Wetting hysteresis induces effective unidirectional water transport through a fluctuating nanochannel.

We propose a water pump that actively transports water molecules through nanochannels. Spatially asymmetric noise fluctuations imposed on the channel radius cause unidirectional water flow without osmotic pressure, which can be attributed to hysteresis in the cyclic transition between the wetting/drying states. We show that the water transport depends on fluctuations, such as white, Brownian, and pink noises. Because of the high-frequency components in white noise, fast switching of open and closed states inhibits channel wetting. Conversely, pink and Brownian noises generate high-pass filtered net flow. Brownian fluctuation leads to a faster water transport rate, whereas pink noise has a higher capability to overcome pressure differences in the opposite direction. A trade-off relationship exists between the resonant frequency of the fluctuation and the flow amplification. The proposed pump can be considered as an analogy for the reversed Carnot cycle, which is the upper limit of the energy conversion efficiency.

Nanotube Active Water Pump Driven by Alternating Hydrophobicity.

Water transport must be efficiency controlled for the future sustainability of life. Various water transport systems using carbon nanotubes have been proposed in recent years. Although these systems are more permeable than aquaporins, their water transport is passive. In this study, we successfully demonstrate an active water pump driven by simple hydrophobic interaction through computer simulation. Even in the absence of a pressure- or density-gradient, the proposed pump can actively transport water molecules by alternately switching the hydrophobicity of the pump surface. The water transport rate can be easily controlled by varying the time interval of switching. The pump with optimized switching time exhibits prominent water permeance. The results obtained herein can be applied in various water transport technologies because of the simple mechanics. The proposed water pump has the potential to realize an effective device such as a low-energy artificial purification system.

Molecular Dynamics Study of the Effect of Water on Hydrophilic and Hydrophobic Ionic Liquids.

We performed molecular dynamics (MD) simulations of ionic liquid (IL)-water mixtures to investigate the effects of water addition. The IL cation 1-butyl-3-methylimidazolium ([C4mim]) and the four anions, nitrate (NO3), tetrafluoroborate (BF4), hexafluorophosphate (PF6), and bis(trifluoromethanesulfonyl)imide (NTf2), were used to examine the effects of differences in hydrophobicity and anion size. The radial distribution functions of water-water have two different water content dependences. In NO3 and BF4 systems, the effect of water-anion-water structures connected by hydrogen bonds due to the strong interaction of anion-water is large. The growth of water clusters changes the peak heights of the radial distribution functions in PF6 and NTf2 systems. The increase in the diffusion constant is small in the NO3 system but is not small in the other systems. The relaxation time of the anion-water hydrogen bonds in the NO3 system is much longer than those in the other systems. It is the reason for the low water content dependency of the diffusion constants of the NO3 system. The water constant dependences of structures in NO3 and BF4 systems are similar, but that in the diffusion constant is not. The BF4 system shows hydrophilic features in the structural change and hydrophobic features in the water content dependences of the diffusion constant. The addition of water molecules provides various hydrophobicity/hydrophilicity of anions.

Molecular Dynamics Simulation of Water Nanodroplet Bounce Back from Flat and Nanopillared Surface.

Molecular dynamics simulations of impinging nanodroplets were performed to study the bounce-back condition for flat and nanopillared surfaces. We found that the bounce-back condition can be closely related to the degree of droplet deformation upon collision with the solid surface. When the droplets have little or small deformation, the bounce-back condition solely depends on the hydrophobicity of the surface. On the other hand, when the droplet deformation is large, the impinging velocity dependence of the bounce-back condition becomes stronger due to the increase of the liquid-vapor interfacial area of colliding droplet, which is proportional to the liquid-vapor surface energy. The impinging droplet simulations with nanopillared hydrophobic surfaces were also performed. The contribution of droplet deformation in this case is relatively small because the surface hydrophobicity is enhanced due to the existence of pillars. Finally, we find that the maximum spreading diameter of the impinging droplets exhibits a consistent trend, in terms of the Weber number dependence, as the experimental measurements with macrodroplets.

Theory of nanobubble formation and induced force in nanochannels.

This paper presents a fundamental theory of nanobubble formation and induced force in confined nanochannels. It is shown that nanobubble formation between hydrophobic plates can be predicted from their surface tension and geometry, with estimated values for the surface free energy and the force acting on the plates in good agreement with the results of molecular dynamics simulation and experimentation. When a bubble is formed between two plates, vertical attractive force and horizontal retract force due to the shifted plates are applied to the plates. The net force exerted on the plates is not dependent on the distance between them. The short-range force between hydrophobic surfaces due to hydrophobic interaction appears to correspond to the force estimated by our theory. We compared between experimental and theoretical values for the binding energy of a molecular motor system to validate our theory. The tendency that the binding energy increases as the size of the protein increases is consistent with the theory.

Understanding molecular motor walking along a microtubule: a themosensitive asymmetric Brownian motor driven by bubble formation.

The "asymmetric Brownian ratchet model", a variation of Feynman's ratchet and pawl system, is invoked to understand the kinesin walking behavior along a microtubule. The model system, consisting of a motor and a rail, can exhibit two distinct binding states, namely, the random Brownian state and the asymmetric potential state. When the system is transformed back and forth between the two states, the motor can be driven to "walk" in one direction. Previously, we suggested a fundamental mechanism, that is, bubble formation in a nanosized channel surrounded by hydrophobic atoms, to explain the transition between the two states. In this study, we propose a more realistic and viable switching method in our computer simulation of molecular motor walking. Specifically, we propose a thermosensitive polymer model with which the transition between the two states can be controlled by temperature pulses. Based on this new motor system, the stepping size and stepping time of the motor can be recorded. Remarkably, the "walking" behavior observed in the newly proposed model resembles that of the realistic motor protein. The bubble formation based motor not only can be highly efficient but also offers new insights into the physical mechanism of realistic biomolecule motors.

Molecular Insight into Different Denaturing Efficiency of Urea, Guanidinium, and Methanol: A Comparative Simulation Study.

We have designed various nanoslit systems, whose opposing surfaces can be either hydrophobic, hydrophilic, or simply a water-vapor interface, for the molecular dynamics simulation of confined water with three different protein denaturants, i.e., urea, guanidinium chloride (GdmCl), and methanol, respectively. Particular attention is placed on the preferential adsorption of the denaturant molecules onto the opposing surfaces and associated resident time in the vicinal layer next to the surfaces, as well as their implication in the denaturing efficiency of different denaturant molecules. Our simulation results show that among the three denaturants, the occupancy of methanol in the vicinal layer is the highest while the residence time of Gdm is the longest. Although the occupancy and the residence time of urea in the vicinal layer is less than those of the other two denaturant molecules, urea entails "all-around" properties for being a highly effective denaturant. The distinct characteristics of three denaturants may suggest a different molecular mechanism for the protein denaturation. This comparative simulation by design allows us to gain additional insights, on the molecular level, into the denaturation effect and related hydrophobic effect.

Measurement of contact-angle hysteresis for droplets on nanopillared surface and in the Cassie and Wenzel states: a molecular dynamics simulation study.

We perform large-scale molecular dynamics simulations to measure the contact-angle hysteresis for a nanodroplet of water placed on a nanopillared surface. The water droplet can be in either the Cassie state (droplet being on top of the nanopillared surface) or the Wenzel state (droplet being in contact with the bottom of nanopillar grooves). To measure the contact-angle hysteresis in a quantitative fashion, the molecular dynamics simulation is designed such that the number of water molecules in the droplets can be systematically varied, but the number of base nanopillars that are in direct contact with the droplets is fixed. We find that the contact-angle hysteresis for the droplet in the Cassie state is weaker than that in the Wenzel state. This conclusion is consistent with the experimental observation. We also test a different definition of the contact-angle hysteresis, which can be extended to estimate hysteresis between the Cassie and Wenzel state. The idea is motivated from the appearance of the hysteresis loop typically seen in computer simulation of the first-order phase transition, which stems from the metastability of a system in different thermodynamic states. Since the initial shape of the droplet can be controlled arbitrarily in the computer simulation, the number of base nanopillars that are in contact with the droplet can be controlled as well. We show that the measured contact-angle hysteresis according to the second definition is indeed very sensitive to the initial shape of the droplet. Nevertheless, the contact-angle hystereses measured based on the conventional and new definition seem converging in the large droplet limit.

Molecular dynamics simulations of urea-water binary droplets on flat and pillared hydrophobic surfaces.

We performed molecular dynamics (MD) simulations to investigate equilibrium behavior of urea-water binary droplets on flat (graphitic) and pillared surfaces. The contact angles as a function of urea concentration on the flat surface are computed. It is found that the contact angle decreases as the urea concentration increases. At the equilibrium state, the urea molecules in the droplet tend to be located near the hydrophobic graphite surface. This behavior is consistent with the denaturing effects of urea in protein solutions. We also performed MD simulations of collision between a urea-water droplet and the pillared surface to examine the tendency for the droplet being in the Cassie state (droplet staying on top of the pillared surface) or in the Wenzel state (droplet staying at the bottom of the pillared surface), at various urea concentrations.

Asymmetric Brownian motor driven by bubble formation in a hydrophobic channel.

The "asymmetric brownian ratchet model" is a variation of Feynman's ratchet and pawl system proposed. In this model, a system consisting of a motor and a rail has two binding states. One is the random brownian state, and the other is the asymmetric potential state. When the system is alternatively switched between these states, the motor can be driven in one direction. This model is believed to explain nanomotor behavior in biological systems. The feasibility of the model has been demonstrated using electrical and magnetic forces; however, switching of these forces is unlikely to be found in biological systems. In this paper, we propose an original mechanism of transition between states by bubble formation in a nanosized channel surrounded by hydrophobic atoms. This amounts to a nanoscale motor system using bubble propulsion. The motor system consists of a hydrophobic motor and a rail on which hydrophobic patterns are printed. Potential asymmetry can be produced by using a left-right asymmetric pattern shape. Hydrophobic interactions are believed to play an important role in the binding of biomolecules and molecular recognition. The bubble formation is controlled by changing the width of the channel by an atomic distance (∼0.1 nm). Therefore, the motor is potentially more efficient than systems controlled by other forces, in which a much larger change in the motor position is necessary. We have simulated the bubble-powered motor using dissipative particle dynamics and found behavior in good agreement with that of motor proteins. Energy efficiency is as high as 60%.

Coexistence and transition between Cassie and Wenzel state on pillared hydrophobic surface.

Water droplets on rugged hydrophobic surfaces typically exhibit one of the following two states: (i) the Wenzel state [Wenzel RN (1936) Ind Eng Chem 28:988-994] in which water droplets are in full contact with the rugged surface (referred as the wetted contact) or (ii) the Cassie state [Cassie, ABD, Baxter S (1944) Trans Faraday Soc 40:546-551] in which water droplets are in contact with peaks of the rugged surface as well as the "air pockets" trapped between surface grooves (the composite contact). Here, we show large-scale molecular dynamics simulation of transition between Wenzel state and Cassie state of water droplets on a periodic nanopillared hydrophobic surface. Physical conditions that can strongly affect the transition include the height of nanopillars, the spacing between pillars, the intrinsic contact angle, and the impinging velocity of water nanodroplet ("raining" simulation). There exists a critical pillar height beyond which water droplets on the pillared surface can be either in the Wenzel state or in the Cassie state, depending on their initial location. The free-energy barrier separating the Wenzel and Cassie state was computed on the basis of a statistical-mechanics method and kinetic raining simulation. The barrier ranges from a few tenths of k(B)T(0) (where k(B) is the Boltzmann constant, and T(0) is the ambient temperature) for a rugged surface at the critical pillar height to approximately 8 k(B)T(0) for the surface with pillar height greater than the length scale of water droplets. For a highly rugged surface, the barrier from the Wenzel-to-Cassie state is much higher than from Cassie-to-Wenzel state. Hence, once a droplet is trapped deeply inside the grooves, it would be much harder to relocate on top of high pillars.

Extended study of molecular dynamics simulation of homogeneous vapor-liquid nucleation of water.

Using the simple point charge/extended water model, we performed molecular dynamics simulations of homogeneous vapor-liquid nucleation at various values of temperature T and supersaturation S, from which the nucleation rate J, critical nucleus size n(*), and the cluster formation free energy DeltaG were derived. As well as providing lots of simulation data, the results were compared with theories on homogeneous nucleation, including the classical, semi-phenomenological, and scaled models, but none of these gave a satisfactory explanation for our results. It was found that two main factors made the theories fail: (1) The average cluster structure including the nonspherical shape and the core structure that is not like the bulk liquid and (2) the forward rate which is larger than assumed by the theories by about one order of magnitude. The quantitative evaluation of these factors is left for future investigations.

Large-scale molecular-dynamics simulation of nanoscale hydrophobic interaction and nanobubble formation.

We performed large-scale molecular-dynamics simulation of nanoscale hydrophobic interaction manifested by the formation of nanobubble between nanometer-sized hydrophobic clusters at constrained equilibrium. Particular attention is placed on the tendency of formation and stability of nanobubbles in between model nanoassemblies which are composed of hydrophobic clusters (or patches) embedded in a hydrophilic substrate. On the basis of physical behavior of nanobubble formation, we observed a change from short-range molecular hydrophobic interaction to midrange nanoscopic interaction when the length scale of hydrophobe approaches to about 1 nm. We investigated the behavior of nanobubble formation with several different patterns of nonpolar-site distribution on the nanoassemblies but always keeping a constant ratio of nonpolar to polar monomer sites. Dynamical properties of confined water molecules in between nanoassemblies are also calculated.

A theory of transport properties in molten salts.

Expressions for diffusion constants in molten salts have been obtained in terms of the inter-ionic pair potentials and the pair distribution functions. Numerical attempts for diffusion constants in molten alkali halides are carried out and results agreed fairly with those obtained by molecular-dynamics simulation and with some experimental data. Based on the coupling of generalized Langevin equation and damped Einstein oscillator equation, ions' velocity autocorrelation functions have also been described and are numerically applied for molten potassium fluoride. The deviation from the Nernst-Einstein relation was also discussed in detail. In Appendixes A x B x C, the short-time expansion of velocity correlation functions in relation to the partial conductivities and the diffusion constants were obtained up to the term of t(4) and these were compared with a model function described by the form of cos(omegat)sech(ttau).

Tyr-317 phosphorylation increases Shc structural rigidity and reduces coupling of domain motions remote from the phosphorylation site as revealed by molecular dynamics simulations.

Activated receptor tyrosine kinases bind the Shc adaptor protein through its N-terminal phosphotyrosine-binding (PTB) and C-terminal Src homology 2 (SH2) domains. After binding, Shc is phosphorylated within the central collagen-homology (CH) linker region on Tyr-317, a residue remote to both the PTB and SH2 domains. Shc phosphorylation plays a pivotal role in the initiation of mitogenic signaling through the Ras/Raf/MEK/ERK pathway, but it is unclear if Tyr-317 phosphorylation affects Shc-receptor interactions through the PTB and SH2 domains. To investigate the structural impact of Shc phosphorylation, molecular dynamics simulations were carried out using special-purpose Molecular Dynamics Machine-Grape computers. After a 1-nanosecond equilibration, atomic motions in the structures of unphosphorylated Shc and Shc phosphorylated on Tyr-317 were calculated during a 2-nanosecond period. The results reveal larger phosphotyrosine-binding domain fluctuations and more structural flexibility of unphosphorylated Shc compared with phosphorylated Shc. Collective motions between the PTB-SH2, PTB-CH, and CH-SH2 domains were highly correlated only in unphosphorylated Shc. Dramatic changes in domain coupling and structural rigidity, induced by Tyr-317 phosphorylation, may alter Shc function, bringing about marked differences in the association of unphosphorylated and phosphorylated Shc with its numerous partners, including activated membrane receptors.