Role of interparticle friction and particle-scale elasticity in the shear-strength mechanism of three-dimensional granular media.

The interlink between particle-scale properties and macroscopic behavior of three-dimensional granular media subjected to mechanical loading is studied intensively by scientists and engineers, but not yet well understood. Here we study the role of key particle-scale properties, such as interparticle friction and particle elastic modulus, in the functioning of dual contact force networks, viz., strong and weak contacts, in mobilizing shear strength in dense granular media subjected to quasistatic shearing. The study is based on three-dimensional discrete element method in which particle-scale constitutive relations are based on well-established nonlinear theories of contact mechanics. The underlying distinctive contributions of these force networks to the macroscopic stress tensor of sheared granular media are examined here in detail to find out how particle-scale friction and particle-scale elasticity (or particle-scale stiffness) affect the mechanism of mobilization of macroscopic shear strength and other related properties. We reveal that interparticle friction mobilizes shear strength through bimodal contribution, i.e., through both major and minor principal stresses. However, against expectation, the contribution of particle-scale elasticity is mostly unimodal, i.e., through the minor principal stress component, but hardly by the major principal stress. The packing fraction and the geometric stability of the assemblies (expressed by the mechanical coordination number) increase for decrease in interparticle friction and elasticity of particles. Although peak shear strength increases with interparticle friction, the deviator strain level at which granular systems attain peak shear strength is mostly independent of interparticle friction. Granular assemblies attain peak shear strength (and maximum fabric anisotropy of strong contacts) when a critical value of the mechanical coordination number is attained. Irrespective of the interparticle friction and elasticity of the particles, the packing fraction and volumetric strain are constant during steady state. Volumetric strain in sheared granular media increases with interparticle friction and elasticity of the particles. We show that the elasticity of the particles does not enhance dilation in frictionless granular media. The results presented here provide additional understanding of the role of particle-scale properties on the collective behavior of three-dimensional granular media subjected to shearing.

Link between single-particle properties and macroscopic properties in particulate assemblies: role of structures within structures.

The prevalence of particulate materials in modern industrial processes and products provides a significant motivation to achieve fundamental understanding of the bulk behaviour of particulate media. The rapid progress being made with atomic force microscopy and related particle characterization techniques pushes the limits of micro- and nanotechnologies such that interparticle interactions can be engineered to fabricate particulate assemblies to deliver specific functionalities. In this paper, primarily based on discrete element method simulations that we performed over the past 10 years, we summarize the key findings on the role of force transmission networks in dense particulate systems subjected to shearing. In general, the macroscopic strength characteristics in particulate systems is dictated by the distribution of heavily loaded contacts, also referred to as 'strong' force chains. Surprisingly, they constitute only a limited proportion of all contacts in particulate systems. They act like a 'granular brain' (memory networks) at particle scale. We show that the structural arrangement of the force chains and their evolution during loading depends on the single-particle properties and the initial packing condition in particulate assemblies. Further, the 'nature' of force chains in sheared granular media induces larger 'solid' grains to behave like 'fluid' particles, retarding their breakage. Later, we probe for ways by which we can control the signature of memory networks in packed beds, for example by applying an external electrical field in a densely packed particulate bed subjected to shearing (combined electromechanical loading). Though further research is required to account for more realistic conditions and preferably to allow particles to self-organize to strength specifications, understanding the hidden memory networks in particulate materials could be exploited to optimize their collective strength.

Force, relative-displacement, and work networks in granular materials subjected to quasistatic deformation.

To describe the heterogeneous nature of stress transmission in granular materials, the concept of the "strong" network consisting of contacts with large normal forces has been proposed by Radjaï [Phys. Rev. Lett. 80, 61 (1998)]. The shear stress is mainly determined by this strong network. The dual viewpoint is adopted here, by not only considering the forces at contacts, but also the deformation. It is shown that the strain increments are determined by the tangential component of the relative displacements at the contacts. A "mobile" network consisting of contacts with large tangential relative displacements is defined that primarily accounts for the strain increments. The investigation of the relation between the strong and the mobile networks shows that these networks are largely unrelated. An alternative network is defined that consists of contacts at which the contribution to the work input is large. It is found that this work input occurs primarily through the tangential forces and tangential relative displacements.

Peritonitis associated with Pasteurella multocida in peritoneal dialysis patients--case report and review of the literature.

Pasteurella multocida is a zoonotic pathogen found in the oral cavities of domestic dogs and cats and other wild and domestic animals. 14 cases ofperitoneal dialysis-associated Pasteurella multocida peritonitis linked to animal contact have been reported in the literature to date. In each case, the source of infection was believed to be a domestic cat or cats. One case of spontaneous bacterial peritonitis has been reported in a cirrhotic patient and was not linked to animal contact. Poor hygiene in relation to pets was considered the source of infection. We describe the first case of Pasteurella multocida peritonitis in a patient undergoing continuous ambulatory peritoneal dialysis (CAPD) believed to be caused by contact with dogs and discuss the relevant literature.

Role of interparticle forces and interparticle friction on the bulk friction in charged granular media subjected to shearing.

We study the consequences of the interplay between electrostatic forces, mechanical contact forces, and frictional properties of grains upon the bulk frictional properties of charged granular media subjected to quasistatic shearing. We show that, the variations in short-range electrostatic forces between the grains (which are often ignored in the existing studies) dominantly affect the bulk friction. Charging enhances the fabric anisotropy of heavily loaded contacts--this enhances the bulk friction, more significantly, in the case of low frictional granular systems.

Clinical experience with linezolid in the treatment of resistant gram-positive infections.

This study presents our clinical experience with linezolid in 19 patients with serious resistant gram-positive infections enrolled as part of the compassionate study. In this prospective, non-randomized, noncomparative study, 19 patients were enrolled as part of the National Compassionate Study Protocol conducted by Pharmacia-Upjohn. At the time of this writing, these patients had not been published in the literature. All of the patients had to have documented evidence of serious gram-positive infections in normally sterile sites and should have been unable to tolerate available antimicrobial therapy or be unresponsive to available drugs. Clinical characteristics, laboratory values, and pharmacokinetic and pharmacodynamic parameters were obtained. Patients were followed both short-term and long-term after completion of therapy. Nineteen patients were enrolled: 13 females and 6 males. The average age was 63 years. The average length of therapy with linezolid was 22 days. Methicillin-resistant Staphylococcus aureus (MRSA) was treated in eight patients, methicillin-resistant Staphylococcus epidermidis (MRSE) in two patients, vancomycin-resistant Enterococcus faecium (VREF) in eight patients, and coagulase-negative Staphylococcus in two patients. Co-infecting organisms include Enterococcus species colonization in six patients, Pseudomonas species in one patient, Serratia marcenens in one patient, and Candida albicans in one patient. Sterile sites that were infected included bone and joint (wounds and septic joints) in six patients, gastrointestinal system (hepatobiliary, liver abscess, Crohn's) in five patients, genitourinary (kidney and urine) in two patients, blood in five patients, respiratory in one patient, and aortic valve in 1 patient. Linezolid was given at 600 mg IV every 12 hours with a mean length of therapy of 22 days. Surgical drainage was used in combination with linezolid in 11 of the patients. Seventy nine percent of these patients achieved clinical and microbiologic cure, and none of the deaths reported in this series were related to the drug. Adverse events included skin rash in one patient, mild bone marrow suppression in two patients, and mild elevation in liver function tests in two patients. No life-threatening adverse events were noted. It appears that linezolid, along with surgical intervention (when necessary), appears to be an effective treatment option for resistant gram-positive infections. Long-term studies evaluating the possible resistance rates are necessary.

Vancomycin-resistant Enterococcus faecium osteomyelitis: successful treatment with quinupristin-dalfopristin.

Vancomycin-resistant enterococci (VRE) have recently emerged as an increasing concern in the management of severe infections. Treatment of these life-threatening infections has been limited to quinupristin-dalfopristin and, more recently, linezolid therapy. We report the first case, to our knowledge, of vancomycin-resistant Enterococcus faecium vertebral osteomyelitis treated successfully with quinupristin-dalfopristin. We review the recent epidemiology of VRE and briefly outline the pharmacology and pharmacokinetics of quinupristin-dalfopristin.

Evolution of force distribution in three-dimensional granular media.

Based on the discrete element method, the nature of normal contact force distribution and the effect of microstructure (contact fabric) on stresses in granular media sheared under constant mean stress condition is analyzed. The particles are tested in a periodic cell, having a nearly monodispersed system of spherical particles ("hard" and "soft"). The granular systems were initially isotropically compressed to have different solid fractions in order to obtain "dense" and "loose" samples. To study the nature of the force distribution, the granular medium was considered as both (i) noncohesive and (ii) with low values of interface energy. For the granular systems considered here, the nature of force distribution is shown to be dependent on shear history. The amount of interface energy introduced in the granular system does not seem to change the nature of normal force distribution significantly. However, it improves the postpeak stability in agreement with previous research [C. Thornton, Geotechnique 50, 43 (2000)]. The simulation of systems subjected to quasistatic shearing, in general, reveals that in a hard system (both dense and loose), the normal contact force distribution (i) at "peak" shear strength is purely an exponential decay throughout the entire range of force scale that is used, and (ii) at "isotropic" and "steady" states, the contact normal force distribution is bimodal with forces greater than average decaying exponentially at both the states, while the forces less than average tend to be half-Gaussian at the "isotropic" state and a second-order polynomial function at the "steady" state. For the soft (dense) system, the normal contact force distribution at "peak" shear strength is bimodal with forces greater than average decaying exponentially while the forces less than average tend to be a second-order polynomial function. However, for the soft system at both "isotropic" and "steady" states, the contact normal force distribution is half-Gaussian throughout the entire range of force scale that is used. It has been pointed out that in a granular system undergoing slow shearing, the shear strength of the system seems to depend on the ability of the material to form strong fabric anisotropy of contacts carrying strong (greater than average) force.

Lactobacillemia: an emerging cause of infection in both the immunocompromised and the immunocompetent host.

The bacterium, lactobacillus, is found in the mucosal surfaces of the mouth and the gastrointestinal and genitourinary tracts. There have been increasing reports of the micro-organism being a cause of serious infection in immunocompromised individuals. This article reviews the clinical presentation, laboratory characteristics and treatment of patients with lactobacillemia.

Actinomyces and nocardia infections in immunocompromised and nonimmunocompromised patients.

A retrospective survey of nocardia and actinomyces infections in five local hospitals was conducted over a 3-year period in El Paso, Texas, a border city, in the southwestern United States. The medical records of 42 patients with suspected nocardiosis or actinomycosis were reviewed. One patient was diagnosed with actinomyces and 12 patients with nocardia. Microbiological data included morphologic characteristics, biochemical profile, and susceptibility testing. Predisposing factors included leukemia, renal insufficiency, renal transplant, and lymphoma. No predisposing factors were found in 67% (n = 8) of patients (including the patient with actinomycosis). Twenty-three percent (n = 3) of patients had disseminated disease without evidence of underlying disease or immunosuppression. The mortality and morbidity of these infections appeared to be low.

Multidrug-resistant enterococci: the dawn of a new era in resistant pathogens.

Resistant enterococci, especially vancomycin-resistant enterococci, have rapidly become an important nosocomial pathogen. They are increasingly prevalent among hospitalized patients, patients with serious chronic illnesses, and immunosuppressed patients. Risk factors identified include previous antibiotics, exposure to contaminated equipment, and close proximity to infected patients. Treatment of multidrug-resistant pathogens has become increasingly difficult, with increased morbidity and mortality in these patients. Strict infection control measures remain the mainstay in the management of these infections.

Use of fluconazole in the treatment of non-AIDS cryptococcal meningitis.

The treatment of non-acquired immunodeficiency syndrome (AIDS) cryptococcal meningitis has not been well defined as yet. The current recommendation is amphotericin B with or without flucytosine. However, due to the many side effects of these drugs, fluconazole is becoming an attractive alternative in patients who cannot tolerate amphotericin B and flucytosine. This case study demonstrates the successful use of fluconazole in a patient with underlying diabetes mellitus and cryptococcal meningitis.

Multidrug-resistant Pneumococcus causing vertebral osteomyelitis.

Primary vertebral osteomyelitis in a rare presentation of pneumococcal infection especially in an asymptomatic patient with no primary focus of infection. This report describes a patient who presented with lower back pain in which the magnetic resonance imaging showed little evidence of L1 and L2 vertebral body destruction. Cultures from these vertebral bodies grew penicillin and third-generation resistant pneumococcus. The patient was treated successfully with 6 weeks of vancomycin and rifampin.

Q fever pneumonia.

Pneumonia is one of the principal manifestations of Q fever, a disease caused by Coxiella burnetii. This bacterium can replicate only within cells, yet it is capable of surviving in the environment because it can withstand drying and substantial temperature variations. Livestock, especially sheep, goats and cattle, are a major reservoir of C burnetii. The organism is transmitted to humans by direct contact with animal products, especially during parturition. Aerosols have transmitted infection over considerable distances. The illness is characterized by an influenza-like syndrome with patchy pulmonary infiltrates. The pneumonia may be accompanied by biochemical evidence of mild hepatitis. The diagnosis is established serologically. Tetracycline or doxycycline provide effective therapy.

Isoniazid hepatotoxicity in renal transplant recipients.

Tuberculosis occurs at higher rates in renal transplant recipients than in the general population. It would be desirable to use isoniazid prophylaxis in renal transplant recipients at risk for reactivation of tuberculosis; yet many transplant centers do not routinely employ INH prophylaxis because they perceive transplant recipients to be an enhanced risk of hepatotoxicity from isoniazid. Data on the risk of isoniazid in renal transplant patients receiving cyclosporine-based immunosuppression are limited. We retrospectively studied 83 renal transplant recipients (mean age 39.1 +/- 11.7 yr) who had received INH prophylaxis between 1985 and 1994. Eight patients had laboratory evidence of chronic hepatitis B or chronic hepatitis C infection. The mean duration of INH therapy was 344 +/- 163 d. The mean serum glutamate oxalacetic transferase (SGOT) at the start of INH therapy was 24.1 +/- 10.9 I.U., and 10% of patients had mildly elevated SGOTs. Mean peak SGOT during therapy was 36.4 +/- 15.3 I.U. (p < 0.001 compared to start (SGOT). In follow up, 31% of patients had an abnormal SGOT (> 40 I.U.); however, the elevations were small (the highest SGOT was 88.I.U.) and never necessitated discontinuation of INH. No patient had jaundice or other evidence of clinical hepatotoxicity. The 95% confidence interval for the observed frequency of clinical hepatitis was 0% to 4.3%. At the end of INH therapy the mean SGOT was 22.7 +/- 6.2 I.U. (p > 0.2, compared with start SGOT) and only one patient had an abnormal SGOT. In conclusion, it appears that the risk of renal transplant recipients developing serious hepatotoxicity with the administration of INH is low and not different from normal individuals.