The effect of surface active solutes on bubbles in an acoustic field

The interaction between gas bubbles and ultrasound in water leads to acoustic cavitation under specific experimental conditions. Acoustic cavitation has been used in a number of areas, these include therapeutic applications, contrast imaging, synthesis of nanomaterials, production of nanoemulsions, treatment of food materials, waste-water treatment, etc. In all these applications, a variety of chemical species are present and most of these chemicals are surface active in nature. There is limited literature available on the effect of surface active solutes on acoustic bubbles. The current review aims to provide a consolidated overview of the recent experimental investigations on the effect of surface active solutes on cavitation bubbles and the relevance of these studies to some applications, including sonochemistry.     

Minimising oil droplet size using ultrasonic emulsification

The efficient production of nanoemulsions, with oil droplet sizes of less than 100 nm would facilitate the inclusion of oil soluble bio-active agents into a range of water based foods. Small droplet sizes lead to transparent emulsions so that product appearance is not altered by the addition of an oil phase. In this paper, we demonstrate that it is possible to create remarkably small transparent O/W nanoemulsions with average diameters as low as 40 nm from sunflower oil. This is achieved using ultrasound or high shear homogenization and a surfactant/co-surfactant/oil system that is well optimised. The minimum droplet size of 40 nm, was only obtained when both droplet deformability (surfactant design) and the applied shear (equipment geometry) were optimal. The time required to achieve the minimum droplet size was also clearly affected by the equipment configuration. Results at atmospheric pressure fitted an expected exponential relationship with the total energy density. However, we found that this relationship changes when an overpressure of up to 400 kPa is applied to the sonication vessel, leading to more efficient emulsion production. Oil stability is unaffected by the sonication process.     

Microbial synthesis of silver nanoparticles by Bacillus sp.

A silver resistant Bacillus sp. was isolated through exposure of an aqueous AgNO₃ solution to the atmosphere. Silver nanoparticles were synthesized using these airborne bacteria (Bacillus sp.). Transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analyses confirmed that silver nanoparticles of 5–15 nm in size were deposited in the periplasmic space of the bacterial cells; a preferable cell surface location for the easy recovery of biogenic nanoparticles.     

Quantification of high-power ultrasound induced damage on potato starch granules using light microscopy

A simple light microscopic technique was developed in order to quantify the damage inflicted by high-power low-frequency ultrasound (0-160 W, 20 kHz) treatment on potato starch granules in aqueous dispersions. The surface properties of the starch granules were modified using ethanol and SDS washing methods, which are known to displace proteins and lipids from the surface of the starch granules. The study showed that in the case of normal and ethanol-washed potato starch dispersions, two linear regions were observed. The number of defects first increased linearly with an increase in ultrasound power up to a threshold level. This was then followed by another linear dependence of the number of defects on the ultrasound power. The power threshold where the change-over occurred was higher for the ethanol-washed potato dispersions compared to non-washed potato dispersions. In the case of SDS-washed potato starch, although the increase in defects was linear with the ultrasound power, the power threshold for a second linear region was not observed. These results are discussed in terms of the different possible mechanisms of cavitation induced-damage (hydrodynamic shear stresses and micro-jetting) and by taking into account the hydrophobicity of the starch granule surface.     

A novel approach for enhancing metal ion separation using acoustic nebulisation

A novel technique for anionic surfactant assisted separation and preconcentration of metal cations was developed using ultrasound induced nebulization at MHz frequency. The ions of copper, zinc, cadmium, and calcium were used as model analytes. Analysis of the aerosol using flame atomic absorption spectrometry showed enrichment factors for the metal ions studied between 5 and 8, when dilute solutions containing sodium dodecylsulfate and the metal ions were nebulized. The mechanism of metal ion enrichment was explained based on surfactant adsorption and the droplet model for aerosol droplets. It was demonstrated that further increase in the enrichment factor could be achieved by increasing the ultrasound frequency, thus producing smaller droplets.     

Ultrasonics in food processing

In recent years, the physical and chemical effects of ultrasound in liquid and solid media have been extensively used in food processing applications. Harnessing the physical forces generated by ultrasound, in the absence and presence of cavitation, for specific food processing applications such as emulsification, filtration, tenderisation and functionality modification have been highlighted. While some applications, such as filtration and emulsification are "mature" industrial processes, other applications, such as functionality modification, are still in their early stages of development. However, various investigations discussed suggest that ultrasonic processing of food and dairy ingredients is a potential and viable technology that will be used by many food industries in the near future.     

Sonochemical production of fluorescent and phosphorescent latex particles

A novel method for producing nanosized polymer latex particles doped with fluorescent and phosphorescent solutes is described. Methyl methacrylate monomer (10 wt %) containing either pyrene, 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DMDP), or 1-bromonaphthalene was ultrasonically dispersed in water and simultaneously polymerized to produce approximately 60 nm diameter latex particles. A fluorescence spectroscopic examination of the latex dispersion containing either pyrene or DMDP showed that the solutes were not covalently bound to the polymer and that they were embedded in a highly viscous environment possessing a low polarity (dielectric constant on the order of 2). The fluorescence lifetime of the pyrene in the core of the poly(MMA) latex was found to be 520 ns, irrespective of the oxygen concentration in the dispersion. Room-temperature phosphorescence was observed from 1-bromonaphthalene, with a lifetime of 2.0 ms in an argon atmosphere. In the presence of air, phosphorescence was still observable although with a partially reduced emission intensity.     

Effect of surfactants on the rate of growth of an air bubble by rectified diffusion

The rectified diffusion growth of a single air bubble levitated in an acoustic field (frequency = 22.35 kHz) in water and in aqueous solutions containing surfactants (sodium dodecyl sulfate and sodium dodecylbenzene sulfonate) was investigated. As reported by Crum (J. Acoust. Soc. Am. 1980, 68, 203), the presence of surfactants at the bubble/liquid interface enhanced the growth rate of the bubble by rectified diffusion. It is suggested in this paper that in addition to the effect of surfactants on the surface tension and interfacial resistance to mass transfer, the effect of surface rheological properties may also contribute to the cause of the enhancement observed in the bubble growth rate.     

Structural and Thermal Investigations of Biomimetically Grown Casein–Soy Hybrid Protein Fibers

A hybrid protein fiber from different protein sources such as casein and soybean using wet-spinning technique was prepared. The casein/soybean hybrid fibers were synthesized at different weight ratios such as 100/0 (casein), 75/25, 50/50, 25/75, and 0/100 (soy) and characterized. Electron microscopic analysis confirmed the growth of pure and hybrid fibers and shows an increased surface roughness as the soy concentration increases in the hybrid fibers. Infrared spectra did not exhibit any significant changes in the functional groups between pure and hybrid fibers. X-ray diffraction pattern indicates slight increase in the diffraction peak values of hybrid fibers compared with the neat fibers. Thermal analyses show a moderate increase in the thermal stability of hybrid fibers when compared with the pure fibers. These results implicitly indicate that the casein and soy proteins are homogeneous in the hybrid fiber form. It has been demonstrated that the hybrid fiber with ≥50 wt.% casein content exhibits better morphology and increased thermal stability, which has scope for application in technical and medical industries.