Non-Ergodic Nuclear Depolarization in Nano-Cavities

Recently, it has been observed that the effective dipolar interactions between nuclear spins of spin-carrying molecules of a gas in a closed nano-cavities are independent of the spacing between all spins. We derive exact time-dependent polarization for all spins in spin-1/2 ensemble with spatially independent effective dipolar interactions. If the initial polarization is on a single (first) spin,$P_1(0)= 1$ then the exact spin dynamics of the model is shown to exhibit a periodical short pulses of the polarization of the first spin, the effect being typical of the systems having a large number, $N$, of spins. If $N \gg 1$, then within the period $4\pi/g$ ($2\pi/g$) for odd (even) $N$-spin clusters, with $g$ standing for spin coupling, the polarization of spin 1 switches quickly from unity to the time independent value, 1/3, over the time interval about $(g\sqrt{N})^{-1}$, thus, almost all the time, the spin 1 spends in the time independent condition $P_1(t)= 1/3$. The period and the width of the pulses determine the volume and the form-factor of the ellipsoidal cavity. The formalism is adopted to the case of time varying nano-fluctuations of the volume of the cavitation nano-bubbles. If the volume $V(t)$ is varied by the Gaussian-in-time random noise then the envelope of the polarization peaks goes irreversibly to 1/3. The polarization dynamics of the single spin exhibits the Gaussian (or exponential) time dependence when the correlation time of the fluctuations of the nano-volume is larger (or smaller) than the $<(\delta g)^2 >^{-1/2}$, where the $$ is the variance of the $g(V(t))$ coupling. Finally, we report the exact calculations of the NMR line shape for the $N$-spin gaseous aggregate.Comment: 26 pages, 3 figure

Cavitation erosion in polymer aqueous solutions

We report the results of experiments designed to test the hypothesis that the enhanced levels of extensional viscosity conferred upon a liquid due to a polymer additive substantially mitigate cavitation damage, in addition to substantially increasing the liquid's cavitation threshold stress. As far as we are aware, these issues have never been directly addressed in a single investigation, involving samples of the same polymer system, in complementary experiments expressly designed for these purposes. The cavitation thresholds of aqueous PAA solutions are measured under dynamic stressing by pulses of tension and cavitation erosion experiments involving solid target specimens are also reported. The cavitation threshold of the solutions is found to be substantially enhanced by the presence of the polymer and the damage patterns recorded by scanning electron microscopy after 80 min exposure to cavitation in polymer solutions differ significantly from those in water. Whereas in water the surface presents heavily eroded areas with deep pitting cavities, in the 1% PAA solution the damage appears only in the form of individual craters that accumulate along specific lines and large undamaged areas (a stringy damage pattern). The weight loss decreases with increasing the polymer concentration and is one order of magnitude smaller in the 1% PAA solution than in the case of water. The present results suggest that the reduction of the maximum pressure inside the bubble at its minimum volume upon addition of polymer is the dominant mechanism of the observed suppression of cavitation damage in polymer solutions. The implications of the results are discussed with respect to the reduction of collateral damage in ultrasound phacoemulsification

Characterization of the interaction of two oscillating bubbles near a thin elastic membrane

10.1007/s00348-012-1389-yExperiments in Fluids5361723-1735EXFL