Macroscopic liquid-state molecular hydrodynamics.

Experimental evidence and theoretical modeling suggest that piles of confined, high-restitution grains, subject to low-amplitude vibration, can serve as experimentally-accessible analogs for studying a range of liquid-state molecular hydrodynamic processes. Experiments expose single-grain and multiple-grain, collective dynamic features that mimic those either observed or predicted in molecular-scale, liquid state systems, including: (i) near-collision-time-scale hydrodynamic organization of single-molecule dynamics, (ii) nonequilibrium, long-time-scale excitation of collective/hydrodynamic modes, and (iii) long-time-scale emergence of continuum, viscous flow. In order to connect directly observable macroscale granular dynamics to inaccessible and/or indirectly measured molecular hydrodynamic processes, we recast traditional microscale equilibrium and nonequilibrium statistical mechanics for dense, interacting microscale systems into self-consistent, macroscale form. The proposed macroscopic models, which appear to be new with respect to granular physics, and which differ significantly from traditional kinetic-theory-based, macroscale statistical mechanics models, are used to rigorously derive the continuum equations governing viscous, liquid-like granular flow. The models allow physically-consistent interpretation and prediction of observed equilibrium and non-equilibrium, single-grain, and collective, multiple-grain dynamics.

Mercury release from dental amalgams into continuously replenished liquids.

OBJECTIVE: Studies have been performed using high- and low-copper amalgams to measure the amounts of mercury dissolution from dental amalgam in liquids such as artificial saliva; however, in most cases, mercury dissolution has been measured under static conditions and as such, may be self-limiting. This study measured the mercury release from low- and high-copper amalgams into flowing aqueous solutions to determine whether the total amounts of dissolution vary under these conditions when tested at neutral and acidic pH. METHODS: High- and low-copper amalgam specimens were prepared and kept for 3 days. They were then longitudinally suspended in dissolution cells with an outlet at the bottom. Deionized water or acidic solution (pH1) was pumped through the cell. Test solutions were collected at several time periods up to 6 days or 1 month and then analyzed with a cold vapor atomic absorption spectrophotometer. After dissolution testing, the specimens were examined using SEM/XEDA for any selective degradation of the phases in the amalgam. RESULTS: Except for the high-copper amalgam in the pH1 solution, the dissolution rates were found to decrease exponentially with time. The rate for the high-copper amalgam in pH1 solution slowly increased for 1 month. The total amounts (microgram/cm(2)) of mercury released over 6 days or 1 month from both types of amalgam in deionized water were not significantly different (p>/=0.05). The high-copper amalgam released significantly more mercury than the low-copper amalgam in the pH1 solution at both time periods. For both amalgams, the dissolution in pH1 was significantly higher than in deionized water. SIGNIFICANCE: Mercury dissolution from amalgam under dynamic conditions is enhanced in an acidic media, and most prominently for a high-copper formulation.

A theoretical model of circulatory interstitial fluid flow and species transport within porous cortical bone.

A three-dimensional model of interstitial fluid flow and passive species transport within mineralized regions surrounding cross-cortical vessel canals is developed. In contrast to earlier studies, the present model applies to circulatory, non-stress-induced interstitial flow in porous cortical bone. Based on previous experimental observations, the canals are modeled as line sources that pass at an oblique angle through the cortex. Cross-cortical interstitial flow from the endosteal surface to the periosteal surface is also taken into account. It is found that model transport characteristics are qualitatively consistent with reported observations. In addition, parametric studies reveal the following: (1) Solute contact with the matrix is maximized when the ratio of canal radius to cortex thickness (R) is near physiological R values. (2) Solute-matrix contact falls to low levels when R falls below the physiological range. (3) Solute-matrix contact is maximized when the cross-cortical velocity is approximately an order of magnitude smaller than the canal outflow velocity. The first and second findings suggest that within porous bone physiological ranges of R promote near optimal species contact with the mineralized matrix. The third finding suggests that relatively impermeable layers of bone within the cortex can effectively promote solute-matrix contact by limiting cross-cortical flow. Finally, the model suggests that intra-canal resorption associated with reduced external loading may effectively compensate for reduced stress-induced interstitial flow by enhancing circulatory interstitial flow and species transport.

In vivo creep and stress relaxation experiments to determine the wall extensibility and yield threshold for the sporangiophores of phycomyces.

The pressure probe was used to conduct in vivo creep and in vivo stress relaxation experiments on the sporangiophores of Phycomyces blakesleeanus. The in vivo creep and in vivo stress relaxation methods are compared with respect to their utility for determining the irreversible wall extensibility and the yield threshold. The results of the in vivo stress relaxation experiments demonstrate that the growth usually does not cease when the external water supply is removed, and the turgor pressure does not decay for hours afterwards. A successful stress relaxation experiment requires that the cell enlargement rate (growth rate) be zero during the turgor pressure decay. In a few experiments, the growth rate was zero during the turgor pressure decay. However, in general only the yield threshold could be determined.In vivo creep experiments proved to be easier to conduct and more useful in determining values for both the irreversible wall extensibility and the yield threshold. The results of the in vivo creep experiments demonstrate that small steps-up in turgor pressure, generally <0.02 MPa, elicit increases in growth rate as predicted by the growth equations and the augmented growth equations. The irreversible wall extensibility and the yield threshold were determined from these results. The results also demonstrate that steps-up in turgor pressure larger than 0.02 MPa, produce a different response; a decrease in growth rate. The decreased growth rate behavior is related to the magnitude of the step-up, and in general, larger steps-up in turgor pressure produce larger decreases in growth rate and longer periods of decreased growth rate. Qualitatively, this growth behavior is very similar to the "stretch response" previously reported by Dennison and Roth (1967).

Pressure probe technique to study transpiration in phycomyces sporangiophores.

The growth equation for the rate of water uptake is augmented with a transpiration term. The obtained augmented growth equations are used to develop methodology which employs the pressure probe to measure transpiration rates from single plant cells. Experiments are conducted on the sporangiophores of Phycomyces blakesleeanus to demonstrate this technique.