Fluctuations in viscous fingering

Our experiments on viscous (Saffman-Taylor) fingering in Hele-Shaw channels reveal finger width fluctuations that were not observed in previous experiments, which had lower aspect ratios and higher capillary numbers Ca. These fluctuations intermittently narrow the finger from its expected width. The magnitude of these fluctuations is described by a power law, Ca^{-0.64}, which holds for all aspect ratios studied up to the onset of tip instabilities. Further, for large aspect ratios, the mean finger width exhibits a maximum as Ca is decreased instead of the predicted monotonic increase.Comment: Revised introduction, smoothed transitions in paper body, and added a few additional minor results. (Figures unchanged.) 4 pages, 3 figures. Submitted to PRE Rapi

Analytical approach to viscous fingering in a cylindrical Hele-Shaw cell

We report analytical results for the development of the viscous fingering instability in a cylindrical Hele-Shaw cell of radius a and thickness b. We derive a generalized version of Darcy's law in such cylindrical background, and find it recovers the usual Darcy's law for flow in flat, rectangular cells, with corrections of higher order in b/a. We focus our interest on the influence of cell's radius of curvature on the instability characteristics. Linear and slightly nonlinear flow regimes are studied through a mode-coupling analysis. Our analytical results reveal that linear growth rates and finger competition are inhibited for increasingly larger radius of curvature. The absence of tip-splitting events in cylindrical cells is also discussed.Comment: 14 pages, 3 ps figures, Revte

Parallel flow in Hele-Shaw cells with ferrofluids

Parallel flow in a Hele-Shaw cell occurs when two immiscible liquids flow with relative velocity parallel to the interface between them. The interface is unstable due to a Kelvin-Helmholtz type of instability in which fluid flow couples with inertial effects to cause an initial small perturbation to grow. Large amplitude disturbances form stable solitons. We consider the effects of applied magnetic fields when one of the two fluids is a ferrofluid. The dispersion relation governing mode growth is modified so that the magnetic field can destabilize the interface even in the absence of inertial effects. However, the magnetic field does not affect the speed of wave propagation for a given wavenumber. We note that the magnetic field creates an effective interaction between the solitons.Comment: 12 pages, Revtex, 2 figures, revised version (minor changes

Rotating Hele-Shaw cells with ferrofluids

We investigate the flow of two immiscible, viscous fluids in a rotating Hele-Shaw cell, when one of the fluids is a ferrofluid and an external magnetic field is applied. The interplay between centrifugal and magnetic forces in determining the instability of the fluid-fluid interface is analyzed. The linear stability analysis of the problem shows that a non-uniform, azimuthal magnetic field, applied tangential to the cell, tends to stabilize the interface. We verify that maximum growth rate selection of initial patterns is influenced by the applied field, which tends to decrease the number of interface ripples. We contrast these results with the situation in which a uniform magnetic field is applied normally to the plane defined by the rotating Hele-Shaw cell.Comment: 12 pages, 3 ps figures, RevTe

A New Class of Nonsingular Exact Solutions for Laplacian Pattern Formation

We present a new class of exact solutions for the so-called {\it Laplacian Growth Equation} describing the zero-surface-tension limit of a variety of 2D pattern formation problems. Contrary to common belief, we prove that these solutions are free of finite-time singularities (cusps) for quite general initial conditions and may well describe real fingering instabilities. At long times the interface consists of N separated moving Saffman-Taylor fingers, with ``stagnation points'' in between, in agreement with numerous observations. This evolution resembles the N-soliton solution of classical integrable PDE's.Comment: LaTeX, uuencoded postscript file

Role of the Laboratory in Ensuring Global Access to ARV Treatment for HIV-Infected Children: Consensus Statement on the Performance of Laboratory Assays for Early Infant Diagnosis

A two day meeting hosted by the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC) was held in May 2006 in Entebbe, Uganda to review the laboratory performance of virologic molecular methods, particularly the Roche Amplicor DNA PCR version 1.5 assay, in the diagnosis of HIV-1 infection in infants. The meeting was attended by approximately 60 participants from 17 countries. Data on the performance and limitations of the HIV-1 DNA PCR assay from 9 African countries with high-burdens of HIV/AIDS were shared with respect to different settings and HIV- subtypes. A consensus statement on the use of the assay for early infant diagnosis was developed and areas of needed operational research were identified. In addition, consensus was reached on the usefulness of dried blood spot (DBS) specimens in childhood as a means for ensuring greater accessibility to serologic and virologic HIV testing for the paediatric population

The Saffman-Taylor problem on a sphere

The Saffman-Taylor problem addresses the morphological instability of an interface separating two immiscible, viscous fluids when they move in a narrow gap between two flat parallel plates (Hele-Shaw cell). In this work, we extend the classic Saffman-Taylor situation, by considering the flow between two curved, closely spaced, concentric spheres (spherical Hele-Shaw cell). We derive the mode-coupling differential equation for the interface perturbation amplitudes and study both linear and nonlinear flow regimes. The effect of the spherical cell (positive) spatial curvature on the shape of the interfacial patterns is investigated. We show that stability properties of the fluid-fluid interface are sensitive to the curvature of the surface. In particular, it is found that positive spatial curvature inhibits finger tip-splitting. Hele-Shaw flow on weakly negative, curved surfaces is briefly discussed.Comment: 26 pages, 4 figures, RevTex, accepted for publication in Phys. Rev.

Can biomedical and traditional health care providers work together? Zambian practitioners' experiences and attitudes towards collaboration in relation to STIs and HIV/AIDS care: a cross-sectional study

BACKGROUND: The World Health Organization's World health report 2006: Working together for health underscores the importance of human resources for health. The shortage of trained health professionals is among the main obstacles to strengthening low-income countries' health systems and to scaling up HIV/AIDS control efforts. Traditional health practitioners are increasingly depicted as key resources to HIV/AIDS prevention and care. An appropriate and effective response to the HIV/AIDS crisis requires reconsideration of the collaboration between traditional and biomedical health providers (THPs and BHPs). The aim of this paper is to explore biomedical and traditional health practitioners' experiences of and attitudes towards collaboration and to identify obstacles and potential opportunities for them to collaborate regarding care for patients with sexually transmitted infections (STIs) and HIV/AIDS. METHODS: We conducted a cross-sectional study in two Zambian urban sites, using structured questionnaires. We interviewed 152 biomedical health practitioners (BHPs) and 144 traditional health practitioners (THPs) who reported attending to patients with STIs and HIV/AIDS. RESULTS: The study showed a very low level of experience of collaboration, predominated by BHPs training THPs (mostly traditional birth attendants) on issues of safe delivery. Intersectoral contacts addressing STIs and HIV/AIDS care issues were less common. However, both groups of providers overwhelmingly acknowledged the potential role of THPs in the fight against HIV/AIDS. Obstacles to collaboration were identified at the policy level in terms of legislation and logistics. Lack of trust in THPs by individual BHPs was also found to inhibit collaboration. Nevertheless, as many as 40% of BHPs expressed an interest in working more closely with THPs. CONCLUSION: There is indication that practitioners from both sectors seem willing to strengthen collaboration with each other. However, there are missed opportunities. The lack of collaborative framework integrating maternal health with STIs and HIV/AIDS care is at odds with the needed comprehensive approach to HIV/AIDS control. Also, considering the current human resources crisis in Zambia, substantial policy commitment is called for to address the legislative obstacles and the stigma reported by THPs and to provide an adequate distribution of roles between all partners, including traditional health practitioners, in the struggle against HIV/AIDS

Planning for Sustainability in Small Municipalities: The Influence of Interest Groups, Growth Patterns, and Institutional Characteristics

How and why small municipalities promote sustainability through planning efforts is poorly understood. We analyzed ordinances in 451 Maine municipalities and tested theories of policy adoption using regression analysis.We found that smaller communities do adopt programs that contribute to sustainability relevant to their scale and context. In line with the political market theory, we found that municipalities with strong environmental interests, higher growth, and more formal governments were more likely to adopt these policies. Consideration of context and capacity in planning for sustainability will help planners better identify and benefit from collaboration, training, and outreach opportunities

Mechanisms of congenital heart disease caused by NAA15 haploinsufficiency

Rationale: NAA15 is a component of the N-terminal (Nt) acetyltransferase complex, NatA. The mechanism by which NAA15 haploinsufficiency causes congenital heart disease (CHD) remains unknown. To better understand molecular processes by which NAA15 haploinsufficiency perturbs cardiac development, we introduced NAA15 variants into human induced pluripotent stem cells (iPSCs) and assessed the consequences of these mutations on RNA and protein expression. Objective: We aim to understand the role of NAA15 haploinsufficiency in cardiac development by investigating proteomic effects on NatA complex activity, and identifying proteins dependent upon a full amount of NAA15. Methods and Results: We introduced heterozygous LoF, compound heterozygous and missense residues (R276W) in iPS cells using CRISPR/Cas9. Haploinsufficient NAA15 iPS cells differentiate into cardiomyocytes, unlike NAA15-null iPS cells, presumably due to altered composition of NatA. Mass spectrometry (MS) analyses reveal ~80% of identified iPS cell NatA targeted proteins displayed partial or complete Nt-acetylation. Between null and haploinsufficient NAA15 cells Nt-acetylation levels of 32 and 9 NatA-specific targeted proteins were reduced, respectively. Similar acetylation loss in few proteins occurred in NAA15 R276W iPSCs. In addition, steady-state protein levels of 562 proteins were altered in both null and haploinsufficient NAA15 cells; eighteen were ribosomal-associated proteins. At least four proteins were encoded by genes known to cause autosomal dominant CHD. Conclusions: These studies define a set of human proteins that requires a full NAA15 complement for normal synthesis and development. A 50% reduction in the amount of NAA15 alters levels of at least 562 proteins and Nt-acetylation of only 9 proteins. One or more modulated proteins are likely responsible for NAA15-haploinsufficiency mediated CHD. Additionally, genetically engineered iPS cells provide a platform for evaluating the consequences of amino acid sequence variants of unknown significance on NAA15 function