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S J Mitchell

Publications and source records attributed to S J Mitchell.

At least 19 recordsLinked to original sources
PubMed2006

Phase separation in a compressible 2D Ising model.

We perform a high precision Monte Carlo study of asymptotic domain growth in a compressible two-dimensional spin-exchange Ising model with continuous particle positions and zero total magnetization, and we investigate the effects of compressibility and lattice mismatch on the late-time domain growth law, R(t) = A + Bt(n). For mismatched systems, we measure significant deviations from the theoretically expected n = 1/3 late-time growth (n = 0.224 +/- 0.004), and for a compressible model with no mismatch, we measure only a slight deviation from n = 1/3. These results strongly suggest that the current understanding of the classes of domain growth is incomplete.

Journal Article↗
PubMed2005

Discontinuities in self-affine functions lead to multiaffinity.

Many systems of both theoretical and applied interest display multiaffine scaling at small length or time scales. We demonstrate analytically and numerically that when vertical discontinuities are introduced into a self-affine function, the function becomes multiaffine. The discontinuities may correspond to surface overhangs or some source of discontinuous noise. Two functions are numerically examined with different distributions of discontinuities. The multiaffinity is shown to arise simply from the function of discontinuities, and the analytic scaling form at small scales for the function of discontinuities is derived and compared to numerical results.

Journal Article↗
PubMed2005

Monte Carlo study of the vector Blume-Emery-Griffiths model for 3He--4He mixtures in three dimensions.

A version of the Vector Blume-Emery-Griffiths model with three-dimensional spins was studied on a simple cubic lattice by Monte Carlo simulations. We obtained the phase diagram, which reproduces, for a range of the parameters of the model, the topology of the one observed for bulk mixtures of 3He--4He. The phase diagram displays a superfluid, 4He-rich phase which undergoes a phase transition to a normal phase. This transition is first or second order, depending on the region of the phase diagram examined. One of the main features of this diagram is the existence of a tricritical point, which, for some values of the parameters of the model, decomposes into a critical endpoint and a double critical endpoint. These points were located with reasonable precision. This study provides the basis for the subsequent study of dynamic properties of 3He--4He mixtures.

Journal Article↗
PubMed2003

Rejection-free Monte Carlo algorithms for models with continuous degrees of freedom.

We construct a rejection-free Monte Carlo algorithm for a system with continuous degrees of freedom. We illustrate the algorithm by applying it to the classical three-dimensional Heisenberg model with canonical Metropolis dynamics. We obtain the lifetime of the metastable state following a reversal of the external magnetic field. Our rejection-free algorithm obtains results in agreement with a direct implementation of the Metropolis dynamic and requires orders of magnitude less computational time at low temperatures. The treatment is general and can be extended to other dynamics and other systems with continuous degrees of freedom.

Journal Article↗
PubMed2003

Scaling analysis of polyacrylamide gel surfaces synthesized in the presence of surfactants.

Surfaces of polyacrylamide hydrogels synthesized in the presence of surfactants were imaged by atomic force microscopy (AFM), and the surface morphology was studied by numerical scaling analysis. The gels were formed by polymerizing acrylamide plus a cross-linker in the presence of surfactants, which were then removed by soaking in distilled water. Gels formed in the presence of over 20% surfactant (by weight) formed clear, but became opaque upon removal of the surfactants. Other gels formed and remained clear. The surface morphology of the gels was studied by several one- and two-dimensional numerical scaling methods. The surfaces were found to be self-affine on short length scales, with a roughness (Hurst) exponent in the range from 0.85 to 1, crossing over to a constant root-mean-square surface width at long scales. Both the crossover length between these two regimes and the saturation value of the surface width increased significantly with increasing surfactant concentration, coincident with the increase in opacity. We propose that the changes in the surface morphology are due to a percolation transition in the system of voids formed upon removal of the surfactants from the bulk.

Acrylic Resins↗
PubMed2002

Surface scaling analysis of a frustrated spring-network model for surfactant-templated hydrogels.

We propose and study a statistical-mechanical model, inspired by recent atomic force microscopy studies of the surface structures of crosslinked polymer gels into which voids are introduced through templating by surfactant micelles [M. Chakrapani et al., e-print cond-mat/0112255]. The gel is represented by a frustrated, triangular network of nodes connected by springs of random equilibrium lengths. The nodes represent crosslinkers, and the springs correspond to polymer chains. The boundaries are fixed at the bottom, free at the top, and periodic in the lateral direction. Voids are introduced by deleting a proportion of the nodes and their associated springs. The model is numerically relaxed to a representative local energy minimum, resulting in an inhomogeneous, "clumpy" bulk structure. The free top surface is defined at evenly spaced points in the lateral (x) direction by the height of the topmost spring, measured from the bottom layer, h(x). Its scaling properties are studied by calculating the root-mean-square surface width and the generalized increment correlation functions C(q)(x)=<|h(x(0)+x)-h(x(0))|(q)>. The surface is found to have a nontrivial scaling behavior on small length scales, with a crossover to scale-independent behavior on large scales. As the vacancy concentration approaches the site-percolation limit, both the crossover length and the saturation value of the surface width diverge in a manner that appears to be proportional to the bulk connectivity length. This suggests that a percolation transition in the bulk also drives a similar divergence observed in surfactant templated polyacrylamide gels at high surfactant concentrations.

Journal Article↗
PubMed2002

Halide adsorption on single-crystal silver substrates: dynamic simulations and ab initio density functional theory.

We investigate the static and dynamic behaviors of a Br adlayer electrochemically deposited onto single-crystal Ag(100) using an off-lattice model of the adlayer. Unlike previous studies using a lattice-gas model, the off-lattice model allows adparticles to be located at any position within a two-dimensional approximation to the substrate. Interactions with the substrate are approximated by a corrugation potential. Using density functional theory (DFT) to calculate surface binding energies, a sinusoidal approximation to the corrugation potential is constructed. A variety of techniques, including Monte Carlo and Langevin simulations, are used to study the behavior of the adlayer. The lateral root-mean-square (rms) deviation of the adparticles from the binding sites is presented along with equilibrium coverage isotherms, and the thermally activated Arrhenius barrier-hopping model used in previous dynamic Monte Carlo simulations is tested.

Journal Article↗
PubMed2002

Early diagnosis of colonic carcinoma: a haemorrhagic complication after the use of tissue plasminogen activator.

Haemorrhagic complications of thrombolytic therapy for myocardial infarction are common. A 68 year old man presented with a myocardial infarction and was started on tissue plasminogen activator. Soon after he experienced rectal bleeding and a barium enema showed a small carcinoma in the colon, which was treated surgically. It is important that gastrointestinal bleeding secondary to thrombolysis is always investigated for an underlying cause.

Aged↗
PubMed2002

Successful use of oral methadone after failure of intravenous morphine and ketamine.

We describe an opioid-tolerant patient with severe acute pain which was unrelieved by morphine and ketamine via intravenous patient-controlled analgesia, but almost totally relieved by methadone. In the previous 24 hours, 509 mg of intravenous morphine and 769 mg of ketamine had been used and this was replaced by 200 mg of oral methadone. This implies that the success of methadone in morphine tolerant patients chiefly involves factors other than its role as an N-methyl-D-aspartate receptor antagonist, and that methadone should be considered as a replacement for morphine when the N-methyl-D-aspartate antagonist ketamine has proved ineffective.

Acute Disease↗
PubMed2001

The physiological kinetics of nitrogen and the prevention of decompression sickness.

Decompression sickness (DCS) is a potentially crippling disease caused by intracorporeal bubble formation during or after decompression from a compressed gas underwater dive. Bubbles most commonly evolve from dissolved inert gas accumulated during the exposure to increased ambient pressure. Most diving is performed breathing air, and the inert gas of interest is nitrogen. Divers use algorithms based on nitrogen kinetic models to plan the duration and degree of exposure to increased ambient pressure and to control their ascent rate. However, even correct execution of dives planned using such algorithms often results in bubble formation and may result in DCS. This reflects the importance of idiosyncratic host factors that are difficult to model, and deficiencies in current nitrogen kinetic models. Models describing the exchange of nitrogen between tissues and blood may be based on distributed capillary units or lumped compartments, either of which may be perfusion- or diffusion-limited. However, such simplistic models are usually poor predictors of experimental nitrogen kinetics at the organ or tissue level, probably because they fail to account for factors such as heterogeneity in both tissue composition and blood perfusion and non-capillary exchange mechanisms. The modelling of safe decompression procedures is further complicated by incomplete understanding of the processes that determine bubble formation. Moreover, any formation of bubbles during decompression alters subsequent nitrogen kinetics. Although these factors mandate complex resolutions to account for the interaction between dissolved nitrogen kinetics and bubble formation and growth, most decompression schedules are based on relatively simple perfusion-limited lumped compartment models of blood: tissue nitrogen exchange. Not surprisingly, all models inevitably require empirical adjustment based on outcomes in the field. Improvements in the predictive power of decompression calculations are being achieved using probabilistic bubble models, but divers will always be subject to the possibility of developing DCS despite adherence to prescribed limits.

Algorithms↗
PubMed2001

Lidocaine in the treatment of decompression illness: a review of the literature.

While recompression and hyperbaric oxygen administration remain the mainstays of treatment for decompression illness (DCI), drugs that might improve outcomes or prove beneficial in first aid management have been sought. There has been much interest in lidocaine, a sodium channel-blocking agent used clinically as an antiarrhythmic and local anesthetic. The relevant literature is reviewed. Lidocaine is neuro-protective in cerebral arterial gas embolism (CAGE) in vivo, and in a variety of in vivo and in vitro models of ischemic brain injury. There has been limited in vivo investigation of efficacy in DCI where bubbles have formed from dissolved nitrogen. Mechanisms of neuro-protection by lidocaine include deceleration of ischemic ion fluxes across the neuronal cell membrane and prevention of the consequent neurotoxic events. In addition, lidocaine lowers neuronal metabolism, exerts advantageous effects on cerebral hemodynamics, and is a potent anti-inflammatory. There is one randomized double blind study that demonstrates improved neuropsychological outcomes in cardiac surgery patients receiving lidocaine. Clinical evidence of efficacy in DCI is limited to anecdotal reports. Expeditious administration of lidocaine is justified in cases of unequivocal CAGE. Speculative use may be justified in severe neurologic DCI after patient counseling and consent.

Cell Hypoxia↗
PubMed2000

GABA spillover from single inhibitory axons suppresses low-frequency excitatory transmission at the cerebellar glomerulus.

GABA type B receptors (GABA(B)-Rs) are present on excitatory terminals throughout the CNS, but surprisingly little is known about their role in modulating neurotransmission under physiological conditions. We have investigated activation of GABA(B)-Rs on excitatory terminals within the cerebellar glomerulus, a structure where glutamatergic excitatory and GABAergic inhibitory terminals are in close apposition and make axodendritic synapses onto granule cells. Application of the GABA(B)-R agonist baclofen depressed evoked mossy fiber EPSCs by 54% at 1 Hz. The amplitude of miniature EPSCs recorded in tetrodotoxin was unchanged in the presence of baclofen, but the frequency was significantly reduced, indicating a purely presynaptic action of baclofen under our recording conditions. At physiological temperature (37 degrees C) presynaptic GABA(B)-Rs were not tonically activated by spontaneous GABA release from Golgi cells, which fire at approximately 8 Hz in slices at this temperature. However, tonic activation could be induced by blocking GABA uptake or by lowering temperature. GABA(B)-Rs were activated at physiological temperature when Golgi cell firing was increased above the basal level by stimulating a single inhibitory Golgi cell input at 50 Hz, suppressing the mossy fiber-evoked EPSC by 24% at 1 Hz. Furthermore, glutamate release was selectively inhibited at low-frequency mossy fiber inputs (<10 Hz) during Golgi cell stimulation. Our findings suggest that GABA spillover in the glomerulus modulates sensory input to the cerebellar cortex.

Animals↗
PubMed2000

Glutamate spillover suppresses inhibition by activating presynaptic mGluRs.

Metabotropic glutamate receptors (mGluRs) found on synaptic terminals throughout the brain are thought to be important in modulating neurotransmission. Activation of mGluRs by synaptically released glutamate depresses glutamate release from excitatory terminals but the physiological role of mGluRs on inhibitory terminals is unclear. We have investigated activation of mGluRs on inhibitory terminals within the cerebellar glomerulus, a structure in which GABA (gamma-aminobutyric acid)-releasing inhibitory terminals and glutamatergic excitatory terminals are in close apposition and make axo-dendritic synapses onto granule cells. Here we show that 'spillover' of glutamate, which is released from excitatory mossy fibres, inhibits GABA release from Golgi cell terminals by activating presynaptic mGluRs under physiological conditions. The magnitude of the depression of the inhibitory postsynaptic current is dependent on the frequency of mossy fibre stimulation, reaching 50% at 100 Hz. Furthermore, the duration of inhibitory postsynaptic current depression mirrors the time course of mossy fibre activity. Our results establish that mGluRs on inhibitory interneuron axons sense the activity of neighbouring excitatory synapses. This heterosynaptic mechanism is likely to boost the efficacy of active excitatory fibres by locally reducing the level of inhibition.

Animals↗
PubMed2000

Cerebral arterial gas embolism by helium: an unusual case successfully treated with hyperbaric oxygen and lidocaine.

A 27-year-old man inhaled helium from an unregulated pressurized cylinder and underwent cerebral arterial gas embolism (CAGE), leaving him blind and with radiologic evidence initially suggesting cortical infarction. There was complete recovery of vision and substantial regression of the radiologic changes after 4 hyperbaric oxygen treatments and a 54-hour lidocaine infusion, which began 6 hours after the accident. This is the second reported case of CAGE occurring by this mechanism and the first case of unequivocal CAGE in which lidocaine has been used as an adjunctive treatment with hyperbaric oxygen.

Adult↗
PubMed1999

Energy metabolism following prolonged hepatic cold preservation: benefits of interrupted hypoxia on the adenine nucleotide pool in rat liver.

The ability of brief hypothermic reperfusion (HtR) to restore hepatic energy metabolism following periods of cold hypoxic preservation was studied in isolated rat livers after storage times of 5, 10, and 24 h. In addition, investigations were performed on the effects of HtR used to restore liver oxidative metabolism in the middle of a prolonged (24 h) hypoxic preservation period. A histidine-lactobionate-raffinose solution was used for the initial cold portal flush in all groups. Results showed that cold hypoxia for either 5 or 10 h yielded livers capable of similar recoveries of ATP, energy charge, and total adenine nucleotides, but that HtR after 24 h cold preservation resulted in reduced regeneration of ATP, a lower energy charge, and a fall in tissue adenine nucleotides. When livers were stored for 24 h but subjected to brief HtR after either 5 or 10 h before return to hypoxic storage, improved recoveries of the energy metabolites were seen over those recorded after 24 h hypoxia alone. The fact that these improvements were not due to an improved supply of adenine nucleotide precursors was demonstrated by studying groups which were given HtR with perfusate containing precursors of adenine nucleotides (adenosine, adenine, and inosine) after 24 h cold hypoxia. These data are consistent with the hypothesis that poor metabolic recovery after long-term hepatic cold preservation results more from decreased mitochondrial oxidative phosphorylation than from a lack of precursors for adenine nucleotide resynthesis. In addition, restoring oxidative metabolism at hypothermia for brief periods can to some extent protect final metabolic status after prolonged storage.

Adenine Nucleotides↗
PubMed1999

Cerebral protection by lidocaine during cardiac operations.

BACKGROUND: Lidocaine improves outcome in animal brain injury models. Cardiac operations often cause postoperative neuropsychological (NP) impairment. We investigated cerebral protection by lidocaine in cardiac surgical patients. METHODS: Sixty-five patients undergoing left heart valve procedures completed 11 preoperative NP tests, a self-rating inventory for memory, and inventories measuring depression and anxiety. These were repeated 10 days, 10 weeks, and 6 months postoperatively. Patients received a 48-hour double-blinded infusion of either lidocaine in a standard antiarrhythmic dose or placebo, beginning at induction of anesthesia. A postoperative deficit in any test was defined as decline by more than or equal to the group preoperative standard deviation. In addition, sequential postoperative percentage change scores were calculated for each patient in all NP tests and the inventories for memory, depression and anxiety. RESULTS: Forty-two patients completed all three reviews, 8 completed two reviews, and 5 patients were reviewed once. Significantly more placebo patients had a deficit in at least one NP test at 10 days (p<0.025) and 10 weeks (p<0.05). The lidocaine group achieved superior sequential percentage change scores in 6 of the 11 NP tests (p<0.05) and in the memory inventory (p<0.025). There were no group differences in the remaining NP tests or the depression and anxiety inventories. CONCLUSIONS: These data show that cerebral protection by lidocaine, which is unrelated to any effect on depression or anxiety, and is at a level that is noticed by the patients.

Adult↗