Continuum Percolation of 2D and 3D Simple Fluids

We use Monte Carlo and Molecular Dynamics computer simulations to investigate the percolation threshold, ρ _p , of d-dimensional Lennard-Jones LJ, and square-well fluids. We find that when the range of the potential well is small compared to the hard-core diameter (in the so-called ‘hard-core’ limit), an attractive well decreases ρ _p below the high temperature limiting value. In contrast, a hard shoulder potential produces the opposite trend. We investigate the structure of the 2D percolating clusters, in particular, the effects of periodic boundaries. We examine the shapes of the 3D clusters at the percolation threshold, resolved as a function of the number of particles in a cluster, s. The asphericity parameter, A₃ , describing the instantaneous shape of the cluster decays slowly from unity, typically only achieving ～ 0.3 by s ～ 100, close to the estimated universal value of 0.312.

We focus especially on the relationship between long and short range structural order as probed by the coordination numbers of the molecules. We also use a cluster-resolution of the co-ordination number to indicate the degree of branching in the clusters, and how it is influenced by the number of atoms in the cluster and temperature for the different potentials. We look forward to future directions for simulation in investigating physical properties in the vicinity of the percolation threshold.     

Brownian Dynamics Simulations of Electro-Rheological Fluids,II

Electro-rheological fluids are technologically relevant colloidal dispersions that on application of an applied electric field manifest a yield stress and dramatic increase in viscosity. To assist optimisation of these fluids we have performed molecular simulations to understand the basic mechanisms operating in these fluids.

In this work a dispersion of field-aligned dipoles is simulated in shear flow by non-equilibrium Brownian dynamics. In agreement with experiment, a plot of the simultated relative, η _r , against a dimensionless characteristic ER parameter, the Mason Number, Mn, exhibits a plateau region (at low electric fields and/or high shear rates) prior to a steep increase in viscoisty for Mn smaller. Although, the simulations exhibit only an approximate data collapse according to the Mn. Following on from the first paper in this series we relate the origins of the ER effect in more detail to the temporal structural changes that take place in the fluid. We find for example, that the fluids reorganise microscopically into layers of strings of particles in the shearing plane at low Mn, a structure which is destroyed on entry onto the plateau in η _r at higher Mn. We suggest how the model could be made more realistic in future studies.     

Colocalization in vivo and association in vitro of perlecan and elastin

We have colocalized elastin and fibrillin-1 with perlecan in extracellular matrix of tensional and weight-bearing connective tissues. Elastin and fibrillin-1 were identified as prominent components of paraspinal blood vessels, and posterior longitudinal ligament in the human fetal spine and outer annulus fibrosus of the fetal intervertebral disc. We also colocalized perlecan with a synovial elastic basal lamina, where the attached synovial cells were observed to produce perlecan. Elastin, fibrillin-1 and perlecan were co-localized in the intima and media of small blood vessels in the synovium and in human fetal paraspinal blood vessels. Elastic fibers were observed at the insertion point of the anterior cruciate ligament to bone in the ovine stifle joint where they colocalized with perlecan. Elastin has not previously been reported to be spatially associated with perlecan in these tissues. Interactions between the tropoelastin and perlecan heparan sulfate chains were demonstrated using quartz crystal microbalance with dissipation solid phase binding studies. Electrostatic interactions through the heparan sulfate chains of perlecan and core protein mediated the interactions with tropoelastin, and were both important in the coacervation of tropoelastin and deposition of elastin onto perlecan immobilized on the chip surface. This may help us to understand the interactions which are expected to occur in vivo between the tropoelastin and perlecan to facilitate the deposition of elastin and formation of elastic microfibrils in situ and would be consistent with the observed distributions of these components in a number of connective tissues.     

The use of Histochoice™® for histological examination of articular and growth plate cartilages,intervertebral disc and meniscus

Histochoice?® is a proprietary nontoxic, non-cross-linking fixative designed by the manufacturer to replace formaldehyde based fixation protocols. We compared Histochoice and formalin fixation for several cartilaginous tissues including, articular and growth plate cartilage, meniscus and intervertebral disc. The tissues were stained with general histology stains including toluidine blue for tissue proteoglycans, picrosirius red to evaluate collagenous organization, and hematoxylin and eosin to assess cell morphology. The chondroitin sulfate and heparin sulfate substituted proteoglycans aggrecan and perlecan were also immunolocalized in some of the tissues to provide a comparison. Histochoice did not fix deep into the tissue blocks resulting in focal loss of aggrecan and other matrix components from the more central regions of the blocks. This was evident in toluidine blue stained sections of immature tibial articular cartilage where loss of glycosaminoglycan was significant in Histochoice?® fixed tissues. Histochoice fixation worked well, however, in the aggrecan and perlecan immunohistology applications where its non-cross-linking traits were conducive to epitope retrieval and identification by primary antibodies to extracellular matrix components.     

An FMRI study of the role of the medial temporal lobe in implicit and explicit sequence learning

fMRI was used to investigate the neural substrates supporting implicit and explicit sequence learning, focusing especially upon the role of the medial temporal lobe. Participants performed a serial reaction time task (SRTT). For implicit learning, they were naive about a repeating pattern, whereas for explicit learning, participants memorized another repeating sequence. fMRI analyses comparing repeating versus random sequence blocks demonstrated activation of frontal, parietal, cingulate, and striatal regions implicated in previous SRTT studies. Importantly, mediotemporal lobe regions were active in both explicit and implicit SRTT learning. Moreover, the results provide evidence of a role for the hippocampus and related cortices in the formation of higher order associations under both implicit and explicit learning conditions, regardless of conscious awareness of sequence knowledge.     

拐芹根化学成分研究Ⅱ

从伞型科当归属植物拐芹(Angelica polymorpha Maxim)的根及根茎中又分得4个结晶性化合物。经物理常数测定、光谱分析,分别鉴定为欧前胡素Ⅰ,异氧化前胡内酯Ⅱ,Pabulenol Ⅲ,Phellopterin Ⅳ。