Session C3: Fluid Dynamics

Retardation of Hydration Dynamics in the Bulk by Disaccharide Osmolytes

The bioprotective nature of disaccharides is hypothesized to come from the modification of the hydrogen bonding network of water which protects biomolecules through lowered water activity at the protein interface. Using ultrafast fluorescence spectroscopy we measured the relaxation of bulk water dynamics around the induced dipole moment of two fluorescent probes (Lucifer yellow ethylenediamine and Tryptophan). Our results indicate a reduction in bulk water reorganization rate of approximately of 30 -70{\%}. We report this retardation to be present in the low concentration regime measured at0.1M and 0.25 M, far below the onset of glassy dynamics. This reduction in water activity could be significant in crowded biological systems, contributing to global change in protein energy landscape, significantly enhancing the stability of proteins under thermal stress. We observed similar dynamic reduction for two disaccharide osmolytes, sucrose and trehalose, with trehalose being the more effective dynamic reducer.   

Theory of mud sediments at the bottom of the ocean

In a large fraction of the world's oceans, the bottom is covered by a layer of mud, which can be regarded as a fluid with unusual properties. In many cases, the density is actually less than that of water, and the density varies with depth below the interface. Theory currently under development is the card house model, where the tiny clay particles (shaped like platelets) in the mud carry a net electronic charge and repel each other when oriented face to face. Experiments suggest that, insofar as the propagation of sound within the mud is concerned, mud behaves as an anisotropic fluid, with the speed of sound in the horizontal direction being greater than that in the vertical direction. Paper attempts to explain this using statistical mechanics and also explains the anomalous density gradients at the water-mud interface. With gravity taken into account, the equilibrium separation distance between two parallel vertically aligned platelets is one to two orders of magnitude greater than a typical length scale of the face of a platelet. When platelets touch, edge to face, there is an attractive force between platelets, and the net effect is that the platelets tend to be separated at a much shorter distance than the stand-off distance deep within the sediment.   

Direct Statistical Simulation: Ensemble Averaging

Low­-order statistics of geophysical fluid models may be directly accessed by solving the equations of motion for the equal-­time cumulants. We investigate a variant of the second ­order cumulant expansion (CE2) in which zonal averaging is replaced by ensemble averaging. The approach is tested on two different highly idealized models of planetary atmospheres on a spherical geodesic grid: A stochastically-forced barotropic jet, and a deterministic jet relaxed toward an unstable profile. The results are compared to the traditional approach of accumulating statistics via numerical simulation, and to zonally-averaged CE2.   

Studies of pH in Attoliter-Volume Droplets of Water

The applications of attoliter-volume droplets suspended in perfluorinated liquids are numerous, for instance, in microfluidics, single molecule biophysics, and the pharmaceutical industry. As we examine droplets of smaller volumes, the effect of the water/perfluorinate interface on droplet pH becomes more pronounced. We studied the attoliter environment using the pH-sensing fluorescent dye fluorescein. Droplet preparation was done by ultrasonication or extrusion with buffer and perfluorinated oil (FC40 or FC77). The emulsions were stabilized with a non-ionic surfactant. By varying the buffer strength, ionic strength and pH of the aqueous phase, the pH of the resulting confined phase, measured in a fluorimeter, was compared to the pH of the original stock buffer. Preliminary data would suggest that the concentration of non-ionic surfactant in the emulsion affects the measured pH of the droplets. For droplets made by ultrasonication with low surfactant concentration, droplet pH is significantly lower than that of the stock buffer.