Possible causes for rippling in a multivacancy graphene system

The occurrence of ripples in an eight‐atom vacancy graphene system was analyzed through first principles calculations based on density functional theory. The study focuses on the conditions that should be considered in order that ripples could occur in the single layer defected graphene. The vacancies concentration, especially the pentagonal figures formed after relaxation of the vacant system, and the distances between neighbor pentagons, are found to be key aspects for rippling occurrence, with no charge distribution consequence. Two different configurations of rippling have been found where the position of the pentagonal figures is discussed as the driving force for any of them to occur.     

多壁碳纳米管弯曲导致的扭转

文章介绍了多壁碳纳米管弯曲的分子动力学模拟,从原子尺度上解释了由于屈曲失稳而导致的起皱现象,并发现多壁碳纳米管弯曲时会呈现出非线性的力学响应。同时,在模拟过程中,观察到了弯曲诱发扭转的现象,并揭示出扭转变形的内在起因是曲率诱导的晶格错配。     

Nanoscale Spontaneous Patterning

The SMB equation describing nanoscale spontaneous patterning is studied both analytically and numerically. In contradiction to the claim in the original SMB paper [D. Srolovitz, A. Mazor, B. Bukiet, J. Vac. Sci. Technol. A6(4) (1988), 2371--2380.] that some steady states are stable, we found that all the steady states are unstable. A dynamical system reason for this is given. We also found that typical small initial data solutions undergo an exponential growth followed by an almost linear growth. Such a feature is consistent with the experimental data in the paper [J. Erlebacher et al., J. Vac. Sci. Technol., A18(1) (2000), 115--120, Figure 3]. On the other hand, we never observed the decay portion of the numerical solution reported in this paper. We invent an elegant energy principle which supports our findings.     

Emerging Patterns in a Hyperbolic Model for Locally Interacting Cell Systems

Summary. Morphogenetic processes such as neurulation and gastrulation involve co- ordinated movements of cells. It is assumed that these processes happen due to long-range signaling, although the detailed mechanisms are not completely understood. Therefore, one is interested in biological ``model-systems' where self-organization of cells and in particular the mechanisms of signaling can be analyzed in greater detail. A major question is whether or not short-range signaling or local interaction of cells can also be the cause of coordinated movement and morphogenetic processes. As a model problem we analyze ripple formation of myxobacteria due to purely local interaction, a hypothesis which is discussed in the biological literature. These ripples can be observed before the final aggregation of the bacteria and fruiting body formation take place. Our basic mathematical model is a one-dimensional hyperbolic system of Goldstein-Kac type with density-dependent coefficients. Conditions for the existence of traveling waves are discussed by means of linear analysis and the construction of invariant domains.     

The effects of power rippling on CO2 laser cutting

A power rippling method was proposed in this study to improve the cut edge quality by disturbing the natural frequency of the periodic striations in high power C0₂ laser cutting. A pilot study was also designed and set up to examine the power responsibility of the laser system, which is externally controlled by a modulated analogue signal from a signal generator. Several aspects were examined in this study. The effects of different power rippling frequencies on the cut surface roughness is the first such aspect, and the different power modulation depth on cut surface roughness is another. The power rippling shape is the third important parameter examined in this study. It was found that the amplitude and frequency of the striation pattern on the cut surface were significantly affected by the characteristics of the rippling power. Cutting quality can be significantly improved by rippling the laser power.