Bidirectional water transport through non-straight carbon nanotubes
Graphical abstract
Introduction
Water, as a key component of cell biology [1,2] plays a crucial role in biological activities [3,4]. It is well established that the ability of living cells in transferring water molecules through their cellular membranes is due to the proteins that act as means of transports and channels. Because of the similarity between the water transfer phenomenon in carbon nanotubes (CNTs) and biological nanochannels, they are often viewed in comparison with each other [[5], [6], [7], [8], [9]]. Materials such as nanoporous and nanochannels, mainly owing to their potential applications, have attracted a considerable amount of interest in recent years [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]. These applications include hydroelectric power production, power converters, [20,21] the revolutionary technologies for chemical separation and desalination [[22], [23], [24], [25], [26], [27]], water pumping [[28], [29], [30], [31], [32], [33]], various medical applications [34], drug delivery [35], biomolecular separation [6] to name just a few.
The molecular transfer of water/gas/ions through nanochannels is the basis for understanding biological activities and designing novel nanofluidic tools [[36], [37], [38], [39], [40], [41], [42], [43], [44]]. Previous studies have shown water transfer through CNTs to be a complex process which is often directed by many factors such as the temperature [[45], [46], [47], [48], [49]], pressure [[50], [51], [52], [53], [54]], water structure [55,56], wall roughness [57] and electric fields [47,[58], [59], [60], [61], [62], [63]]. Also, the water molecules are able to penetrate through CNTs at a fast rate [[64], [65], [66], [67], [68], [69], [70], [71]].
The molecular dynamics (MD) simulation is a powerful tool for predicting the structure and dynamic characteristics of mass transfer in nanometer scale, in conjunction with the experimental data. Numerous studies have verified the impeccability and accuracy of MD simulations for studying the fluid behavior in nanoscale system. For example, Hummer et al. [64] examined the transportation of single-file water molecules through a single-walled carbon nanotube (SWCNT) with a diameter of 8.1 nm.
Water infiltration capacity in CNTs often depends on the ambient conditions of the nanotube. Many external variables such as pressure gradient, mechanical forces and electric fields have been investigated by researchers on CNTs that are mostly straight. As an instance, in our previous study, we proposed the blueprint of novel nanoscale water pump for unidirectional transport of water using charged nanochannels [72]. Nanotubes that are commonly utilized for experimental purposes are mainly built in the scale of several nanometers, therefore making the study of geometrical parameters such as length and diameter of the nanotube, which are even often available in the order of several micrometers, as vital as the analysis of the effects of angle in increasing the amount of water flow rate.
However, due to the exorbitant cost of complex computations in MD simulations, nanometers long CNTs are considered and studied. For instance, Mattia and Gogotsi [73] have suggested that nanotube's length is a determinant and a deciding factor of the fluid flow behavior within it. They also observed the fluid flow in short nanotubes due to thermal fluctuations. In another related research, Su and Guo [74] reported that the relation between the water flow and CNTs can be represented in terms of a power law of the length. These results enlighten us about the effect that size of a channel has on water transport phenomenon, which has considerable implications for designing devices that operate based on the principles of fluids transport or Nano fluidic systems.
Recently T. Qiu et al. [75] suggested that the water transfer rate in the non-straight CNTs with the bending angle of 35° in the SWCNTs may increase 3.5 times relative to the straight system. In the present study, we further pursue such an investigation. Notwithstanding, we rather consider the cases in which the water transport is not single-file, as most natural biological nanochannels exhibit non single-file transport.
Since nanotubes in nature, have relatively large diameters which, in turn, make fluid flow cease to be single-file and passing of molecules beside one other possible. Due to the fact that no external force can solely determine the direction of the flow, bidirectional flow is ubiquitous in most biological membranes. As a consequence, in the current study we, for the first time, concentrated on the often-neglected bidirectional flow both in straight and non-straight CNTs and were able to observe numerous unexpected results.
Due to the many applications of CNTs, such as energy storage, drug delivery, water filters, and desalination [76] it is necessary to conduct a study that is representative of the widely used natural or experimental CNTs in which the predominant mode of transfer is non single-file. Such investigation will be greatly helpful in explaining the fluid flow behavior in various cases that nanotubes systems are implemented. Therefore, in order to have such a realistic and accurate study while achieving optimal working conditions for the nanotube (i.e. maximum water transfer rate and minimized cost) not only do we focus on the effect of the bending angle on the transfer rate but also we inspect the effect of other major parameters such as length, diameter and the temperature that have not been collectively studied as of now.
Section snippets
The method
On the basis of the work of Ref. [75] we consider a nanotube as depicted in Fig. 1 that has a 1.331 nm diameter and two water reservoirs on the top and bottom of graphene sheets. There is no water movement between two reservoirs except in the nanotubes.
The CNTs considered for the study of water transport within them, have different geometrical characteristics (length, diameter and bending angle). First, by using VMD software a CNT and two graphene sheets were generated and the output file was
Results and discussion
Fig. 2 shows the transport rate of water molecules in the non-straight CNTs. A molecule transport is deemed complete when a molecule enters the system from one side and the same molecule leaves the system from the other side. In order to calculate the rate accurately for every water molecule in the system, an identity number was assigned to each one and its position was tracked during the simulation. The number of the transported molecules in the duration of simulation divided by the time
Conclusion
In summary, as water movement in biological nanochannels is of paramount importance and as most of the nanochannels are non-straight with non-single-file flow within them due to their large diameters, this study focused on bidirectional water transport in non-straight CNTs. We changed three primary geometrical characteristics of the system, namely, the length, the diameter and the bending angle of the nanotubes along with the water temperature to determine their effects on the transfer rate of
Acknowledgments
The authors gratefully acknowledge the Sheikh Bahaei National High Performance Computing Center (SBNHPCC) for providing us with their computing facilities and precious time. SBNHPCC is supported by scientific and technological department of the presidential office and Isfahan University of Technology (IUT). We would also like to express our gratitude to Sharif University of Technology's Supercomputer Center for their support and authorization to access their supercomputers to perform advanced
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