Mixed equilibrium/nonequilibrium effects govern surface mobility in polymer glasses.

Using angle-resolved X-ray photoelectron spectroscopy, sum-frequency generation vibrational spectroscopy, contact angle measurements, and molecular dynamics simulations, we verify that the glass transition temperature (Tg) of polymer glass is lower near the free surface. However, the experimental Tg-gradients showed a linear variation with depth (z) from the free surface, while the simulated equilibrium Tg-gradients exhibited a double exponential z-dependence. In typical simulations, Tg is determined based on the relaxation time of the system reaching a prescribed threshold value at equilibrium. Conversely, the experiments determined Tg by observing the unfreezing of molecular mobility during heating from a kinetically arrested, nonequilibrium glassy state. To investigate the impact of nonequilibrium effects on the Tg-gradient, we reduced the thermal annealing time in simulations, allowing the system to fall out of equilibrium. We observe a decrease in the relaxation time and the emergence of a modified z-dependence consistent with a linear Tg-gradient near the free surface. We further validate the impact of nonequilibrium effects by studying the dependence of the Tg on the heating/cooling rate for polymer films of varying thickness (h). Our experimental results reveal significant variations in the Tg-heating/cooling rate dependence with h below the bulk Tg, which are also observed in simulation when the simulated system is not equilibrated. We explain our findings by the reduction in mass density within the inner region of the system under nonequilibrium conditions, as observed in simulation, and recent research indicating a decrease in the local Tg of a polymer when placed next to a softer material.

Probing Surface Dynamics of SiOx Thin-Film Electrodes during Cycling through X-Ray Photoemission Spectroscopy and Operando X-Ray Reflectivity.

SiOx electrodes are promising for high-energy-density lithium-ion batteries (LIBs) due to their ability to mitigate volume expansion-induced degradation. Here, we investigate the surface dynamics of SiOx thin-film electrodes cycled in different carbonate-based electrolytes using a combination of ex situ X-ray photoelectron spectroscopy (XPS) and operando synchrotron X-ray reflectivity analyses. The thin-film geometry allows us to probe the depth-dependent chemical composition and electron density from surface to current collector through the solid electrolyte interphase (SEI), the active material, and the thickness evolution during cycling. Results reveal that SiOx lithiation initiates below 0.4 V vs Li+/Li and indicate a close relationship between SEI formation and SiOx electrode lithiation, likely due to the high resistivity of SiOx. We find similar chemical compositions for the SEI in FEC-containing and FEC-free electrolytes but observe a reduced thickness in the former case. In both cases, the SEI thickness decreases during delithiation due to the removal or dissolution of some carbonate species. These findings give insights into the (de)lithiation of SiOx, in particular, during the formation stage, and the effect of the presence of FEC in the electrolyte on the evolution of the SEI during cycling.

(Sub-)Picosecond Surface Correlations of Femtosecond Laser Excited Al-Coated Multilayers Observed by Grazing-Incidence X-ray Scattering.

Femtosecond high-intensity laser pulses at intensities surpassing 1014 W/cm2 can generate a diverse range of functional surface nanostructures. Achieving precise control over the production of these functional structures necessitates a thorough understanding of the surface morphology dynamics with nanometer-scale spatial resolution and picosecond-scale temporal resolution. In this study, we show that single XFEL pulses can elucidate structural changes on surfaces induced by laser-generated plasmas using grazing-incidence small-angle X-ray scattering (GISAXS). Using aluminium-coated multilayer samples we distinguish between sub-picosecond (ps) surface morphology dynamics and subsequent multi-ps subsurface density dynamics with nanometer-depth sensitivity. The observed subsurface density dynamics serve to validate advanced simulation models representing matter under extreme conditions. Our findings promise to open new avenues for laser material-nanoprocessing and high-energy-density science.

Design and Construction of a Multi-Tiered Minimal Actin Cortex for Structural Support in Lipid Bilayer Applications.

Artificial lipid bilayers have revolutionized biochemical and biophysical research by providing a versatile interface to study aspects of cell membranes and membrane-bound processes in a controlled environment. Artificial bilayers also play a central role in numerous biosensing applications, form the foundational interface for liposomal drug delivery, and provide a vital structure for the development of synthetic cells. But unlike the envelope in many living cells, artificial bilayers can be mechanically fragile. Here, we develop prototype scaffolds for artificial bilayers made from multiple chemically linked tiers of actin filaments that can be bonded to lipid headgroups. We call the interlinked and layered assembly a multiple minimal actin cortex (multi-MAC). Construction of multi-MACs has the potential to significantly increase the bilayer's resistance to applied stress while retaining many desirable physical and chemical properties that are characteristic of lipid bilayers. Furthermore, the linking chemistry of multi-MACs is generalizable and can be applied almost anywhere lipid bilayers are important. This work describes a filament-by-filament approach to multi-MAC assembly that produces distinct 2D and 3D architectures. The nature of the structure depends on a combination of the underlying chemical conditions. Using fluorescence imaging techniques in model planar bilayers, we explore how multi-MACs vary with electrostatic charge, assembly time, ionic strength, and type of chemical linker. We also assess how the presence of a multi-MAC alters the underlying lateral diffusion of lipids and investigate the ability of multi-MACs to withstand exposure to shear stress.

Unveiling PET Hydrolase Surface Dynamics through Fluorescence Microscopy.

PET hydrolases are an emerging class of enzymes that are being heavily researched for their use in bioprocessing polyethylene terephthalate (PET). While work has been done in studying the binding of PET oligomers to the active site of these enzymes, the dynamics of PET hydrolases binding to a bulk PET surface is an unexplored area. Here, methods were developed for total internal reflection fluorescence (TIRF) microscopy and fluorescence recovery after photobleaching (FRAP) microscopy to study the adsorption and desorption dynamics of these proteins onto a PET surface. TIRF microscopy was employed to measure both on and off rates of two of the most commonly studied PET hydrolases, PHL7 and LCC, on a PET surface. It was found that these proteins have a much slower off rates on the order of 10-3  s-1 , comparable to non-productive binding in enzymes such as cellulose. In combination with FRAP microscopy, a dynamic model is proposed in which adsorption and desorption dominates over lateral diffusion over the surface. The results of this study could have implications for the future engineering of PET hydrolases, either to target them to a PET surface or to modulate interaction with their substrate.

3He spin-echo scattering indicates hindered diffusion of isolated water molecules on graphene-covered Ir(111).

The dynamics of water diffusion on carbon surfaces are of interest in fields as diverse as furthering the use of graphene as an industrial-coating technology and understanding the catalytic role of carbon-based dust grains in the interstellar medium. The early stages of water-ice growth and the mobility of water adsorbates are inherently dependent on the microscopic mechanisms that facilitate water diffusion. Here, we use 3He spin-echo quasi-inelastic scattering to probe the microscopic mechanisms responsible for the diffusion of isolated water molecules on graphene-covered and bare Ir(111). The scattering of He atoms provides a non-invasive and highly surface-sensitive means to measure the rate at which absorbates move around on a substrate at very low coverage. Our results provide an approximate upper limit on the diffusion coefficient for water molecules on GrIr(111) of <10-12 m2/s, an order of magnitude lower than the coefficient that describes the diffusion of water molecules on the bare Ir(111) surface. We attribute the hindered diffusion of water molecules on the GrIr(111) surface to water trapping at specific areas of the corrugated moiré superstructure. Lower mobility of water molecules on a surface is expected to lead to a lower ice nucleation rate and may enhance the macroscopic anti-icing properties of a surface.

The dominant instability of near-extreme Stokes waves.

The instability of Stokes waves, steady propagating waves on the surface of an ideal fluid of infinite depth, is a fundamental problem in the field of nonlinear science. The dominant instability of these waves depends on their steepness. For small amplitude waves, it is well known that the Benjamin-Feir or modulational instability dominates the dynamics of a wave train. We demonstrate that for steeper waves, an instability caused by disturbances localized at the wave crest vastly surpasses the growth rate of the modulational instability. These dominant localized disturbances are either coperiodic with the Stokes wave or have twice its period. In either case, the nonlinear evolution of the instability leads to the formation of plunging breakers. This phenomenon explains why long propagating ocean swell consists of small-amplitude waves.

Monitoring surface dynamics of electrodes during electrocatalysis using in situ synchrotron FTIR spectroscopy.

Monitoring the surface dynamics of catalysts under working conditions is important for a deep understanding of the underlying electrochemical mechanisms towards efficient energy conversion and storage. Fourier transform infrared (FTIR) spectroscopy with high surface sensitivity has been considered as a powerful tool for detecting surface adsorbates, but it faces a great challenge when being adopted in surface dynamics investigations during electrocatalysis due to the complication and influence of aqueous environments. This work reports a well designed FTIR cell with tunable micrometre-scale water film over the surface of working electrodes and dual electrolyte/gas channels for in situ synchrotron FTIR tests. By coupling with a facile single-reflection infrared mode, a general in situ synchrotron radiation FTIR (SR-FTIR) spectroscopic method is developed for tracking the surface dynamics of catalysts during the electrocatalytic process. As an example, in situ formed key *OOH is clearly observed on the surface of commercial benchmark IrO2 catalysts during the electrochemical oxygen evolution process based on the developed in situ SR-FTIR spectroscopic method, which demonstrates its universality and feasibility in surface dynamics studies of electrocatalysts under working conditions.

Surface engineering of mixed conjugated/polyelectrolyte brushes - Tailoring interface structure and electrical properties.

HYPOTHESIS: Mixed polymer brushes (MPBs) could be synthesized by surface dilution of homopolymer brushes and subsequent grafting of other type of chains in the formed voids. Nanophase separation and dynamics of surface-grafted chains could be tailored by modification of their molecular architecture. Mixed polyelectrolyte and conjugated chains contribute synergistically to tailor properties of the coating. EXPERIMENTS: A new synthetic strategy that allowed spatially controlled grafting of poly(sodium 4-styrenesulfonate) chains (PSSNa) in close neighborhood of poly(3-methylthienyl methacrylate) (PMTM) brushes (precursors of the conjugated chains) using surface-initiated polymerizations was developed. The final mixed conjugated/polyelectrolyte brushes were prepared by template polymerization of pendant thiophene groups in PMTM chains. Surface dynamics and nanophase separation of MPBs were studied by nanoscale resolution IR imaging, SIMS profiling and AFM mapping in selective solvents. FINDINGS: Unconjugated MPBs were shown to undergo vertical, and horizontal nanophase separation, while the size and shape of the nanodomains were dependent on molar ratio of the mixed chains and their relative lengths. Generation of the conjugated chains led to diminishing of nanophase separation thanks to stronger mutual interactions of conjugated PMTM and PSSNa (macromolecular mixing). The obtained systems demonstrated tunable interfacial structure and resistance switching phenomenon desired in construction of smart surfaces or memristive devices.

The Influence of the Surface Chemistry of Cellulose Nanocrystals on Ethyl Lauroyl Arginate Foam Stability.

Guanidine-based surfactant ethyl lauroyl arginate (LAE) and cellulose nanocrystals (CNCs) form complexes of enhanced surface activity when compared to pure surfactants. The LAE-CNC mixtures show enhanced foaming properties. The dynamic thin-film balance technique (DTFB) was used to study the morphology, drainage and rupture of LAE-CNC thin liquid films under constant driving pressure. A total of three concentrations of surfactant and the corresponding mixtures of LAE with sulfated (sCNC) and carboxylated (cCNC) cellulose nanocrystals were studied. The sCNC and cCNC suspension with LAE formed thin films, with stability increasing with surfactant concentration and with complex rheological properties. In the presence of LAE, the aggregation of CNC was observed. While the sCNC aggregates were preferentially present in the film volume with a small fraction at the surface, the cCNC aggregates, due to their higher hydrophobicity, were preferentially located at film interfaces, forming compact layers. The presence of both types of aggregates decreased the stability of the thin liquid film compared to the one for the LAE solution with the same concentration. The addition of CNC to LAE was critical for foam formation, and foam stability was in qualitative agreement with the thin films' lifetimes. The foam volume increased with the LAE concentration. However, there was an optimum surfactant concentration to achieve stable foam. In particular, the very resistant foam was obtained with cCNC suspensions that formed the interfaces with a complex structure and rheology. On the other hand, at high LAE concentrations, the aggregates of CNC may exhibit antifoaming properties.

Structure and surface dynamics of genomic DNA as probed with surface-enhanced Raman spectroscopy: Trace level sensing of nucleic acids extracted from plants.

In this work surface-enhanced Raman spectra of nucleic acids from in vitro grown Solanum tuberosum L. cultivars and populations (Buzau population, Lazarea population, Patraque d'Auvergne, RFA Roclas Clone 2.6 Ferma, Vitelotte Negresse, Roclas Clone C, Blue Congo) were measured with 532 nm laser line. Main surface-enhanced Raman modes of these DNAs have been analyzed. Also, DNA from two grapevine (Vitis vinifera L.) varieties were studied at acidic pHs by surface-enhanced Raman spectroscopy. Modified SERS intensities and wavenumber shifts of nucleic acids bands were observed upon lowering the pH, being a proof of binding affinity changes of DNA with silver nanoparticles (AgNPs) and of structural modifications induced at acidic pHs in DNA molecular groups. Furthermore, the (sub)picosecond surface dynamics of DNA extracted from leaf tissues of grapevine (Vitis vinifera L.) varieties was investigated. In this work, the bands full widths at half-maximum (FWHMs) have values in the wavenumber range from 8 to 34 cm-1. (Sub)picosecond molecular dynamics of DNA groups with global relaxation times between 0.31 ps - 1.33 ps has been found.

Manipulation of Subwavelength Periodic Structures Formation on 4H-SiC Surface with Three Temporally Delayed Femtosecond Laser Irradiations.

Controlling laser-induced periodic surface structures on semiconductor materials is of significant importance for micro/nanophotonics. We here demonstrate a new approach to form the unusual structures on 4H-SiC crystal surface under irradiation of three collinear temporally delayed femtosecond laser beams (800 nm wavelength, 50 fs duration, 1 kHz repetition), with orthogonal linear polarizations. Different types of surface structures, two-dimensional arrays of square islands (670 nm periodicity) and one-dimensional ripple structures (678 nm periodicity) are found to uniformly distribute over the laser-exposed areas, both of which are remarkably featured by the low spatial frequency. By altering the time delay among three laser beams, we can flexibly control the transition between the two surface structures. The experimental results are well explained by a physical model of the thermally correlated actions among three laser-material interaction processes. This investigation provides a simple, flexible, and controllable processing approach for the large-scale assembly of complex functional nanostructures on bulk semiconductor materials.

Cell surface fluctuations regulate early embryonic lineage sorting.

In development, lineage segregation is coordinated in time and space. An important example is the mammalian inner cell mass, in which the primitive endoderm (PrE, founder of the yolk sac) physically segregates from the epiblast (EPI, founder of the fetus). While the molecular requirements have been well studied, the physical mechanisms determining spatial segregation between EPI and PrE remain elusive. Here, we investigate the mechanical basis of EPI and PrE sorting. We find that rather than the differences in static cell surface mechanical parameters as in classical sorting models, it is the differences in surface fluctuations that robustly ensure physical lineage sorting. These differential surface fluctuations systematically correlate with differential cellular fluidity, which we propose together constitute a non-equilibrium sorting mechanism for EPI and PrE lineages. By combining experiments and modeling, we identify cell surface dynamics as a key factor orchestrating the correct spatial segregation of the founder embryonic lineages.

Water adsorption and dynamics on graphene and other 2D materials: Computational and experimental advances.

The interaction of water and surfaces, at molecular level, is of critical importance for understanding processes such as corrosion, friction, catalysis and mass transport. The significant literature on interactions with single crystal metal surfaces should not obscure unknowns in the unique behaviour of ice and the complex relationships between adsorption, diffusion and long-range inter-molecular interactions. Even less is known about the atomic-scale behaviour of water on novel, non-metallic interfaces, in particular on graphene and other 2D materials. In this manuscript, we review recent progress in the characterisation of water adsorption on 2D materials, with a focus on the nano-material graphene and graphitic nanostructures; materials which are of paramount importance for separation technologies, electrochemistry and catalysis, to name a few. The adsorption of water on graphene has also become one of the benchmark systems for modern computational methods, in particular dispersion-corrected density functional theory (DFT). We then review recent experimental and theoretical advances in studying the single-molecular motion of water at surfaces, with a special emphasis on scattering approaches as they allow an unparalleled window of observation to water surface motion, including diffusion, vibration and self-assembly.

Time-Resolved Scanning Ion Conductance Microscopy for Three-Dimensional Tracking of Nanoscale Cell Surface Dynamics.

Nanocharacterization plays a vital role in understanding the complex nanoscale organization of cells and organelles. Understanding cellular function requires high-resolution information about how the cellular structures evolve over time. A number of techniques exist to resolve static nanoscale structure of cells in great detail (super-resolution optical microscopy, EM, AFM). However, time-resolved imaging techniques tend to either have a lower resolution, are limited to small areas, or cause damage to the cells, thereby preventing long-term time-lapse studies. Scanning probe microscopy methods such as atomic force microscopy (AFM) combine high-resolution imaging with the ability to image living cells in physiological conditions. The mechanical contact between the tip and the sample, however, deforms the cell surface, disturbs the native state, and prohibits long-term time-lapse imaging. Here, we develop a scanning ion conductance microscope (SICM) for high-speed and long-term nanoscale imaging of eukaryotic cells. By utilizing advances in nanopositioning, nanopore fabrication, microelectronics, and controls engineering, we developed a microscopy method that can resolve spatiotemporally diverse three-dimensional (3D) processes on the cell membrane at sub-5-nm axial resolution. We tracked dynamic changes in live cell morphology with nanometer details and temporal ranges of subsecond to days, imaging diverse processes ranging from endocytosis, micropinocytosis, and mitosis to bacterial infection and cell differentiation in cancer cells. This technique enables a detailed look at membrane events and may offer insights into cell-cell interactions for infection, immunology, and cancer research.

n-Butane, iso-Butane and 1-Butene Adsorption on Imidazolium-Based Ionic Liquids Studied with Molecular Beam Techniques.

The interaction of molecules, especially hydrocarbons, at the gas/ionic liquid (IL) surface plays a crucial role in supported IL catalysis. The dynamics of this process is investigated by measuring the trapping probabilities of n-butane, iso-butane and 1-butene on a set of frozen 1-alkyl-3-methylimidazolium-based ILs [Cn C1 Im]X, where n=4, 8 and X- =Cl- , Br- , [PF6 ]- and [Tf2 N]- . The decrease of the initial trapping probability with increasing surface temperature is used to determine the desorption energy of the hydrocarbons at the IL surfaces. It increases with increasing alkyl chain length n and decreasing anion size for the ILs studied. We attribute these effects to different degrees of alkyl chain surface enrichment, while interactions between the adsorbate and the anion do not play a significant role. The adsorption energy also depends on the adsorbing molecule: It decreases in the order n-butane>1-butene>iso-butane, which can be explained by different dispersion interactions.

Fast Surface Dynamics on a Metallic Glass Nanowire.

The dynamics near the surface of glasses can be much faster than in the bulk. We studied the surface dynamics of a Pt-based metallic glass using electron correlation microscopy with sub-nanometer resolution. Our studies show an ∼20 K suppression of the glass transition temperature at the surface. The enhancement in surface dynamics is suppressed by coating the metallic glass with a thin layer of amorphous carbon. Parallel molecular dynamics simulations on Ni80P20 show a similar temperature suppression of the surface glass transition temperature and that the enhanced surface dynamics are arrested by a capping layer that chemically binds to the glass surface. Mobility in the near-surface region occurs via atomic caging and hopping, with a strong correlation between slow dynamics and high cage-breaking barriers and stringlike cooperative motion. Surface and bulk dynamics collapse together as a function of temperature rescaled by their respective glass transition temperatures.

Acidic pH-responsive changes of DNA structure and surface dynamics as probed with ultrasensitive Raman spectroscopy.

In this work structural and (sub)picosecond surface dynamical changes of genomic DNA isolated from different medicinal plants (Hyssopus officinalis, Majorana hortensis, Melissa officinalis, Mentha piperita, Mentha piperita cv "Cristal", Monarda didyma and Matricaria chamomilla), as probed with surface-enhanced Raman spectroscopy (SERS), are discussed upon modifying the acidic pH of mixtures consisting of silver colloidal suspension and DNA samples, respectively. Binding affinity changes of DNA with silver NPs and nucleic acids protonation are supposed to take place upon lowering the pH. A small percentage of Hoogsteen GC basepairs was found in Mentha piperita cv "Cristal" DNA, at low acidic pH. As a general observation, the global relaxation times corresponding to different functional groups of the investigated genomic DNAs, respectively, show a decrease of their values upon lowering the pH.

Medial Surface Dynamics as a Function of Subglottal Pressure in a Canine Larynx Model.

During vocal fold vibration, there may be a mucosal wave in the superior-inferior (vertical) direction, resulting in a convergent shape during opening and a divergent shape during closing. Most of our understanding of the converging/diverging shape of the glottis has come from studies in a hemilarynx model. Previous work has shown that vibratory patterns in the full excised larynx are different than the hemilarynx. This study characterized the dynamics of the medial glottal wall geometry during vibrations in the full excised canine larynx model. Using particle image velocimetry, the intraglottal geometry was measured at the midmembranous coronal plane in an excised canine larynx model. Measurements of the glottal area were taken simultaneously using high-speed imaging. The results show that skewing of the glottal area waveform occurs without the presence of a vocal tract and that the phase-lag of the superior edge relative to the inferior edge is smaller than reported and depends on the subglottal pressure. In addition, it shows that the glottal divergence angle during closing is proportional to the magnitude of the acoustic intensity and the intraglottal negative pressure. This preliminary data suggests that more studies are needed to determine the important mechanisms determining the relationship between intraglottal flow, intraglottal geometry, and acoustics.

Te-Vacancy-Induced Surface Collapse and Reconstruction in Antiferromagnetic Topological Insulator MnBi2Te4.

MnBi2Te4 is an antiferromagnetic topological insulator that has stimulated intense interest due to its exotic quantum phenomena and promising device applications. The surface structure is a determinant factor to understand the magnetic and topological behavior of MnBi2Te4, yet its precise atomic structure remains elusive. Here we discovered a surface collapse and reconstruction of few-layer MnBi2Te4 exfoliated under delicate protection. Instead of the ideal septuple-layer structure in the bulk, the collapsed surface is shown to reconstruct as a Mn-doped Bi2Te3 quintuple layer and a MnxBiyTe double layer with a clear van der Waals gap in between. Combined with first-principles calculations, such surface collapse is attributed to the abundant intrinsic Mn-Bi antisite defects and the tellurium vacancy in the exfoliated surface, which is further supported by in situ annealing and electron irradiation experiments. Our results shed light on the understanding of the intricate surface-bulk correspondence of MnBi2Te4 and provide an insightful perspective on the surface-related quantum measurements in MnBi2Te4 few-layer devices.