Sonoluminescence and sonochemiluminescence from a microreactor

Micromachined pits on a substrate can be used to nucleate and stabilize microbubbles in a liquid exposed to an ultrasonic field. Under suitable conditions, the collapse of these bubbles can result in light emission (sonoluminescence, SL). Hydroxyl radicals (OH*) generated during bubble collapse can react with luminol to produce light (sonochemiluminescence, SCL). SL and SCL intensities were recorded for several regimes related to the pressure amplitude (low and high acoustic power levels) at a given ultrasonic frequency (200 kHz) for pure water, and aqueous luminol and propanol solutions. Various arrangements of pits were studied, with the number of pits ranging from no pits (comparable to a classic ultrasound reactor), to three-pits. Where there was more than one pit present, in the high pressure regime the ejected microbubbles combined into linear (two-pits) or triangular (three-pits) bubble clouds (streamers). In all situations where a pit was present on the substrate, the SL was intensified and increased with the number of pits at both low and high power levels. For imaging SL emitting regions, Argon (Ar) saturated water was used under similar conditions. SL emission from aqueous propanol solution (50 mM) provided evidence of transient bubble cavitation. Solutions containing 0.1 mM luminol were also used to demonstrate the radical production by attaining the SCL emission regions.Comment: http://www.sciencedirect.com/science/article/pii/S1350417712000855; ISSN 1350-417

Frequency effects during acoustic cavitation in surfactant solutions

The acoustic cavitation-induced events, multibubble sonoluminescence (MBSL) and initial growth of MBSL have been studied in surfactant solutions and correlated with bubble coalescence data at three different ultrasound frequencies. For an ionic surfactant, both the number of ultrasonic pulses required to reach a steady state MBSL intensity (Ncrit) and the magnitude of this intensity increases to a maximum as the surfactant concentration increases and then falls again. The total bubble volume generated for a fixed sonication time, which is indirectly related to bubble coalescence, similarly falls as surfactant concentration increases and then rises again. These effects are caused by a combination of electrostatic and coalescence factors at relatively low surfactant concentrations and the screening of the electrostatic factor as surfactant concentration increases further. The peak in coalescence inhibition occurs almost at the same surfactant concentrations as the acoustic frequency is increased; however, the concentrations at which peaks in MBSL and Ncrit occur vary at different frequencies. These results have been discussed in terms of coalescence, electrostatic interactions, rectified diffusion growth and the adsorption kinetics of the surfactants

Growth of bubbles by rectified diffusion in aqueous surfactant solutions

Bubbles grow by the rectified diffusion process in an acoustic field. While there is a thorough understanding of this process for the air-water system, only limited experimental data is available in the literature for aqueous solutions containing surfactants. In order to expand the experimental database, we have determined the bubble growth rate by the rectified diffusion process in aqueous solutions containing sodium dodecyl sulfate (SDS) at various concentrations. Compared to water, the growth rate is higher in SDS solutions. The addition of 0.1 M sodium chloride to SDS results in a further enhanced growth rate at lower bulk concentrations of the surfactant. These results suggest that the surface loading of surfactant molecules plays a key role in enhancing the growth rate, likely due to an increase in the mass transfer resistance during the compression phase of the bubble oscillations. This is supported by results for the growth rates determined for dodecyl trimethylammonium chloride with the growth rate for a given equilibrium surface concentration higher than that of SDS. The experimentally determined bubble oscillation amplitudes for both surfactants decline relative to that of water, consistent with previously published models

Quantification of Cavitation Activity by Sonoluminescence To Study the Sonocrystallization Process under Different Ultrasound Parameters

In this study, both the antisolvent sonocrystallization process of sodium chloride and cavitation activity were investigated as a function of frequency (22–1080 kHz) and acoustic calorimetric power (0–30 W). For frequencies between 20 and 139 kHz, the size of the sodium chloride crystals decreased sharply with increasing power. For frequencies 647 and 1080 kHz, a certain power threshold needs to be exceeded before a decrease in the crystal size was observed. This power threshold coincided with the power threshold for sonoluminescence emission from cavitation bubbles. It was found that the onset of cavitation bubble activity, irrespective of the magnitude (measured in terms of sonoluminescence), enhanced the crystal nucleation rate and decreased crystal size. The minimum crystal size obtained was found to decrease with increasing maximum total integrated sonoluminescence intensity. The results suggest sonoluminescence could be used as a measure to evaluate the sonocrystallization process and that a greater collapse intensity would yield the smallest crystals. In addition, photographs of the sonocrystallization process are reported, suggesting a link between nonsymmetrical transient cavitation activity and crystal nucleation

The effect of condensable minor components on the gas separation performance of polymeric membranes for carbon dioxide capture

AbstractPolymeric membranes as a carbon dioxide capture technology have a number of advantages over other approaches, including their low cost, high performance separation, ease of synthesis, as well as mechanical and thermal stability. However, condensable components in flue gas, in particular water, undergo competitively adsorption with carbon dioxide within the membranes, resulting in a reduction in CO2 permeability. Furthermore, on a longer timescale plasticization of the membrane can occur, turning the glassy polymer to a more rubbery state, which alters both gas permeability and selectivity. Here, the impact of water on three glassy polymeric membranes are studied; polysulfone, Matrimid and 6FDA-TMPDA (a polyimide). The purpose of this work is to model the behavior of gas separation membranes under humid conditions that mimic real flue gas. This will assist in analyzing the performance of glassy gas separation membranes in planned CO2 capture trials on both pre- and post-combustion carbon capture.Upon exposure to water in the feed, all three membranes, Matrimid, polysulfone and 6FDA-TMPDA experience reduced CO2 permeability indicative of competitive adsorption. Over a longer timescale, both polysulfone and 6FDA-TMPDA recover some of the loss in permeability performance, due to plasticization by water. Matrimid displays no plasticization behavior

Membrane adhesives

The adhesive seal between membranes and their housing has a vital role to play in any membrane application, as it ensures the product and feed streams do not mix and that pressure can be maintained. In many applications, the adhesive seal is a major source of membrane module failure and can dictate the operational life of a module. Hence, understanding adhesives in membrane systems is fundamental in ensuring both separation performance and durability; however, this field has been widely overlooked in the literature. This paper attempts to rectify this by discussing in depth adhesive theory and factors that will maximize the adhesion strength, relative to membrane technology. Also highlighted are specific membrane factors that lead to adhesive failure, important when designing a module. The performance of different adhesives is then presented, based on their ability to adhere to different substrates and their resistance to environmental factors. They are discussed and compared relative to the wide range of polymeric and inorganic membrane systems that are currently commercialized or under research. The conclusion raises the possibility of future research in membrane adhesives, as membrane specific developments in the adhesion field have the potential to increase the durability and environmental resistance of membrane modules

Potential uses of ultrasound in the dairy ultrafiltration processes (A)

There has been a growing interest in the industrial application of ultrasound, especially in the food industry