增龄对人牙本质失水收缩的重要性

牙本质内的矿物质随着年龄的增长呈现增长的趋势．由于矿物质与蛋白胶原比例的改变,可能导致牙本质脱水时氢键强度和收缩程度的变化．因此,该研究的目的是确定牙本质收缩随年龄增长的量变．将磨牙牙本质的冠部制成试件,青年组:23≤age≤34,老年组:52≤age≤62,贮存在水或Hanks平衡盐溶液（HBSS）中．利用数字图像相关法(DIC)观察牙本质试件在自由空气状态下的尺寸变化过程．结果表明青年人的牙本质不管在哪个方向或存储时间上,其收缩都明显大于老年人(p<0.01)．平行于牙本质小管方向上的应变从牙髓腔侧到釉牙本质界(DEJ)侧依次递增,而垂直于牙本质小管方向的应变则依次递减．即从牙髓腔侧到DEJ牙本质收缩的各向异性增强,该现象在青年人牙本质中尤其明显．     

Functional Reduction and Mental Causation

Over the past few decades, Jaegwon Kim has argued that non-reductive physicalism is an inherently unstable position. In his view, the most serious problem is that non-reductive physicalism leads to type epiphenomenalism—the causal inefficacy of mental properties. Kim suggests that we can salvage mental causation by endorsing functional reduction. Given the fact that Kim’s goal in formulating functional reduction is to provide a robust account of mental causation it would be surprising if his position implies eliminativism about mental properties or leads to a view that is similar to one of the versions of non-reductive physicalism that he criticizes. We will show that depending on how certain key claims are interpreted, there are reasons for thinking functional reduction has these implications, in which case either Kim fails to provide a robust account of mental causation or there is reason to suspect that some of his criticisms of non-reductive physicalism are misguided.     

Evaluating the mechanical behavior of arterial tissue using digital image correlation

In this study, digital image correlation (DIC) was adopted to examine the mechanical behavior of arterial tissue from bovine aorta. Rectangular sections comprised of the intimal and medial layers were excised from the descending aorta and loaded in displacement control uniaxial tension up to 40 percent elongation. Specimens of silicon rubber sheet were also prepared and served as a benchmark material in the application of DIC for the evaluation of large strains; the elastomer was loaded to 50 percent elongation. The arterial specimens exhibited a non-linear hyperelastic stress-strain response and the stiffness increased with percent elongation. Using a bilinear model to describe the uniaxial behavior, the average minor and major elastic modulii were 192±8 KPa and 912±40 KPa, respectively. Poisson's ratio of the arterial sections increased with the magnitude of axial strain; the average Poisson's ratio was 0.17±0.02. Although the correlation coefficient obtained from image correlation decreased with the percent elongation, a correlation coefficient greater than 0.8 was achieved for the tissue experiments and exceeded that obtained in the evaluation of the elastomer. Based on results from this study, DIC may serve as a valuable method for the determination of mechanical properties of arteries and other soft tissues.     

A Mössbauer effect study of the magnetic ordering in the Y6(FexMn1−x)23 solid solutions

The ternary alloys, Y₆ (Fe_xMn_1–2)₂₃, exhibit interesting magnetic behavior over a range of solid solutions of iron and manganese. The transition metal atoms occupy four crystallographically different sites. For x less than ca. 0.27, the transition metal atoms couple ferromagnetically, whereas for greater values of x they couple antiferromagnetically. By using the iron populations for each site, obtained from neutron diffraction studies, it is possible to determine the Mössbauer effect hyperfine parameter fields for each site. As x increases, the ordering temperature decreases dramatically. At 1.3K in Y₆Fe₂₃, the four hyperfine fields range from 372kOe to 250kOe, whereas in Y₆(Fe_0.52Mn_0.48)₂₃ they decrease to values between 100kOe and 10kOe. This decrease apparently results from the increasing frustration of the ferromagnetic iron-iron coupling as additional manganese, which would prefer antiferromagnetic coupling, is introduced into the alloys.     

Femtosecond electron pulse characterization using laser ponderomotive scattering

We demonstrate a method for the measurement of the instantaneous duration of femtosecond electron pulses using the ponderomotive force of an intense ultrashort laser pulse. An analysis procedure for the extraction of the electron pulse duration from the transient change of the transverse electron beam profile is proposed. The durations of the electron pulses generated in our setup were determined to be 410+/-30 fs.     

Electronically driven structure changes of Si captured by femtosecond electron diffraction

The excitation of a high density of carriers in semiconductors can induce an order-to-disorder phase transition due to changes in the potential-energy landscape of the lattice. We report the first direct resolution of the structural details of this phenomenon in freestanding films of polycrystalline and (001)-oriented crystalline Si, using 200-fs electron pulses. At excitation levels greater than approximately 6% of the valence electron density, the crystalline structure of the lattice is lost in <500 fs, a time scale indicative of an electronically driven phase transition. We find that the relaxation process along the modified potential is not inertial but rather involves multiple scattering towards the disordered state.     

Hamiltonian Chaos in Homogeneous ADM Cosmologies?

Chaos in dynamical systems has best been understood in terms of Hamiltonian systems. A primary method of diagnosis of chaos in these systems is the Lyapunov exponent. According to general relativity, space-time is itself a dynamical system. When the evolution of a model universe is expressed in the ADM form it can be described as a Hamiltonian system. Among the various model cosmologies, the Mixmaster or Bianchi IX cosmology has been extensively studied as a candidate to exhibit chaos. However, the Lyapunov exponents in this system have shown contradictory properties, including a seeming dependence on the coordinates used to describe space-time. Such dependencies, if true, would be surprising as the time coordinate of space-time is unrelated to the parameterization of phase space. Further, this sort of dependence would relegate chaos to a bad coordinate choice rather than a dynamic property of the system. The problem with the Lyapunov exponent lies in the ambiguities remaining in the ADM action integral. The current interpretation involves an arbitrary Lagrange multiplier—thought to be necessary for the coordinate invariance of space-time. An arbitrary multiplier turns out to be unnecessary for coordinate invariance, and in addition destroys the symplectic structure of phase space. In reality, the geometry selects the parameterization of phase space, and any change in the parameter results in a changed Hamiltonian system. It must be emphasized that the fixing of the phase space parameter does NOT impose a coordinate choice on space-time. The parameter is selected by the symplectic structure of phase space and full coordinate invariance of space-time is left intact. Once the demands of both geometries, space-time and phase space, have been satisfied, the Lyapunov exponent becomes independent of the coordinate imposed on space-time. Additionally, the correction of the phase space structure leads to a Hamiltonian that is more general, in that it describes a gravitational system with a cosmological constant, than is currently the case.     

Time evolution of synchronously pumped dye laser pulses: Picosecond pulse formation studies

The pulse-by-pulse evolution and compression of the output of a synchronously pumped dye laser as a function of the number of pump pulses was investigated with a high resolution, variably triggered streak camera. The measured pulse durations and energies for the pulse compression process are demonstrated to be in disagreement with the predictions of recent classical rate equation models. This is shown not to be due to the complete breakdown of the model but, rather, is caused by the failure to include two key loss terms in the rate equation treatment, namely, excited state absorption and thermal lensing. In addition, a highly durable dye laser compatible with the 100 MHz output of commercially available QS and ML YAG lasers is presented which is capable of 50 μJ, 20–30 ps transform limited pulses without the need of amplification. Further refinements of this laser system are discussed, based on the experimental work reported, which will lead to tunable ～ 100 fs pulses with energies in the μJ range from a single laser pump source.