Viscoelastic relaxation and topological fluctuations in glass-forming liquids.

A method for characterizing the topological fluctuations in liquids is proposed. This approach exploits the concept of the weighted gyration tensor of a collection of particles and permits the definition of a local configurational unit (LCU). The first principal axis of the gyration tensor serves as the director of the LCU, which can be tracked and analyzed by molecular dynamics simulations. Analysis of moderately supercooled Kob-Andersen mixtures suggests that orientational relaxation of the LCU closely follows viscoelastic relaxation and exhibits a two-stage behavior. The slow relaxing component of the LCU corresponds to the structural, Maxwellian mechanical relaxation. Additionally, it is found that the mean curvature of the LCUs is approximately zero at the Maxwell relaxation time with the Gaussian curvature being negative. This observation implies that structural relaxation occurs when the configurationally stable and destabilized regions interpenetrate each other in a bicontinuous manner. Finally, the mean and Gaussian curvatures of the LCUs can serve as reduced variables for the shear stress correlation, providing a compelling proof of the close connection between viscoelastic relaxation and topological fluctuations in glass-forming liquids.

Potential energy landscape activations governing plastic flows in glass rheology.

While glasses are ubiquitous in natural and manufactured materials, the atomic-level mechanisms governing their deformation and how these mechanisms relate to rheological behavior are still open questions for fundamental understanding. Using atomistic simulations spanning nearly 10 orders of magnitude in the applied strain rate we probe the atomic rearrangements associated with 3 characteristic regimes of homogeneous and heterogeneous shear flow. In the low and high strain-rate limits, simulation results together with theoretical models reveal distinct scaling behavior in flow stress variation with strain rate, signifying a nonlinear coupling between thermally activated diffusion and stress-driven motion. Moreover, we find the emergence of flow heterogeneity is closely correlated with extreme values of local strain bursts that are not readily accommodated by immediate surroundings, acting as origins of shear localization. The atomistic mechanisms underlying the flow regimes are interpreted by analyzing a distance matrix of nonaffine particle displacements, yielding evidence of various barrier-hopping processes on a fractal potential energy landscape (PEL) in which shear transformations and liquid-like regions are triggered by the interplay of thermal and stress activations.

Understanding the mechanisms of amorphous creep through molecular simulation.

Molecular processes of creep in metallic glass thin films are simulated at experimental timescales using a metadynamics-based atomistic method. Space-time evolutions of the atomic strains and nonaffine atom displacements are analyzed to reveal details of the atomic-level deformation and flow processes of amorphous creep in response to stress and thermal activations. From the simulation results, resolved spatially on the nanoscale and temporally over time increments of fractions of a second, we derive a mechanistic explanation of the well-known variation of creep rate with stress. We also construct a deformation map delineating the predominant regimes of diffusional creep at low stress and high temperature and deformational creep at high stress. Our findings validate the relevance of two original models of the mechanisms of amorphous plasticity: one focusing on atomic diffusion via free volume and the other focusing on stress-induced shear deformation. These processes are found to be nonlinearly coupled through dynamically heterogeneous fluctuations that characterize the slow dynamics of systems out of equilibrium.

Mesoscale texture of cement hydrates.

Strength and other mechanical properties of cement and concrete rely upon the formation of calcium-silicate-hydrates (C-S-H) during cement hydration. Controlling structure and properties of the C-S-H phase is a challenge, due to the complexity of this hydration product and of the mechanisms that drive its precipitation from the ionic solution upon dissolution of cement grains in water. Departing from traditional models mostly focused on length scales above the micrometer, recent research addressed the molecular structure of C-S-H. However, small-angle neutron scattering, electron-microscopy imaging, and nanoindentation experiments suggest that its mesoscale organization, extending over hundreds of nanometers, may be more important. Here we unveil the C-S-H mesoscale texture, a crucial step to connect the fundamental scales to the macroscale of engineering properties. We use simulations that combine information of the nanoscale building units of C-S-H and their effective interactions, obtained from atomistic simulations and experiments, into a statistical physics framework for aggregating nanoparticles. We compute small-angle scattering intensities, pore size distributions, specific surface area, local densities, indentation modulus, and hardness of the material, providing quantitative understanding of different experimental investigations. Our results provide insight into how the heterogeneities developed during the early stages of hydration persist in the structure of C-S-H and impact the mechanical performance of the hardened cement paste. Unraveling such links in cement hydrates can be groundbreaking and controlling them can be the key to smarter mix designs of cementitious materials.

Use of the International Prostate Symptom Score (IPSS) in Chinese male patients with benign prostatic hyperplasia.

PURPOSE: To test the psychometric properties of the International Prostate Symptom Score (Hong Kong Chinese version 2) (IPSS) in Chinese male patients with benign prostatic hyperplasia (BPH) under secondary care. METHODS: A prospective longitudinal study was done by interviewing subjects at baseline, at 2 week after baseline for assessing test-retest reliability and at 26 week after baseline for assessing responsiveness. All subjects were interviewed to complete a structured questionnaire including IPSS, Short Form-12 Health Survey version 2 (SF-12v2) and Depression Anxiety Stress Scale (DASS). RESULTS: The IPSS HRQOL score had weak correlations with SF-12v2 summary and DASS domain scores. For reliability analysis, Cronbach's alpha coefficient was 0.90 for the seven symptom-related items. The intraclass correlation coefficients of the IPSS total symptom score and HRQOL score were 0.90 and 0.86, respectively. For sensitivity, statistically significant differences were detected between the subjects with BPH and those without for IPSS total symptom score (effect size = 0.68) but not the IPSS HRQOL score. The areas under ROC curves for the IPSS total symptom and HRQOL scores were 0.67 and 0.60, respectively. CONCLUSIONS: The IPSS was valid, reliable instrument in Chinese patients with BPH. The IPSS total symptom score, but not the HRQOL score, is sensitive in differentiating subgroups.

Mapping strain rate dependence of dislocation-defect interactions by atomistic simulations.

Probing the mechanisms of defect-defect interactions at strain rates lower than 10(6) s(-1) is an unresolved challenge to date to molecular dynamics (MD) techniques. Here we propose an original atomistic approach based on transition state theory and the concept of a strain-dependent effective activation barrier that is capable of simulating the kinetics of dislocation-defect interactions at virtually any strain rate, exemplified within 10(-7) to 10(7) s(-1). We apply this approach to the problem of an edge dislocation colliding with a cluster of self-interstitial atoms (SIAs) under shear deformation. Using an activation-relaxation algorithm [Kushima A, et al. (2009) J Chem Phys 130:224504], we uncover a unique strain-rate-dependent trigger mechanism that allows the SIA cluster to be absorbed during the process, leading to dislocation climb. Guided by this finding, we determine the activation barrier of the trigger mechanism as a function of shear strain, and use that in a coarse-graining rate equation formulation for constructing a mechanism map in the phase space of strain rate and temperature. Our predictions of a crossover from a defect recovery at the low strain-rate regime to defect absorption behavior in the high strain-rate regime are validated against our own independent, direct MD simulations at 10(5) to 10(7) s(-1). Implications of the present approach for probing molecular-level mechanisms in strain-rate regimes previously considered inaccessible to atomistic simulations are discussed.

Nano-scale mechanics of colloidal C-S-H gels.

Gels of calcium-silicate-hydrates (C-S-H) are the glue that is largely responsible for the mechanical properties of cement. Despite their practical relevance, their nano-scale structure and mechanics are still mainly unexplored, because of the difficulties in characterizing them in a complex material like cement. We propose a colloidal model to investigate the gel mechanics emerging in the critical range of length-scales from several tens to hundreds of nanometers. We show that the size polydispersity of the hydrates and size-dependent effective interactions can explain the mechanical heterogeneities detected in nano-indentation experiments. We also show how these features control the arising of irreversible structural rearrangements under deformation, which are good candidates as nano-scale mechanisms underlying mechanical aging and slow structural relaxation in the gels.

Analogy between glass rheology and crystal plasticity: yielding at high strain rate.

An abrupt increase of the yield stress at sufficiently high strain rate, seen in glassy as well as crystalline structures, signifies a transition from classical thermal fluctuation to stress activated processes. For crystals this behavior has been recently explained using transition-state-theory with a stress-dependent activation barrier for dislocation glide. An equivalent approach, developed independently for amorphous solids, suggests the physical basis of the upturn behavior of the yield stress is more general. Insights into the interplay between thermal and stress activation processes can contribute to the current efforts toward identifying materials science frontiers at the mesoscale.

Onset mechanism of strain-rate-induced flow stress upturn.

The strain-rate response of flow stress in a plastically deforming crystal is formulated through a stress-sensitive dislocation mobility model that can be evaluated by atomistic simulation. For the flow stress of a model crystal of bcc Fe containing a 1/2[111] screw dislocation, this approach describes naturally a non-Arrhenius upturn at high strain rate, an experimentally established transitional behavior for which the underlying mechanism has not been clarified. Implications of our findings regarding the previous explanations of strain-rate effects on flow stress are discussed.

Evaluation of the clinical value of a simple flowmeter in the management of male lower urinary tract symptoms.

UNLABELLED: Study Type - Diagnostic (exploratory cohort) Level of Evidence 3b What's known on the subject? and What does the study add? Electronic uroflowmetry reasonably predicts the likelihood of bladder outlet obstruction (BOO) and risk of AUR. This low-cost device, Uflowmeter(™) , allows men to perform uroflowmetry at home with ease and the results are compatible with that of electronic uroflowmentry. It can also estimates risk of AUR and the need for TURP to relieve LUTS. OBJECTIVE: To show the clinical value of a simple flowmeter, which has been devised to measure uroflow on an ordinal scale (<10, 10-15, 15-19 and >19 mL/s) at home, for the management of male lower urinary tract symptoms (LUTS). PATIENTS AND METHODS: A total of 186 men with LUTS were enrolled in the study. The mean (range) follow-up was 220 (68-431) days. The men's mean (range) age was 65.5 (46-83) years, mean (range) maximum urinary flow rate (Qmax) 12.8 (4.3-39.5) mL/s, mean (range) voided volume 294.8 (151-686) mL; mean (range) postvoid residual urine volume (PVR) 50 (0-303) mL and mean (range) International Prostate Symptom Score (IPSS) 13.5 (1-31). The men underwent electronic uroflowmetry ('clinic uroflowmetry') and completed an IPSS questionnaire in the clinic. They then conducted 10 measurements with the device at home ('home uroflowetry'). The uroflowmetry and IPSS questionnaire were repeated 2 weeks later. Quadratically weighted Kappa analysis (κ) of the home uroflowmetry vs. clinic uroflowmetry, and of the sensitivity and specificity of the home uroflowmetry values to correspond to the mean Qmax of clinic uroflowmetry (<10, 10-15, 15-19 and >19 mL/s) was performed. Similar analyses were performed for the IPSS. Kaplan-Meier analysis was performed to evaluate whether home uroflowmetry was able to prognosticate acute urinary retention (AUR) or the need for transurethral resection of the prostate (TURP). RESULTS: The home uroflowmetry values (κ= 0.84, 95% confidence interval [CI]: 0.78-0.90) were superior to the IPSS (κ= 0.083; 95% CI: 0-0.173) in correlating with the mean Qmax of clinic uroflowmetry. Home uroflowmetry was most sensitive in identifying a mean Qmax of >19 mL/s (sensitivity: 0.99; 95% CI:0.97-1.00) and most specific in identifying a mean Qmax of <10 mL/s (specificity: 0.90; 95% CI:0.83-0.94). The home uroflowmetry works best in ruling out a mean Qmax of <19 mL/s (diagnostic odds ratio [DOR]= 349.3; 95% CI:40.24-3037.7), followed by a mean Qmax of <15 mL/s (DOR = 91.02; 95% CI:31.23-265.23) and a mean Qmax of <10 mL/s (DOR = 32.04; 95% CI:14.0-73.19). Men with a home uroflowmetry value ≤10 mL/s were more likely (n= 6; 8.8%) than those with a home uroflowmetry value >10 mL/s (n= 2; 1.7%) to develop AUR or require TURP (log-rank test: P= 0.017; hazard ratio:5.61(95% CI:1.10-28.64)). The IPSS failed to display the same discriminative capability. CONCLUSION: Home uroflowmetry using this simple device is a satisfactory estimation of clinic uroflowmetry using an electronic flowmeter and can predict the significant progression of male LUTS.

A realistic molecular model of cement hydrates.

Despite decades of studies of calcium-silicate-hydrate (C-S-H), the structurally complex binder phase of concrete, the interplay between chemical composition and density remains essentially unexplored. Together these characteristics of C-S-H define and modulate the physical and mechanical properties of this "liquid stone" gel phase. With the recent determination of the calcium/silicon (C/S = 1.7) ratio and the density of the C-S-H particle (2.6 g/cm(3)) by neutron scattering measurements, there is new urgency to the challenge of explaining these essential properties. Here we propose a molecular model of C-S-H based on a bottom-up atomistic simulation approach that considers only the chemical specificity of the system as the overriding constraint. By allowing for short silica chains distributed as monomers, dimers, and pentamers, this C-S-H archetype of a molecular description of interacting CaO, SiO2, and H2O units provides not only realistic values of the C/S ratio and the density computed by grand canonical Monte Carlo simulation of water adsorption at 300 K. The model, with a chemical composition of (CaO)(1.65)(SiO2)(H2O)(1.75), also predicts other essential structural features and fundamental physical properties amenable to experimental validation, which suggest that the C-S-H gel structure includes both glass-like short-range order and crystalline features of the mineral tobermorite. Additionally, we probe the mechanical stiffness, strength, and hydrolytic shear response of our molecular model, as compared to experimentally measured properties of C-S-H. The latter results illustrate the prospect of treating cement on equal footing with metals and ceramics in the current application of mechanism-based models and multiscale simulations to study inelastic deformation and cracking.

Photoselective vaporisation prostatectomy using a GreenLight High Performance System for patients with bleeding tendency.

OBJECTIVES: To report the results of a modified vaporisation incision technique using a GreenLight High Performance System in the treatment of benign prostatic disease in men receiving anticoagulants. DESIGN: Case series. SETTING: Regional hospital, Hong Kong. PATIENTS: From January 2007 to April 2010, 48 patients with a bleeding tendency or on oral anticoagulants who underwent photoselective vaporisation prostatectomy with a GreenLight High Performance System in the North District Hospital were studied. Data collected prospectively were analysed to determine perioperative and postoperative outcomes, including uroflowmetry parameters, serum prostate-specific antigen level, prostate volume, and complications at 1, 3, 6, and 12 months post-surgery. RESULTS: The patients' mean age was 76 (standard deviation, 7; range 62-94) years. The mean follow-up period was 13 (standard deviation, 9) months. Thirty-six (75%) patients had urinary retention prior to surgery. Bleeding tendencies were due to receipt of aspirin (n=36), two antiplatelet agents (n=6), warfarin (n=4) and clopidogrel (n=1), and to thrombocytopaenia (n=1). Preoperative transrectal ultrasonography showed a mean prostate size of 58 (standard deviation, 30; range, 18-154) mL. Of the patients, 81% were discharged without a catheter and their mean hospital stay was 3 days. Five patients were readmitted for secondary haemorrhage, two had a drop of more than 10 g/L in their haemoglobin level, but only one received a blood transfusion. Mean uroflowmetry parameters, namely, peak flow rate and residual volume, were 8.7 mL/s and 199 mL preoperatively and 14.7 mL/s and 50 mL 1 year after the operation. CONCLUSION: With an ageing population in which patients with various co-morbidities receive anticoagulant/antiplatelet therapy, photoselective vaporisation prostatectomy using a GreenLight High Performance System is a safe treatment option.

Mechanism of void nucleation and growth in bcc Fe: atomistic simulations at experimental time scales.

Evolution of small-vacancy clusters in bcc Fe is simulated using a multiscale approach coupling an atomistic activation-relaxation method for sampling transition-state pathways with environment-dependent reaction coordinate calculations and a kinetic Monte Carlo simulation to reach time scales on the order of ~104 s. Under vacancy-supersaturated condition, di- and trivacancy clusters form and grow by coalescence (Ostwald ripening). For cluster size greater than four we find a transition temperature of 150 °C for accelerated cluster growth, as observed in positron annihilation spectroscopy experiments. Implications for the mechanism of stage-IV radiation-damage-recovery kinetics are discussed.

An unusual cause of retention of urine after intravesical Bacillus Calmette-Guérin therapy for superficial bladder cancer.

We report an unusual complication of intravesical Bacillus Calmette-Guérin therapy for superficial bladder cancer, namely, retention of urine secondary to prostatic Bacillus Calmette-Guérin infection and abscess. A summary of the literature together with a review of its management is discussed.

Atomistic simulation of creep in a nanocrystal.

We describe a method to simulate on macroscopic time scales the stress relaxation in an atomistic nanocrystal model under an imposed strain. Using a metadynamics algorithm for transition state pathway sampling we follow the full evolution of a classical anelastic relaxation event, with relaxation times governed by the nanoscale microstructure imperfections in the solid. We show that probing this sensitive variation leads to mechanistic insights that reveal a direct correlation between system-level relaxation behavior and localized atomic displacements in the vicinity of the nanostructured defects, in turn implying a unit mechanism for self-organized plastic response. This suggests a new class of measurements in which the microstructure imperfections are characterized and matched to predictive simulations enabled by the present method.

Predicting dislocation climb and creep from explicit atomistic details.

Here we report kinetic Monte Carlo simulations of dislocation climb in heavily deformed, body-centered cubic iron comprising a supersaturation of vacancies. This approach explicitly incorporates the effect of nonlinear vacancy-dislocation interaction on vacancy migration barriers as determined from atomistic calculations, and enables observations of diffusivity and climb over time scales and temperatures relevant to power-law creep. By capturing the underlying microscopic physics, the calculated stress exponents for steady-state creep rates agree quantitatively with the experimentally measured range, and qualitatively with the stress dependence of creep activation energies.

Robotic radical prostatectomy in east Asia: development, surgical results and challenges.

PURPOSE OF REVIEW: The shift toward robot-assisted laparoscopic radical prostatectomy has reshaped the surgical approach for localized prostate cancer in America and many parts of Europe. Its impact on Asia, however, has been somewhat delayed and less widespread compared with western countries. We reviewed and surveyed the evolving trends in robotic prostatectomy in east Asia and describe how the influence of cancer demography, financial reimbursement models, refinements in robotic technology and robotic surgical training will alter the future direction of the procedure in this region. RECENT FINDINGS: There are about 50 systems installed in east Asia. Numerous centers have reported successful implementation of robotic prostatectomy program, with transfusion rate of 7-26.4%. Margin positivity for T2 disease ranges from 9.8 to 24%, whereas continence rates range from 75 to 94% over 3-6 months. Significant increase in number of prostatectomy has been observed in some centers. SUMMARY: The outlook for robotic prostatectomy in east Asia remains rosy despite the obstacles in financial reimbursement, patient volume and surgical skill development. Future robotic systems with smaller footprint, leaner instrument arms and lower costs will help to accelerate its integration into more Asian hospitals.

Progressive increase of genetic alteration in urinary bladder cancer by combined allelotyping analysis and comparative genomic hybridization.

Bladder cancer is the ninth most common cancer in the world. Urothelial carcinoma (formerly known as transitional cell carcinoma) comprises the majority of bladder cancers. In order to decipher the genetic alteration leading to the carcinogenesis of urothelial cancer, we performed genome-wide allelotyping analysis using 384 microsatellite markers spanning 22 autosomes together with comparative genomic hybridization (CGH) in 21 urothelial cancer. High frequency of allelic imbalance was observed in chromosome arm 1q (61.9%), 3p (61.9%), 4q (66.67%), 8p (57.14%), 9p (76.2%) and 9q (66.67%). Allelic imbalance with frequency above average was also observed in chromosome arm 2q, 10p, 10q, 11p, 11q, 12q, 13q, 15q, 17p and 19q. The allelic imbalance of each case and fractional allelic loss for each chromosome was associated with higher tumor grade and stage (P<0.05). We have also delineated several minimal deletion regions on chromosome 3p, 4q, 8p, 9p, 9q, 11p, 13q, 16q and 17p. By CGH analysis, common chromosomal alterations included gain of 1p, 1q, 12q, 16p, 17q and 19p as well as loss of 4q and 9p in most of the cases. Our findings may provide valuable information to locate putative oncogenes and tumor suppressor genes in the carcinogenesis of bladder cancer in this locality.

Computing the viscosity of supercooled liquids.

We describe an atomistic method for computing the viscosity of highly viscous liquids based on activated state kinetics. A basin-filling algorithm allowing the system to climb out of deep energy minima through a series of activation and relaxation is proposed and first benchmarked on the problem of adatom diffusion on a metal surface. It is then used to generate transition state pathway trajectories in the potential energy landscape of a binary Lennard-Jones system. Analysis of a sampled trajectory shows the system moves from one deep minimum to another by a process that involves high activation energy and the crossing of many local minima and saddle points. To use the trajectory data to compute the viscosity we derive a Markov Network model within the Green-Kubo formalism and show that it is capable of producing the temperature dependence in the low-viscosity regime described by molecular dynamics simulation, and in the high-viscosity regime (10(2)-10(12) Pa s) shown by experiments on fragile glass-forming liquids. We also derive a mean-field-like description involving a coarse-grained temperature-dependent activation barrier, and show it can account qualitatively for the fragile behavior. From the standpoint of molecular studies of transport phenomena this work provides access to long relaxation time processes beyond the reach of current molecular dynamics capabilities. In a companion paper we report a similar study of silica, a representative strong liquid. A comparison of the two systems gives insight into the fundamental difference between strong and fragile temperature variations.