Entropic Force In Nanoscale Adhesion

Due to its crucial role in theoretical researches and its widely potential applications, graphene—the first two dimensional material which was unexpectedly discovered by two researchers at University of Manchester in 2004, has been a rising star at nanoscience. Over the past decade, about one hundred thousand papers from experiments and theoretical researches were published regarding graphene. Although widely studied, graphene seems not going to stop surprising us by its continually emerging novel physics. So the "golden rush" of the graphene is continuing.There are few studies showing graphene has unique thermal properties, e.g., the thermophoresis along a thermal gradient—which is the first time found in crystal besides that in gas or liquid, and the thermal induced reversible dominoes in carbon nanotubes. Although they are obviously theoretical important and may have many potential applications, the origin of these thermal properties has not be well studied.In this dissertation, we concern the process of adhesion of graphene layer with substrates. Molecular dynamic simulations show that the process of attaching a graphene layer onto a substrate is not solely a mechanical process, it also involves entropy variation. As a consequence, it needs less work to peel off graphene from substrate than only considering the mechanical forces. We further found that the entropy variation has fundamental influence on interlayer motions between graphene layers as well as on carbon nanotubes. Thus the thermal effect we mentioned above may be explained by the concept of entropic force.The dissertation begins with a brief introduction of two dimensional material and its properties in Chapter 1. The main body of the dissertation is in the following Chapters 2—7 and ends with a few concluding remarks in Chapter 8. The main results and contributions of this dissertation are as follows.Chapter 2 studied the process of peeling graphene layer off diamond substrate by using classical molecular dynamic simulations. The temperature is found decreasing during this process, indicating an entropic force may assist the peel force, and required less mechanical work. In contrast, during the process of attaching a graphene layer onto a diamond substrate, the system temperature is found increasing. Same phenomena are also found during the processes of peeling/attaching hexagon boron nitride layer off/on diamond substrate.Chapter 3 focuses on the entropic force during telescopic motions of multi-walled carbon nanotubes. The entropic effect causes the temperature decreasing during the telescopic extension of walls. Direct calculating of force on the axis show that the entropic force is proportional to the system temperature. Moreover, when the inter-wall distance reduced to 0.31 nm, the entropic force dominates the telescopic motion. Specifically, the competition between entropic force and Van der Waals forces makes the telescopic motion tunable. When the system temperature is lower than a critical value, the telescopic motion is extensional. When the system temperature is higher than the critical value, the telescopic motion is retractile.Chapter 4 proposes an analytical model to study the entropic forces. We firstly study a one dimensional chain partially confined in a potential valley and directly derived the entropic force as a function of temperature and valley curvature. The results confirmed the linearly dependence of entropic force on temperature. The analytic solution of two dimensional graphene in three dimensional potential valley is also obtained.Chapter 5 proposes a new dissipation mechanism for the frictional motion of relative wall in multi-walled carbon nanotubes. In the studying of a two-nanotube consisted oscillator, we found that the periodical energy conversion between heat and mechanical energy induced by the entropic force during the oscillation is the main source of irreversible entropy production.Chapter 6 focuses on the sliding frictional energy dissipation between graphene layers. The directly calculation of frictional forces on edge and inner zone shows the edge zoon contributes mainly to the total friction. Thus it confirm that the entropic force causes the main dissipation as shown in the multi-walled carbon nanotubes. We also confirm that the friction is proportional to the system temperature and the sliding velocity.Chapter 7 presents the study of defect effect on interlayer friction in polycrystalline graphene layers. The molecular dynamic simulation shows that the friction simply linearly depends on temperature and sliding velocity. However, the friction shows complex dependence on the vertical loading. At the low pressure, the friction decreases with increasing pressure. At the high pressure, the friction increases with the pressure increasing. To explain the negative frictional coefficient, we proposed that friction is dominated by two competing dissipation mechanisms.