Controlled spherical deuterium droplets as Lagrangian tracers for cryogenic turbulence experiments.

The study of the smallest scales of turbulence by (Lagrangian) particle tracking faces two major challenges: the requirement of a 2D or 3D optical imaging system with sufficiently high spatial and temporal resolution and the need for particles that behave as passive tracers when seeded into the flow. While recent advances in the past decade have led to the development of fast cameras, there is still a lack of suitable methods to seed cryogenic liquid helium flows with mono-disperse particles of sufficiently small size, of the order of a few micrometers, and a density close enough to that of helium. Taking advantage of the surface tension, we propose two different techniques to generate controlled liquid spherical droplets of deuterium over a liquid helium bath. The first technique operates in a continuous mode by fragmenting a liquid jet, thanks to the Rayleigh-Taylor instability. This results in the formation of droplets with a diameter distribution of 2 ± 0.25DN, where DN is the diameter of the jet nozzle (DN = 20 µm in the present experiment). This method offers a high production rate, greater than 30 kHz. The second technique operates in a drop-on-demand mode by detaching droplets from the nozzle using pressure pulses generated using a piezoelectric transducer. This approach yields a much narrower diameter distribution of 2.1 ± 0.05DN but at a smaller production rate, in the range 500 Hz-2 kHz. The initial trajectories and shapes of the droplets, from the moment they are released from the nozzle until they fall 3 mm below, are investigated and discussed based on back-light illumination images.

Luminescence from Collapsing Centimeter Bubbles Expanded by Chemical Reaction.

We report on a new method for realizing an exceptionally strong inertial confinement of a gas in a liquid: A centimetric spherical bubble filled with a reactive gaseous mixture in a liquid is expanded by an exothermic chemical reaction whose products condense in the liquid at the bubble wall. Hence, the cavity formed in this way is essentially empty as it collapses. The temperatures reached at maximum compression, inferred from the cavity radius dynamics and further confirmed by spectroscopic measurements exceed 20 000 K. Because the cavity is typically big, our findings also provide unique space and time resolved sequences of the events accompanying the collapse, notably the development of the inertial instability notoriously known to deter strong compression.

Superdiffusive trajectories in Brownian motion.

The Brownian motion of a microscopic particle in a fluid is one of the cornerstones of statistical physics and the paradigm of a random process. One of the most powerful tools to quantify it was provided by Langevin, who explicitly accounted for a short-time correlated "thermal" force. The Langevin picture predicts ballistic motion, ~t(2) at short-time scales, and diffusive motion ~t at long-time scales, where x is the displacement of the particle during time t, and the average is taken over the thermal distribution of initial conditions. The Langevin equation also predicts a superdiffusive regime, where ~t(3), under the condition that the initial velocity is fixed rather than distributed thermally. We analyze the motion of an optically trapped particle in air and indeed find t(3) dispersion. This observation is a direct proof of the existence of the random, rapidly varying force imagined by Langevin.