Glass Microbubbles Research Articles

Extension of logarithmic growth ofThiobacillus ferrooxidans by potential controlled electrochemical reduction of Fe(III)

In this study, we demonstrated that the period of logarithmic growth for Thiobacillus ferrooxidans could be extended when optimal conditions for cell growth were maintained using potential controlled electrochemical cultivation with sufficient aeration. The optimal pH and Fe(II) concentration for the electrolytic cultivation were determined to be 2.0 and 150 mM, respectively. When the potential was set to 0.0V vs Ag/AgCl, the Pt electrode reduced Fe(III) to Fe(II) with an efficiency of 95%. A porous glass microbubble generator was used to maintain adequate levels of dissolved oxygen, which was the electron acceptor for T. ferrooxidans when the cell density in the medium was high. Under these conditions, cells at an initial density of 10(7) cells/mL grew logarithmically for 4days until the cell density was 4 x 10(9) cells/mL. This corresponded to a period of logarithmic growth that was 3 times longer than was observed in batch cultures without electrolysis. In addition, the final cell density reached 10(10) cells/mL after 6 days of electrochemical cultivation, which was a 50-fold increase over conventional batch culture. Under conditions of increasing cell density, potentiostatic electrolysis made it possible to remove Fe(III), which causes product inhibition, at an increasing rate and to correspondingly increase the production rate of Fe(II), which is the electron donor for T. ferrooxidans. Thus, our cultivation system provides a sufficient supply of electron donor and acceptor for T. ferrooxidans, thereby elongating the period of logarithmic growth and producing very high cell densities.

Low-density granular fill for damping structural vibrations

Granular materials have been used for many years to damp structural vibrations. Often these treatments incorporate sand or lead shot. Both are heavy and provide some of their damping effect through mass loading. This paper discusses the damping properties of a low-density granular material, 3M glass microbubbles (tradename Scothlite). A paste was made using water and Scothlite and placed in an aluminum free-free beam. Resonant peaks of the beam were reduced by 10 dB, and in some cases more. The specific gravity of the Scothlite is about 0.1, so mass loading effects cannot account for the damping. Further, glass is not normally considered to be highly viscoelastic at room temperature. Rather, the attenuation mechanism is thought to be activated by the low bulk sound speed of the granular fill. With a low sound speed, the wavelength is short, and incipient attenuation in the fill becomes important. The mechanism is a combination of four possibilities: (1) small but finite intrinsic material attenuation, (2) frictional losses between rubbing grains, (3) nonlinear hystere viscous losses of the fluid flow between grains. [Research sponsored by ARPA/ONR.] a)E-mail: fricke@mit.edu

Brownian motion of a particle levitated in vacuum

A glass microbubble (diameter ∼15 μm, wall 0.65 μm) was levitated in high vacuum by an actively controlled electric field. While held vertically, the particle moved freely in the horizontal plane under the influence of a central restoring force. The Brownian motion of the particle was observed and quantified by successive measurements of the oscillation amplitude in a horizontal axis. Brownian motion was exhibited throughout the pressure range from 7×10−6 to 7×10−4 Torr. The statistical behavior of the oscillation amplitude is consistent with the theoretical prediction for Brownian motion in a low pressure gas. These measurements constitute the first direct observation of Brownian motion of a levitated particle in an environment below 1 Torr. In addition to the experimental verification of low pressure Brownian motion theory, this work serves as an important step in the development of a standardizing high vacuum gauge.

Stability of compression of shell targets in the Progress facility

The six-beam Progress laser facility was used in a study of compression and heating of glass microbubbles filled with a DT gas and coated outside by a thin (~0.1?) aluminum film. The power density of Nd laser radiation (?=1.054?) reaching the target surface was (1?2)?1015 W/cm2 and the specific energy deposited in the target was 0.1-0.25 J/ng. Two-dimensional images of a laser plasma were recorded using x-ray lines of multiply charged Si and Al ions. Strong Al lines observed at distances close to the target center were evidence of a strongly unstable nature of compression of the laser plasma. This was confirmed also by a weak dependence of the neutron yield on the specific input energy, which did not agree with the results of one-dimensional calculations carried out using the Zarya program.

Polishing of glass microbubbles

A method for polishing glass microbubbles into hemispheres is described. The method consists of potting the microbubble, polishing, dissolving the potting material, and searching for a good finished product.

Permeation Eccentricities of He, Ne, and D‐T from Soda‐Lime Glass Microbubbles

Permeation of 3He, Ne, and D‐T (deuterium‐tritium) was studied through batches of several million glass microbubbles. Although the calculated permeabilities agree roughly with literature data, the bubbles show several vagaries. Isothermal outgassing is not exponential but shows “tails” that become more pronounced at lower temperatures. An isotopic disproportionation effect occurs in the bubble loading at 698 K. Gas that has permeated the glass walls is more deuterium‐rich and the effect is increased by large amounts of 3He in the feed gas. Finally, a slow and unexplained leakage occurs when loaded bubbles are stored for many months at 243 K. Data are also presented on bubble size distributions and mean wall thicknesses.

Reply to ’’Comment on ’Permeation of helium and hydrogen from glass-microsphere laser targets’ ’’

An ensemble of glass microbubbles at constant temperature does not show straight exponential outgassing, but instead tails out. The curvature of this tail may be related to the distribution of permeability time constants in the ensemble.

Distribution of Ne gas permeabilities in glass laser targets

A 3.6 million sample of sized, soda-lime glass microbubbles was filled to various total percent fills with Ne. The batches were outgassed isothermally at 498K. The partially filled batches outgassed faster because initially only the more permeable microbubbles were filled. Two models were used to study permeation, with values of tau (the time constant) ranging from 1-100. The data agree better with the model skewed to the fast time constant side. An analysis of the dimensions indicates that the permeability must be responsible for most of the distribution. The reason for this is not known; however, it may be due to variability of chemical composition or to varying degrees of phase separation.

Prediction of a Forbidden Zone in Light Interference Fringes of Glass Microbubbles

Light interference microscopy has been used to characterize 50 to 100 μm o.d. glass microbubbles for use as laser fusion targets. In support of this programme, we have written a parallel-light, ray-trace code that follows the refracted paths of rays passing through the microbubbles. The code ultimately calculates the increase in optical path length as one moves from rays passing through the centre of the microbubble to those at the edge. The code also predicts four types of ray paths, of which only two are strong enough to produce measurable light in the interference microscope. Most interesting is the code's prediction of a forbidden zone in which no interference fringes appear. Attempts to verify this effect with laser target microbubbles gave inconclusive results because of improper dimensions and lack of sphericity in the microbubbles. Experiments with high-refractive-index, thick-walled glass cylinders might confirm the code's predictions.