surfactant molecules. We observe a significant decrease in interfacial tension with increasing particle/molecule concentration in all three systems. We analyse the surface tension data using the Gibbs adsorption isotherm and the Langmuir equation of state for the surface, resulting in surprisingly high adsorption densities for the particle-based systems. These seem to behave very much like the surfactant system: the decrease in tension is due to the presence of many particles at the interface, each with an adsorption energy of a few kBT. Dynamic interfacial tension measurements show that the systems are in equilibrium, and that the characteristic time scale for adsorption is much longer for particle-based systems than for surfactants, in line with their size difference. In addition, the particle-based emulsion is shown to be less stable against coalescence than the surfactant-stabilised emulsion. This leaves us with the conclusion that we are not able to make a clear distinction between the surfactant-stabilised and Pickering emulsions.

Infrared Diffusion-Ordered Spectroscopy Reveals Molecular Size and Structure.

Inspired by ideas from NMR, we have developed Infrared Diffusion-Ordered Spectroscopy (IR-DOSY), which simultaneously characterizes molecular structure and size. We rely on the fact that the diffusion coefficient of a molecule is determined by its size through the Stokes-Einstein relation, and achieve sensitivity to the diffusion coefficient by creating a concentration gradient and tracking its equilibration in an IR-frequency resolved manner. Analogous to NMR-DOSY, a two-dimensional IR-DOSY spectrum has IR frequency along one axis and diffusion coefficient (or equivalently, size) along the other, so the chemical structure and the size of a compound are characterized simultaneously. In an IR-DOSY spectrum of a mixture, molecules with different sizes are nicely separated into distinct sets of IR peaks. Extending this idea to higher dimensions, we also perform 3D-IR-DOSY, in which we combine the conformation sensitivity of femtosecond multi-dimensional IR spectroscopy with size sensitivity.

Nonmonotonic Friction due to Water Capillary Adhesion and Hydrogen Bonding at Multiasperity Interfaces.

Capillary adhesion due to water adsorption from the air can contribute to friction, especially for smooth interfaces in humid environments. We show that for multiasperity (naturally oxidized) Si-on-Si interfaces, the friction coefficient goes through a maximum as a function of relative humidity. An adhesion model based on the boundary element method that takes the roughness of the interfaces into account reproduces this nonmonotonic behavior very well. Remarkably, we find the dry friction to be significantly lower than the lubricated friction with macroscopic amounts of water present. The difference is attributed to the hydrogen-bonding network across the interface. Accordingly, the lubricated friction increases significantly if the water is replaced by heavy water (D_{2}O) with stronger hydrogen bonding.

Universal Aspects of Droplet Spreading Dynamics in Newtonian and Non-Newtonian Fluids.

Droplet impacts are common in many applications such as coating, spraying, or printing; understanding how droplets spread after impact is thus of utmost importance. Such impacts may occur with different velocities on a variety of substrates. The fluids may also be non-Newtonian and thus possess different rheological properties. How the different properties such as surface roughness and wettability, droplet viscosity, and rheology as well as interfacial properties affect the spreading dynamics of the droplets and the eventual drop size after impact are unresolved questions. Most recent work focuses on the maximum spreading diameter after impact and uses scaling laws to predict this. In this paper, we show that a proper rescaling of the spreading dynamics with the maximum radius attained by the drop and the impact velocity leads to a unique single and thus universal curve for the variation of diameter versus time. The validity of this universal functional shape is validated for different liquids with different rheological properties as well as substrates with different wettabilities. This universal function agrees with a recent model that proposes a closed set of differential equations for the spreading dynamics of droplets.

Transition from Dendritic to Cell-like Crystalline Structures in Drying Droplets of Fetal Bovine Serum under the Influence of Temperature.

The desiccation of biofluid droplets leads to the formation of complex deposits which are morphologically affected by the environmental conditions, such as temperature. In this work, we examine the effect of substrate temperatures between 20 and 40 °C on the desiccation deposits of fetal bovine serum (FBS) droplets. The final dried deposits consist of different zones: a peripheral protein ring, a zone of protein structures, a protein gel, and a central crystalline zone. We focus on the crystalline zone showing that its morphological and topographical characteristics vary with substrate temperature. The area of the crystalline zone is found to shrink with increasing substrate temperature. Additionally, the morphology of the crystalline structures changes from dendritic at 20 °C to cell-like for substrate temperatures between 25 and 40 °C. Calculation of the thermal and solutal Bénard-Marangoni numbers shows that while thermal effects are negligible when drying takes place at 20 °C, for higher substrate temperatures (25-40 °C), both thermal and solutal convective effects manifest within the drying drops. Thermal effects dominate earlier in the evaporation process leading, we believe, to the development of instabilities and, in turn, to the formation of convective cells in the drying drops. Solutal effects, on the other hand, are dominant toward the end of drying, maintaining circulation within the cells and leading to crystallization of salts in the formed cells. The cell-like structures are considered to form because of the interplay between thermal and solutal convection during drying. Dendritic growth is associated with a thicker fluid layer in the crystalline zone compared to cell-like growth with thinner layers. For cell-like structures, we show that the number of cells increases and the area occupied by each cell decreases with temperature. The average distance between cells decreases linearly with substrate temperature.

Transition from viscoelastic to fracture-like peeling of pressure-sensitive adhesives.

We investigate the process of the slow unrolling of a roll of typical pressure-sensitive adhesive, Scotch tape, under its own weight. Probing the peeling velocities down to nm s-1 resolution, which is three orders of magnitudes lower than earlier measurements, we find that the speed is still non-zero. Moreover, the velocity is correlated to the relative humidity. A humidity increase leads to water uptake, making the adhesive weaker and easier to peel. At very low humidity, the adhesive becomes so stiff that it mainly responds elastically, leading to a peeling process akin to interfacial fracture. We provide a quantitative understanding of the peeling velocity in the two regimes.

Fluorescent molecular rotor probes nanosecond viscosity changes.

Viscosity is a key property of liquids, but it is difficult to measure in short-lived, metastable samples due to the long measuring times required by conventional rheology. Here, we show how this problem can be solved by using fluorescent molecular rotors. The excited-state fluorescence decay rate of these molecules is sensitive to the viscosity of their local environment, and by combining pulsed laser excitation with time-resolved fluorescence detection, we can measure viscosities with a time resolution of a few ns. We demonstrate this by measuring in real time the viscosity change in glycerol induced by a nanosecond temperature jump. This new approach makes it possible to measure the viscosity of extremely short-lived states of matter.

Molecular rotors to probe the local viscosity of a polymer glass.

We investigate the local viscosity of a polymer glass around its glass transition temperature by using environment-sensitive fluorescent molecular rotors embedded in the polymer matrix. The fluorescence of the rotors depends on the local viscosity, and measuring the fluorescence intensity and lifetime of the probe therefore allows us to measure the local free volume in the polymer glass when going through the glass transition. This also allows us to study the local viscosity and free volume when the polymer film is put under an external stress. We find that the film does not flow homogeneously but undergoes shear banding that is visible as a spatially varying free volume and viscosity.

Needle-free jet injection-induced small-droplet aerosol formation during intralesional bleomycin therapy.

OBJECTIVES: Needle-free jet injectors are frequently used in dermatological practice. Injection-generated small-droplet aerosols could be harmful upon inhalation when chemotherapeutics, like bleomycin, are used. Here, we aim to explore jet injector-induced small-droplet aerosol formation of bleomycin in relation to air ventilation and to provide safety measures for clinical practice. MATERIALS AND METHODS: With a professional particle sensor, we measured airborne aerosol particles (0.2-10.0 µm) after electronic pneumatic injection (EPI), spring-loaded jet injection (SLI), and needle injection (NI) of bleomycin and saline (100 µl) on ex vivo human skin. Three levels of air ventilation were explored: no ventilation, room ventilation, and room ventilation with an additional smoke evacuator. RESULTS: EPI and SLI induced significant small-droplet aerosol formation compared with none after NI (0.2-1.0 µm; no ventilation). The largest bleomycin aerosol generation was observed for the smallest particles (0.2-1.0 µm) with 673.170 (528.802-789.453) aerosol particles/liter air (EPI; no ventilation). Room ventilation and smoke evacuation led to a reduction of ≥99% and 100% of measured aerosols, respectively. CONCLUSION: Jet injectors generate a high number of small-droplet aerosols, potentially introducing harmful effects to patients and healthcare personnel. Room ventilation and smoke evacuation are effective safety measures when chemotherapeutics are used in clinical practice.

Risk of aerosol transmission of SARS-CoV-2 in a clinical cardiology setting.

Cardiac exercise stress testing (CEST) is an important diagnostic tool in daily cardiology practice. However, during intense physical activity microdroplet aerosols, potentially containing SARS-CoV-2 particles, can persist in a room for a long time. This poses a potential infection risk for the medical staff involved in CEST, as well as for the patients entering the same room afterwards. We measured aerosol generation and persistence, to perform a risk assessment for SARS-CoV-2 transmission through aerosols during CEST. We find that during CEST, the aerosol levels remain low enough that SARS-CoV-2 transmission through aerosols is unlikely, with the room ventilation system producing 14 air changes per hour. A simple measurement of CO2 concentration gives a good indication of the ventilation quality.

Lubricated Friction and the Hersey Number.

Most frictional contacts are lubricated in some way, but is has proven difficult to measure and predict lubrication layer thicknesses and assess how they influence friction at the same time. Here we study the problem of rigid-isoviscous lubrication between a plate and a sphere, both experimentally and theoretically. The liquid layer thickness is measured by a novel method using inductive sensing, while the friction is measured simultaneously. The measured values of the layer thickness and friction on the disk are well described by the hydrodynamic description of liquid flowing through a contact area. This allows us to propose a modified version of the Hersey number that compares viscous to normal forces and allows us to rescale data for different geometries and systems. The modification overcomes the shortcomings of the commonly used Hersey number, adds the effects of the geometry of the configuration on the friction, and successfully predicts the lubrication layer thickness.