Several studies have reported the use of high intensity ultrasound (HIU) to induce the crystallization of lipids. The effect that HIU has on lipid crystallization is usually attributed to the generation of cavities but acoustic cavitation has never been fully explored in lipids. The dynamics of a particular cavitation cluster next to a piston like emitter (PLE) in an oil, was investigated in this study. The lipid systems, which are important in food processing, are studied with high-speed camera imaging, laser scattering and acoustic pressure measurements. A sequence of stable clusters were noted. In addition, a bifurcated streamer was detected which exists within a sequence of clusters. This is shown to originate from two clusters on the PLE tip oscillating with a 180° phase shift in time with respect to one another. Finally, the collapse phase of the cluster is shown to involve a rapid (< 10 µs) two stage process. These results show that the dynamics of cluster formation and collapse is driven by HIU power levels and might have implications in lipid sonocrystallization.

Improving the sensitivity and ultimately the range of particle sizes that can be detected with a single pore extends the versatility of the Coulter counting technique. Here, to enable a pore to have greater sensitivity, we have developed and tested a novel differential resistive pulse sensing (DiS) system for sizing particles. To do this, the response was generated through a time shift approach utilizing a “self-servoing regime” to enable the final signal to operate with a zero background in the absence of particle translocation. The detection and characterization of a series of polystyrene particles, forced to translocate through a cylindrical glass microchannel (GMC) by a suitable static pressure difference using this approach, is demonstrated. An analytical response, which scales with the size of the particles employed, was verified. Parasitic capacitive effects are discussed; however, translocations on the millisecond time scale can be detected with high sensitivity and accuracy using the approach described.

Nanobubbles are fascinating but controversial objects. Although there is strong evidence for the existence of surface bound nanobubbles, the possibility of stable nanobubbles in the bulk remains in question. In this work, we show how ultrasonication of electrolytes can create transient bulk nanobubbles. To do this, glass nanopores are used as Coulter counters in order to detect nanobubbles. During ultrasonication, these transient bulk nanobubbles are shown to exist in relatively high concentrations while bubble activity on the surface of a solid media close to the pore is driven by ultrasound. However, the transient nature of these bubbles is evident upon termination of the ultrasonic source. Highspeed imaging suggests that these transient nanobubbles originate from the fragmentation of larger bubbles, which skate over the surface of the structure in the acoustic field present. Transient nanobubbles as small as ~100 nm diameter are detected. In contrast to previous work with microbubbles, no evidence for the oscillation of these nanobubbles during translocation was found. The novel experimental approach presented here provides strong evidence for the existence of transient nanobubbles in bulk solution.

No abstract

The type of environment generated by an ultrasonic source is shown to alter the crystallisation of an oil. In particular a bifurcated streamer, with its unusual dual cluster, is surprisingly efficient at accelerating crystallisation.

Oscillating microbubbles, driven by the local sound field, and crystals are detected and sized in oils as they pass through an optical detector.

The objective of this research was to evaluate if cavitation events generated during sonication (20 kHz, 216 μm amplitude, 10 s) are responsible for changes in physical properties of a fat with low levels of saturated fatty acids and if these changes are maintained during storage. The fat was crystallized at 24 and 34 C and stored at 25 C for up to 24 weeks. An increase in solid fat content and melting enthalpy was observed for sonicated samples crystallized at 34 C and an increase in elasticity was observed for sonicated samples crystallized at 24 C (P < 0.05). Hardness increased in sonicated samples crystallized at 24 and 34 C (P < 0.05) after 60 min of crystallization and after 24 weeks storage. Elasticity of nonsonicated samples crystallized at 24 C decreased (P < 0.05) after storage at 25 C for 48 h while it remained constant in sonicated samples. Sonicated samples had more, and smaller crystals compared to the non-sonicated ones. No significant change was observed in physical properties of sonicated samples crystallized at 24 C and 34 C during the 24 weeks of storage. Sonication at 24 C was less efficient at changing the physical properties of the fat compared to 34 C; however, the number of subharmonic components generated during sonication at these two temperatures was not affected by crystallization temperature. These results suggest that changes in physical properties are associated with secondary effects of sonication such as bubble streamers rather than changes in cluster dynamics.

Both bubbles and associated mechanical forces, enhance crystallisation of a lipid sample through primary and secondary effects.