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David Halpern

Publications and source records attributed to David Halpern.

6 recordsLinked to original sources
PubMed2005

Linear stability of a surfactant-laden annular film in a time-periodic pressure-driven flow through a capillary.

This paper analyzes the effect of surfactant on the linear stability of an annular film in a capillary undergoing a time-periodic pressure gradient force. The annular film is thin compared to the radius of the tube. An asymptotic analysis yields a coupled set of equations with time-periodic coefficients for the perturbed fluid-fluid interface and the interfacial surfactant concentration. Wei and Rumschitzki (submitted for publication) previously showed that the interaction between a surfactant and a steady base flow could induce a more severe instability than a stationary base state. The present work demonstrates that time-periodic base flows can modify the features of the steady-flow-based instability, depending on surface tension, surfactant activity, and oscillatory frequency. For an oscillatory base flow (with zero mean), the growth rate decreases monotonically as the frequency increases. In the low-frequency limit, the growth rate approaches a maximum corresponding to the growth rate of a steady base flow having the same amplitude. In the high-frequency limit, the growth rate reaches a minimum corresponding to the growth rate in the limit of a stationary base state. The underlying mechanisms are explained in detail, and extension to other time-periodic forms is further exploited.

Journal Article↗
PubMed2005

Effect of inertia on the insoluble-surfactant instability of a shear flow.

We study, for the case of the two-layer plane Couette flow, the effects of inertia on the recently found instability due to insoluble surfactants. The insoluble-surfactant instability takes place even when inertia is absent provided an interface or a free surface under a nonzero shear is laden with an insoluble surfactant. Considering a normal mode of the streamwise wave number alpha , the perturbation theory we construct is good for any alpha provided the Reynolds number is correspondingly small. Inertia is responsible for some notable effects, including the appearance of new regions of instability and stability. For long--and only for long--waves, the following growth-rate additivity property for the inertia and interfacial instabilities holds: the growth rate corresponding to some nonzero values of the Marangoni number M and the Reynolds number Re is the sum of two contributions, one corresponding to the same value of M but zero Re, and the other corresponding to the same (nonzero) Re but zero M. This violation of the additivity property is in contrast to the case of a surfactantless Couette flow where this property holds for all wave numbers. Thus these results provide a counterexample to a conjecture that this additivity property is a universal principle. Among other results, when the thinner layer is the less viscous one, there is a nonzero critical Marangoni number Mc for the onset of instability; this (long-wave) threshold Mc grows from zero with the Reynolds number. Also, varying the ratio of viscosities through certain characteristic values leads to changes in the topology of marginal-stability curves.

Journal Article↗
PubMed2005

Comparative analysis of the roles of HtrA-like surface proteases in two virulent Staphylococcus aureus strains.

The HtrA surface protease is involved in the virulence of many pathogens, mainly by its role in stress resistance and bacterial survival. Staphylococcus aureus encodes two putative HtrA-like proteases, referred to as HtrA(1) and HtrA(2). To investigate the roles of HtrA proteins in S. aureus, we constructed htrA(1), htrA(2), and htrA(1) htrA(2) insertion mutants in two genetically different virulent strains, RN6390 and COL. In the RN6390 context, htrA(1) inactivation resulted in sensitivity to puromycin-induced stress. The RN6390 htrA(1) htrA(2) mutant was affected in the expression of several secreted virulence factors comprising the agr regulon. This observation was correlated with the disappearance of the agr RNA III transcript in the RN6390 htrA(1) htrA(2) mutant. The virulence of this mutant was diminished in a rat model of endocarditis. In the COL context, both HtrA(1) and HtrA(2) were essential for thermal stress survival. However, only HtrA(1) had a slight effect on exoprotein expression. The htrA mutations did not diminish the virulence of the COL strain in the rat model of endocarditis. Our results indicate that HtrA proteins have different roles in S. aureus according to the strain, probably depending on specific differences in the regulation of virulence factor and stress protein expression. We propose that HtrA(1) and HtrA(2) contribute to pathogenicity by controlling the production of certain extracellular factors that are crucial for bacterial dissemination, as revealed in the RN6390 background. We speculate that HtrA proteins act in the agr-dependent regulation pathway by assuring folding and/or maturation of some surface components of the agr system.

Animals↗
PubMed2004

rexAB mutants in Streptococcus pneumoniae.

Streptococcus pneumoniae is a human pathogen that is naturally transformable. In this study a major component of the homologous recombination pathway, the RexAB exonuclease/helicase, was characterized. rexA and rexB insertional mutants were constructed using mariner mutagenesis and found to have identical phenotypes. Both rexAB mutants displayed poor cell viability, reduced double-strand exonuclease activity, UV sensitivity and a reduced level of gene conversion compared to the wild-type strain. No effect was observed on plasmid and chromosomal transformation efficiencies. These results indicate that in S. pneumoniae, RexAB is required for DNA repair, but not for chromosomal transformation and plasmid establishment.

DNA Helicases↗
PubMedDate not supplied

Radiology-Pathology Conference: carcinosarcoma of the colon.

Carcinosarcomas are very uncommon tumors, which are comprised of both malignant epithelial and mesenchymal elements. They occur most commonly in the head and neck, respiratory tract, and female reproductive organs. In the gastrointestinal tract, they are most often found in the oropharynx, esophagus, and, to a lesser extent, in the stomach. Carcinosarcomas rarely originate from the colon, but when they do, they are extremely aggressive malignancies. We report the radiologic and pathologic findings of a patient with a carcinosarcoma believed to have arisen from the colon and which involved the adjacent mesentery and omentum.

Aged↗