Very intense pulse in the groundwater flow in fissurized-porous stratum

An asymptotic solution is obtained corresponding to a very intense pulse: a sudden strong increase and fast subsequent decrease of the water level at the boundary of semi-infinite fissurized-porous stratum. This flow is of practical interest: it gives a model of a groundwater flow after a high water period or after a failure of a dam around a collector of liquid waste. It is demonstrated that the fissures have a dramatic influence on the groundwater flow, increasing the penetration depth and speed of fluid penetration into the stratum. A characteristic property of the flow in fissurized-porous stratum is the rapid breakthrough of the fluid at the first stage deeply into the stratum via a system of cracks, feeding of porous blocks by the fluid in cracks, and at a later stage feeding of advancing fluid flow in fissures by the fluid, accumulated in porous blocks.

Water does not flow across the tight junctions of MDCK cell epithelium

Although it has been known for decades that the tight junctions of fluid-transporting epithelia are leaky to ions, it has not been possible to determine directly whether significant transjunctional water movement also occurs. An optical microscopic technique was developed for the direct visualization of the flow velocity profiles within the lateral intercellular spaces of a fluid-absorptive, cultured renal epithelium (MDCK) and used to determine the velocity of the fluid flow across the tight junction. The flow velocity within the lateral intercellular spaces fell to near zero adjacent to the tight junction, showing that significant transjunctional flow did not occur, even when transepithelial fluid movement was augmented by imposition of osmotic gradients.

The mechanical properties of Saccharomyces cerevisiae

Cell-wall mechanical properties play an integral part in the growth and form of Saccharomyces cerevisiae. In contrast to the tremendous knowledge on the genetics of S. cerevisiae, almost nothing is known about its mechanical properties. We have developed a micromanipulation technique to measure the force required to burst single cells and have recently established a mathematical model to extract the mechanical properties of the cell wall from such data. Here we determine the average surface modulus of the S. cerevisiae cell wall to be 11.1 ± 0.6 N/m and 12.9 ± 0.7 N/m in exponential and stationary phases, respectively, giving corresponding Young's moduli of 112 ± 6 MPa and 107 ± 6 MPa. This result demonstrates that yeast cell populations strengthen as they enter stationary phase by increasing wall thickness and hence the surface modulus, without altering the average elastic properties of the cell-wall material. We also determined the average breaking strain of the cell wall to be 82% ± 3% in exponential phase and 80% ± 3% in stationary phase. This finding provides a failure criterion that can be used to predict when applied stresses (e.g., because of fluid flow) will lead to wall rupture. This work analyzes yeast compression experiments in different growth phases by using engineering methodology.

A novel microbial habitat in the mid-ocean ridge subseafloor

The subseafloor at the mid-ocean ridge is predicted to be an excellent microbial habitat, because there is abundant space, fluid flow, and geochemical energy in the porous, hydrothermally influenced oceanic crust. These characteristics also make it a good analog for potential subsurface extraterrestrial habitats. Subseafloor environments created by the mixing of hot hydrothermal fluids and seawater are predicted to be particularly energy-rich, and hyperthermophilic microorganisms that broadly reflect such predictions are ejected from these systems in low-temperature (≈15°C), basalt-hosted diffuse effluents. Seven hyperthermophilic heterotrophs isolated from low-temperature diffuse fluids exiting the basaltic crust in and near two hydrothermal vent fields on the Endeavour Segment, Juan de Fuca Ridge, were compared phylogenetically and physiologically to six similarly enriched hyperthermophiles from samples associated with seafloor metal sulfide structures. The 13 organisms fell into four distinct groups: one group of two organisms corresponding to the genus Pyrococcus and three groups corresponding to the genus Thermococcus. Of these three groups, one was composed solely of sulfide-derived organisms, and the other two related groups were composed of subseafloor organisms. There was no evidence of restricted exchange of organisms between sulfide and subseafloor habitats, and therefore this phylogenetic distinction indicates a selective force operating between the two habitats. Hypotheses regarding the habitat differences were generated through comparison of the physiology of the two groups of hyperthermophiles; some potential differences between these habitats include fluid flow stability, metal ion concentrations, and sources of complex organic matter.

Using three-dimensional microfluidic networks for solving computationally hard problems

This paper describes the design of a parallel algorithm that uses moving fluids in a three-dimensional microfluidic system to solve a nondeterministically polynomial complete problem (the maximal clique problem) in polynomial time. This algorithm relies on (i) parallel fabrication of the microfluidic system, (ii) parallel searching of all potential solutions by using fluid flow, and (iii) parallel optical readout of all solutions. This algorithm was implemented to solve the maximal clique problem for a simple graph with six vertices. The successful implementation of this algorithm to compute solutions for small-size graphs with fluids in microchannels is not useful, per se, but does suggest broader application for microfluidics in computation and control.

Illite and hydrocarbon exploration

Illite is a general term for the dioctahedral mica-like clay mineral common in sedimentary rocks, especially shales. Illite is of interest to the petroleum industry because it can provide a K-Ar isotope date that constrains the timing of basin heating events. It is critical to establish that hydrocarbon formation and migration occurred after the formation of the trap (anticline, etc.) that is to hold the oil. Illite also may precipitate in the pores of sandstone reservoirs, impeding fluid flow. Illite in shales is a mixture of detrital mica and its weathering products with diagenetic illite formed by reaction with pore fluids during burial. K-Ar ages are apparent ages of mixtures of detrital and diagenetic end members, and what we need are the ages of the end members themselves. This paper describes a methodology, based on mineralogy and crystallography, for interpreting the K-Ar ages from illites in sedimentary rocks and for estimating the ages of the end members.

What electrical measurements can say about changes in fault systems.

Earthquake zones in the upper crust are usually more conductive than the surrounding rocks, and electrical geophysical measurements can be used to map these zones. Magnetotelluric (MT) measurements across fault zones that are parallel to the coast and not too far away can also give some important information about the lower crustal zone. This is because the long-period electric currents coming from the ocean gradually leak into the mantle, but the lower crust is usually very resistive and very little leakage takes place. If a lower crustal zone is less resistive it will be a leakage zone, and this can be seen because the MT phase will change as the ocean currents leave the upper crust. The San Andreas Fault is parallel to the ocean boundary and close enough to have a lot of extra ocean currents crossing the zone. The Loma Prieta zone, after the earthquake, showed a lot of ocean electric current leakage, suggesting that the lower crust under the fault zone was much more conductive than normal. It is hard to believe that water, which is responsible for the conductivity, had time to get into the lower crustal zone, so it was probably always there, but not well connected. If this is true, then the poorly connected water would be at a pressure close to the rock pressure, and it may play a role in modifying the fluid pressure in the upper crust fault zone. We also have telluric measurements across the San Andreas Fault near Palmdale from 1979 to 1990, and beginning in 1985 we saw changes in the telluric signals on the fault zone and east of the fault zone compared with the signals west of the fault zone. These measurements were probably seeing a better connection of the lower crust fluids taking place, and this may result in a fluid flow from the lower crust to the upper crust. This could be a factor in changing the strength of the upper crust fault zone.

Vascular MADs: Two novel MAD-related genes selectively inducible by flow in human vascular endothelium

Vascular endothelium is an important transducer and integrator of both humoral and biomechanical stimuli within the cardiovascular system. Utilizing a differential display approach, we have identified two genes, Smad6 and Smad7, encoding members of the MAD-related family of molecules, selectively induced in cultured human vascular endothelial cells by steady laminar shear stress, a physiologic fluid mechanical stimulus. MAD-related proteins are a recently identified family of intracellular proteins that are thought to be essential components in the signaling pathways of the serine/threonine kinase receptors of the transforming growth factor β superfamily. Smad6 and Smad7 possess unique structural features (compared with previously described MADs), and they can physically interact with each other, and, in the case of Smad6, with other known human MAD species, in endothelial cells. Transient expression of Smad6 or Smad7 in vascular endothelial cells inhibits the activation of a transfected reporter gene in response to both TGF-β and fluid mechanical stimulation. Both Smad6 and Smad7 exhibit a selective pattern of expression in human vascular endothelium in vivo as detected by immunohistochemistry and in situ hybridization. Thus, Smad6 and Smad7 constitute a novel class of MAD-related proteins, termed vascular MADs, that are induced by fluid mechanical forces and can modulate gene expression in response to both humoral and biomechanical stimulation in vascular endothelium.

The leukocyte response to fluid stress

Leukocyte migration from a hemopoietic pool across marrow endothelium requires active pseudopod formation and adhesion. Leukocytes rarely show pseudopod formation while in circulation. At question then is the mechanism that serves to minimize leukocyte pseudopod formation in the circulation. We tested the hypothesis that fluid shear stress acts to prevent pseudopod formation. When individual human leukocytes (neutrophils, monocytes) spreading on glass surfaces in vitro were subjected to fluid shear stress (≈1 dyn/cm2), an instantaneous retraction of pseudopods was observed. Removal of the fluid shear stress in turn led to the return of pseudopod projection and cell spreading. When steady shear stress was prolonged over several minutes, leukocyte swelling occurs together with an enhanced random motion of cytoplasmic granules and a reduction of cytoplasmic stiffness. The response to shear stress could be suppressed by K+ channel blockers and chelation of external Ca2+. In rat mesentery microvessels after occlusion, circulating leukocytes project pseudopods in free suspension or when attached to the endothelium, even though immediately after occlusion only few pseudopods were present. When flow was restored, pseudopods on adhering leukocytes were retracted and then the cells began to roll and detach from the endothelium. In conclusion, plasma shear stress in the circulation serves to reduce pseudopod projection and adhesion of circulating leukocytes and vice versa reduction of shear stress leads to pseudopod projection and spreading of leukocytes on the endothelium.

Behavior of several two-dimensional fluid equations in singular scenarios

We give conditions that rule out formation of sharp fronts for certain two-dimensional incompressible flows. We show that a necessary condition of having a sharp front is that the flow has to have uncontrolled velocity growth. In the case of the quasi-geostrophic equation and two-dimensional Euler equation, we obtain estimates on the formation of semi-uniform fronts.

Fluid shear stress inhibits TNF-α activation of JNK but not ERK1/2 or p38 in human umbilical vein endothelial cells: Inhibitory crosstalk among MAPK family members

Atherosclerosis preferentially occurs in areas of turbulent flow and low fluid shear stress, whereas laminar flow and high shear stress are atheroprotective. Inflammatory cytokines, such as tumor necrosis factor-α (TNF-α) and IL-1 stimulate expression of endothelial cell (EC) genes that may promote atherosclerosis. TNF-α and IL-1 regulate gene expression in ECs, in part, by stimulating mitogen-activated protein kinases (MAPK), which phosphorylate transcription factors. We hypothesized that steady laminar flow inhibits cytokine-mediated activation of MAPK in EC. To test this hypothesis, we determined the effects of flow (shear stress = 12 dynes/cm2) on TNF-α and IL-1-stimulated activity of three MAPK in human umbilical vein ECs (HUVEC): extracellular signal-regulated kinase (ERK1/2), p38, and c-Jun N-terminal kinase (JNK). Flow alone stimulated ERK1/2 and p38 activity but decreased JNK activity compared with static controls. TNF-α or IL-1 alone activated ERK1/2, p38, and JNK maximally at 15 min in HUVEC. Preexposing HUVEC for 10 min to flow inhibited TNF-α and IL-1 activation of JNK by 46% and 49%, respectively, but had no significant effect on ERK1/2 or p38 activation. Incubation of HUVEC with PD98059, which inhibits flow-mediated ERK1/2 activation, prevented flow from inhibiting cytokine activation of JNK. Phorbol 12-myristate 13-acetate, which strongly activates ERK1/2, also inhibited TNF-α activation of JNK. These findings indicate that fluid shear stress inhibits TNF-α-mediated signaling events in HUVEC via the activation of the ERK1/2 signaling pathway. Inhibition of TNF-α signal transduction represents a mechanism by which steady laminar flow may exert atheroprotective effects on the endothelium.

Nitric oxide mediates the increase in local cerebral blood flow during focal seizures.

The role of nitric oxide (NO) in the increase in local cerebral blood flow (LCBF) elicited by focal cortical epileptic seizures was investigated in anesthetized adult rats. Seizures were induced by topical bicuculline methiodide applied through two cranial windows drilled over homotopic sites of the frontal cortex, and LCBF was measured by quantitative autoradiography by using 4-iodo[N-methyl-14C]antipyrine. Superfusion of an inhibitor of NO synthase, N omega-nitro-L-arginine (NA; 1 mM), for 45 min abolished the increase of LCBF induced by topical bicuculline methiodide (10 mM) [164 +/- 18 ml/100 g per min in the artificial cerebrospinal fluid (aCSF)-superfused side and 104 +/- 12 ml/100 g per ml in the NA-superfused side; P < 0.005]. This effect was reversed by coapplication of an excess of L-arginine substrate (10 mM) (218 +/- 22 ml/100 g per min in the aCSF-superfused side and 183 +/- 31 ml/100 g per min in the NA + L-Arg-superfused side) but not by 10 mM D-arginine, a stereoisomer with poor affinity for NO synthase (193 +/- 17 ml/100 g per min in the aCSF-superfused side and 139 +/- 21 ml/100 g per min in the NA + D-Arg-superfused side; P < 0.005). Superfusion of the guanylyl cyclase inhibitor methylene blue attenuated the LCBF increase elicited by topical bicuculline methiodide by 25% +/- 16% (P < 0.05). The present findings suggest that NO is the mediator of the vasodilation in response to focal epileptic seizures.