Computer Simulation Of Carbon Based Materials And Their Appli- Cations In Nanofluid Field

Carbon nanotube (CNT) and Graphene are two kinds of famous carbon-based ma-terials that discovered in recent twenty five years. Due to their extraordinary proper-ties such as high strength, high thermal conductivity, superhydrophobicity and unique electronic properties, they have shown promising applications in many aspects, such as constructing high strength materials, electrical cables, water treatment and biomedical field. In detail, CNT shows high water flow rate inside it because of its smooth inner surface. So, many attentions have been paid on the mechanisms of water flow inside CNT and its potential applications. In this thesis, we mainly focus on the water and ion transportation behaviors inside CNTs by using molecular dynamics simulations. Besides, we also discuss the geometries of graphene under various conditions.In chapter 1, we briefly introduce the structure and properties of CNT. Then, we describe the water structure and water transportation behaviors inside CNTs. Further, the recent investigations about how to control the rate and direction of water flow inside CNTs are illustrated. Lastly, we mainly focus on the ion inside CNTs. We discuss two aspects, i.e., the hydration shell of ion and ion transportation properties.In chapter 2, we present the method of molecular dynamics simulations used in our studies. We first introduce the basic theory and the mathematics behind the method. We also show the procedure and some important algorithms when performing a simu-lation. Then, some mostly used force fields (i.e., CHARMM and AIREBO) are intro-duced. Finally, we describe the basic way to construct a CNT in our simulations.In chapter 3, we systematically investigate the water transportation behaviors (i.e., water flow rate) inside CNTs in the presence of lipid membranes by using all atom molecular dynamics simulations. Our results show that the hydrophilicity of CNT as well as membrane thickness can have important impacts on the water flow rate. Interestingly, since the membrane thickness is temperature-dependent, the water flow rate can exhibit thermo-responsive behaviors. Further, we also provide insights into the effect of CNT on lipid membranes. It is found that all CNTs can increase the lipid tail order parameters and thicken the membrane at 320 K; while these effects are not obvious at 290 K. Importantly, we observe that the CNT with specific hydrophobicity has the least effect on membranes.In chapter 4, we investigate the pumping of water by rotating a chiral CNT. It is found that the chirality and rotation of CNT are two preconditions of stable water flux inside it. Besides, we find that the water flux shows an approximately logarithmic dependence of the angular velocity of the rotation, a linear dependence of the radius of 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 well fit the results obtained from simulations.In chapter 5, we propose a new method of sea water desalination using CNT. The two ends of CNT are decorated by oscillating charges. When the amplitude of charge vibration is larger than 0.04 e and the frequency is between 10 THz and 20 THz, the oscillating electric field induced by the charge can totally prevent the translocation of ions through CNT. The potential of mean force of ion transporting through CNT confirms that there exists a high energy barrier. The ion transport rate increases when the charge decreases to zero or the frequency deviates from the appropriated range. Further, we find that the resonance between hydrogen bonds inside CNT and the oscil-lating electric field accounts for this phenomenon. When the amplitude is large enough and the frequency is similar to the intrinsic frequency of hydrogen bonds, the hydrogen bonds break under the resonance. So, ions will lose their hydration shells when inside the CNT, which will increase their free energy. Finally, we compare our results using different ions and find this mechanism is independent of the type or charge of the ion.In chapter 6, we systematically investigate the self-assembly of fullerenes (diam-eter from 0.7 to 2.3 nm) and graphene at room temperature (300 K). It is found that single fullerene can be wrapped by graphene nanoribbon (GNR) due to the van der Waals interaction between them. However, if the GNR is wide enough, the fullerene will only bind to the surface of GNR. To overcome the bending energy of wide GNR, we further use multiple fullerenes, and find that they can self-assemble into various structures. Importantly, fullerenes show dramatically different behaviors as the size changes. Giant fullerenes can work together to scroll a very large graphene from the corner, while this effect is weakened or even disappears in the cases of smaller ones. Finally, we also find that the wrapping process can be completely retarded by adding a substrate below the graphene flake.Finally, we give a brief summary of the thesis, and an outlook for future works on carbon-based materials is described.