Hydraulic Fracture

We consider a variation of Griffith's analysis of rupture, one which simulates the process of hydrofracturing, where fluid forced into a crack raises the fluid pressure until the crack begins to grow. Unlike that of Griffith, in this analysis fluid pressure drops as a hydrofracture grows. We find that growth of the fracture depends on the ratio of the compliances of the fluid and the fracture, a non-dimensional parameter called $\alpha_0$ here. The pressure needed to initiate a hydrofracture is found to be the same as that derived by Griffith. Once a fracture initiates, for relatively low values of the model parameter $\alpha_0$ ($\alpha_0 \leq 0.2$) the fracture advances spontaneously to a new radius which depends on the value of $\alpha_0$. For $\alpha_0 \leq 0.2$ further fluid injection is required to increase the fracture radius after spontaneous growth stops. For the case where $\alpha_0 > 0.2$ each increment of fracture growth requires injection of more fluid. For the extreme case where $\alpha_0 = 0$ our results are the same as Griffith's, i.e., a fracture once initiated grows without limit.Comment: 10 pages, 4 figure

\u3cem\u3eIn vivo\u3c/em\u3e Imaging of Human Retinal Microvasculature Using Adaptive Optics Scanning Light Ophthalmoscope Fluorescein Angiography

The adaptive optics scanning light ophthalmoscope (AOSLO) allows visualization of microscopic structures of the human retina in vivo. In this work, we demonstrate its application in combination with oral and intravenous (IV) fluorescein angiography (FA) to the in vivo visualization of the human retinal microvasculature. Ten healthy subjects ages 20 to 38 years were imaged using oral (7 and/or 20 mg/kg) and/or IV (500 mg) fluorescein. In agreement with current literature, there were no adverse effects among the patients receiving oral fluorescein while one patient receiving IV fluorescein experienced some nausea and heaving. We determined that all retinal capillary beds can be imaged using clinically accepted fluorescein dosages and safe light levels according to the ANSI Z136.1-2000 maximum permissible exposure. As expected, the 20 mg/kg oral dose showed higher image intensity for a longer period of time than did the 7 mg/kg oral and the 500 mg IV doses. The increased resolution of AOSLO FA, compared to conventional FA, offers great opportunity for studying physiological and pathological vascular processes

Classification of Human Retinal Microaneurysms Using Adaptive Optics Scanning Light Ophthalmoscope Fluorescein Angiography

Purpose. Microaneurysms (MAs) are considered a hallmark of retinal vascular disease, yet what little is known about them is mostly based upon histology, not clinical observation. Here, we use the recently developed adaptive optics scanning light ophthalmoscope (AOSLO) fluorescein angiography (FA) to image human MAs in vivo and to expand on previously described MA morphologic classification schemes. Methods. Patients with vascular retinopathies (diabetic, hypertensive, and branch and central retinal vein occlusion) were imaged with reflectance AOSLO and AOSLO FA. Ninety-three MAs, from 14 eyes, were imaged and classified according to appearance into six morphologic groups: focal bulge, saccular, fusiform, mixed, pedunculated, and irregular. The MA perimeter, area, and feret maximum and minimum were correlated to morphology and retinal pathology. Select MAs were imaged longitudinally in two eyes. Results. Adaptive optics scanning light ophthalmoscope fluorescein angiography imaging revealed microscopic features of MAs not appreciated on conventional images. Saccular MAs were most prevalent (47%). No association was found between the type of retinal pathology and MA morphology (P = 0.44). Pedunculated and irregular MAs were among the largest MAs with average areas of 4188 and 4116 μm2, respectively. Focal hypofluorescent regions were noted in 30% of MAs and were more likely to be associated with larger MAs (3086 vs. 1448 μm2, P = 0.0001). Conclusions. Retinal MAs can be classified in vivo into six different morphologic types, according to the geometry of their two-dimensional (2D) en face view. Adaptive optics scanning light ophthalmoscope fluorescein angiography imaging of MAs offers the possibility of studying microvascular change on a histologic scale, which may help our understanding of disease progression and treatment response

Fracture Blisters

Fracture blisters are a relatively uncommon complication of fractures in locations of the body, such as the ankle, wrist elbow and foot, where skin adheres tightly to bone with little subcutaneous fat cushioning. The blister that results resembles that of a second degree burn

A new model for analyzing the effect of fractures on triaxial deformation

Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in International Journal of Rock Mechanics and Mining Sciences 43 (2006): 1241-1255, doi:10.1016/j.ijrmms.2006.04.002.Rock is porous, with a connected network of cracks and pores. The static and dynamic behaviors of a rock sample under load depend on both the solid mineral matrix and the porous phase. In general, the configuration of the pore phase is complex; thus most studies on the effect of the porous phase on rock deformation are conducted numerically and theoretical analyses of the constitutive relations are scarce. We have studied rock deformation under axially-symmetric loading by analyzing a model where the pore phase is approximated by rough planes, randomly spaced and oriented, extending through the sample. The roughness is caused by asperities, all with the same tip radii, but having heights h with a probability density distribution given by the negative exponential e-h/λ where λ is a length parameter. Slip at contacts under local shear stress is resisted by simple Coulomb friction, with friction coefficient f. Both static and dynamic deformation were analyzed. The effect of porosity on deformation for both modes was found to be given by the non-dimensional parameter λαj, where αj is the total area of the fault planes per unit volume. We demonstrate that stress-induced microfracturing begins as randomly oriented microslip throughout the sample. As axial load increases, microslip occurs along preferred orientations and locations, which finally leads to deformation on a single fault. The model was found to fault under static loading conditions---the axial load at faulting and the angle of the “fracture” plane agree with values of those parameters given by Coulomb’s theory of fracture. Dynamic moduli and Poisson’s ratio are found to be virtually elastic and independent of the friction coefficient acting at contacts. The attenuation for uniaxial dynamic loading is a strong function of the friction coefficient and increases linearly with strain amplitude, in agreement with laboratory measurements.W.Z. was partially supported by the Department of Energy under grant #DEFGO200ER15058 and the National Science Foundation under grant NSF-OCE0221436 (WHOI). Financial support for J.B.W. was provided by NSF grants EAR-9903217 and EAR- 0336840

A comparison of computational and experimental lift-off heights of coflow laminar diffusion flames

As a sensitive marker of changes in flame structure, the number densities of excited-state CH (denoted CH*), and excited-state OH (denoted OH*) are imaged in coflow laminar diffusion flames. Measurements are made both in normal gravity and on the NASA KC-135 reduced-gravity aircraft. The spatial distribution of these radicals provides information about flame structure and lift-off heights that can be directly compared with computational predictions. Measurements and computations are compared over a range of buoyancy and fuel dilution levels. Results indicate that the lift-off heights and flame shapes predicted by the computations are in excellent agreement with measurement for both normal gravity (1g) and reduced gravity flames at low dilution levels. As the fuel mixture is increasingly diluted, however, the 1g lift-off heights become underpredicted. This trend continues until the computations predict stable flames at highly dilute fuel mixtures beyond the 1g experimental blow-off limit. To better understand this behavior, an analysis was performed, which indicates that the lift-off height is sensitive to the laminar flame speed of the corresponding premixed mixture at the flame edge. By varying the rates of two key "flame speed" controlling reactions, we were able to modify the predicted lift-off heights so as to be in closer agreement with the experiments. The results indicate that reaction sets that work well in low dilution systems may need to be modified to accommodate high dilution flames

Emergency Department and Radiological Cost of Delayed Diagnosis of Cannabinoid Hyperemesis.

Background. Chronic cannabis use has become prevalent with decriminalization, medical prescription, and recreational legalization in numerous US states. With this increasing incidence of chronic cannabis use a new clinical syndrome has become apparent in emergency departments and hospitals across the country, termed Cannabinoid Hyperemesis (CH). CH has been described as cyclical vomiting and abdominal pain in the setting of chronic cannabis use, which is often temporarily relieved by hot showers. CH presents a diagnostic challenge to clinicians who do not have a high clinical suspicion for the syndrome and can result in high costs and resource utilization for hospitals and patients. This study investigates the expenditures associated with delayed CH evaluation and delayed diagnosis. Methods. This is a retrospective observational study of 17 patients diagnosed with CH at three medical centers in the United States from 2010 to 2015, consisting of two academic centers and a community hospital. Emergency department (ED) costs were calculated and analyzed for patients eventually diagnosed with CH. Results. For the 17 patients treated, the total cost for combined ED visits and radiologic evaluations was an average of $76,920.92 per patient. On average these patients had 17.9 ED visits before the diagnosis of CH was made. Conclusion. CH provides a diagnostic challenge to clinicians without a high suspicion of the syndrome and may become increasingly prevalent with current trends toward cannabis legalization. The diagnosis of CH can be made primarily through a thorough history and physical examination. Awareness of this syndrome can save institutions money, prevent inappropriate utilization of healthcare resources, and save patients from unnecessary diagnostic tests

Sliding of a rough surface under oblique loading

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): B05208, doi:10.1029/2004JB003027.Sliding of a rough surface having a range of asperity heights is a gradual process, starting at contacts under relatively low normal shear load and spreading until the surface slides as a unit. We analyze this process theoretically for asperities with spherical tips, with heights having a probability density distribution given by a negative exponential. The case where applied normal traction increases concurrently with applied shear is treated in detail, resulting in analytical expressions for the normal and shear displacements. These results are used to show limitations on constitutive behavior for more complex normal stress-shear stress histories.J. B. Walsh was supported by grant EAR- 9903217 from the National Science Foundation. W. Zhu gratefully acknowledges the support of the Department of Energy through grant DE-FG02-00ER15058 and WHOI Mellon Independent Study Award (2000)

Physical and social environmental characteristics of physical activity for Mexican-origin children: examining differences between school year and summer perceptions

BACKGROUND: Colonias are substandard residential areas along the U.S.-Mexico border. Families of Mexican-origin living in colonias face health burdens characterized by environmental and socioeconomic hardships. Mexican Americans and low-income families, including colonias children, do not frequently participate in physical activity despite the known link to disease risk reduction. For colonias children, schools are the most commonly reported location for physical activity. School closures and extreme temperatures during summer months create a need to explore seasonal differences in environmental supports and barriers in this population. The purpose of this study was to examine the effect of seasonality on perceived environmental barriers, opportunities, and social support for physical activity among colonias children. As a secondary aim, mother-child discordance for each factor was analyzed. METHODS: Promotora-researchers recruited mother-child dyads (n=101 dyads, n=202 participants) from colonias in Hidalgo County, Texas. Mothers and children were separately administered surveys at two time points to capture perceived barriers, opportunities, and social support for physical activity (school-year: February-May; summertime: July-August). Summative scores for each outcome were calculated and three multilevel longitudinal models for continuous outcomes were examined; children were nested within households. Mother-child discordance was measured using Cohen’s Kappa statistic. RESULTS: Physical activity barriers and environmental opportunities (household and neighborhood) increased from school-year to summer by 1.16 and 2.83 points respectively (p≤0.01), after adjusting for covariates. Significant predictors of increased barriers included household income of >$900/month and having more household members. Children of mothers with significant others who were employed part-time or full-time saw significant decreases in barriers. Mother-child agreement of barriers, environmental opportunities, and social support across seasons was slight to fair (range: median κ=0.047 to κ=0.262). CONCLUSIONS: These results suggest a complex relationship between dimensions of economic hardship (employment status, household income, etc…) and perceived opportunities and barriers of children’s physical activity engagement during the school-year and summer. In this study, both barriers and opportunities increased from school-year to summer, further demonstrating that interactions among these characteristics need to be better understood and addressed when considering physical activity initiatives for colonias and other Mexican-American children, specifically during summer when school-based physical activity resources are unavailable

High Glucose-enhanced Acetylcholine Stimulated CGMP Masks Impaired Vascular Reactivity in Tail Arteries from Short-Term Hyperglycemic Rats

Impaired vascular endothelium-dependent relaxation and augmented contractile responses have been reported in several models of long-term hyperglycemia. However, the effects of short-term ambient hyperglycemia are poorly understood. Since oxidative stress has been implicated as a contributor to impaired vascular function, we investigated the following