Viscosity of deeply supercooled water and its coupling to molecular diffusion

Remarkable Insensitivity of Protein Diffusion to Protein Charge

Friction to translational diffusion of ionic particles in polar liquids should scale linearly with the squared ion charge, according to standard theories. Substantial slowing of translational diffusion is expected for proteins in water. In contrast, our simulations of charge mutants of green fluorescent proteins in water show remarkable insensitivity of the translational diffusion constant to protein's charge in the range of charges between -29 and +35. The friction coefficient is given as a product of the force variance and the memory function relaxation time. We find remarkably accurate equality between the variance of the electrostatic force and the negative cross-correlation of the electrostatic and van der Waals forces. The charge invariance of the diffusion constant is a combined effect of the force variance and relaxation time invariances with the protein charge. The temperature dependence of the protein diffusion constant is highly non-Arrhenius, with a fragile-to-strong crossover at the glass transition.

Bridging classical nucleation theory and molecular dynamics simulation for homogeneous ice nucleation

Water freezing, initiated by ice nucleation, occurs widely in nature, ranging from cellular to global phenomena. Ice nucleation has been experimentally proven to require the formation of a critical ice nucleus, consistent with classical nucleation theory (CNT). However, the accuracy of CNT quantitative predictions of critical cluster sizes and nucleation rates has never been verified experimentally. In this study, we circumvent this difficulty by using molecular dynamics (MD) simulation. The physical properties of water/ice for CNT predictions, including density, chemical potential difference, and diffusion coefficient, are independently obtained using MD simulation, whereas the calculation of interfacial free energy is based on thermodynamic assumptions of CNT, including capillarity approximation among others. The CNT predictions are compared to the MD evaluations of brute-force simulations and forward flux sampling methods. We find that the CNT and MD predicted critical cluster sizes are consistent, and the CNT predicted nucleation rates are higher than the MD predicted values within three orders of magnitude. We also find that the ice crystallized from supercooled water is stacking-disordered ice with a stacking of cubic and hexagonal ices in four representative types of stacking. The prediction discrepancies in nucleation rate mainly arise from the stacking-disordered ice structure, the asphericity of ice cluster, the uncertainty of ice-water interfacial free energy, and the kinetic attachment rate. Our study establishes a relation between CNT and MD to predict homogeneous ice nucleation.

Ruptures of mixed lipid monolayers under tension and supercooling: implications for nanobubbles in plants

Mixed phospholipid and glycolipid monolayers likely coat the surfaces of pressurised gas nanobubbles within the hydraulic systems of plants. The lipid coatings bond to water under negative pressure and are thus stretched out of equilibrium. In this work, we have used molecular dynamics simulations to produce trajectories of a biologically relevant mixed monolayer, pulled at mild negative pressures (-1.5 to -4.5 MPa). Pore formation within the monolayer is observed at both 270 and 310 K, and proceeds as an activated process once the lipid tails fully transition from the two dimensional liquid condensed to liquid expanded phase. Pressure:area isotherms showed reduced surface pressure under slight supercooling (T = 270 K) at all observed areas per lipid. Finally, Rayleigh-Plesset simulations were used to predict evolving nanobubble size using the calculated pressure:area isotherms as dynamic surface tensions. We confirm the existence of a second critical radius with respect to runaway growth, above the homogeneous cavitation radius.

Mode coupling behavior and fragile to strong transition of trehalose in a binary mixture with water upon supercooling

We perform molecular dynamics simulations of a binary mixture of water and trehalose with the TIP4P/Ice water model. We analyze the slow dynamics of trehalose molecules in the mildly supercooled region for concentrations of 3.66 and 18.57 wt. %. We previously studied the dynamics of water in the same mixtures. Supercooled TIP4P/Ice water solvating trehalose molecules was found to follow the Mode Coupling Theory (MCT) and to undergo a transition from a fragile to a strong behavior for both concentrations. Here, we show that also the dynamics of trehalose molecules follows the MCT and displays a fragile to strong crossover (FSC). The results show that trehalose in binary mixtures with water shares with it the dynamical behavior typical of glass forming liquids. Moreover, the FSC for trehalose structural relaxation times is found to occur at temperatures close to those previously obtained for water in the same solutions, showing that the dynamics of the solute is strongly coupled to that of the solvent. We also perform a MCT test showing that the trehalose dynamics obeys the MCT time-temperature superposition principle and that the exponents derived from the theory and the ones obtained from fitting procedure of the relaxation times are comparable, confirming that trehalose molecules in supercooled water solutions follow the MCT of glassy dynamics. Moreover, as predicted by the theory, trehalose particles have MCT parameters comparable to those of water in the same mixtures. This is an important result, given that MCT was originally formulated for monoatomic particles.

Structural effects of water clusters on viscosity at high shear rates

In this study, we use molecular dynamics simulations of liquid water to investigate how shear thinning affects the viscosity of liquid water by structural changes of the hydrogen bond network. The effect of shear on viscosity can be divided into two parts: shear-induced destruction of the hydrogen bond network and the influence of the water structure on shear viscosity. First, strong shear destroys tetrahedral structures and thus reduces the connectivity of the hydrogen bond network. It is mainly because shear deformation, characterized by compression and expansion axes, respectively, triggers the destruction and formation of hydrogen bonds, resulting in anisotropic effects on water structures. At the same time, shear destroys large clusters and enhances the formation of small ones, resulting in a decrease in average cluster sizes. Second, the change of viscosity obeys a power law relationship with the change of hydrogen bond structures, highlighting a one-to-one correspondence between structure and property. Meanwhile, in order to explain why the structure affects viscosity, we define hydrogen-bond viscosity and find that the cooperative motion of the water structures can promote momentum transfer in the form of aggregations. Hydrogen-bond viscosity accounts for 5%-50% of the total viscosity. Our results elucidate that water structures are the important structural units to explain the change of water properties.

Translational and reorientational dynamics in carboxylic acid-based deep eutectic solvents

The glass formation and the dipolar reorientational motions in deep eutectic solvents (DESs) are frequently overlooked, despite their crucial role in defining the room-temperature physiochemical properties. To understand the effects of these dynamics on the ionic conductivity and their relation to the mechanical properties of the DES, we conducted broadband dielectric and rheological spectroscopy over a wide temperature range on three well-established carboxylic acid-based natural DESs. These are the eutectic mixtures of choline chloride with oxalic acid (oxaline), malonic acid (maline), and phenylacetic acid (phenylaceline). In all three DESs, we observe signs of a glass transition in the temperature dependence of their dipolar reorientational and structural dynamics, as well as varying degrees of motional decoupling between the different observed dynamics. Maline and oxaline display a breaking of the Walden rule near the glass-transition temperature, while the relation between the dc conductivity and dipolar relaxation time in both maline and phenylaceline is best described by a power law. The glass-forming properties of the investigated systems not only govern the orientational dipolar motions and rheological properties, which are of interest from a fundamental point of view, but they also affect the dc conductivity, even at room temperature, which is of high technical relevance.

Enhanced fluidity of water in superhydrophobic nanotubes: estimating viscosity using jump-corrected confined Stokes-Einstein approach

Accurately predicting the viscosity of water confined within nanotubes is vital for various technological applications. Traditional methods have failed in this regard, necessitating a novel approach. We introduced the jump-corrected confined Stokes-Einstein (JCSE) method and now employ the same to estimate the viscosity and diffusion in superhydrophobic nanotubes. Our study covers a temperature range of 230-300 K and considers three nanotube diameters. Results show that water inside superhydrophobic nanotubes exhibits a significantly lower viscosity and higher diffusion than those inside hydrophobic nanotubes. Narrower nanotubes and lower temperatures accentuate these effects. Furthermore, water inside superhydrophobic nanotubes display a lower viscosity than bulk water, with the difference increasing at lower temperatures. This reduction is attributed to weaker water-water interactions caused by a lower water density in the interfacial region. These findings highlight the importance of interfacial water density and its influence on nanotube viscosity, shedding light on nanoscale fluid dynamics and opening avenues for diverse applications.

Viscosity and Stokes-Einstein relation in deeply supercooled water under pressure

We report measurements of the shear viscosity η in water up to 150 MPa and down to 229.5 K. This corresponds to more than 30 K supercooling below the melting line. The temperature dependence is non-Arrhenius at all pressures, but its functional form at 0.1 MPa is qualitatively different from that at all pressures above 20 MPa. The pressure dependence is non-monotonic, with a pressure-induced decrease of viscosity by more than 50% at low temperature. Combining our data with literature data on the self-diffusion coefficient Ds of water, we check the Stokes-Einstein relation which, based on hydrodynamics, predicts constancy of Dsη/T, where T is the temperature. The observed temperature and pressure dependence of Dsη/T is analogous to that obtained in simulations of a realistic water model. This analogy suggests that our data are compatible with the existence of a liquid-liquid critical point at positive pressure in water.

Accumulation of Antifreeze Proteins on Ice Is Determined by Adsorption

Antifreeze proteins (AFPs) facilitate the survival of diverse organisms in frigid environments by adsorbing to ice crystals and suppressing their growth. The rate of AFP accumulation on ice is determined by an interplay between AFP diffusion from the bulk solution to the ice-water interface and the subsequent adsorption of AFPs to the interface. To interrogate the relative importance of these two processes, here, we combine nonequilibrium fluorescence experiments with a reaction-diffusion model. We find that as diverse AFPs accumulate on ice, their concentration in the aqueous solution does not develop a gradient but remains equal to its bulk concentration throughout our experiments. These findings lead us to conclude that AFP accumulation on ice crystals, which are smaller than 100 μm in radius, is not limited by the diffusion of AFPs, but by the kinetics of AFP adsorption. Our results imply that mass transport limitations do not hinder AFPs from performing their biological function.

Effect of Concentration, Cooling, and Warming Rates on Glass Transition Temperatures for NaClO4, Ca(ClO4)2, and Mg(ClO4)2 Brines with Relevance to Mars and Other Cold Bodies

The hygroscopic and supercooling properties of perchlorates make them potentially important for sustaining liquid water on Mars. To understand the possibility for supercooled liquids and glasses on Mars and other cold bodies, we have characterized the supercooling and vitrification features using differential scanning calorimetry for Na, Ca, and Mg perchlorate brines in a temperature range relevant to Mars. Results show that the glass transition temperature (T_g) depends on the salt composition, concentration, and cooling or warming rate. The difference in T_g may be significant even in a single composition, producing glass transitions with over 40 K difference. A new model was developed to describe these T_g dependencies, with the warmest T_g values found for high concentrations and fast cooling rates. These results emphasize the importance of considering T_g as a range rather than a discrete temperature. For all perchlorates measured, the degree of supercooling was extensive at high concentrations, exceeding 100 K from the liquidus. With a highly reduced glass temperature (T_g/liquidus temperature) and low critical rate of temperature change to avoid crystallization, concentrated perchlorate brines are strong glass formers when compared to other glass-forming materials. The consideration of cooling rates in the context of cellular cryopreservation suggests that cooling and warming rates may be an important astrobiological factors in a diverse set of planetary environments. These findings provide additional constraints on the possibility of liquid water on Mars in terms of concentration, different latitudes, seasons, and times of day.

Relaxation time scales of interfacial water upon fluid to ripple to gel phase transitions of bilayers

The slow relaxation of interface water (IW) across three primary phases of membranes is relevant to understand the influence of IW on membrane functions at supercooled conditions. To this objective, a total of ∼16.26μs all-atom molecular dynamics simulations of 1,2-dimyristoyl-sn-glycerol-3-phosphocholine lipid membranes are carried out. A supercooling-driven drastic slow-down in heterogeneity time scales of the IW is found at the fluid to the ripple to the gel phase transitions of the membranes. At both fluid-to-ripple-to-gel phase transitions, the IW undergoes two dynamic crossovers in Arrhenius behavior with the highest activation energy at the gel phase due to the highest number of hydrogen bonds. Interestingly, the Stokes-Einstein (SE) relation is conserved for the IW near all three phases of the membranes for the time scales derived from the diffusion exponents and the non-Gaussian parameters. However, the SE relation breaks for the time scale obtained from the self-intermediate scattering functions. The behavioral difference in different time scales is universal and found to be an intrinsic property of glass. The first dynamical transition in the α relaxation time of the IW is associated with an increase in the Gibbs energy of activation of hydrogen bond breaking with locally distorted tetrahedral structures, unlike the bulk water. Thus, our analyses unveil the nature of the relaxation time scales of the IW across membrane phase transitions in comparison with the bulk water. The results will be useful to understand the activities and survival of complex biomembranes under supercooled conditions in the future.

Using 4D STEM to Probe Mesoscale Order in Molecular Glass Films Prepared by Physical Vapor Deposition

Physical vapor deposition can be used to prepare highly stable organic glass systems where the molecules show orientational and translational ordering at the nanoscale. We have used low-dose four-dimensional scanning transmission electron microscopy (4D STEM), enabled by a fast direct electron detector, to map columnar order in glassy samples of a discotic mesogen using a 2 nm probe. Both vapor-deposited and liquid-cooled glassy films show domains of similar orientation, but their size varies from tens to hundreds of nanometers, depending on processing. Domain sizes are consistent with surface-diffusion-mediated ordering during film deposition. These results demonstrate the ability of low-dose 4D STEM to characterize a mesoscale structure in a molecular glass system which may be relevant to organic electronics.

How Do Cyclopropane Fatty Acids Protect the Cell Membrane of Escherichia coli in Cold Shock?

The cyclopropanation of unsaturated lipid acyl chains of some bacterial cell membranes is an important survival strategy to protect the same against drastic cooling. To elucidate the role of cyclopropane ring-containing lipids, we have simulated the lipid membrane of Escherichia coli (E. coli) and two modified membranes by replacing the cyclopropane rings with either single or double bonds at widely different temperatures. It has been observed that the cyclopropane rings provide more rigid kinks in the lipid acyl chain compared to the double bonds and therefore further reduce the packing density of the membrane and subsequently enhance the membrane fluidity at low temperatures. They also inhibit the close packing of other lipids and deleterious phase separation by strongly interacting with them. Therefore, this study has explained why E. coli bacterial strain, susceptible to freezing environments, relies on the cyclopropanation of an unsaturated chain.

Self-diffusion and shear viscosity for the TIP4P/Ice water model

With an ever-increasing interest in water properties, many intermolecular force fields have been proposed to describe the behavior of water. Unfortunately, good models for liquid water usually cannot provide simultaneously an accurate melting point for ice. For this reason, the TIP4P/Ice model was developed for targeting the melting point and has become the preferred choice for simulating ice at coexistence. Unfortunately, available data for its dynamic properties in the liquid state are scarce. Therefore, we demonstrate a series of simulations aimed at the calculation of transport coefficients for the TIP4P/Ice model over a large range of thermodynamic conditions, ranging from T = 245 K to T = 350 K, for the temperature, and from p = 0 to p = 500 MPa, for the pressure. We have found that the self-diffusion (shear viscosity) exhibits smaller (increased) values than TIP4P/2005 and experiments. However, rescaling the temperature with respect to the triple point temperature, as in a corresponding states plot, we find that TIP4P/Ice compares very well with TIP4P/2005 and experiment. Such observations allow us to infer that despite the different original purposes of these two models examined here, one can benefit from a vast number of reports regarding the behavior of transport coefficients for the TIP4P/2005 model and utilize them following the routine described in this paper.

Microscopic mechanism study of the rheological behavior of non-Newtonian fluids based on dissipative particle dynamics

Non-Newtonian fluid rheological properties are a hot research topic for realizing intelligent applications. In order to investigate the microscopic mechanism and structural evolution process of the nonlinear rheological behavior of non-Newtonian fluids, this paper systematically investigates two continuous nonlinear rheological behaviors of non-Newtonian fluids, namely shear-thickening and shear-thinning rheological properties, using a non-Newtonian fluid system composed of polyethylene glycol (PEG) mixed with nano-silica (Nano-SiO₂) by a dissipative particle dynamics (DPD) method. It is shown that at low shear rates, the molecular chains of PEG in the fluid are stretched due to shear flow and the molecular structure is transformed into an ordered state; and the effective hydrodynamic radius of Nano-SiO₂ beads decreases, which makes the translational friction coefficient of the beads decrease and the system mobility increases, exhibiting shear-thinning behavior. When the shear rate exceeds the critical value, the contact and collision probability between Nano-SiO₂ beads in the non-Newtonian fluid increases; a large number of silicon hydroxyl groups exist on the surface of Nano-SiO₂, which form a large number of hydrogen bonds when they are close to each other and constrain the particle separation, resulting in a large aggregation of Nano-SiO₂ beads, leading to an increase in the effective kinetic radius of Nano-SiO₂ beads and an increase in the coefficient of translational friction, forming a blockage of the fluid system and exhibiting a shear-thickening behavior. Our study provides insights for understanding the rheological behavior of non-Newtonian fluids from a microscopic perspective, and contributes to the intelligent application of non-Newtonian fluids.

The Modification of Polyvinyl Alcohol for Ice Nucleation Based upon the Structures of Antifreeze Glycoproteins Found in Antarctic Fish

Various alternative compounds have been investigated to prevent icing, one of which includes poly(vinyl) alcohol (PVA), which has shown promising anti-freeze effects. However, determining the optimal structures and formulations of PVA for anti-icing applications has remained a challenge. Building upon our previous work, which used molecular dynamics simulations to assess the effects of hydroxyl group separation distance on ice nucleation, in this work, PVA was modified based upon the structures of antifreeze glycoproteins (AFGPs) found in Antarctic fish, and examined as a potential antifreeze compound. Four different PVA samples with different degrees of hydrolysis were fabricated and subsequently examined for their effects on ice crystallization. The results showed that the modified PVA samples with degrees of hydrolysis of 76% and 66% had an effect on ice crystallization, delaying ice crystallization by an average of approximately 20 min, and even preventing ice crystallization altogether in a small portion of the sample. Meanwhile, other samples with degrees of hydrolysis of 100% and 34% either showed no effect on ice crystallization, shortened the ice crystallization time, and appeared to promote ice nucleation.

Effect of diffusion kinetics on the ice nucleation temperature distribution

The nucleation behavior of water is crucial in many fields, spanning meteorology, glaciology, biology, and astrophysics. We report observations suggesting an effect of diffusion kinetics in water on the heterogeneous immersion/contact mode nucleation temperature distribution of ice. We performed differential scanning calorimetry analyses of repeated freeze/thaw cycles and investigated the effect of several variables on the regularity of the nucleation temperature distributions obtained. We observed that the thawing temperature and residence time above 0 °C affect the width of the measured distributions. We explain the observed phenomena according to the diffusion behavior of an external nucleator. Specifically, conditions of enhanced diffusion of the nucleator translated into broader, more scattered distributions, while conditions of limited diffusion translated into narrower, more regular distributions. Lastly, based on our experimental findings, we propose a theoretical explanation centered on the temperature dependence of diffusion kinetics in water.

Homeoviscous Adaptation of the Lipid Membrane of a Soil Bacterium Surviving under Diurnal Temperature Variation: A Molecular Simulation Perspective

A recent experiment has reported the lipidome remodeling of a soil-based plant-associated bacterium Methylobacterium extorquens due to diurnal temperature variations. The key adaptation strategy is the headgroup-specific remodeling of the acyl chain. To understand the idiosyncratic adaptation at the molecular level, we simulate the model membrane of the same bacterium using the reported lipidome compositions at four different experimental temperatures. We investigate the temperature-dependent packing density and fluidity of the membrane, the constancy of which is key to the homeoviscous adaptation. The results show that complex lipidome remodeling approximately preserves membrane properties under heat and cold stress. The headgroup-specific remodeling of the acyl chain serves to fine-tune the packing density and fluidity of the membrane at different temperatures. While lipids with strongly interacting headgroups are more abundant at higher temperatures, the lipidome is more dominated by lipids with weaker interacting headgroups at lower temperatures. This adaptation alleviates lipid membrane disruption caused by heat and cold stress. This study provides a molecular picture of the homeoviscous adaptation of the realistic lipid membrane of a soil-based bacterium.

Viscosity Measurement in Biocondensates Using Deep-Learning-Assisted Single-Particle Rotational Analysis

Viscoelastic characterization is of great importance for the investigation of biomolecular condensates. Single-particle-tracking-based rotational diffusion analysis of single nanorods is an effective approach for quantitative viscosity measurement. However, in the case of high background and noise with high-speed image acquisition, accurate extraction of diffusivity from the data is a challenging task. Here, we develop a novel frequency-domain-based deep learning (DL) method for single nanorod rotational tracking analysis. We synthesized Brownian rotational time-series data for training, designed a data preprocessing module to reduce the effect of noise, and extracted rotational diffusion coefficient using recurrent neural networks in the frequency domain. Compared with the traditional curve-fitting-based methods, our method shows higher accuracy and a wider detection range for viscosity measurement. We verified our method using experimental data from plasmonic imaging of single gold nanorods (AuNRs) in glycerol solution and PGL droplets. Our method can be potentially applied to the viscosity measurement of different biomolecular condensates in vitro and in vivo.

Non-monotonic composition dependence of the breakdown of Stokes-Einstein relation for water in aqueous solutions of ethanol and 1-propanol: explanation using translational jump-diffusion approach

A series of experimental and simulation studies examined the validity of the Stokes-Einstein relationship (SER) of water in binary water/alcohol mixtures of different mixture compositions. These studies revealed a strong non-monotonic composition dependence of the SER with maxima at the specific alcohol mole fraction where the non-idealities of the thermodynamic and transport properties are observed. The translational jump-diffusion (TJD) approach elucidated the breakdown of the SER in pure supercooled water as caused by the jump translation of molecules. The breakdown of SER in the supercooled water/methanol binary mixture was successfully explained using the same TJD approach. To further generalize the picture, here we focus on the non-monotonic composition dependence of SER breakdown of water in two water/alcohol mixtures (water/ethanol and water/propanol) for a broad temperature range. In agreement with previous studies, maximum breakdown of SER is observed for the mixture with alcohol mole fraction x = 0.2. Diffusion of the water molecules at the maximum SER breakdown point is largely contributed by jump-diffusion. The residual-diffusion, obtained by subtracting the jump-diffusion from the total diffusion, approximately follows the SER for different compositions and temperatures. We also performed hydrogen (H-)bond dynamics and observed that the contribution of jump-diffusion is proportional to the total free energy of activation of breaking all H-bonds that exist around a molecule. This study, therefore, suggests that the more a molecule is trapped by H-bonding, the more likely it is to diffuse through the jump-diffusion mechanism, eventually leading to an increasing degree of SER breakdown.

Shear viscosity and Stokes-Einstein violation in supercooled light and heavy water

We report shear viscosity of heavy water supercooled 33K below its melting point, revealing a 15-fold increase compared to room temperature. We also confirm our previous data for the viscosity of supercooled light water and reach a better accuracy. Our measurements, based on the spontaneous Brownian motion of 350nm spheres, disagree at the lowest temperature with the only other available data, based on Poiseuille flow in a narrow capillary, which may have been biased by electro-osmotic effects. Here we provide a detailed description of the experiment and its analysis. We review the literature data about dynamic properties of water (viscosity, self-diffusion coefficient, and rotational correlation time), discuss their temperature dependence, and compare their decoupling in the two isotopes.

Single molecule demonstration of Debye-Stokes-Einstein breakdown in polystyrene near the glass transition temperature

Rotational-translational decoupling, in which translational motion is apparently enhanced over rotational motion in violation of Stokes-Einstein (SE) and Debye-Stokes-Einstein (DSE) predictions, has been observed in materials near their glass transition temperatures (T_g). This has been posited to result from ensemble averaging in the context of dynamic heterogeneity. In this work, ensemble and single molecule experiments are performed in parallel on a fluorescent probe in high molecular weight polystyrene near its T_g. Ensemble results show decoupling onset at approximately 1.15T_g, increasing to over three orders of magnitude at T_g. Single molecule measurements also show a high degree of decoupling, with typical molecules at T_g showing translational diffusion coefficients nearly 400 times higher than expected from SE/DSE predictions. At the single molecule level, higher degree of breakdown is associated with particularly mobile molecules and anisotropic trajectories, providing support for anomalous diffusion as a critical driver of rotational-translational decoupling and SE/DSE breakdown.

Experiments with high-molecular-weight polystyrene provide insights into the mechanisms behind rotational-translational decoupling in glassy systems. Specifically, particularly mobile molecules exhibiting anisotropic trajectories are found to play a key role in Debye-Stokes-Einstein breakdown.

Low- and High-Density Unknown Waters at Ice-Water Interfaces

Experimental confirmation of liquid polymorphs of water, high-density liquid (HDL) and low-density liquid (LDL), is desired for understanding not only the liquid state of matter but also the origin of the mysterious properties of water. However, this remains challenging because the liquid-liquid critical point of water lies in experimentally inaccessible supercooling conditions known as "no man's land". Here, we show by in situ optical microscopy that droplets and layers of low- and high-density unknown waters (LDUW and HDUW) appear macroscopically depending upon ice polymorphs at non-equilibrium interfaces between water and ices under experimentally accessible (de)pressurization conditions. These unknown waters were found to have characteristic velocities (about 20 and 100 m/s for LDUW and HDUW, respectively) different from water (about 40 m/s) and quasi-liquid layers (QLLs) (about 2 and 0.2 m/s for droplet and layer forms of QLLs, respectively). Our discoveries provide insight on liquid polymorphism of water.

Translational Jump-Diffusion of Hydroxide Ion in Anion Exchange Membrane: Deciphering the Nature of Vehicular Diffusion

Earlier, ab initio and reactive force-field-based molecular dynamics (MD) simulation studies suggested an overwhelming contribution of the vehicular diffusion in the total diffusion of hydroxide ions rather than structural diffusion. But does the vehicular diffusion occur via small-step displacement? This question is important to have an understanding of the real characteristics of vehicular diffusion. To answer this question, we perform a classical molecular dynamics simulation of a system containing a hydroxide ion exchange membrane polymer and hydroxide ion at different hydration levels and temperatures using the same molecular force field (Dubey, V. Chem. Phys. Lett. 2020, 755, 137802), which successfully captured the microscopic structure and dynamics of the system. We use the translational jump-diffusion approach, used previously in supercooled water for understanding the origin of breakdown of the Stokes-Einstein relation, to calculate the jump-diffusion coefficient of hydroxide ion and water in the anion exchange membrane. We have seen a significant role of hydration level and temperature in the mechanism of vehicular diffusion. In overhydrated membrane, both hydroxide ions and water molecules diffuse via both small- and large-step displacement. With decreasing hydration level and temperature, the diffusion is increasingly governed by the jump-diffusion mechanism. The larger contribution of jump-diffusion comes from the stronger caging of the diffusing species by the solvent at lower hydration levels and temperature. These results, therefore, suggest that the hydration level and temperature of the hydroxide ion exchange membrane determine the detailed mechanism of the vehicular diffusion of hydroxide ion, especially whether the diffusion follows hydrodynamics or not.

Enhanced Surface Icephobicity on an Elastic Substrate

Ice accumulation on exposed surfaces is unavoidable as time elapses and the temperature decreases sufficiently. To mitigate icing problems, various types of icephobic substrates have been rationally designed, including superhydrophobic substrates (SHSs), aqueous lubricating layers, organic lubricating layers, organogels, polyelectrolyte brush layers, electrolyte-based hydrogels, elastic substrates, and multicrack initiator-promoted surfaces. Among these surfaces, elastic substrates show excellent enhanced surface icephobicity during dynamic processes (i.e., water-impacting and de-icing tests). Herein, we summarize recent progress in elastic icephobic substrates and discuss the reasons that surface icephobicity can be enhanced on elastic substrates in terms of enhanced water repellency and further lowering the ice adhesion strength. For enhanced water repellency, we focus on reducing the contact time of water impacting such that water droplets can be easily shed from an elastic substrate before ice occurs. Reducing the contact time of water impacting various substrates (i.e., micro/nanostructured rigid SHSs, macrotextured rigid SHSs, and elastic SHSs) is discussed, followed by exploring their mechanisms. We argue that the ice adhesion strength can be further lowered on an elastic substrate by rationally tuning the elastic modulus and surface textures (i.e., surface textured and hollow subsurface textured) and combining elastic substrate with other passive anti-icing strategies (or functioning passive icephobic substrates with an electrothermal or photothermal stimulus). In short, the introduction of an elastic substrate into a passive or active icephobicity surface opens an avenue toward designing a versatile icephobic surface, providing great potential for outdoor anti-icing applications.

Kinetics of nonisothermal phase change with arbitrary temperature-time history and initial transformed phase distributions

This paper describes the extension of the classic Avrami equation to nonisothermal systems with arbitrary temperature-time history and arbitrary initial distributions of transformed phase. We start by showing that through examination of phase change in Fourier space, we can decouple the nucleation rate, growth rate, and transformed fraction, leading to the derivation of a nonlinear differential equation relating these three properties. We then consider a population balance partial differential equation (PDE) on the phase size distribution and solve it analytically. Then, by relating this PDE solution to the transformed fraction of phase, we are able to derive initial conditions to the differential equation relating nucleation rate, growth rate, and transformed fraction.

Advances in the study of supercooled water

In this review, we report recent progress in the field of supercooled water. Due to its uniqueness, water presents numerous anomalies with respect to most simple liquids, showing polyamorphism both in the liquid and in the glassy state. We first describe the thermodynamic scenarios hypothesized for the supercooled region and in particular among them the liquid-liquid critical point scenario that has so far received more experimental evidence. We then review the most recent structural indicators, the two-state model picture of water, and the importance of cooperative effects related to the fact that water is a hydrogen-bonded network liquid. We show throughout the review that water's peculiar properties come into play also when water is in solution, confined, and close to biological molecules. Concerning dynamics, upon mild supercooling water behaves as a fragile glass former following the mode coupling theory, and it turns into a strong glass former upon further cooling. Connections between the slow dynamics and the thermodynamics are discussed. The translational relaxation times of density fluctuations show in fact the fragile-to-strong crossover connected to the thermodynamics arising from the existence of two liquids. When considering also rotations, additional crossovers come to play. Mobility-viscosity decoupling is also discussed in supercooled water and aqueous solutions. Finally, the polyamorphism of glassy water is considered through experimental and simulation results both in bulk and in salty aqueous solutions. Grains and grain boundaries are also discussed.

Exploring the validity of the Stokes-Einstein relation in supercooled water using nanomolecular probes

The breakdown of Stokes-Einstein relation in liquid water is one of the many anomalies that take place upon cooling and indicates the decoupling of diffusion and viscosity. It is hypothesized that these anomalies manifest due to the appearance of nanometer-scale spatial fluctuations, which become increasingly pronounced in the supercooled regime. Here, we explore the validity of the Stokes-Einstein relation in supercooled water using nanomolecular probes. We capture the diffusive dynamics of the probes using dynamic light scattering and target dynamics at different length scales by varying the probe size, from ≈100 nm silica spheres to molecular-sized polyhydroxylated fullerenes (≈1 nm). We find that all the studied probes, independent of size, display similar diffusive dynamics with an Arrhenius activation energy of ≈23 kJ mol^-1. Analysis of the diffusion coefficient further indicates that the probes, independent of their size, experience similar dynamic environment, which coincides with the macroscopic viscosity, while single water molecules effectively experience a comparatively lower viscosity. Finally, we conclude that our results indicate that the Stokes-Einstein relation is preserved for diffusion of probes in supercooled water T ≥ 260 K with size as small as ≈1 nm.

Conservation of the Stokes-Einstein relation in supercooled water

The Stokes-Einstein (SE) relation is commonly regarded as being breakdown in supercooled water. However, this conclusion is drawn by testing the validity of some variants of the SE relation rather than its original form, and it appears conflicting with the fact that supercooled water is in its local equilibrium. In this work, we show by molecular dynamics simulations that the Stokes-Einstein relation is indeed conserved in supercooled water. The inconsistency between the original SE relation and its variants comes from two facts: (1) the substitutes of the shear viscosity in the SE variants are only approximate relations; and (2) the effective hydrodynamic radius actually decreases with decreasing temperature, instead of being a constant as assumed in the SE variants.

Breakdown of the Stokes-Einstein relation in supercooled water: the jump-diffusion perspective

Although water is the most ubiquitous liquid it shows many thermodynamic and dynamic anomalies. Some of the anomalies further intensify in the supercooled regime. While many experimental and theoretical studies have focused on the thermodynamic anomalies of supercooled water, fewer studies explored the dynamical anomalies very extensively. This is due to the intricacy of the experimental measurement of the dynamical properties of supercooled water. Violation of the Stokes-Einstein relation (SER), an important relation connecting the diffusion of particles with the viscosity of the medium, is one of the major dynamical anomalies. In absence of experimentally measured viscosity, researchers used to check the validity of SER indirectly using average translational relaxation time or α-relaxation time. Very recently, the viscosity of supercooled water was accurately measured at a wide range of temperatures and pressures. This allowed direct verification of the SER at different temperature-pressure thermodynamic state points. An increasing breakdown of the SER was observed with decreasing temperature. Increasing pressure reduces the extent of breakdown. Although some well-known theories explained the above breakdown, a detailed molecular mechanism was still elusive. Recently, a translational jump-diffusion (TJD) approach has been able to quantitatively explain the breakdown of the SER in pure supercooled water and an aqueous solution of methanol. The objective of this article is to present a detailed and state-of-the-art analysis of the past and present works on the breakdown of SER in supercooled water with a specific focus on the new TJD approach for explaining the breakdown of the SER.

Regulation of microtubule dynamics, mechanics and function through the growing tip

Microtubule dynamics and their control are essential for the normal function and division of all eukaryotic cells. This plethora of functions is, in large part, supported by dynamic microtubule tips, which can bind to various intracellular targets, generate mechanical forces and couple with actin microfilaments. Here, we review progress in the understanding of microtubule assembly and dynamics, focusing on new information about the structure of microtubule tips. First, we discuss evidence for the widely accepted GTP cap model of microtubule dynamics. Next, we address microtubule dynamic instability in the context of structural information about assembly intermediates at microtubule tips. Three currently discussed models of microtubule assembly and dynamics are reviewed. These are considered in the context of established facts and recent data, which suggest that some long-held views must be re-evaluated. Finally, we review structural observations about the tips of microtubules in cells and describe their implications for understanding the mechanisms of microtubule regulation by associated proteins, by mechanical forces and by microtubule-targeting drugs, prominently including cancer chemotherapeutics.

Dynamical crossover and its connection to the Widom line in supercooled TIP4P/Ice water

We perform molecular dynamics simulations with the TIP4P/Ice water model to characterize the relationship between dynamics and thermodynamics of liquid water in the supercooled region. We calculate the relevant properties of the phase diagram, and we find that TIP4P/Ice presents a retracing line of density maxima, similar to what was previously found for atomistic water models and models of other tetrahedral liquids. For this model, a liquid-liquid critical point between a high-density liquid and a low-density liquid was recently found. We compute the lines of the maxima of isothermal compressibility and the minima of the coefficient of thermal expansion in the one phase region, and we show that these lines point to the liquid-liquid critical point while collapsing on the Widom line. This line is the line of the maxima of correlation length that emanates from a second order critical point in the one phase region. Supercooled water was found to follow mode coupling theory and to undergo a transition from a fragile to a strong behavior right at the crossing of the Widom line. We find here that this phenomenology also happens for TIP4P/Ice. Our results appear, therefore, to be a general characteristic of supercooled water, which does not depend on the interaction potential used, and they reinforce the idea that the dynamical crossover from a region where the relaxation mechanism is dominated by cage relaxation to a region where cages are frozen and hopping dominates is correlated in water to a phase transition between a high-density liquid and a low-density liquid.

Advances and challenges in the high-pressure rheology of complex fluids

Complex fluids and soft materials are ubiquitous in nature and industry. In industrial processes, these materials often get exposed to high hydrostatic pressures. Some examples include polymer melts, crude oils, gas hydrates, food systems, foams, motor oils, lubricants, etc. In spite of the relevance and utilization of hydrostatic pressure in many industrial applications, the role of pressure on the rheological properties has not been examined extensively in the literature. We review the high-pressure rheometric systems and present advantages and drawbacks of various kinds of rheometers such as capillary rheometer, sliding plate rheometer, falling ball viscometer, and rotational rheometer. By outlining the design complexities, precision, low-torque resolution limits and the inherent error sources of each type are critically evaluated. Furthermore, the high-pressure rheology data, chosen to cover a broad range of pressures and material class ranging from simple Newtonian fluids (incompressible), complex non-Newtonian fluids and compressible fluids featuring various key applications from different industries, are reviewed. The literature suggests, while effect of pressure on the rheological behavior is vital for many applications, compared to the effects of temperature on the rheological behavior, knowledge of the effect of pressure is still in its infancy.

Transition pathway of hydrogen bond switching in supercooled water analyzed by the Markov state model

In this work, we examine hydrogen-bond (H-bond) switching by employing the Markov State Model (MSM). During the H-bond switching, a water hydrogen initially H-bonded with water oxygen becomes H-bonded to a different water oxygen. MSM analysis was applied to trajectories generated from molecular dynamics simulations of the TIP4P/2005 model from a room-temperature state to a supercooled state. We defined four basis states to characterize the configuration between two water molecules: H-bonded ("H"), unbound ("U"), weakly H-bonded ("w"), and alternative H-bonded ("a") states. A 16 × 16 MSM matrix was constructed, describing the transition probability between states composed of three water molecules. The mean first-passage time of the H-bond switching was estimated by calculating the total flux from the HU to UH states. It is demonstrated that the temperature dependence of the mean first-passage time is in accordance with that of the H-bond lifetime determined from the H-bond correlation function. Furthermore, the flux for the H-bond switching is decomposed into individual pathways that are characterized by different forms of H-bond configurations of trimers. The dominant pathway of the H-bond switching is found to be a direct one without passing through such intermediate states as "w" and "a," the existence of which becomes evident in supercooled water. The pathway through "w" indicates a large reorientation of the donor molecule. In contrast, the pathway through "a" utilizes the tetrahedral H-bond network, which is revealed by the further decomposition based on the H-bond number of the acceptor molecule.

Universal Nucleation Behavior of Sheared Systems

Using molecular simulations and a modified classical nucleation theory, we study the nucleation, under flow, of a variety of liquids: different water models, Lennard-Jones, and hard sphere colloids. Our approach enables us to analyze a wide range of shear rates inaccessible to brute-force simulations. Our results reveal that the variation of the nucleation rate with shear is universal. A simplified version of the theory successfully captures the nonmonotonic temperature dependence of the nucleation behavior, which is shown to originate from the violation of the Stokes-Einstein relation.

Testing the Stokes-Einstein relation with the hard-sphere fluid model

The Stokes-Einstein (SE) relation has been widely applied to quantitatively describe the Brownian motion. Notwithstanding, here we show that even for a simple fluid, the SE relation may fail over a wide range of the Brownian particle's size. Namely, although the SE relation could be a good approximation for a large enough Brownian particle, a significant error may appear when decreasing the Brownian particle's size down to several hundred times the size of the fluid molecules, and the error increases with the decrease of the Brownian particle's size. The cause is rooted in the fact that the kinetic contribution to the diffusion coefficient is inversely proportional to the squared radius of the Brownian particle. After excluding the kinetic contribution, we show that the applicable range of the SE relation is expanded significantly.

Tailoring the nuclear Overhauser effect for the study of small and medium-sized molecules by solvent viscosity manipulation

The nuclear Overhauser effect (NOE) is a consequence of cross-relaxation between nuclear spins mediated by dipolar coupling. Its sensitivity to internuclear distances has made it an increasingly important tool for the determination of through-space atom proximity relationships within molecules of sizes ranging from the smallest systems to large biopolymers. With the support of sophisticated FT-NMR techniques, the NOE plays an essential role in structure elucidation, conformational and dynamic investigations in liquid-state NMR. The efficiency of magnetization transfer by the NOE depends on the molecular rotational correlation time, whose value depends on solution viscosity. The magnitude of the NOE between ¹H nuclei varies from +50% when molecular tumbling is fast to -100% when it is slow, the latter case corresponding to the spin diffusion limit. In an intermediate tumbling regime, the NOE may be vanishingly small. Increasing the viscosity of the solution increases the motional correlation time, and as a result, otherwise unobservable NOEs may be revealed and brought close to the spin diffusion limit. The goal of this review is to report the resolution of structural problems that benefited from the manipulation of the negative NOE by means of viscous solvents, including examples of molecular structure determination, conformation elucidation and mixture analysis (the ViscY method).

Structural relaxation and crystallization in supercooled water from 170 to 260 K

The origin of water's anomalous properties has been debated for decades. Resolution of the problem is hindered by a lack of experimental data in a crucial region of temperatures, T, and pressures where supercooled water rapidly crystallizes-a region often referred to as "no man's land." A recently developed technique where water is heated and cooled at rates greater than 10⁹ K/s now enables experiments in this region. Here, it is used to investigate the structural relaxation and crystallization of deeply supercooled water for 170 K < T < 260 K. Water's relaxation toward a new equilibrium structure depends on its initial structure with hyperquenched glassy water (HQW) typically relaxing more quickly than low-density amorphous solid water (LDA). For HQW and T > 230 K, simple exponential relaxation kinetics is observed. For HQW at lower temperatures, increasingly nonexponential relaxation is observed, which is consistent with the dynamics expected on a rough potential energy landscape. For LDA, approximately exponential relaxation is observed for T > 230 K and T < 200 K, with nonexponential relaxation only at intermediate temperatures. At all temperatures, water's structure can be reproduced by a linear combination of two, local structural motifs, and we show that a simple model accounts for the complex kinetics within this context. The relaxation time, τ _rel , is always shorter than the crystallization time, τ _xtal For HQW, the ratio, τ _xtal /τ _rel , goes through a minimum at ∼198 K where the ratio is about 60.

Brownian dynamics simulation of protofilament relaxation during rapid freezing

Electron cryo-microscopy (Cryo-EM) is a powerful method for visualizing biological objects with up to near-angstrom resolution. Instead of chemical fixation, the method relies on very rapid freezing to immobilize the sample. Under these conditions, crystalline ice does not have time to form and distort structure. For many practical applications, the rate of cooling is fast enough to consider sample immobilization instantaneous, but in some cases, a more rigorous analysis of structure relaxation during freezing could be essential. This difficult yet important problem has been significantly under-reported in the literature, despite spectacular recent developments in Cryo-EM. Here we use Brownian dynamics modeling to examine theoretically the possible effects of cryo-immobilization on the apparent shapes of biological polymers. The main focus of our study is on tubulin protofilaments. These structures are integral parts of microtubules, which in turn are key elements of the cellular skeleton, essential for intracellular transport, maintenance of cell shape, cell division and migration. We theoretically examine the extent of protofilament relaxation within the freezing time as a function of the cooling rate, the filament's flexural rigidity, and the effect of cooling on water's viscosity. Our modeling suggests that practically achievable cooling rates are not rapid enough to capture tubulin protofilaments in conformations that are incompletely relaxed, suggesting that structures seen by cryo-EM are good approximations to physiological shapes. This prediction is confirmed by our analysis of curvatures of tubulin protofilaments, using samples, prepared and visualized with a variety of methods. We find, however, that cryofixation may capture incompletely relaxed shapes of more flexible polymers, and it may affect Cryo-EM-based measurements of their persistence lengths. This analysis will be valuable for understanding of structures of different types of biopolymers, observed with Cryo-EM.

Towards molecular movies with X-ray photon correlation spectroscopy

In this perspective article we highlight research opportunities and challenges in probing structural dynamics of molecular systems using X-ray Photon Correlation Spectroscopy (XPCS). The development of new X-ray sources, such as 4th generation storage rings and X-ray free-electron lasers (XFELs), provides promising new insights into molecular motion. Employing XPCS at these sources allows to capture a very broad range of timescales and lengthscales, spanning from femtoseconds to minutes and atomic scales to the mesoscale. Here, we discuss the scientific questions that can be addressed with these novel tools for two prominent examples: the dynamics of proteins in biomolecular condensates and the dynamics of supercooled water. Finally, we provide practical tips for designing and estimating feasibility of XPCS experiments as well as on detecting and mitigating radiation damage.

Diffusion-controlled and `diffusionless' crystal growth: relation between liquid dynamics and growth kinetics of griseofulvin

Understanding how liquid dynamics govern crystallization is critical for maintaining the physical stability of amorphous pharmaceutical formulations. In the present study, griseofulvin (GSF), a classic antifungal drug, was used as the model system to investigate the correlations between crystal growth kinetics and liquid dynamics. The temperature dependence of the kinetic part of the bulk crystal growth in a supercooled liquid of GSF was weaker than that of the structural relaxation time τα and scaled as τα −0.69. In the glassy state, GSF exhibited the glass-to-crystal (GC) growth behavior, whose growth rate was too fast to be under the control of the α-relaxation process. Moreover, from the perspective of τα, the GC growth of GSF also satisfied the general condition for GC growth to exist: D/u < 7 pm, where D is the diffusion coefficient and u the speed of crystal growth. Also compared were the fast surface crystal growth rates u s and surface relaxation times τsurface predicted by the random first-order transition theory. Here, the surface crystal growth rate u s of GSF exhibited a power-law dependence upon the surface structural relaxation time: u s ∝ τsurface −0.71, which was similar to that of the bulk growth rate and τα. These findings are important for understanding and predicting the crystallization of amorphous pharmaceutical solids both in the bulk and at the surface.

Dynamic heterogeneity and viscosity decoupling: origin and analytical prediction

The molecular-level structure and dynamics decide the functionality of solvent media. Therefore, a significant amount of effort is being dedicated continually over time in understanding their structural and dynamical features. One intriguing aspect of solvent structure and dynamics is heterogeneity. In these systems, the dynamics follow , where p is the measure of viscosity decoupling. We analytically predicted that in such cases, the Stokes-Einstein relationship is modified to due to microdomain formation, and the second term on the right-hand side leads to viscosity decoupling. We validated our prediction by estimating the p values of a few solvents, and they matched well with the literature. Overall, we believe that our approach gives a simple yet unique physical picture to help us understand the heterogeneity of solvent media.

Breakdown of the Stokes-Einstein Relation in Supercooled Water/Methanol Binary Mixtures: Explanation Using the Translational Jump-Diffusion Approach

A recent experiment has directly checked the validity of the Stokes-Einstein (SE) relation for pure water, pure methanol, and their binary mixtures of three different compositions at different temperatures. The effect of composition on the nature of breakdown of the SE relation is interesting. While in the majority of the systems, an increasing SE breakdown is observed with decreasing temperature, the breakdown is already significant at higher temperatures for the equimolar mixture. Violations of the SE relation in pure supercooled water at different temperatures and pressures have been previously explained using the translational jump-diffusion (TJD) approach, which provides a fundamental molecular basis, by directly connecting the SE breakdown with jump-diffusion of the molecules. We have used the same TJD approach for explaining the SE breakdown for the methanol/water binary mixtures of compositions studied in the experiment over a wide range of temperatures between 220 K and 300 K. We have understood that the jump-diffusion is the key responsible factor for the SE breakdown. The maximum jump-diffusion contribution gives rise to the early SE breakdown observed for the equimolar mixture observed in the experiment. This study, therefore, provides molecular insight into the SE breakdown for the supercooled water/methanol binary mixture, as found in the experiment.

Fast increase of nanofluidic slip in supercooled water: the key role of dynamics

Nanofluidics is an emerging field offering innovative solutions for energy harvesting and desalination. The efficiency of these applications depends strongly on liquid-solid slip, arising from a favorable ratio between viscosity and interfacial friction. Using molecular dynamics simulations, we show that wall slip increases strongly when water is cooled below its melting point. For water on graphene, the slip length is multiplied by up to a factor of five and reaches 230 nm at the lowest simulated temperature, T ∼ 225 K; experiments in nanopores can reach much lower temperatures and could reveal even more drastic changes. The predicted fast increase in water slip can also be detected at supercoolings reached experimentally in bulk water, as well as in droplets flowing on anti-icing surfaces. We explain the anomalous slip behavior in the supercooled regime by a decoupling between viscosity and bulk density relaxation dynamics, and we rationalize the wall-type dependence of the enhancement in terms of interfacial density relaxation dynamics. While providing fundamental insights on the molecular mechanisms of hydrodynamic transport in both interfacial and bulk water in the supercooled regime, this study is relevant to the design of anti-icing surfaces, could help explain the subtle phase and dynamical behaviors of supercooled confined water, and paves the way to explore new behaviors in supercooled nanofluidic systems.