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We present a measurement of molecular orientation of water at charged surfactant aqueous interfaces as a function of surface charge density. The polarization dependent spectral line shapes of the water bend mode were measured by vibrational sum-frequency generation at the positively charged surfactant cetyltrimethylammonium bromide (CTAB)/water interface and negatively charged surfactant sodium dodecyl sulfate/water interface. Orientational analysis using the water bend mode as a vibrational probe, within the electric dipole approximation, reveals structural differences between these surfaces and quantifies how different hydrogen bonded species re-orient around the surfactant head groups as the surface charge density changes. As the concentration of the positively charged surfactant (CTAB) increases, the surface water molecules with free-OH groups reorient their hydrogen away from the bulk water and the C2v axis closer to the surface normal. This suggests that these free-OH molecules are in general located above the positively charged head groups of CTAB, and thus, the charge-dipole interaction pulls their oxygen "down" and pushes hydrogen "up." On the contrary, water molecules with two donor hydrogen bonds re-orient their hydrogen toward the bulk water, likely because most of these molecules are below the CTAB surfactant head groups. [ABSTRACT FROM AUTHOR]

Laser ablation of copper and silver targets immersed in bulk normal and superfluid 4He was studied through time-resolved shadowgraph photography. In normal fluid, only a sub-millimeter cavitation bubble is created and immediate formation of metal clusters is observed within a few hundred microseconds. The metal clusters remain spatially tightly focused up to 15 ms, and it is proposed that this observation may find applications in particle image velocimetry. In superfluid helium, the cavitation bubble formation process is distinctly different from the normal fluid. Due to the high thermal conductivity and an apparent lag in the breakdown of superfluidity, about 20% of the laser pulse energy was transferred directly into the liquid and a large gas bubble, up to several millimeters depending on laser pulse energy, is created. The internal temperature of the gas bubble is estimated to exceed 9 K and the following bubble cool down period therefore includes two separate phase transitions: gas-normal liquid and normal liquid-superfluid. The last stage of the cool down process was assigned to the superfluid lambda transition where a sudden formation of large metal clusters is observed. This is attributed to high vorticity created in the volume where the gas bubble previously resided. As shown by theoretical bosonic density functional theory calculations, quantized vortices can trap atoms and dimers efficiently, exhibiting static binding energies up to 22 K. This, combined with hydrodynamic Bernoulli attraction, yields total binding energies as high as 35 K. For larger clusters, the static binding energy increases as a function of the volume occupied in the liquid to minimize the surface tension energy. For heliophobic species an energy barrier develops as a function of the cluster size, whereas heliophilics show barrierless entry into vortices. The present theoretical and experimental observations are used to rationalize the previously reported metal nanowire assembly in both superfluid bulk liquid helium and helium droplets, both of which share the common element of a rapid passage through the lambda point. The origin of vorticity is tentatively assigned to the Zurek-Kibble mechanism. Implications of the large gas bubble formation by laser ablation to previous experiments aimed at implanting atomic and dimeric species in bulk superfluid helium are also discussed, and it is proposed that the developed visualization method should be used as a diagnostic tool in such experiments to avoid measurements in dense gaseous environments. [ABSTRACT FROM AUTHOR]