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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.

In this work, high resolution integrated AFM–EC/SECM was used to reveal the spatially heterogeneous electroactivity of microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) surfaces. During electrochemical corrosion, NCD surfaces undergo a stronger corrosion reaction than MCD because of the higher amount of sp2 hybridized carbon. In-situ EC-AFM imaging shows no significant change in surface morphology, while corroded surfaces become more hydrophilic due to the oxidation reactions that occur in the outermost layer. On non-corroded MCD and NCD surfaces, intercrystallite boundaries exhibit stronger localized (electro)chemical reactivity than crystallites. However, after electrochemical corrosion, both MCD and NCD surfaces become thermodynamically stabilized by corrosion products that passivate the surface and inhibit further corrosion. In this way, the (electro)chemical reactivity of the intercrystallite boundaries is reduced to a greater extent by electrochemical corrosion than the (electro)chemical reactivity of the crystallites due to the more intense electrochemical oxidation reactions taking place at these boundaries. After corrosion, this results in a comparatively greater (electro)chemical reactivity on the crystallites than at the boundaries. This behavior suggests the following order of (electro)chemical reactivity: sp2 > sp3 > oxidized/passivated structures.

Silicon nanoparticles (SiNPs) have been explored intensively for their use in applications requiring efficient fluorescence for LEDs, lasers, displays, photovoltaic spectral-shifting filters, and biomedical applications. High radiative rates are essential for such applications, and theoretically these could be achieved via quantum confinement and/or straining. Wet-chemical methods used to synthesize SiNPs are under scrutiny because of reported contamination by fluorescent carbon species. To develop a cleaner method, we utilize a specially designed attritor type high-energy ball-mill and use a high-purity (99.999%) Si microparticle precursor. The mechanochemical process is used under a continuous nitrogen gas atmosphere to avoid oxidation of the particles. We confirm the presence of quantum-confined NPs (

A high degree of spin polarization in half-metallic double perovskites is a prerequisite for several applications in spintronics, which depends crucially on the cationic order of the systems. This paper reports a study on tailoring the structure and morphology of nano-sized Sr2Fe1-xZnxMoO6 (x = 0.05, 0.1, 0.15) materials to improve their spin polarization. The combined analysis of synchrotron X-ray diffraction and magnetization data shows that Zn replaces Fe in the B sites. Although the majority of particles have lateral dimensions in the range 30–60 nm as observed by scanning electron microscope, the samples with x = 0.1 and 0.15 show finite-size effects with superparamagnetism below room temperature and a reduced Curie temperature (from 410 K for x = 0.05–390 K for x = 0.15). The results are due to the formation of networks of insulating Mo–O–Zn–O–Mo linkages and anti-phase boundaries, which divide the particles into smaller domains with a mean diameter of ∼11 nm as determined via a Langevin fit. The almost perfectly ordered structure in the nanodomains is responsible for a high magnetoresistance ratio. A value of -42% at 5 K in 50 kOe is recorded for the sample x = 0.15. Via fitting the magnetoresistance curve using the Inoue-Mekagawa theory, the spin polarization of 99% is determined.

Once doped away from their parent Mott insulating state, the hole-doped cuprates enter into many varied and exotic phases. The onset temperature of each phase is then plotted versus p-the number of doped holes per copper atom - to form a representative phase diagram. Apart from differences in the absolute temperature scales among the various families, the resultant phase diagrams are strikingly similar. In particular, the p values corresponding to optimal doping (popt ~ 0.16) and to the end of the pseudogap phase (p* ~ 0.19-0.20) are essentially the same for all cuprate families bar one: the single-layer Bi-based cuprate Bi2+z-yPbySr2-x-zLaxCuO6+d (Bi2201). This anomaly arises partly due to the complex stoichiometry of this material and also to the different p values inferred from disparate (e.g., bulk or surface) measurements performed on samples with comparable superconducting transition temperatures Tc. Here, by combining measurements of the in-plane resistivity in zero and high magnetic fields with angle-resolved photoemission spectroscopy studies in the superconducting and normal state, we argue that the phase diagram of Bi2201 may in fact be similar to that realized in other families. This study therefore brings Bi2201 into the fold and supports the notion of universality of popt and p* in all hole-doped cuprates.

Hydrodynamic lubrication is studied for both shear thinning and viscoelastic polymer solutions. We find that elasticity, notably strong normal stresses, does not change the friction significantly for the range of parameters tested in this manuscript. Shear-thinning properties, on the other hand, do change the formation of the lubricating layer thickness and the dependence of friction on velocity relative to Newtonian fluids. A hydrodynamic model that includes shear thinning is developed and compared to experimental data. The model describes the dependence on lubrication parameters well, but underestimates the lubricating layer thickness by a constant factor of roughly 1.5. The theory allows us to define a Hersey-like number for shear-thinning fluids that describes the lubricating layer thickness as a result of the balance between normal load and viscous force. For each tested liquid it succeeds in collapsing friction measurements onto the same curve. The friction analysis for both lubrication theory and experiments then reveals that shear thinning mainly changes the layer thickness, which is the main determinant of the friction coefficient.

Environmentally sensitive molecular rotors are widely used to probe the local molecular environment in e.g. polymer solutions, polymer glasses, and biological systems. These applications make it important to understand its fluorescence properties in the vicinity of a solid surface, since fluorescence microscopy generically employs cover slides, and measurements are often done in its immediate vicinity. Here, we use a confocal microscope to investigate the fluorescence of (4-daspi) in glycerol/water solutions close to the interface using hydrophilic or hydrophobic cover slips. Despite the dye’s high solubility in water, the observed lengthening of the fluorescence lifetime close to the hydrophobic surface, implies a surprising affinity of the dye with the surface. Because the homogeneous solution and the refractive index mismatch reduces the optical sectioning power of the microscope, we quantify the affinity with the help of a simple model of the signal vs. depth of focus, exhibiting surface and bulk contributions. The model reduces artefacts due to refractive index mismatch, as supported by Monte Carlo ray tracing simulations.

Pathways to tune the electronic, chemical, mechanical, and optical properties of solids without modifying their composition represent a new paradigm in the design of functional and sustainable materials. The level of structural disorder - from perfectly crystalline to fully amorphous – for example, induces remarkable changes in material properties. Typically, disorder is introduced by altering the composition of a material, adding to the misconception that these two properties cannot be decoupled. Here, we demonstrate that striking differences in the optical, electronic, and corrosion properties of CuZr are achieved by deliberately and reproducibly engineering the level of structural disorder in pulsed laser deposited thin films of a constant composition. This approach allows tuning the structure of CuZr from polycrystalline to fully amorphous, switching the nature of charge transport from metallic to semiconductor-like, the optical properties in the visible regime from opaque to transparent, and the corrosion behavior in air from mixed oxidation to the formation of a protective Zr oxide overlayer. Our results highlight the tunability of structural disorder in alloys and its remarkable effect on material properties, providing the opportunity to design sustainable functional materials based on customizing properties beyond their composition.

The narrow band gap semiconductors Bi2Te3 and Sb2Te3 are well known for their room temperature thermoelectric performance. Recently, they were shown to serve as model systems of three-dimensional topological insulators with a bulk band gap and very robust, spin-resolved surface states due to a special band inversion in their periodic bulk. Evidently, it is of interest to investigate the special surface states with a local probe like nuclear magnetic resonance (NMR). However, especially the bulk NMR of these materials shows peculiar and rather unexpected phenomena, e. g., a magnetic field induced deformation of the local charge symmetry and excessive line broadening due to a special internuclear coupling. Here we report a comprehensive account of the room temperature NMR properties of the spin-1/2 nucleus 125Te of single crystalline Bi2Te3 and Sb2Te3. This includes a very unusual spin-lattice relaxation that seems not to be reflected in the NMR shift, as well as an exchange enhanced NMR line broadening.

The charging of poorly conducting liquids due to flows is a well-known phenomenon, yet the precise charging mechanism is not fully understood. This is especially relevant for sprays, where the spray plume dynamics and maximum distance travelled of a spray dramatically changes for different levels of charging: charging of the droplets makes them repel, thereby preventing drop coalescence and altering the shape of the spray plume. As the charging depends on many factors including the flow and the interactions between the liquid and the nozzle, many models and scaling laws exist in the literature. In this work we focus on perhaps the simplest flow regime, laminar jets created by ultra short channels, and quantify the charging as a function of the different parameters. We present a simple model that collapses all the data for over 4 orders of magnitude difference in streaming currents for various nozzle sizes, flow velocities and surface treatments. We further show that the charging polarity can even be reversed by applying an oppositely charged coating to the nozzle, an important step for any application.

During the high velocity impact of an object on a solid covered with a thin fluid layer, a lubricated contact exists within the short time in which the liquid is squeezed out from the contact. This is important for e.g. the grip of shoes on wet surfaces. We experimentally study the squeeze flow of such layers and find that the amount of viscous dissipation determines how much fluid remains in the contact after the kinetic energy of the impacting object is absorbed. For impacts with sufficient amount of kinetic energy, it is possible to completely drain a Newtonian or shear thinning fluid from the contact on a short time scale. Viscoelastic liquids, however, cannot be drained by increasing impact velocity, because of the fluids’ elastic tendency to retract back into the contact after rapid squeezing. This explains why the presence of polymeric fluids can lead to extreme slipperiness of surfaces. Furthermore, we show that all our experimental results agree with the predictions given by hydrodynamic theory applied to the fluid flow in the gap.

Magnetic dilution of a well-established Kitaev candidate system is realized in the substitutional Ru$_{1-x}$Rh$_x$Cl$_3$ series ($x = 0.02-0.6$). Optimized syntheses protocols yield uniformly-doped single crystals and polycrystalline powders that are isostructural to the parental $α$-RuCl$_3$ as per X-ray diffraction. The Rh content $x$ is accurately determined by the quantitative energy-dispersive X-ray spectroscopy technique with standards. We determine the magnetic phase diagram of Ru$_{1-x}$Rh$_x$Cl$_3$ for in-plane magnetic fields from magnetization and specific-heat measurements as a function of $x$ and stacking periodicity, and identify the suppression of the magnetic order at $x \approx 0.2$ towards a disordered phase, which does not show any clear signature of freezing into a spin glass. Comparing with previous studies on the substitution series Ru$_{1-x}$Ir$_x$Cl$_3$, we propose that chemical pressure would contribute to the suppression of magnetic order especially in Ru$_{1-x}$Ir$_x$Cl$_3$ and that the zigzag magnetic ground state appears to be relatively robust with respect to the dilution of the Kitaev--$Γ$--Heisenberg magnetic lattice. We also discovered a slight dependence of the magnetic properties on thermal cycling, which would be due to an incomplete structural transition.

High entropy materials (HEMs) are of great interest for their mechanical, chemical and electronic properties. In this paper we analyse (TaNbHfTiZr)C, a carbide type of HEM, both in crystalline and amorphous phases, using density functional theory (DFT). We find that the relaxed lattice volume of the amorphous phase is larger, while its bulk modulus is lower, than that of its crystalline counterpart. Both phases are metallic with all the transition metals contributing similarly to the density of states (DOS) close to the Fermi level, with Ti and Nb giving the proportionally largest contribution of states. We confirm that despite its great structural complexity, 2x2x2 supercells are large enough for reliable simulation of the presented mechanical and electronic properties by DFT., Comment: 10 pages, 6 figures

The interface of manganese(II) phtalocyanine (MnPc) with an α-RuCl3 single crystal is studied by means of photoelectron spectroscopy. The organic molecule was evaporated on top of an in situ cleaved α-RuCl3 bulk crystal in steps. The work function and electron affinity of α-RuCl3 is determined to be 6.3 and 5.4 eV, respectively, in line with common fluorinated acceptor molecules, but with the potential to absorb more electrons per area at interfaces with other materials. These favorable electronic properties lead to a charge transfer from MnPc to α-RuCl3 which manifests itself mainly in peak shape changes of core level photoemission spectra and further filling of Ru 4d states as revealed by valence band photoemission. This work demonstrates the promising potential of α-RuCl3 as a strong acceptor material for interfaces in general, and for organic molecules in particular.

Living microorganisms in confined systems typically experience an affinity to populate boundaries. The reason for such affinity to interfaces can be a combination of their directed motion and hydrodynamic interactions at distances larger than their own size. Here we will show that self-propelled Janus particles (polystyrene particles partially coated with platinum) immersed in droplets of water and hydrogen peroxide tend to accumulate in the vicinity of the liquid/gas interface. Interestingly, the interfacial accumulation occurs despite the presence of an evaporation-driven flow caused by a solutal Marangoni flow, which typically tends to redistribute the particles within the droplet's bulk. By performing additional experiments with passive colloids (flow tracers) and comparing with numerical simulations for both particle active motion and the fluid flow, we disentangle the dominating mechanisms behind the observed interfacial particle accumulation. These results allow us to make an analogy between active Janus particles and some biological microswimmers concerning how they interact with their environment., 9 pages, 7 figures

We report a detailed experimental and theoretical study on the effect of hydrostatic pressure on various structural and magnetic aspects of the layered honeycomb antiferromagent α-RuCl3. Through measurements of the magnetic susceptibility χ performed under almost ideal hydrostatic-pressure conditions by using helium as a pressure-transmitting medium, we find that the phase transition to zigzag-type antiferromagnetic order at TN = 7.3 K can be rapidly suppressed to about 6.1 K at a weak pressure of about 94 MPa. A further suppression of TN with increasing pressure is impeded, however, due to the occurrence of a pressure-induced structural transition at p≥ 104 MPa, accompanied by a strong dimerization of Ru-Ru bonds, which gives rise to a collapse of the magnetic susceptibility. Whereas the dimerization transition is strongly first order, the magnetic transition under varying pressure and magnetic field also reveals indications for a weakly first-order transition. We assign this observation to a strong magnetoelastic coupling in this system. Measurements of χ under varying pressure in the paramagnetic regime (T>TN) and before dimerization (p< 100 MPa) reveal a considerable increase of χ with pressure. These experimental observations are consistent with the results of ab initio density functional theory (DFT) calculations on the pressure-dependent structure of α-RuCl3 and the corresponding pressure-dependent magnetic model. We find that pressure strengthens the nearest-neighbor Heisenberg J and off-diagonal anisotropic Γ coupling and simultaneously weakens the Kitaev K and anisotropic Γ′ coupling. Comparative susceptibility measurements on a second crystal showing two consecutive magnetic transitions instead of one, indicating the influence of stacking faults, reveal that by the application of different temperature-pressure protocols the effect of these stacking faults can be temporarily overcome.

Active solids consist of elastically coupled out-of-equilibrium units performing work. They are central to autonomous processes, such as locomotion, self-oscillations and rectification, in biological systems,designer materials and robotics. Yet, the feedback mechanism between elastic and active forces, and the possible emergence of collective behaviours in a mechanically stable elastic solid remains elusive. Here we introduce a minimal realization of an active elastic solid, in which we characterize the emergence of selective and collective actuation and fully map out the interplay between activity, elasticity and geometry. Polar active agents exert forces on the nodes of a two dimensional elastic lattice. The resulting displacement field nonlinearly reorients the active agents. For large enough coupling, a collective oscillation of the lattice nodes around their equilibrium position emerges. Only a few elastic modes are actuated and, crucially, they are not necessarily the lowest energy ones. Combining experiments with the numerical and theoretical analysis of an agents model, we unveil the bifurcation scenario and the selection mechanism by which the collective actuation takes place. Our findings may provide a new mechanism for oscillatory dynamics in biological tissues and specifically confluent cell monolayers. The present selection mechanism may also be advantageous in providing meta-materials, with bona fide autonomy., 6 pages, 4 figures

Lattice defects such as stacking faults may obscure electronic topological features of real materials. In fact, defects are a source of disorder that can enhance the density of states and conductivity of the bulk of the system and they break crystal symmetries that can protect the topological states. On the other hand, in recent years it has been shown that lattice defects can act as a source of nontrivial topology. Motivated by recent experiments on three-dimensional (3D) topological systems such as Bi$_2$TeI and Bi$_{14}$Rh$_3$I$_9$, we examine the effect of stacking faults on the electronic properties of weak topological insulators (WTIs). Working with a simple model consisting of a 3D WTI formed by weakly-coupled two-dimensional (2D) topological layers separated by trivial spacers, we find that 2D stacking faults can carry their own, topologically nontrivial gapless states. Depending on the WTI properties, as well as the way in which the stacking fault is realized, the latter can form a topologically protected 2D semimetal, but also a 2D topological insulator which is embedded in the higher-dimensional WTI bulk. This suggests the possibility of using stacking faults in real materials as a source of topologically nontrivial, symmetry-protected conducting states., Comment: 9 pages, 7 figures

The influence of corrosion upon the nanoscale topography and friction response of a hydrogenated amorphous carbon film (a-C:H) was investigated. Electrochemical atomic force microscopy was used to characterise topographical changes to the coating at two oxidative potentials. Corrosion of the coating at 1.5 V (corrosion rate 0.5 nm h−1) resulted in no changes to the nanoscale topography; whereas corrosion at 2.5 V (corrosion rate 26.4 nm h−1) caused the root mean square roughness of the a-C:H film topography to decrease, but the local fine-scale irregularity or ‘jaggedness’ of the surface to increase. X-ray photoelectron spectroscopy revealed that corrosion at both potentials oxidised the a-C:H surface to form alcohol, carbonyl and carboxyl groups. Lateral force microscopy and adhesion force measurements showed that both the friction force and surface adhesion of the coating increased upon corrosion. The outcome was attributed to the surface oxidation that had occurred at both oxidative potentials, resulting in several potential mechanisms including increased attractive intermolecular interactions and capillary forces. The highest friction coefficient was observed for the a-C:H film corroded at 2.5 V, and identified as a consequence of the jagged surface topography promoting an interlocking friction mechanism.

BACKGROUND: There is a persistent concern over the risk of respiratory pathogen transmission, including SARS-CoV-2, via the formation of aerosols (ie, a suspension of microdroplets and residual microparticles after evaporation) generated during high-flow nasal cannula (HFNC) oxygen therapy in critically ill patients. This concern is fueled by limited available studies on this subject. In this study, we tested our hypothesis that HFNC treatment is not associated with increased aerosol formation as compared to conventional oxygen therapy. METHODS: We used laser light scattering and a handheld particle counter to detect and quantify aerosols in healthy subjects and in adults with acute respiratory disease, including COVID-19, during HFNC or conventional oxygen therapy.RESULTS: The use of HFNC was not associated with increased formation of aerosols as compared to conventional oxygen therapy in both healthy subjects (n = 3) and subjects with acute respiratory disease, including COVID-19 (n = 17).CONCLUSIONS: In line with scarce previous clinical and experimental findings, our results indicate that HFNC itself does not result in overall increased aerosol formation as compared to conventional oxygen therapy. This suggests there is no increased risk of respiratory pathogen transmission to health care workers during HFNC.

We study drop impact for the case where the impacted surface is cooled below the freezing temperature of the liquid droplet. The freezing is found to affect the spreading dynamics of the impacting drops and, thus, the degree of surface coverage. The cooling of the surface leads to the arrest of the three-phase contact line, impeding droplet spreading and, thus, drastically reducing the maximum spreading diameter. Besides the surface temperature, the impact speed is also an important parameter: the higher the impact speed, the more the droplet spreads before arrest. Based on experimental observations of droplet impacts using two different liquids and two different substrates, we show using a combination of experiments and a one-dimensional freezing model, that droplet arrest occurs when a solid layer of the liquid forms on the substrate: droplet arrest occurs when this solid layer reaches a well-defined critical thickness. We then devise a simple model that efficiently predicts the maximum spreading diameter of droplets impinging, at different velocities, and freezing onto surfaces maintained at different temperatures below the liquid freezing point.

Unlocking the mystery of the strange metal state has become the focal point of high-Tc research, not because of its importance for superconductivity, but because it appears to represent a truly novel phase of matter dubbed "quantum supreme matter."Detected originally through high magnetic field, transport experiments, signatures of this phase have now been uncovered with a variety of probes. Our high resolution optical data of the low-Tc cuprate superconductor, Bi2-xPbxSr2-yLayCuO6+δ allows us to probe this phase over a large energy and temperature window. We demonstrate that the optical signatures of the strange metal phase persist throughout the phase diagram. The strange metal signatures in the optical conductivity are twofold: (i) a low energy Drude response with Drude width on the order of temperature and (ii) a high energy conformal tail with a doping dependent power-law exponent. While the Drude weight evolves monotonically throughout the entire doping range studied, the spectral weight contained in the high energy conformal tail appears to be doping and temperature independent. Our analysis further shows that the temperature dependence of the optical conductivity is completely determined by the Drude parameters. Our results indicate that there is no critical doping level inside the superconducting dome where the carrier density starts to change drastically and that the previously observed "return to normalcy"is a consequence of the increasing importance of the Drude component relative to the conformal tail with doping. Importantly, both the doping and temperature dependence of the resistivity are largely determined by the Drude width.

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.

Active enhancement of the optical absorption coefficient to improve the light converting efficiency of thin-film solar cell materials is crucial to develop the next-generation solar cell devices. Here we report first-principles calculations with generalized gradient approximation to study the optoelectronic properties of pristine and divacancy (DV) blue phosphorene (BlueP) thin films under structural deformation. We show that instead of forming sp-like covalent bonds as in the pristine BlueP layer, a DV introduces two particular dangling bonds between the voids. Using a microscopic (non-) affine deformation model, we reveal that the orbital hybridization of these dangling bonds is strongly modified in both the velocity and vorticity directions depending on the type of deformation, creating an effective light trap to enhance the material absorption efficiency. Furthermore, this successful light trap is complemented by a clear signature of σ + π plasmon when a DV BlueP layer is slightly compressive. These results demonstrate a practical approach to tailor the optoelectronic properties of low-dimensional materials and to pave a novel strategy to design functionalized solar cell devices from the bottom-up with selective defects.

The effect of viscoelasticity on sprays produced from agricultural flat fan nozzles is investigated experimentally using dilute aqueous solutions of polyethylene oxide (PEO). Measurements of the droplet size distribution using laser diffraction reveal that polymer addition to water results in the formation of overall bigger droplets with a broader size distribution. The median droplet size $D_{50}$ is found to increase linearly with the extensional relaxation time of the liquid. The non-dimensional median droplet sizes of different polymer solutions, sprayed at different operating pressures from nozzles of different sizes, rescale on a single master curve when plotted against an empirical function of the Weber and Deborah numbers. Using high-speed photography of the spraying process, we show that the increase in droplet size with viscoelasticity can be partly attributed to an increase of the wavelength of the flapping motion responsible for the sheet breakup. We also show that droplet size distributions, rescaled by the average drop size, are well described by a compound gamma distribution with parameters $n$ and $m$ encoding for the ligament corrugation and the width of the ligament size distribution, respectively. These parameters are found to saturate to values $n=4$ and $m=4$ at high polymer concentrations., Comment: 21 pages, 11 figures

Wear of multi-asperity interfaces remains difficult to predict from first principles, in part because improvements are required in our ability to quantify and track wear across the micro-to nanometer scale. In this work, we developed a 6° of freedom topographical difference method based on large atomic force microscopy (AFM) measurements, up to 90 × 90 μm2 in size. We detect wear volumes as small as 1.6 × 10−11 ± 3.7 × 10−12 mm3 (0.016 μm3), beyond the sensitivity of many existing techniques for the quantification of wear at multi-asperity interfaces. We show that our wear detection technique can be combined with 100 mN normal force ball-on-flat friction experiments to track nanoscale wear across the entire area of apparent contact.

Low-dimensional and lead-free halide perovskites are of great interest for their wide application potential for optoelectronic applications. We report on the successful synthesis of novel lead-free colloidal Cs3BiBr6 nanocrystals (NCs) with an ultra-small size of ∼1.5–3 nm by a room temperature antisolvent process. From crystallographic characterization we show that it is critical to precisely control the ratio of precursors to obtain the pure 3-1-6 phase. The synthesis process is facile and repeatable and results in Cs3BiBr6 NCs that display stable blue emission around 438 nm with a relatively broad linewidth of 92.1 nm. Low-temperature photoluminescence (PL) measurements displayed a red-shift of bandgap with decreasing temperature, which might be attributed to the thermal expansion of the lattice. In addition, the NCs demonstrate high stability at ambient conditions., We are grateful for the Dutch Technology Foundation STW, The Netherlands Organization for Scientific Research (NWO), and the Joint Solar Program (JSP III, 680-91-011) of The NWO for financial support.

Silicon nanoparticles (Si-NPs) represent one of many types of nanomaterials, where the origin of emission is difficult to assess due to a complex interplay between the core and surface chemistry. Band-gap tunability in Si-NPs is predicted to span from the infrared to the ultraviolet spectral range, which is rarely observed in practice. In this work, we directly assess the size dependence of the optical band gap using a single-dot correlative microscopy tool, where the size of the individual NPs is measured using atomic force microscopy (AFM) and the optical band gap is evaluated from single-dot photoluminescence measured on the very same NPs. We analyze 2–8 nm alkyl-capped Si-NPs prepared by a sol–gel method, followed by annealing at 1300 °C. Surprisingly, we find that the optical band gap is given by the amorphous shell, as evidenced by the convergence of the optical band gap size dependence toward the amorphous Si band gap of ∼1.56 eV. We propose that the structural disorder might be the reason behind the often reported limited emission tunability from various Si-NPs in the literature. We believe that our message points toward a pressing need for development and broader use of such direct correlative single-dot microscopy methods to avoid possible misinterpretations that could arise from attempts to recover size–band gap relation from ensemble methods, as practiced nowadays.

We report on enhanced detection of strain waves in gold gratings, using a probe laser pulse tuned at and close to the surface plasmon-polariton (SPP) resonance of the gratings. The enhancement, of up to twenty times, is observed in the strain-wave-induced reflection changes and depends on the type of acoustic wave. For the surface acoustic wave, the enhancement of the optical reflection is greatest at the peak of the resonance, while strain-wave-induced reflection changes of other types of strain waves show their strongest enhancement when the probe pulse is tuned to the wings of the SPP resonance.

The transition from static to dynamic friction is often described as a fracture instability. However, studies on slow sliding processes aimed at understanding frictional instabilities and earthquakes report slow friction transients that are usually explained by empirical rate-and-state formulations. We perform very slow ($$\sim$$ ∼ nm/s) macroscopic-scale sliding experiments and show that the onset of frictional slip is governed by continuous non-monotonic dynamics originating from a competition between contact aging and shear-induced rejuvenation. This allows to describe both our non-monotonic dynamics and the simpler rate-and-state transients with a single evolution equation.

Free energy is a key thermodynamic observable that controls the elusive physics of the glass transition. However, measuring the free energy of colloidal glasses from microscopy images is challenging due to the difficulty of measuring the individual particle size in the slightly polydisperse glassy systems. In this paper, we carry out experiments and numerical simulations of colloidal glasses with the aim to find a practical approach to measure the free energy from colloidal particles at mild polydispersity. We propose a novel method which requires only the particle coordinates from a few confocal microscopy snapshots to estimate the average particle diameter and use it as an input for our experimental free energy measurements. We verify our free energy calculations from Cell Theory with the free energy obtained by Thermodynamic Integration. The excellent agreement between the free energies measured using the two methods close to the glass transition packing fraction highlights the dominant role played by vibrational entropy in determining a colloidal glass’s free energy. Finally, the noticeable free energy difference calculated from uniform and conjectured particle sizes emphasizes the sensitivity on particle free volumes when measuring free energy in the slightly polydisperse colloidal glass.

We study the correlation clustering problem using the quantum approximate optimization algorithm (QAOA) and qudits, which constitute a natural platform for such non-binary problems. Specifically, we consider a neutral atom quantum computer and propose a full stack approach for correlation clustering, including Hamiltonian formulation of the algorithm, analysis of its performance, identification of a suitable level structure for ${}^{87}{\rm Sr}$ and specific gate design. We show the qudit implementation is superior to the qubit encoding as quantified by the gate count. For single layer QAOA, we also prove (conjecture) a lower bound of $0.6367$ ($0.6699$) for the approximation ratio on 3-regular graphs. Our numerical studies evaluate the algorithm's performance by considering complete and Erd\H{o}s-R\'enyi graphs of up to 7 vertices and clusters. We find that in all cases the QAOA surpasses the Swamy bound $0.7666$ for the approximation ratio for QAOA depths $p \geq 2$. Finally, by analysing the effect of errors when solving complete graphs we find that their inclusion severely limits the algorithm's performance., Comment: 30+12 pages, 14 figures, accepted into Quantum

Cesium lead halides are a family of bright, visible-light emitting materials with near-unity photoluminescence quantum yield (PLQY) in nanocrystals (NCs). The usual way to achieve visible light-emission tunability is by mixing halides, which often leads to phase separation and poor stability. While the NCs should also show size-dependent PL emission, reports on strong quantum confinement in these materials are scarce. Here, we report the synthesis of quantum-confined cesium lead bromide (CsPbBr3) NCs via a facile, environment-friendly, and scalable high-energy mechanochemical synthesis route. The PLQY measured is ∼85%, even after 90 days of synthesis, and the emission wavelength is shifted from green, 520 nm, to blue, 460 nm by quantum confinement in NCs of size 3–5 nm. Micro-PL optical spectroscopy and atomic force microscopy confirm the size tunability of PL on a single-dot scale. Our work demonstrates the potential of mechanochemical synthesis in the medium-scale production of bright luminescent quantum-confined NCs that could be extended to other materials as well.

We measure chemical reactions between a single trapped $^{174}$Yb$^+$ ion immersed in an ultracold bath of $^6$Li atoms containing trace amounts of Li$_2$ dimers. This produces LiYb$^+$ molecular ions that we detect via mass spectrometry. We explain the reaction rates by modelling the dimer density as a function of the magnetic field and obtain excellent agreement when we assume the reaction to follow the Langevin rate. Our results present a novel approach towards the creation of cold molecular ions and point to the exploration of ultracold chemistry in ion molecule collisions. What is more, with a detection sensitivity below molecule densities of $10^{14}\,\mathrm{m}^{-3}$, we provide a new method to detect low-density molecular gases., Comment: 10 pages, 8 figures

Reaching ultracold temperatures within hybrid atom–ion systems is a major limiting factor for control and exploration of the atom–ion interaction in the quantum regime. In this work, we present results on numerical simulations of trapped ion buffer gas cooling using an ultracold atomic gas in a large number of experimentally realistic scenarios. We explore the suppression of micromotion-induced heating effects through optimization of trap parameters for various radio-frequency (rf) traps and rf driving schemes including linear and octupole traps, digital Paul traps, rotating traps and hybrid optical/rf traps. We find that very similar ion energies can be reached in all of them even when considering experimental imperfections that cause so-called excess micromotion. Moreover we look into a quantum description of the system and show that quantum mechanics cannot save the ion from micromotion-induced heating in an atom–ion collision. The results suggest that buffer gas cooling can be used to reach close to the ion’s groundstate of motion and is even competitive when compared to some sub-Doppler cooling techniques such as Sisyphus cooling. Thus, buffer gas cooling is a viable alternative for ions that are not amenable to laser cooling, a result that may be of interest for studies into cold controlled quantum chemistry and charged impurity physics.

We investigate the H/D isotope effect in a liquid-liquid transition in crystallization-resistant supercooled aqueous solution. Using steady-state infrared spectroscopy and measuring thermal conductivities, we observe a ≈5 K shift in the onset temperature of the liquid-liquid transition, a value that is similar to the H/D isotope shifts of the HDA-LDA transition and of the Widom line of pure supercooled water [Kim et al., Science 358, 1589 (2017)], and that highlights the importance of isotope effects in water's supercooled regime. By employing multivariate data analysis we find isosbestic point related to the phase transformation in both isotopic systems, indicating phase coexistence during the transition. This article is dedicated to the memory of Austen Angell, and we begin with some personal memories of his two stays at the University of Amsterdam.

Proceedings of the National Academy of Sciences of the United States of America 117(48), 30220 - 30227 (2020). doi:10.1073/pnas.2005701117, Using inelastic X-ray scattering beyond the dipole limit and hard X-ray photoelectron spectroscopy we establish the dual nature of the U 5f electrons in UM$_2$Si$_2$ (M = Pd, Ni, Ru, Fe), regardless of their degree of delocalization. We have observed that the compounds have in common a local atomic-like state that is well described by the U 5f$^2$ configuration with the $Γ^{(1)}_1$ and $Γ_2$ quasi-doublet symmetry. The amount of the U 5f$^3$ configuration, however, varies considerably across the UM$_2$Si$_2$ series, indicating an increase of U 5f itineracy in going from M = Pd to Ni to Ru and to the Fe compound. The identified electronic states explain the formation of the very large ordered magnetic moments in UPd$_2$Si$_2$ and UNi$_2$Si$_2$, the availability of orbital degrees of freedom needed for the hidden order in URu2Si2 to occur, as well as the appearance of Pauli paramagnetism in UFe$_2$Si$_2$. A unified and systematic picture of the UM$_2$Si$_2$ compounds may now be drawn, thereby providing suggestions for additional experiments to induce hidden order and/or superconductivity in U compounds with the tetragonal body-centered ThCr$_2$Si$_2$ structure., Published by National Acad. of Sciences, Washington, DC

We present an experimental study in which we compare two different pump–probe setups to generate and detect high-frequency laser-induced ultrasound for the detection of gratings buried underneath optically opaque metal layers. One system is built around a high-fluence, low-repetition-rate femtosecond laser (1 kHz) and the other around a low-fluence, high-repetition-rate femtosecond laser (5.1 MHz). We find that the signal diffracted by the acoustic replica of the grating as a function of pump–probe time delay is very different for the two setups used. We attribute this difference to the presence of a constant background field due to optical scattering by interface roughness. In the low-fluence setup, the optical field diffracted by the acoustic replica is significantly weaker than the background optical field, with which it can destructively or constructively interfere. For the right phase difference between the optical fields, this can lead to a significant “amplification” of the weak field diffracted off the grating-shaped acoustic waves. For the high-fluence system, the situation is reversed because the field diffracted off the acoustic-wave-induced grating is significantly larger than the background optical field. Our measurements show that optical scattering by interface roughness must be taken into account to properly explain experiments on laser-induced ultrasound performed with high-repetition-rate laser systems and can be used to enhance signal strength.

Background When spraying simple liquids through a hydraulic nozzle, the mechanisms that affect breakup into droplets have recently been described. However, it is unknown how the droplet size distribution changes when surfactant‐based adjuvants are added to the spray. Results When spraying different surfactant‐based solutions containing commercial adjuvants, the breakup of the different liquids behaves in the same way as for water‐only sprays, but the droplet sizes are smaller. By replacing the equilibrium surface tension with the dynamic surface tension at a surface age of ~20 ms, the volume mean droplet size variation with the Weber number, flow rate and nozzle geometry follows the predictions established for pure water sprays. When we rescale the droplet size distribution with the mean droplet size, all distributions collapse onto a single curve and can be described by a compound Gamma function. Conclusion Addition of a number of surfactant‐based adjuvants to an agricultural spray is observed to lead to a slight decrease in the volume mean droplet size. We show that the effects of these adjuvants on the drop size can be understood by taking into account the nozzle geometry, the flow rate, the liquid and air densities and the dynamic surface tension of the surfactant solutions at a surface age of approximately 20 ms. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry., Surfactant additives are often used for agricultural sprays; we show that this leads to smaller droplets due to the dynamic surface tension. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Inorganic perovskites display an enticing foreground for their wide range of optoelectronic applications. Recently, supercrystals (SCs) of inorganic perovskite nanocrystals (NCs) have been reported to possess highly ordered structure as well as novel collective optical properties, opening new opportunities for efficient films. Here, we report the large-scale assembly control of spherical, cubic, and hexagonal SCs of inorganic perovskite NCs through templating by oil-in-oil emulsions. We show that an interplay between the roundness of the cubic NCs and the tension of the confining droplet surface sets the superstructure morphology, and we exploit this interplay to design dense hyperlattices of SCs. The SC films show strongly enhanced stability for at least two months without obvious structural degradation and minor optical changes. Our results on the controlled large-scale assembly of perovskite NC superstructures provide new prospects for the bottom-up production of optoelectronic devices based on the microfluidic production of mesoscopic building blocks., We are grateful for the Dutch Technology Foundation STW, The Netherlands Organization for Scientific Research (NWO), and the Joint Solar Program (JSP III, 680-91-011) of The NWO for financial support.

A recently developed emulsion-templated assembly method promises the scalable, low-cost, and reproducible fabrication of hierarchical nanocrystal (NC) superstructures. These superstructures derive properties from the unique combination of choices of NC building blocks and superstructure morphology and therefore realize the concept of "artificial solids". To control the final properties of these superstructures, it is essential to control the assembly conditions that yield distinct architectural morphologies. Here, we explore the phase-space of experimental parameters describing the emulsion-templated assembly including temperature, interfacial tension, and NC polydispersity and demonstrate which conditions lead to the growth of the most crystalline NC superstructures or supercrystals. By using a combination of electron microscopy and small-angle X-ray scattering, we show that slower assembly kinetics, softer interfaces, and lower NC polydispersity contribute to the formation of supercrystals with grain sizes up to 600 nm, while reversing these trends yields glassy solids. These results provide a clear path to the realization of higher-quality supercrystals, necessary to many applications.

Imaging and manipulating individual atoms with submicrometer separation can be instrumental for quantum simulation of condensed matter Hamiltonians and quantum computation with neutral atoms. Quantum gas microscope experiments in most cases rely on quite costly solutions. Here we present an open-source design of a microscope objective for atomic strontium consisting solely of off-the-shelf lenses that is diffraction-limited for 461${\,}$nm light. A prototype built with a simple stacking design is measured to have a resolution of 0.63(4)${\,\mu}$m, which is in agreement with the predicted value. This performance, together with the near diffraction-limited performance for 532${\,}$nm light makes this design useful for both quantum gas microscopes and optical tweezer experiments with strontium. Our microscope can easily be adapted to experiments with other atomic species such as erbium, ytterbium, and dysprosium, as well as Rydberg experiments with rubidium.

We report on femtosecond laser ablation experiments on percolating gold layers deposited on a glass substrate. In our experiments, we measure changes in optical transmission and reflection induced by single, high-intensity infrared laser pulses as a function of the time delay between the pump and the probe. For the highest pump intensities we find that on a time scale of about 150 ps after excitation, the transmission and reflection approach values close to the substrate values. We attribute this rapid ablation to vaporization of the entire layer when the injected energy exceeds the cohesive energy of the material. This vaporization results in the rapid transformation of the gold layer into a sufficiently dilute mist of atoms and nano-particles which renders the material almost optically transparent to the probe pulse. SEM images of the surfaces show how the morphology of the films changes at relatively low excitation intensities and show the complete removal of the gold at high intensities. We find that the ablation threshold for percolating Au on glass is 2.3 × 1011 W/cm2, which is two orders of magnitude lower than the damage threshold for continuous gold layers as reported in the literature.

Everyone wants clean air, peace and other public goods but is tempted to freeride on others’ efforts. The usual way out of this dilemma is to impose norms, maintain reputations and incentivize individuals to contribute. In situations of high uncertainty, however, such as confrontations of protesters with a dictatorial regime, the usual measures are not feasible, but cooperation can be achieved nevertheless. We use an Ising model with asymmetric spins that represent cooperation and defection to show numerically how public goods can be realized. Under uncertainty, people use the heuristic of conformity. The turmoil of a confrontation causes some individuals to cooperate accidentally, and at a critical level of turmoil, they entail a cascade of cooperation. This critical level is much lower in small networks.

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.

We studied the nature of excitons in the transition metal dichalcogenide alloy Mo0.6W0.4S2 compared to pure MoS2 and WS2 grown by atomic layer deposition (ALD). For this, optical absorption/transmission spectroscopy and time-dependent density functional theory (TDDFT) were used. The effects of temperature on A and B exciton peak energies and line widths in optical transmission spectra were compared between the alloy and pure MoS2 and WS2. On increasing the temperature from 25 to 293 K, the energy of the A and B exciton peaks decreases, while their line width increases due to exciton-phonon interactions. The exciton-phonon interactions in the alloy are closer to those for MoS2 than those for WS2. This suggests that exciton wave functions in the alloy have a larger amplitude on Mo atoms than that on W atoms. The experimental absorption spectra could be reproduced by TDDFT calculations. Interestingly, for the alloy, the Mo and W atoms had to be distributed over all layers. Conversely, we could not reproduce the experimental alloy spectrum by calculations on a structure with alternating layers, in which every other layer contains only Mo atoms and the layers in between also contain W atoms. For the latter atomic arrangement, the TDDFT calculations yielded an additional optical absorption peak that could be due to excitons with some charge transfer character. From these results, we conclude that ALD yields an alloy in which Mo and W atoms are distributed uniformly among all layers.

We show that the surface of ice is scratch healing: micrometer-deep scratches in the ice surface spontaneously disappear by thermal relaxation on the time scale of roughly an hour. Following the dynamics and comparing it to different mass transfer mechanisms, we find that sublimation from and condensation onto the ice surface is the dominant scratch-healing mechanism. The scratch-healing kinetics shows a strong temperature dependence, following an Arrhenius behavior with an activation energy of ΔE = 58.6 ± 4.6 kJ/mol, agreeing with the proposed sublimation mechanism and at odds with surface diffusion or fluid flow or evaporation-condensation from a quasi-liquid layer.

We report a doping study directed to intentionally induce disorder in PdTe$_2$ by the isoelectronic substitution of Pt. Two single-crystalline batches Pd$_{1-x}$Pt$_x$Te$_2$ have been prepared with nominal doping concentrations x = 0.05 and x = 0.10. Sample characterization by energy dispersive x-ray spectroscopy (EDX) revealed Pt did not dissolve homogeneously in the crystals. For the nominal value x = 0.10 small single crystals cut from the batch appeared to have x = 0.09, as well as the non stoichiometric composition Pd$_{0.97}$Pt$_{, Comment: 7 pages, 5 figures. SM: 6 pages, 3 figures

In this paper, we report on the time-dependent strain-wave-induced changes in the reflection and diffraction of a gold plasmonic grating. We demonstrate efficient excitation of strain waves using enhanced absorption at and around the surface plasmon polariton resonance. In addition, we observe that the strain-wave-induced changes in the reflection and diffraction of the grating show an approximately quadratic dependence on pump fluence when probed at a wavelength of 400 nm. We tentatively attribute this non-linear behavior to strain-induced nonlinear changes of the interband transition energy. Using a model that calculates the permittivity of the gold taking into account the d to s/p interband transition, we deduce that the interband transition energy would have to change by about 0.013 eV to account for the measured changes in reflection.

High quality thin films of the topological insulator (Bi$_{0.4}$Sb$_{0.6}$)$_2$Te$_3$ have been deposited on SrTiO$_3$ (111) by molecular beam epitaxy. Their electronic structure was investigated by in situ angle-resolved photoemission spectroscopy and in situ scanning tunneling spectroscopy. The experimental results reveal striking similarities with relativistic ab-initio tight binding calculations. We find that ultrathin slabs of the three-dimensional topological insulator (Bi$_{0.4}$Sb$_{0.6}$)$_2$Te$_3$ display topological surface states, surface states with large weight on the outermost Te atomic layer, and dispersive bulk energy levels that are quantized. We observe that the bandwidth of the bulk levels is strongly reduced. These bunched bulk states as well as the surface states give rise to strong peaks in the local density of states., Comment: 8 pages, 4 figures, 4 pages of supplemental material