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

Collapse of cavitation bubbles in blood

The behaviour of a single bubble in blood and in water is studied by using a non-Newtonian model of spherical bubble dynamics. This model considers the compressibility of the liquid surrounding the bubble, the shear-thinning characteristic of liquid viscosity, liquid density and surface tension. It was found that, for values of the maximum bubble radius larger than 10^{-1}\un{mm}, the collapse of a bubble in a constant pressure field in blood is more violent than in water. It suggests that the amount of collateral damage of the biological tissue induced by bubble collapse during high-speed rotational angioplasty and laser-induced angioplasty can be underestimated by experiments in vitro using water as ambient liquid

Dynamics of shock waves and cavitation bubbles in bilinear elastic-plastic media, and the implications to short-pulsed laser surgery

The dynamics of shock waves and cavitation bubbles generated by short laser pulses in water and elastic-plastic media were investigated theoretically in order to get a better understanding of their role in short-pulsed laser surgery. Numerical simulations were performed using a spherical model of bubble dynamics which include the elastic-plastic behaviour of the medium surrounding the bubble, compressibility, viscosity, density and surface tension. Breakdown in water produces a monopolar acoustic signal characterized by a compressive wave. Breakdown in an elastic-plastic medium produces a bipolar acoustic signal, with a leading positive compression wave and a trailing negative tensile wave. The calculations revealed that consideration of the tissue elasticity is essential to describe the bipolar shape of the shock wave emitted during optical breakdown. The elastic-plastic response of the medium surrounding the bubble leads to a significant decrease of the maximum size of the cavitation bubble and pressure amplitude of the shock wave emitted during bubble collapse, and shortening of the oscillation period of the bubble. The results are discussed with respect to collateral damage in short-pulsed laser surgery

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