Characterization of periodic cavitation in an optical tweezer

Microscopic vapor explosions or cavitation bubbles can be generated periodically in an optical tweezer with a microparticle that partially absorbs at the trapping laser wavelength. In this work we measure the size distribution and the production rate of cavitation bubbles for microparticles with a diameter of 3 $\mu$m using high speed video recording and a fast photodiode. We find that there is a lower bound for the maximum bubble radius $R_{max}\sim 2~\mu$m which can be explained in terms of the microparticle size. More than $94 \%$ of the measured $R_{max}$ are in the range between 2 and 6 $\mu$m, while the same percentage of the measured individual frequencies $f_i$ or production rates are between 10 and 200 Hz. The photodiode signal yields an upper bound for the lifetime of the bubbles, which is at most twice the value predicted by the Rayleigh equation. We also report empirical relations between $R_{max}$, $f_i$ and the bubble lifetimes.Comment: 5 pages, 3 figure

Single-bubble and multi-bubble cavitation in water triggered by laser-driven focusing shock waves

In this study a single laser pulse spatially shaped into a ring is focused into a thin water layer, creating an annular cavitation bubble and cylindrical shock waves: an outer shock that diverges away from the excitation laser ring and an inner shock that focuses towards the center. A few nanoseconds after the converging shock reaches the focus and diverges away from the center, a single bubble nucleates at the center. The inner diverging shock then reaches the surface of the annular laser-induced bubble and reflects at the boundary, initiating nucleation of a tertiary bubble cloud. In the present experiments, we have performed time-resolved imaging of shock propagation and bubble wall motion. Our experimental observations of single-bubble cavitation and collapse and appearance of ring-shaped bubble clouds are consistent with our numerical simulations that solve a one dimensional Euler equation in cylindrical coordinates. The numerical results agree qualitatively with the experimental observations of the appearance and growth of bubble clouds at the smallest laser excitation rings. Our technique of shock-driven bubble cavitation opens novel perspectives for the investigation of shock-induced single-bubble or multi-bubble cavitation phenomena in thin liquids

Birth and growth of cavitation bubbles within water under tension confined in a simple synthetic tree

Water under tension, as can be found in several systems including tree vessels, is metastable. Cavitation can spontaneously occur, nucleating a bubble. We investigate the dynamics of spon- taneous or triggered cavitation inside water filled microcavities of a hydrogel. Results show that a stable bubble is created in only a microsecond timescale, after transient oscillations. Then, a diffusion driven expansion leads to filling of the cavity. Analysis reveals that the nucleation of a bubble releases a tension of several tens of MPa, and a simple model captures the different time scales of the expansion process

Isotopic difference in the heteronuclear loss rate in a two-species surface trap

We have realized a two-species mirror-magneto-optical trap containing a mixture of $^{87}$Rb ($^{85}$Rb) and $^{133}$Cs atoms. Using this trap, we have measured the heteronuclear collisional loss rate $\beta_{Rb-Cs}'$ due to intra-species cold collisions. We find a distinct difference in the magnitude and intensity dependence of $\beta_{Rb-Cs}'$ for the two isotopes $^{87}$Rb and $^{85}$Rb which we attribute to the different ground-state hyperfine splitting energies of the two isotopes.Comment: 4 pages, 2 figure

Competing mechanisms and scaling laws for carbon nanotube scission by ultrasonication

Dispersion of carbon nanotubes (CNTs) into liquids typically requires ultrasonication to exfoliate individuals CNTs from bundles. Experiments show that CNT length drops with sonication time (or energy) as a power law t?m. Yet the breakage mechanism is not well understood, and the experimentally reported power law exponent m ranges from approximately 0.2 to 0.5. Here we simulate the motion of CNTs around cavitating bubbles by coupling Brownian dynamics with the Rayleigh-Plesset equation. We observe that, during bubble growth, CNTs align tangentially to the bubble surface. Surprisingly, we find two dynamical regimes during the collapse: shorter CNTs align radially, longer ones buckle.We compute the phase diagram for CNT collapse dynamics as a function of CNT length, stiffness, and initial distance from the bubble nuclei and determine the transition from aligning to buckling. We conclude that, depending on their length, CNTs can break due to either buckling or stretching. These two mechanisms yield different power laws for the length decay (0.25 and 0.5, respectively), reconciling the apparent discrepancy in the experimental data

Interferometric measurement of arbitrary propagating vector beams that are tightly focused

In this work we demonstrate a simple setup to generate and measure arbitrary vector beams that are tightly focused. The vector beams are created with a spatial light modulator and focused with a microscope objective with an effective numerical aperture of 1.2. The transverse polarization components ($E_x$, $E_y$) of the tightly focused vector beams are measured with 3 step interferometry. The axial component $E_z$ is reconstructed using the transverse fields with Gauss law. We measure beams with the following polarization states: circular, radial, azimuthal, spiral, flower and spider web.Comment: 4 pages, 4 figure