INTRODUCTION AND MOTIVATION
Sonoluminescence is the term used to describe the emission of light from a
violently collapsing bubble. Sonoluminescence ("light from sound") is the result of extremely
nonlinear pulsations of gas/vapor bubbles in liquids when subject to sufficiently high amplitude
acoustic pressures. In a single collapse, a bubble’s volume can be compressed more than a
thousand-fold in the span of less than a microsecond. Even the simplest consideration of the
thermodynamics yields pressures on the order of 10,000 ATM. and temperatures of at least
10,000K. On the face of things, it is not surprising that light should be emitted from such an
extreme process. Since 1990 (the year that Gaitan discovered light from a single bubble) there has been
a tremendous amount of experimental and theoretical research in stable, single-bubble
sonoluminescence (SBSL).
Yet there remain at least four unexplained phenomena associated with SBSL in 1g:
· the light emission mechanism itself,
· the existence of anisotropies in the emitted light,
· the disappearance of the bubble at some critical acoustic pressure, and
· the appearance of quasiperiodic and chaotic oscillations in the flash timing.
Gravity, in the context of the buoyant force, is implicated in all four of
these.
We are developing microgravity experiments probing the effect of gravity on
single bubble sonoluminescence. By determining the stability boundaries experimentally in
microgravity, and measuring not only light emission but mechanical bubble response, we will be
able to directly test the predictions of existing theories. By exploiting the microgravity
environment we will gain new knowledge impossible to obtain in earth-based labs which will enable
explanations for the above mysteries. We will also be in a position to make new discoveries about
bubbles which emit light.
OBJECTIVES
The objectives of the planned investigation are:
(1) To develop an experimental apparatus to perform controlled experiments
aboard parabolic flight aircraft to attempt to quantify the effect of gravity on SBSL.
Measurements of the light emission and especially the mechanical oscillations of bubbles in 1g, mg,
and 2g will be performed. We will investigate the possibility of mg experimentation.
(2) To model the hydrodynamic effects of acceleration on bubble dynamics and
SBSL in realistic acoustic resonators. The primary Bjerknes force, buoyancy, drag, mass diffusion,
shape stability and (empirically) light emission will be accounted for in the model.
(3) To measure (as a function of acceleration during parabolic flight) a
bubble’s position, equilibrium radius, maximum radius, oscillatory radius, and spatially and
temporally resolved light emission. This will be done for a range of dissolved gas concentrations in
order to compare with predictions of our hydrodynamic model.
(4) To measure (as a function of acceleration during parabolic flight) the
precise values of acoustic pressure and equilibrium radius that leads to the extinction of a
light-emitting bubble, a phenomenon which occurs at a well defined critical acoustic pressure in 1g
experiments. This will test theories which postulate either a nonlinear levitation instability, or
a Rayleigh-Taylor instability mechanism for the bubble disappearance.
(5) To test the prediction that chaotic and quasiperiodic timing of the flashes
observed in 1g are due to buoyancy-related effects, which could either be induced shape
oscillations or a global levitator instability due to time-varying detuning of the levitation cell
resonance resulting from the nonlinear bubble oscillations.
RESULTS
We have completed 1 KC-135 flight campaign in August, 1999 utilizing a cubic test chamber
operating at 14 kHz. We attempted to test the prediction of our model [1] that hydrodynamics
alone dictate a 5 – 35 % change in SBSL intensity, (depending approximately linearly on the dissolved
gas concentration) for the 10-4g to 1.8g swing typical of a K-135 parabolic maneuver. The driving
mechanism for this effect is the small change in head pressure experienced by the bubble when the
acceleration changes. Figure 1 shows the measured bubble dynamics during a single parabola for the
August flights. The main result is that for nearly 0g, the bubble grows and emits more light, while at
2g the bubble shrinks and emits less light. The results are in rough agreement with our model.
MICROGRAVITY RELEVANCE
SBSL bubbles experience a time-varying buoyancy (quantified by the oscillatory volume ratio
Vmax/V0, see Fig. 2) which reaches maximal excursions precisely where sonoluminescence is observed. This results in a strong nonlinear coupling between volume and translatory motions. Removing the acceleration of gravity from the system will eliminate buoyancy-driven translatory oscillations of the bubble. This would be a decisive test of light emission mechanisms, and will also shed light on the chemical reaction theory of mass flux for volume stability as well as the resonance-controlled shape oscillation instability. Thus, a microgravity environment will change the geography of the parameter space, and the only hope for a clear understanding of the SBSL phenomenon is to perform experiments which locate light emission within the context of the instability boundaries that exist in the parameter space.
Holt, R.G., Roy, R.A., Sonoluminescence in Space: The Critical Role of Buoyancy in Stability and Emission Mechanisms, Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference, NASA Glenn Research Center, Cleveland, OH, CP-2000-210470, pp. 1610-1640, August 9, 2000.