Thin Water Layer Research Articles

C-terminal tail insertion of Bcl-xL in membrane occurs via partial unfolding and refolding cycle associating microsolvation.

Bcl-xL, a member of the Bcl-2 family of proteins, remains distributed over the cytosol and the mitochondrial membrane, maintaining a balance between apoptosis and the survival of the cell. Passage to the membrane is essential for its biological functions (e.g. to antagonize pro-apoptotic proteins of the Bcl2 family), which is known to be initiated by the insertion of the C-terminal segment into the membrane. This tail, composed of ∼24 residues, is reported to act as a pseudo-inhibitor of the protein itself, adapting a helical conformation. It gets released from the confinement when Bcl-xL approaches the membrane. This article reports the events associated with the insertion of the helical tail into an explicitly modeled all-atom membrane, which reveals a partial unfolding to refolding cycle of the peptide, correlating with the early insertion, to a fully inserted state. The polar interactions have been found to have a dominant role in steering the peptide towards the membrane at the desired orientation. The landscape of the potential of mean force (PMF) is consistent with the proposed mechanism. Molecular dynamics further brings the insight that the peptide insertion is associated with the encapsulation of a thin water layer around the peptide throughout the course of insertion, which motivates the protein to refold once the insertion is complete.

Direct numerical simulation of gas transfer across the air–water interface driven by buoyant convection

A series of direct numerical simulations of mass transfer across the air–water interface driven by buoyancy-induced convection have been carried out to elucidate the physical mechanisms that play a role in the transfer of heat and atmospheric gases. The buoyant instability is caused by the presence of a thin layer of cold water situated on top of a body of warm water. In time, heat and atmospheric gases diffuse into the uppermost part of the thermal boundary layer and are subsequently transported down into the bulk by falling sheets and plumes of cold water. Using a specifically designed numerical code for the discretization of scalar convection and diffusion, it was possible to accurately resolve this buoyant-instability-induced transport of atmospheric gases into the bulk at a realistic Prandtl number ($\mathit{Pr}=6$) and Schmidt numbers ranging from$\mathit{Sc}=20$to$\mathit{Sc}=500$. The simulations presented here provided a detailed insight into instantaneous gas transfer processes. The falling plumes with highly gas-saturated fluid in their core were found to penetrate deep inside the bulk. With an initial temperature difference between the water surface and the bulk of slightly above$2$ K, peaks in the instantaneous heat flux in excess of$1600~\text{W}~\text{m}^{-2}$were observed, proving the potential effectiveness of buoyant-convective heat and gas transfer. Furthermore, the validity of the scaling law for the ratio of gas and heat transfer velocities$K_{L}/H_{L}\propto (\mathit{Pr}/\mathit{Sc})^{0.5}$for the entire range of Schmidt numbers considered was confirmed. A good time-accurate approximation of$K_{L}$was found using surface information such as velocity fluctuations and convection cell size or surface divergence. A reasonable time accuracy for the$K_{L}$estimation was obtained using the horizontal integral length scale and the root mean square of the horizontal velocity fluctuations in the upper part of the bulk.

Long-term effects of multiply pulsed dielectric barrier discharges in air on thin water layers over tissue: stationary and random streamers

Tissue covered by thin liquid layers treated by atmospheric pressure plasmas for biomedical applications ultimately requires a reproducible protocol for human healthcare. The desired outcomes of wet tissue treatment by dielectric barrier discharges (DBDs) depend on the plasma dose which determines the integral fluence of radicals, ions, electric fields and UV/VUV photons incident onto the tissue. These fluences are controlled by power, frequency and treatment time. To first order, these parameters determine the energy deposition (J cm−2) onto the tissue. However, energy deposition may not be the only parameter that determines the fluences of reactants to the underlying tissue. In this paper, we report on a computational investigation of multipulse DBDs interacting with wet tissue. The DBDs were simulated for 100 pulses at different repetition rates and liquid thicknesses followed by 10 s or more of afterglow. Two schemes were investigated—stationary and random. In the stationary scheme, the DBD plasma streamer continues to strike at the same location on the liquid layer, whereas in the random scheme the plasma streamer strikes at random locations on the liquid layer. These differences in streamer locations strongly affect the spatial distribution of solvated species such as OHaq and H2O2aq (‘aq’ represents an aqueous species), which have high rates of solvation. The spatial distribution of species such as NOaq, which have low rates of solvation, are less affected by the location of the streamer due to the remediating effects of diffusion in the air. The end result is that fluences to the tissue are sensitive to the spatial location of the streamer due to the ensuing reactions in the liquid between species that have low and high rates of solvation. These reactions can be controlled not only through location of the streamer, but also by repetition rate and thickness of the liquid layer.

Elimination of Clogging in PMMA Microchannels Using Water Assisted CO<sub>2</sub> Laser Micromachining

The accuracy and clogging of microchannels are important for assessing the quality of lab on chip (L-O-C) devices. The clogging affects the fluid mixing efficiency and influences the bonding of substrate. In this paper, inexpensive and quick method for microchannel fabrication in polymethyl methacrylate (PMMA) while reducing the thermal damage is introduced. Accordingly, the substrate was covered with a thin layer of water during CO2 laser ablation. The effect of water cooling on the clogging formation, heat affected zone and the microchannel geometry in terms of depth and width is investigated. Clogging formation mechanism in the intersection of Y-channel is studied to improve its quality for microfluidics applications. During the experimental work, the CO2 laser power was varied from 2.4 to 6 W at scanning speed from 5 to 12.5 mm/s. The results showed that covering the PMMA substrate with a thin layer of water prevented clogging formation and reduced the heat affected zone.

Bubble dynamics in a compressible liquid in contact with a rigid boundary.

A bubble initiated near a rigid boundary may be almost in contact with the boundary because of its expansion and migration to the boundary, where a thin layer of water forms between the bubble and the boundary thereafter. This phenomenon is modelled using the weakly compressible theory coupled with the boundary integral method. The wall effects are modelled using the imaging method. The numerical instabilities caused by the near contact of the bubble surface with the boundary are handled by removing a thin layer of water between them and joining the bubble surface with its image to the boundary. Our computations correlate well with experiments for both the first and second cycles of oscillation. The time history of the energy of a bubble system follows a step function, reducing rapidly and significantly because of emission of shock waves at inception of a bubble and at the end of collapse but remaining approximately constant for the rest of the time. The bubble starts being in near contact with the boundary during the first cycle of oscillation when the dimensionless stand-off distance γ = s/R m < 1, where s is the distance of the initial bubble centre from the boundary and R m is the maximum bubble radius. This leads to (i) the direct impact of a high-speed liquid jet on the boundary once it penetrates through the bubble, (ii) the direct contact of the bubble at high temperature and high pressure with the boundary, and (iii) the direct impingement of shock waves on the boundary once emitted. These phenomena have clear potential to damage the boundary, which are believed to be part of the mechanisms of cavitation damage.

Direct probing of contact electrification by using optical second harmonic generation technique.

Contact electrification between two different materials is one of the oldest fields of study in solid-state physics. Here, we introduced an innovative system based on optical electric-field-induced second harmonic generation (EFI-SHG) technique that can directly monitor the dynamic performance of the contact electrification on the surface of polyimide film. After the contact, the EFI-SHG system visualized briefly three relaxations of the tribo-induced charges on the surface of a polyimide film, a fast relaxation within 3 min followed by two much slower relaxations, which were possibly related to different charge diffusion routes. The contact electrification under several special experimental conditions (wind, water and steam) was studied to demonstrate the high flexibility and material selectivity of the EFI-SHG. The EFI-SHG studies confirmed the motion of the water can remove the surface charge, while the appearance and the evaporation of a thin water layer cannot enhance the charge diffusion. We anticipate that this experimental technique will find a variety of applications in the field of contact electrification and the development of the recently invented triboelectric nano generator.

Stress Corrosion Cracking of Sensitized 304 Stainless Steel Under MgCl2 Droplets

Stainless steels have been widely applied in the building, chemical plants and other industries due to their excellent weldability, superior anticorrosion properties, and so on. However, stainless steels suffer from localized corrosion in chloride-containing environments. Especially, in an atmospheric environments near the seashore, localized corrosion can be initiated easily because a very severe corrosion condition, a thin water-layer containing very high concentration of chloride ions, can be made on stainless steels. Moreover, local corrosion sites such as pits can become initiation sites for crack initiation of stress corrosion cracking (SCC) when the steels are subjected to tensile stress. Therefore, it can be considered that atmospheric SCC is one of the most severe degradations for stainless steels in atmospheric corrosion environments. There are some researches to investigate SCC in the atmospheric corrosion environments with chloride deposits1, 2. This study is aimed at investigating initiation and propagation of stress corrosion cracking of sensitized 304 stainless steel under tensile stress in MgCl2droplets. Austenitic 304 stainless steel sheet of 1 mm in thickness were used as a material. Tensile specimens with a gauge part of L15 mm x W8 mm were cut out from the sheet. The specimens were heated for 90 min at 700 oC and followed by furnace cooling for sensitization. The sensitized specimens were abraded to JIS #1000 grit with SiC papers. Then they were degreased with ethanol in an ultrasonic bath and rinsed with Milli-Q water (18 MWcm). SCC behavior under MgCl2 droplets was investigated by a constant strain tensile test with a horizontal tensile test machine. In the test, a specimen was first elongated to a certain stress value and then kept at a constant strain. After applying the constant strain, 20 mL droplets of 250 mM MgCl2 were placed on the gauge part of the specimen. The gauge part with the droplets was exposed to an environment at 50 OC and ca. 35 RH% for 3 days in a small chamber. During the test, in-situ surface observation was carried out with a digital microscope. After the test, surface of the specimen was observed by SEM-EDX. Fig. 1 shows surface and cross-sectional images of sensitized 304 stainless steel with 8.5% strain under MgCl2droplets. When the droplets are place on the gauge part of the specimen, the droplets evaporate and reach to an equilibrium state with the concentration of 10 M at 35%RH. At this time, the thickness of the solution film of the droplets can be estimated to be 70 mm. Actually pitting corrosion was initiated within 1 h after the droplets were applied. As shown in Fig. 1(a), which is taken at 12 h after a pit is formed in a droplet, some tiny cracks initiate from the edge of the pit. As the test time advances, the cracks propagate in the normal direction to the tensile axis and reach ca. 600 mm after 72 h. During the propagation of the cracks, hydrogen bubbles gushing from the inside of the cracks were observed. Cross-sectional observation in Fig. 1(d) reveals that cracks initiate from the bottom of the pit and propagate in the bulk direction both with transgranular and intergranular cracking modes. In the presentation, crack propagation mechanism of sensitized 304 stainless steel will be discussed.

Water-assisted pulsed Er:YAG laser interaction with silicon

Silicon is virtually transparent to the Er:YAG laser with a wavelength of 2.94 μm. In this study, we report that moderately doped silicon (1–10 Ω cm) can be processed by a pulsed Er:YAG laser with a pulse duration of 350 μs and a peak laser intensity of 1.7 × 105 W/cm2 by applying a thin water layer on top of silicon as a light absorbing medium. In this way, water is heated first by strongly absorbing the laser energy and then heats up the silicon wafer indirectly. As the silicon temperature rises, the free carrier concentration and therefore the absorption coefficient of silicon will increase significantly, which may enable the silicon to get directly processed by the Er:YAG laser when the water is vaporized completely. We also believe that the change in surface morphology after melting could contribute to the increase in the laser beam absorptance. It was observed that 525 nm-thick p-type wafer specimens were fully penetrated after 15 laser pulses were irradiated. Bright yellow flames were observed duri...

Synthesis and Transfer of Large-Area Monolayer WS2 Crystals: Moving Toward the Recyclable Use of Sapphire Substrates.

Two-dimensional layered transition metal dichalcogenides (TMDs) show intriguing potential for optoelectronic devices due to their exotic electronic and optical properties. Only a few efforts have been dedicated to large-area growth of TMDs. Practical applications will require improving the efficiency and reducing the cost of production, through (1) new growth methods to produce large size TMD monolayer with less-stringent conditions, and (2) nondestructive transfer techniques that enable multiple reuse of growth substrate. In this work, we report to employ atmospheric pressure chemical vapor deposition (APCVD) for the synthesis of large size (>100 μm) single crystals of atomically thin tungsten disulfide (WS2), a member of TMD family, on sapphire substrate. More importantly, we demonstrate a polystyrene (PS) mediated delamination process via capillary force in water which reduces the etching time in base solution and imposes only minor damage to the sapphire substrate. The transferred WS2 flakes are of excellent continuity and exhibit comparable electron mobility after several growth cycles on the reused sapphire substrate. Interestingly, the photoluminescence emission from WS2 grown on the recycled sapphire is much higher than that on fresh sapphire, possibly due to p-type doping of monolayer WS2 flakes by a thin layer of water intercalated at the atomic steps of the recycled sapphire substrate. The growth and transfer techniques described here are expected to be applicable to other atomically thin TMD materials.

Nanoparticle Generation From Nitinol Target Using Pulsed Laser Ablation

The influence of experimental conditions (wavelength, liquid environment) on the properties of Ni-Ti nanoparticles generated by nanosecond laser ablation of NiTinol (Shape Memory Alloy) target and the elemental distribution of the irradiated surface were investigated. The studied laser wavelengths were 248 (KrF) and 1064 nm (Q-switched Nd:YAG). Nitinol targets were covered with thin liquid layer of distilled water and ethanol, respectively. The samples were irradiated with various numbers of pulses (500 in case of the investigation of the alloy surface and 20000 for the particle generation) and fluences (2 and 5 J/cm2). The morphology of laser treated NiTi surface and the size of the generated nanoparticles were studied as a function of both the laser fluence and the type of the applied liquids using scanning electron microscopy (SEM). Element mappings were realized by energy-dispersive X-ray spectroscopy (EDX). Our results clearly show that when using ethanol both the laser treated surface and the generated particles had a relatively homogeneous elemental distribution. However, under distilled water the irradiated surface and the generated particles showed a separated presence of Ni and Ti elements. For the higher fluence, formation of core-shell structured particles were observed in water environment.

Wet adhesion of graphene

Interfacial adhesion between graphene and various substrate materials is essential for practical applications of graphene. To date, most of the studies on adhesion of graphene have assumed dry adhesion of van der Waals type. In this paper, we conduct molecular dynamics simulations to study the traction–separation behaviors for wet adhesion of graphene on amorphous silicon oxide covered by a thin layer of water. Three stages of the traction–separation relations are identified and they are analyzed by simple, approximate continuum models. The work of separation is found to be close to the theoretical value dictated by the interaction potential between graphene and water. The maximum traction is found to be set by the critical stress for cavitation at the water/graphene interface. With morphological evolution of water from cavitation to capillary bridging, the range of interaction extends to about 3 nm before complete separation of graphene. Compared to van der Waals interactions for dry adhesion of graphene, the work of separation for wet adhesion is smaller, the maximum traction is lower, but the interaction range is longer. It is noted that the properties of wet adhesion depend sensitively on the graphene–water interactions, which may vary considerably from hydrophobic to hydrophilic interactions.

Atom-resolved analysis of an ionic KBr(001) crystal surface covered with a thin water layer by frequency modulation atomic force microscopy.

An ionic KBr(001) crystal surface covered with a thin water layer was observed with a frequency modulation atomic force microscope (FM-AFM) with atomic resolution. By immersing only the tip apex of the AFM cantilever in the thin water layer, the Q-factor of the cantilever in probing the solid-liquid interface can be maintained as high as that of FM-AFM operation in air, leading to improvement of the minimum detection of a differential force determined by the noise. Two types of images with atom-resolved contrast were observed, possibly owing to the different types of ions (K(+) or Br(-)) adsorbed on the tip apex that incorporated into the hydration layers on the tip and on the sample surface. The force-distance characteristics at the solid-water interface were analyzed by taking spatial variation maps of the resonant frequency shift of the AFM cantilever with the high Q-factor. The oscillatory frequency shift-distance curves exhibited atomic site dependence. The roles of hydration and the ions on the tip and on the sample surface in the measurements were discussed.

Stick-slip control in nanoscale boundary lubrication by surface wettability.

We study the effect of atomic-scale surface-lubricant interactions on nanoscale boundary-lubricated friction by considering two example surfaces-hydrophilic mica and hydrophobic graphene-confining thin layers of water in molecular dynamics simulations. We observe stick-slip dynamics for thin water films confined by mica sheets, involving periodic breaking-reforming transitions of atomic-scale capillary water bridges formed around the potassium ions of mica. However, only smooth sliding without stick-slip events is observed for water confined by graphene, as well as for thicker water layers confined by mica. Thus, our results illustrate how atomic-scale details affect the wettability of the confining surfaces and consequently control the presence or absence of stick-slip dynamics in nanoscale friction.

Demonstration and challenges for joining technology based on direct bonding usable for construction of (large) structures in space

A direct bonding process relying on van der Waals forces offers an ideal combination of easiness to assemble and material compatibility. Such a bonding procedure, also denoted as optical bonding, is already known and used frequently in different applications. However, there are strict requirements to achieve optical bonding: (1) a high level of cleanliness of the surfaces and (2) low roughness (RMS roughness <2 nm and preferably <0.5 nm). The first condition will be possible to realize in space, since the vacuum prevents the absorption of a thin water layer present on all surfaces under atmospheric conditions and forces a desorption of all residual water. Also, particle contamination will be minimized, due to the absence of strong capillary forces which attract particles if present and the absence of particles in space, providing a suitable and without residue removable protection prior to bonding. The second condition is now state of the art of silicon carbide (SiC) polishing and can hence be realized. While meeting the requirements, theoretically very large adhesion forces can be realized in vacuum. Surfaces have been polished according to the requirements. The forces measured on these surfaces are nearly as high as theoretically predicted and demonstrate the proof of principle of direct bonding of SiC under ambient conditions and in vacuum. However, the realization of the required flatness over large contact areas is still a challenge. Furthermore, since the surfaces display a really low roughness, extremely clean handling and bonding conditions need to be realized to avoid the spontaneous adhesion of small particles which would as such prevent direct bonding of larger areas.

A mist pluviation method for reconstituting silty sand specimens

A mist pluviation (MP) technique was proposed in an effort to develop a practical method for reconstituting silty sand specimens without inducing severe particle segregation. The mist pluviation method includes both air and water pluviation processes, with the addition of a mist zone that mixes soil particles with water droplets while falling through air. The soil and water mixtures have similar particle sizes, thus resulting in significantly reduced particle segregation. A thin water layer on top of the deposited soil mimics the water deposition process, and saturates the soil with little chance for particle segregation. The performance of the MP method was evaluated with regard to homogeneity, repeatability, undrained monotonic compression, and cyclic strength of triaxial specimens with various fines contents. Results have indicated that the MP method can be a promising technique to prepare reconstituted silty sand specimens.

Coupled diffusion in lipid bilayers upon close approach.

Biomembrane interfaces create regions of slowed water dynamics in their vicinity. When two lipid bilayers come together, this effect is further accentuated, and the associated slowdown can affect the dynamics of larger-scale processes such as membrane fusion. We have used molecular dynamics simulations to examine how lipid and water dynamics are affected as two lipid bilayers approach each other. These two interacting fluid systems, lipid and water, both slow and become coupled when the lipid membranes are separated by a thin water layer. We show in particular that the water dynamics become glassy, and diffusion of lipids in the apposed leaflets becomes coupled across the water layer, while the “outer” leaflets remain unaffected. This dynamic coupling between bilayers appears mediated by lipid–water–lipid hydrogen bonding, as it occurs at bilayer separations where water–lipid hydrogen bonds become more common than water–water hydrogen bonds. We further show that such coupling occurs in simulations of vesicle–vesicle fusion prior to the fusion event itself. Such altered dynamics at membrane–membrane interfaces may both stabilize the interfacial contact and slow fusion stalk formation within the interface region.

Structural and Dynamical Properties of POPC Bilayers Supported on Nanoporous Substrates

The structure, dynamics and organization of lipid bilayers depend on a variety of environmental factors. Here we examine, using molecular dynamics (MD) simulations, the specific effects of nanoporous substrates on palmitoyl-oleyl phosphatidylcholine (POPC) bilayers. We expose POPC bilayers unilaterally and separately to various model Lennard-Jones (LJ) solid substrates differing in surface chemistry. In an earlier study, we had found that substrates with surface hydroxyl densities in the range 10-20% kept POPC bilayers juxtaposed to the substrates. While the bilayers did not interact directly with these hydroxylated substrates, they were separated from substrates by thin (< 0.6 nm) layers of water. Additionally, despite such buffered interactions, these supported bilayers exhibited physical properties notably different from unsupported bilayers. Recent experiments show that zwitterionic lipid bilayers supported on partially charged, nanoporous silica wafers also do not interact with silica directly. However, in such cases the bilayer is separated from the silica substrate by a relative thicker (∼1.5 nm) layer of solvent. We report here results from our MD simulation that reconcile these two seemingly disparate observations. We find that LJ nano-substrates whose porosity and surface hydroxyl density match those of nanoporous silica wafers do not increase the substrate-bilayer distance. Instead, the introduction of partial charge on the LJ substrate increases the substrate-bilayer distance to 1.5 nm. The negative charge on the substrate attracts hydrated ions, creating an electric double layer, which is a sufficient condition to increase the substrate-bilayer distance. While the properties of the bilayer supported on a charged substrate are also different from those of unsupported bilayers, the extent of the perturbation is not as large compared to that induced by uncharged hydroxylated substrates.

Resettable, Low-temperature Accumulation Gas Sensors Based on Hydrogenated Diamond Transducers

Diamonds with hydrogenated surfaces feature a gas sensitivity which is mediated by a thin layer of BET-adsorbed water (BET: Brunauer, Emmet, Teller). Gases capable of undergoing acid-base reactions in liquid water are absorbed in the BET adsorbate, producing pH changes there and easily observable changes in the sub-surface hole conductivity of the diamond substrates. In this contribution we report on vapor sensing tests with diamond sensors which have been performed at normal ambient temperatures in a free-space environment. We show that such sensors exhibit an integrating dosimeter type response, which follows the temporal vapor concentrations with a time resolution of the order of one second or less. Upon saturation the sensors can be repeatedly reset to baseline by replacing contaminated BET adsorbates by fresh-water ones.

Experimental study of evaporation of horizontal films of water–salt solutions

The present studies were carried out for the horizontal films (thin layers) of water and water solutions of NaCl, CaCl2 , LiCl, and LiBr with different solubility characteristics, as well as with specific features of formation and decay of water hydrates. Required volume of solution V o of given weight concentration ξo , preliminary heated to the working surface temperature, was put in one step on the horizontal bottom of the bowl, heated to working temperature t CT , by means of volume batchers Thermo Scientific. After evaporation completion, the final mass of solution and form of their residue were registered. At the final stage of evaporation formation of NaCl crystals and water hydrates of CaCl2 · 2H2 O, LiCl · H2 O, and LiBr · 2H2 O occurred.

New insight into icing and de-icing properties of hydrophobic and hydrophilic structured surfaces based on core-shell particles.

Icing is an important problem, which often leads to emergency situations in northern countries. The reduction of icing requires a detailed understanding of this process. In this work, we report on a systematic investigation of the effects of geometry and chemical properties of surfaces on the formation of an ice layer, its properties, and thawing. We compare in detail icing and ice thawing on flat and rough hydrophilic and hydrophobic surfaces. We also show advantages and disadvantages of the surfaces of each kind. We demonstrate that water condenses in a liquid form, leading to the formation of a thin continuous water layer on a hydrophilic surface. Meanwhile, separated rounded water droplets are formed on hydrophobic surfaces. As a result of slower heat exchange, the freezing of rounded water droplets on a hydrophobic surface occurs later than the freezing of the continuous water layer on a hydrophilic one. Moreover, growth of ice on hydrophobic surfaces is slower than on the hydrophilic ones, because ice grows due to the condensation of water vapor on already formed ice crystals, and not due to the condensation on the polymer surface. Rough hydrophobic surfaces also demonstrate a very low ice adhesion value, which is because of the reduced contact area with ice. The main disadvantage of hydrophobic and superhydrophobic surfaces is the pinning of water droplets on them after thawing. Flat hydrophilic poly(ethylene glycol)-modified surfaces also exhibit very low ice adhesion, which is due to the very low freezing point of the water-poly(ethylene glycol) mixtures. Water easily leaves from flat hydrophilic poly(ethylene glycol)-modified surfaces, and they quickly become dry. However, the ice growth rate on poly(ethylene glycol)-modified hydrophilic surfaces is the highest. These results indicate that neither purely (super)hydrophobic polymeric surfaces, nor "antifreeze" hydrophilic ones provide an ideal solution to the problem of icing.