School of Chemistry - Research Publications

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    A Comparison of the Effectiveness of Sonication, High Shear Mixing and Homogenisation on Improving the Heat Stability of Whey Protein Solutions
    Koh, LLA ; Chandrapala, J ; Zisu, B ; Martin, GJO ; Kentish, SE ; Ashokkumar, M (SPRINGER, 2014-02)
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    THE FUNDAMENTALS OF POWER ULTRASOUND - A REVIEW
    Leong, T ; Ashokkumar, M ; Kentish, S (SPRINGER SINGAPORE PTE LTD, 2011-08)
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    Experimental and Theoretical Studies on the Movements of Two Bubbles in an Acoustic Standing Wave Field
    Jiao, J ; He, Y ; Leong, T ; Kentish, SE ; Ashokkumar, M ; Manasseh, R ; Lee, J (AMER CHEMICAL SOC, 2013-10-17)
    When subjected to an ultrasonic standing-wave field, cavitation bubbles smaller than the resonance size migrate to the pressure antinodes. As bubbles approach the antinode, they also move toward each other and either form a cluster or coalesce. In this study, the translational trajectory of two bubbles moving toward each other in an ultrasonic standing wave at 22.4 kHz was observed using an imaging system with a high-speed video camera. This allowed the speed of the approaching bubbles to be measured for much closer distances than those reported in the prior literature. The trajectory of two approaching bubbles was modeled using coupled equations of radial and translational motions, showing similar trends with the experimental results. We also indirectly measured the secondary Bjerknes force by monitoring the acceleration when bubbles are close to each other under different acoustic pressure amplitudes. Bubbles begin to accelerate toward each other as the distance between them gets shorter, and this acceleration increases with increasing acoustic pressure. The current study provides experimental data that validates the theory on the movement of bubbles and forces acting between them in an acoustic field that will be useful in understanding bubble coalescence in an acoustic field.
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    Influence of acoustic pressure and bubble sizes on the coalescence of two contacting bubbles in an acoustic field
    Jiao, J ; He, Y ; Yasui, K ; Kentish, SE ; Ashokkumar, M ; Manasseh, R ; Lee, JY (Elsevier, 2015)
    In this study, the coalescence time between two contacting sub-resonance size bubbles was measured experimentally under an acoustic pressure ranging from 10kPa to 120kPa, driven at a frequency of 22.4kHz. The coalescence time obtained under sonication was much longer compared to that calculated by the film drainage theory for a free bubble surface without surfactants. It was found that under the influence of an acoustic field, the coalescence time could be probabilistic in nature, exhibiting upper and lower limits of coalescence times which are prolonged when both the maximum surface approach velocity and secondary Bjerknes force increases. The size of the two contacting bubbles is also important. For a given acoustic pressure, bubbles having a larger average size and size difference were observed to exhibit longer coalescence times. This could be caused by the phase difference between the volume oscillations of the two bubbles, which in turn affects the minimum film thickness reached between the bubbles and the film drainage time. These results will have important implications for developing film drainage theory to account for the effect of bubble translational and volumetric oscillations, bubble surface fluctuations and microstreaming.
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    Experimental and theoretical analysis of secondary Bjerknes forces between two bubbles in a standing wave
    Jiao, J ; He, Y ; Kentish, SE ; Ashokkumar, M ; Manasseh, R ; Lee, J (ELSEVIER, 2015-04)
    Bubbles in an acoustic field are affected by forces such as primary and secondary Bjerknes forces, which have been shown to be influenced by acoustic pressure, frequency, bubble size and separation distance between bubbles. However, such studies are predominantly theoretical, and are mostly focused on the sign reversal of the secondary Bjerknes force. This study provides experimental data on the effect of a range of bubble sizes (8-30 μm), distances (⩽0.2 mm), acoustic pressures (20-40 kPa) and frequencies (40-100 kHz) on the relative acceleration of two approaching bubbles. Under these conditions, only variations in the magnitude of the attractive force were observed. Using coupled equations of radial and translational motions, the acceleration and secondary Bjerknes force were calculated and compared to the experimental data. The variations in the magnitude of the secondary Bjerknes forces were explained by simulating bubble radius and coupled volume oscillation as a function of time.
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    The use of ultrasonic feed pre-treatment to reduce membrane fouling in whey ultrafiltration
    Koh, LLA ; Hanh, THN ; Chandrapala, J ; Zisu, B ; Ashokkumar, M ; Kentish, SE (ELSEVIER SCIENCE BV, 2014-03-01)