Vibrating-Charge-Driven Water Pump Controlled by the Deformation of the Carbon Nanotube

Ultrahigh-Flux Water Nanopumps Generated by Asymmetric Terahertz Absorption

Controlling active transport of water through membrane channels is essential for advanced nanofluidic devices. Despite advancements in water nanopump design using techniques like short-range invasion and subnanometer-level control, challenges remain facilely and remotely realizing massive waters active transport. Herein, using molecular dynamic simulations, we propose an ultrahigh-flux nanopump, powered by frequency-specific terahertz stimulation, capable of unidirectionally transporting massive water through asymmetric-wettability membrane channels at room temperature without any external pressure. The key physics behind this terahertz-powered water nanopump is revealed to be the energy flow resulting from the asymmetric optical absorption of water.

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.

Modeling water purification by an aquaporin-inspired graphene-based nano-channel

Understanding the mechanism of water and particle transport through thin-film membranes is essential to improve the water permeability and the salt rejection rate of the purification progress. In this research, mimicking from the structure and operation of the aquaporin channel, graphene-based nano-channels were designed to be used as a water filter. The effects of variation of the channel's main elements, such as the width of the bottleneck and charges attached to the channel on its efficiency, were investigated via molecular dynamics simulations. We observe that the water flow through the channel decreases by increasing the charge, while the ion rejection rate of the channel is enhanced. Moreover, we find that the geometry and shape of the bottleneck part of the channel can affect the channel water flow and its selectivity. Finally, the pressure and the flow velocity in the channel were considered by using finite element models, and the results indicate that they are high at the entrance of the channel. The outcomes of this study can be used to improve the molecular knowledge of water desalination, which might be helpful in designing more efficient membranes. Graphical abstract As the piston pushed the solution to pass through the nano-channel, positive and negative ions are remained in the first box, by sensing electric field generated from the attached charges to the bottleneck part of the channel. Atomistic structure of channel is shown in the right part of the figure and the generated electric field is shown in the left part of the figure.

Strain effects on rotational property in nanoscale rotation system

This paper presents a study of strain effects on nanoscale rotation system consists of double-walls carbon nanotube and graphene. It is found that the strain effects can be a real-time controlling method for nano actuator system. The strain effects on rotational property as well as the effect mechanism is studied systematically through molecular dynamics simulations, and it obtains valuable conclusions for engineering application of rotational property management of nanoscale rotation system. It founds that the strain effects tune the rotational property by influencing the intertube supporting effect and friction effect of double-walls carbon nanotube, which are two critical factors of rotational performance. The mechanism of strain effects on rotational property is investigated in theoretical level based on analytical model established through lattice dynamics theory. This work suggests great potentials of strain effects for nanoscale real-time control, and provides new ideas for design and application of real-time controllable nanoscale rotation system.

Confined Water: Structure, Dynamics, and Thermodynamics

Understanding the properties of strongly confined water is important for a variety of applications such as fast flow and desalination devices, voltage generation, flow sensing, and nanofluidics. Confined water also plays an important role in many biological processes such as flow through ion channels. Water in the bulk exhibits many unusual properties that arise primarily from the presence of a network of hydrogen bonds. Strong confinement in structures such as carbon nanotubes (CNTs) substantially modifies the structural, thermodynamic, and dynamic (both translational and orientational) properties of water by changing the structure of the hydrogen bond network. In this Account, we provide an overview of the behavior of water molecules confined inside CNTs and slit pores between graphene and graphene oxide (GO) sheets. Water molecules confined in narrow CNTs are arranged in a single file and exhibit solidlike ordering at room temperature due to strong hydrogen bonding between nearest-neighbor molecules. Although molecules constrained to move along a line are expected to exhibit single-file diffusion in contrast to normal Fickian diffusion, we show, from a combination of molecular dynamics simulations and analytic calculations, that water molecules confined in short and narrow CNTs with open ends exhibit Fickian diffusion because of their collective motion as a single unit due to strong hydrogen bonding. Confinement leads to strong anisotropy in the orientational relaxation of water molecules. The time scale of relaxation of the dipolar correlations of water molecules arranged in a single file becomes ultraslow, of the order of several nanoseconds, compared with the value of 2.5 ps for bulk water. In contrast, the relaxation of the vector that joins the two hydrogens in a water molecule is much faster, with a time scale of about 150 fs, which is about 10 times shorter than the corresponding time scale for bulk water. This is a rare example of confinement leading to a speedup of orientational dynamics. The orientational relaxation of confined water molecules proceeds by angular jumps between two locally stable states, making the relaxation qualitatively different from that expected in the diffusive limit. The spontaneous entry of water inside the hydrophobic cavity of CNTs is primarily driven by an increase in the rotational entropy of water molecules inside the cavity, arising from a reduction in the average number of hydrogen bonds attached to a water molecule. From simulations using a variety of water models, we demonstrate that the relatively simple SPC/E water model yields results in close agreement with those obtained from polarizable water models. Finally, we provide an account of the structure and thermodynamics of water confined in the slit pore between two GO sheets with both oxidized and reduced parts. We show that the potential of mean force for the oxidized part of GO sheets in the presence of water exhibits two local minima, one corresponding to a dry cavity and the other corresponding to a fully hydrated cavity. The coexistence of these two regimes provides permeation pathways for water in GO membranes.

Investigation of Transport Properties of Water-Methanol Solution through a CNT with Oscillating Electric Field

Molecular dynamics simulations were used to investigate the transport properties of water-methanol solution getting through a carbon nanotube (CNT) with an oscillating electric field. Eight alternating electric fields with different oscillation periods were used in this work. Under the oscillating electric field, water molecules have the advantage of occupying a CNT over methanol molecules. Meanwhile, the space occupancy of water-methanol solution in the CNT increases as the oscillating period increases. More importantly, we found that the oscillating period of electric field affects the van der Waals interaction of the solution inside the CNT and the shell of the CNT, which results in the change in the number of hydrogen bonds in the water-methanol solution confined in the CNT. And the change in the hydrogen-bond network leads to the change in transport properties of water-methanol solution.

Tuning water transport through nanochannels by changing the direction of an external electric field

The transport properties of water through a nanochannel influenced by the direction of an external electric field has been investigated by using molecular dynamics simulations. Water molecules flow unidirectionally across the nanochannel under a uniform external electric field without an osmotic pressure. It is found that the direction of the external field plays an important role in the interactions and dipole orientations of water molecules in the nanochannel, accordingly changing the net water flux dramatically. Most importantly, a critical angle (θC) between the external field and the nanochannel axis is found. The average net water flux increases as θ increases for θ≤θC but decreases sharply to a near-zero value for a further increase of θ. The maximum value of the average net water flux is 7.33 times as high as the value when the electric field is along the nanochannel axis. Our findings are of great practical importance for nanomolecular engineering, which provide a possible strategy for designing novel controllable water nanopumps.

Current inversions induced by resonant coupling to surface waves in a nanosized water pump

We conducted a molecular dynamics simulation to investigate current inversions in a nanosized water pump based on a single-walled carbon nanotube powered by mechanical vibration. It was found that the water current depended sensitively on the frequency of mechanical vibration. Especially in the resonance region, the nanoscale pump underwent reversals of the water current. This phenomenon was attributed to the dynamics competition of the water molecules in the two sections (the left and right parts) divided by the vibrating atom and the differences in phase and decay between the two mechanical waves generated by mechanical vibration and propagating in opposite directions toward the two ends of the carbon nanotube. Our findings provide an insight into water transportation through nanosized pumps and have potential in the design of high-flux nanofluidic systems and nanoscale energy converters.

Pumping of water by rotating chiral carbon nanotube

Water transportation inside carbon nanotubes is of great importance for designing novel nanodevices. In this article, by using molecular dynamics simulations, we systematically investigate the pumping of water by rotating carbon nanotube (CNT). It is found that the chirality and rotation of the CNT are two preconditions for stable water flux inside it. Besides, we find that the water flux shows an approximately logarithmic dependence on the angular velocity of the rotation, a linear dependence on the radius of the CNT, and interestingly, independence of its length within a certain range of CNT size and angular velocity. Further, we also use a dragging theory which successfully describes the water flux behaviors inside the CNT and can fit well with the results obtained from simulations. The present study provides insight into the designing of nanodevices based on the CNT for real applications.