Nucleation Sites For Bubble Formation Research Articles

Micro-cavitation on demand via the nanoparticle mediated interaction of light and sound

The safe utilization of controlled cavitation for HIFU therapy and ultrasound assisted drug delivery requires nucleation sites for bubble formation. We consider the potential for nucleating transient vapor cavities using laser-illuminated gold nanoparticles combined with high-intensity focused ultrasound. An transparent polyacrylamide gel phantom was seeded with 82-nm diameter gold particles and exposed to 20 ns pulses from a 532 nm Nd:Yag laser. Laser firing was synchronized with the arrival of a burst of 1.1 MHz focused ultrasound. Acoustic emissions from ensuing inertial cavitation were detected passively using a 15 MHz focused transducer. At a laser energy of 0.10 mJ/pulse, the resulting inertial cavitation nucleation threshold pressure (peak-negative focal pressure) was as low as 0.92 MPa. In comparison, a peak-negative focal pressure of 4.50 MPa was required to nucleate detectable cavitation without laser illumination (nano-particles were present in both cases). Experimental results agree well with a simple model for transient heating and cavity formation. Since the particles are durable, one can re-activate them as needed, essentially yielding cavitation nuclei “on demand.” [Work supported by the Dept. of the Army (Award No. DAMD17-02-2-0014) and the Center for Subsurface Sensing and Imaging Systems (NSF ERC Award No. EEC-9986821).]

Cryogenic quenching of rock using liquid nitrogen as a coolant: Investigation of surface effects

Cryogenic quenching experiments were conducted using sandstone rock samples and liquid nitrogen (LN2) under ambient pressure. Four different rock surface conditions were prepared to investigate the surface effects on quenching, as compared to the baseline case where rock surface was polished by 1500 grit sandpaper. A copper sample was also made to reveal the unique property of rock surface with respect to quenching. It is found that a 130 °C increase of LFP was achieved by rock surface than copper, when both were polished by 1500 grit sandpaper. The scanning electron microscope (SEM) images showed that numerous pores and cavities with hierarchical dimensions were present on rock surface. These cavities contributed to the LFP enhancement by providing massive nucleation sites for bubble formation when intermittent solid–liquid contact occurs during film boiling thereby destabilizing the vapor film. Rock surface polished by 36 grit sandpaper was found to be superhydrophilic and rougher than the baseline case but produced no enhancement on cryogenic quenching. Quenching rates were remarkably accelerated on rock surfaces with sand layer coating and with orthogonally intersected grooves. The initial room temperature was not enough to maintain stable film boiling on these two surfaces and transitional film boiling dominated the boiling regime after quenching started. Further increase of LFP was achieved on rock surface covered by crude oil, which served as a low thermal conductivity coating. The present results demonstrated that the unique surface micro structure of rocks can significantly raise LFP temperature thus shortening film boiling duration. In addition, by appropriately modifying the rock surface condition, cryogenic quenching rates and LFP can be further enhanced.

Effect of Dual Frequency Ultrasound on the Bubble Formation in a Capillary Tube

A visual experimental investigation was conducted to determine the effect of dual frequency ultrasound on the bubble formation and growth in a capillary quartz tube. Two piezoelectric ceramics were used in this experiment. They were made of Pb-based lanthanum-doped zirconate titanates (PLZTs). The PLZTs were placed on a quartz tube with an inner diameter of 2 mm and an outer diameter of 3 mm. The capillary tube was vacuumed first and then charged with water using a filling ratio of 70%. The ultrasonic sound was applied to the heating section of a capillary tube. The bubble formation and growth were recorded by a high speed camera. As shown in figures, when the ultrasound with a single frequency of either 154 kHz or 474 kHz was applied, only one bubble was generated. When the dual frequencies of 154 kHz and 474 kHz were applied, more bubbles were generated. The speed of the bubble growth with dual frequency ultrasound was much higher than that with a single frequency. When a dual frequency ultrasound (154 kHz and 474 kHz) was used, the nucleation sites for bubble formation were significantly increased and the bubble growth rate enhanced.

Microstructured silver surfaces produced by freeze casting for enhanced phase change heat transfer

Microporous silver surface layers were formed on copper substrates by means of a modified freeze casting method. The structure of such layers is the result of the templating action of the ice crystals and layers were found to contain a hierarchical porous structure. Three different pore morphologies were present in the microstructure with pore sizes ranging from approximately 0.5 to 200 μm. The application of these surface structures was found to considerably enhance the heat flux during the nucleate phase of pool boiling, with heat fluxes up to five times higher from a microporous surface compared to a bare copper substrate. Bubble formation and departure was found to be significantly different on the two types of surfaces, with smaller bubbles formed with a high density on the microporous surface. The enhancement in heat flux by these structures is most likely due to the combined effect of an increased surface area with high thermal conductivity, an increase in nucleation sites for bubble formation as well as effective wicking from micropores to sustain bubble growth and departure.

Novel Desaturation Cell to Quantify Gushing Intensity: A Preliminary Study on Model Solutions

Beer gushing is an unwanted phenomenon that occurs when the beer foam spontaneously gushes out when a bottle or can is opened. Despite long-term research, gushing still has not been fully explained. To our knowledge, there is no simple, quick, and inexpensive method for reliably measuring gushing intensity. In this article, a desaturation cell enabling saturation, step-wise pressure drop, and image analysis of foam formation is presented. The gushing intensity was evaluated through foam formation rate and relative final foam volume. To study the physicochemical mechanism of gushing, the measurements were carried out in a simple gushing matrix made of water and bovine serum albumin (BSA), the aggregates or micelles of which serve as nucleation sites for bubble formation. The effect of hop bitter compounds (iso-α-acids and tetrahydro-iso-α-acids) and essential oils (linalool) in the BSA matrix was also tested. The hop bitter compounds increased the foam formation rate significantly while affecting the foam volume less. Linalool at low concentration caused an extremely high foam formation rate whereas, at higher concentration, did not affect the foaming of the BSA matrix.

Controlled nucleation of microcavitation with laser-illuminated nano-particles

The safe utilization of controlled cavitation in HIFU therapy requires the presence of nucleation sites for bubble formation. Effective cavitation nuclei do not exist in most tissues; nucleation thresholds pressures in excess of 4–5 MPa have been reported. We investigate the efficacy of transient vapor cavity generation from laser-illuminated gold nano-particles as a means for nucleating cavitation with high-intensity focused ultrasound. An acrylamide tissue phantom seeded with 82-nm diameter gold particle was exposed to 20 ns pulses from a 532 nm Nd:Yag laser. Laser firing was precisely synchronized with a pulsed 1.1 MHz HIFU pressure field. Acoustic emissions from inertial cavitation were detected by a 15 MHz focused transducer at a laser energy of 0.10 mJ/pulse and a HIFU peak-negative focal pressure as low as 0.92 MPa. In comparison, a peak-negative focal pressure of 4.50 MPa was required to nucleate detectable cavitation without laser illumination; nano-particles were present in both cases. Since the particles are durable, one can re-activate them as needed, essentially yielding cavitation nuclei on demand. [Work supported by the Dept. of the Army (award No. DAMD17-02-2-0014) and the Center for Subsurface Sensing and Imaging Systems (NSF ERC Award No. EEC-9986821).]

Bubble formation with pool boiling on tubes with or without basic surface modifications for enhancement

Bubble formation is investigated on a single, horizontal copper tube ( D=25.4 mm) before and after applying surface modifications in the form of macrocavities with comparatively simple shapes (size: 200 × 100 μm on the tube surface and 50 μm in depth) in order to link bubble formation and heat transfer to the geometric features of the cavities. Two kinds of cavities exist in parallel, one with reduced mouth width (so-called “main structure”) and one without this reduction (“secondary structure”). Pool boiling experiments with different organic liquids have been performed with variation of heat flux and saturation pressure. The resulting bubble formation on the tubes with and without surface modifications is discussed in terms of characteristic parameters such as local distribution and temporal activation behavior of the nucleation sites for bubble formation at active nucleation sites (“primary bubbles”) or for bubbles released from the macrocavities (“secondary bubbles”).

Formation of Micronuclei Responsible for Decompression Sickness

In this report, the origin of micronuclei responsible for the onset of decompression sickness is investigated. The formation of cavities by tribonucleation between hydrophobic and hydrophilic surfaces has been studied using a new force measuring device, LLIFE (Light Lever Instrument for Force Evaluation). Using this instrument, the interaction force between silica surfaces in equilibrium with the cationic surfactant cetylpyridinium chloride has been measured above and below the critical micelle concentration. A long range hydrophobic attraction is evident between adsorbed monolayers and a DLVO repulsion is apparent between adsorbed bilayers. It is shown that the formation of cavities on separation occurs not only between hydrophobic monolayers but between hydrophilic bilayers, should the bilayers be squeezed sufficiently to cause rupture. The demonstration that cavities may arise upon separation between bilayers is new and leads to the postulate that cavities are formedin vivoby the rupture of cell membrane bilayers, thereby providing the nucleation sites for bubble formation should sufficient gas supersaturation occur.

Susceptibility of the human fetus to physical damage

<h2>Abstract</h2> The known causes of developmental defects in man were given as: Genetic Transmission (20%); Chromosomal Aberration (5%); and Environmental Causes (including radiation, drugs and chemicals) (10%). The extent of embryonic damage due to the action of dysmorphological agents depends on the developmental stage of the embryo, its genetic susceptibility, and the physiological or pathological status of the mother. The maximum adverse effects occur when embryos are treated during the embryogenesis stage (5–6 wk gestation), whereas exposure during the fetal period (8 wk after organogenesis) has not produced major malformations, but can impair the differentiation of external genitalia and interfere with development of the central nervous system. Reaction of animals to chemical agents depends on their genetic constitution and varies between different animal species. Cortisone is a strong dysmorphogenic agent in rabbits and mice but does not produce malformations in rats. Also the type of malformation can be different for each species. The major mechanism of action in ultrasound irradiation is thermal where a temperature rise of 2.4°C is required to produce a teratological change (Lele, 1975) which occurs at intensity of 100mW/cm²SATA. Little is known about the state of gas nucleation needed for cavitation in animal tissue but decompression studies have shown that nucleation sites for bubble formation do exist and it is known that bubbles can develop in human tissue that are subjected to rapid reduction in ambient pressure when diving. The production of bubbles has been observed in animal tissues during exposure to continuous wave ultrasound 80 mW/cm²SATA (ter Haar and Daniels, 1981). Epidemiological studies published in the scientific literature have indicated no effects of diagnostic ultrasound on the fetus or mother. No confirmed biological effects on patients have been reported. It is difficult and inappropriate to make conclusions or determinations regarding the clinical safety of diagnostic ultrasound based on the current experimental data available. A responsible and vigilant scientific community must continue the search for effects.