Peter R.
Birkin
*a,
Jack J.
Youngs
a,
Tadd T.
Truscott
b and
Silvana
Martini
c
aDepartment of Chemistry, University of Southampton, Southampton, UK, SO17 1BJ, UK. E-mail: prb2@soton.ac.uk
bDepartment of Mechanical and Aerospace Engineering, Utah State University, Logan, UT, 84322-4130, USA
cDepartment of Nutrition, Dietetics, and Food Sciences, Utah State University, Logan, UT, 84322-8700, USA
First published on 4th May 2022
Understanding the origins of the enhancement of crystallisation of a lipid (all-purpose shortening, APS) through the application of ultrasound is a fundamental pre-requisite for the exploitation of this technique in a wider context. To this end, we show here a number of measurements designed to probe the mechanisms responsible for this effect. For example, we show how the type of bubble cluster, produced at the sound source, alters the bubble population and residency time. In addition, to probe the various contributions to the enhanced crystallisation rate, isolation of the cluster environment below the piston like emitter (PLE) used as the ultrasonic source was shown to reduce the enhancement observed, but did not remove it entirely. This implied that the exposure of the liquid to pressure shocks and the environment around the cluster has a positive effect on the crystallisation kinetics. In turn the addition of extra seed crystals and mechanical agitation also enhances the rate of crystallisation. Finally, the time at which ultrasonic irradiation of the fluid is applied is shown to alter the kinetics observed. These observations suggest that two components are important: large bubble populations and mechanical effects on pre-existing crystals. These findings suggest that maximising these effects could be an eloquent way to enhance and control the material characteristics of materials produced in this manner.
While much progress has been made, several key questions remain. First, why does cavitation generated in this manner alter the kinetics of crystallisation so effectively? Second, what are the root causes of this alteration? Third, why does the particular cluster present give rise to varying degrees of effect observed? Some insight into the answers to these questions will now be given using experimental observations related to the lifetime of bubbles within the media and the effect of different treatment protocols on the crystallisation kinetics.
In these experiments the population of bubbles within the media was followed as a function of time over a ∼300 s time window after the exposure of the liquid to HIU was terminated. This is limited by the crystallisation of the media. Fig. 1 shows the results of these experiments for an APS sample exposed to different cavitation environments (specifically different cluster types generated by applying different HIU power levels to the sample). Fig. 1(a), for example, shows the response for the bifurcated streamer (BiS) generated under these conditions. In this case the smallest size range was observed. Note, the sensor and analysis used limits the lower detectable radius to the order of 45 μm. Hence, bubbles smaller that this will not be detected. This is particularly relevant to the BiS event as this appears to produce a mist of small bubbles. Fig. 1 also shows that as the power of the HIU source was increased the number and size of the detectable bubbles within the fluid increased. Fig. 2 shows how the number of events (determined as detectable transients above the baseline) within the media changed as the cluster type and the temperature of the APS sample were altered. It is also important to note that these measurements were performed after the HIU source had been terminated. This indicates that the bubble population is long lived and of varied size.
Turning to the lifetime of the events within the media, Fig. 2 shows how this varies with both the temperature and the HIU power (and type of cluster dynamics generated). Fig. 2 shows that the number of events rises with HIU power and the bubble retention time (defined as the time when transients above the baseline could be recorded) climbs as the HIU power was increased. The temperature of the media also has an effect. For example, Fig. 2(a) shows that the numbers of bubbles recorded are higher at lower temperatures while the retention time was generally smaller at higher temperature. This observation is presumably a result of differing generation conditions and retention effects within the lipid. The decreased viscosity of the lipid at higher temperature is expected to allow bubbles, particularly large ones in the population, to be lost through buoyancy effects over the time course of the experiment. For example, in APS the terminal velocity21–24 of a bubble with a radius of 100 μm can be calculated to be ∼0.91 mm s−1 at 30 °C while it is ∼0.64 mm s−1 at 20 °C (see SI data, Fig. S3, ESI†). In turn the smaller the bubble, the slower its rise time (see SI data, Fig. S2, ESI†). This implies that small bubbles produced in a lipid are slow moving over the timeframe of these experiments. However, other effects are worth consideration. For example, dissolution of bubbles in a gas saturated media is driven by a number of effects including the Ostwald coefficient25 of the gas in question, the diffusion coefficient of the gas in the media and the surface tension26 of the gas/liquid interface. Epstein and Plesset reported that the dissolution time under these conditions can be calculated.27 If we compare a bubble with a radius of 10 μm in water and oil, the dissolution times are significantly different (see SI data, Fig. S4, ESI†). For example, in water this can be calculated as 7.59 s while in oil the same sized bubble will have a dissolution lifetime of 168 s. This longer lifetime (which may be extended through shell effects28 in the crystallising media) of these particularly small bubbles suggests that cluster environments that produce these (e.g., the BiS system) may be more efficient in terms of primary nucleation processes in these lipid environments.
If the bubble population alone is the important factor, the results contained in Fig. 2 suggest that the f/4 cluster should behave very similarly to the BiS system. However, for the BiS event (produced at the lowest power, 10 W) an unusually high crystallisation rate was observed. This suggests that there may be other effects that come into play. In order to probe these effects, a series of experiments were performed. In the first set, the cluster was isolated from the crystallising media to separate the cluster and its bubble population from the main crystallising media. In the second set of experiments, the time of HIU application and the effect of added crystals were investigated.
To isolate the cluster/HIU tip from the crystallising media a dual compartment cell was utilised. Fig. 3 shows a schematic representation of the cell employed. This cell had two compartments. The PLE, cluster and associated bubbles were contained in the upper compartment. This contained sunflower oil. The lower, main chamber contained the APS sample. Fig. 4 shows the response of the hydrophone as the conditions were altered. In the absence of the upper containment chamber, Fig. 4 shows a typical response for the effect of HIU on a crystallising APS sample. The amplitude of the signal () falls with time. Fig. 4 also shows the effect of the upper chamber and the materials that are deployed. If the walls of the upper chamber are all 3 mm acrylic, the signal detected by the hydrophone (
) is severely reduced in comparison to the chamber free example. This indicates that as well as containing the bubble population and cluster in the upper chamber, sound propagation into the APS sample is severely restricted. However, if the base of the containment cell is changed to 0.3 mm ABS, the signal recorded by the hydrophone (
) is significantly higher. This change in behaviour is likely to be as a result of the material characteristics including the acoustic impedance of the materials involved and their relative attenuation effects. For example, the acoustic impedance of ACR is higher than ABS which would suggest more sound energy was reflected from the lipid/ACR interface. In addition, the attenuation coefficient of ABS is ∼11 dB cm−1vs. ∼12.4 dB cm−1 for ACR.29 These properties, in addition to the change of thickness of the samples, will contribute significantly to the observed change in sound transmission into the lower chamber. The effect on the crystallisation kinetics of the APS sample was also assessed under these conditions. Table 1 shows how the conditions employed affected the process.
Conditions | a G (%) | μ max (% min−1) | λ (min) |
---|---|---|---|
wHIU – no inner cell | 7.7 ± 0.3a,b | 0.63 ± 0.02c | 14.2 ± 0.6b |
wHIU – ACR inner cell | 8.0 ± 0.4a,b | 0.30 ± 0.03a | 27.1 ± 0.9a |
wHIU – ABS inner cell | 8.1 ± 0.1a | 0.43 ± 0.05b | 27.0 ± 0.5a |
woHIU – ABS inner cell | 8.2 ± 0.2a | 0.27 ± 0.01a | 28.4 ± 0.7a |
woHIU – no inner cell | 7.3 ± 0.5b | 0.29 ± 0.04a | 29 ± 1a |
These results were obtained from analysing the solid fat content as a function of time over a 60 minutes period using a pNMR approach. The results in Table 1 show that, while we restrict the cluster and bubble population to the upper chamber, transmission of sound to the APS has an effect on the crystallisation kinetics with the degree of transmission affecting the enhancement observed. First, the ABS cell gave a greater enhancement compared to the acrylic case and the without HIU (woHIU) controls in agreement with the sonic transmission data shown in Fig. 4. Second, and significantly, the isolation of the bubble population from the main APS chamber did not completely remove the enhanced crystallisation kinetics observed. This suggests that the progress of the shock like emission into the sample had an effect. This is presumably linked to the action of this physical effect on the crystallites within the media. Third, this physical effect is supported by observing the changes in the crystal structure of the resultant material. Fig. 5 shows the microstructure of samples taken under the various conditions explored.
Fig. 5 shows that the microstructure of the sample changes from the control woHIU towards the wHIU (no isolation cell) as expected from the kinetics data reported in Table 1. These results suggest that a clear effect of ultrasound is associated with the progression of the sound waves through the media. It is postulated here that the intense ‘shock like’ emissions which are associated with the cluster collapse at the PLE tip cause fragmentation of the pre-existing crystals in the sample and lead to secondary nucleation effects. To support this, a set of experiments were performed to investigate the effect of the presence of pre-existing nuclei on the process. Fig. 6 shows the acoustic data gathered from samples cooled to a set temperature and then exposed to ultrasound. However, the delay between the sample reaching the desired crystallisation temperature (here 30 °C) and the application of HIU was varied.
Fig. 6 shows that as the time period resting at 30 °C was increased (see Fig. S1, ESI† for cooling profile), the attenuation rate of the acoustic signal detected by the hydrophone increased.
The effective attenuation coefficient of the crystallising media (see ref. 18 for details) was calculated by fitting the hydrophone positive envelope to a second order model. Table 2 shows how the attenuation coefficient changes in response to the time at which HIU was applied to the sample. The data suggests that the attenuation of the hydrophone signal depends strongly on the time period spent at the crystallisation temperature. In turn, this indicates that the attenuation of the signal is largely in response to the interaction of the HIU with the crystal population. As a result, little attenuation was observed for the data recorded at 7 minutes. This is unsurprising as the temperature of the sample was ∼35 °C at this point. Under these conditions no crystals are expected and the attenuation coefficient, which appears strongly linked to the crystal population, is close to zero. However, as the sample temperature was reduced further, a greater crystal population (which has been associated with an increased effectiveness of HIU30–32) and hence acoustic attenuation was observed.
t cooling to 30 °C min−1 | k 2/V−1 s−1 | r 2 |
---|---|---|
7 | 0.03 ± 0.03a | 0.95 |
10 | 0.13 ± 0.05b | 0.97 |
15 | 0.28 ± 0.02c | 0.97 |
18 | 0.31 ± 0.02c | 0.97 |
Turning to the kinetics of crystallisation, Table 3 shows how the different conditions affect the rates observed. These results indicate that all periods tested showed faster crystallisation kinetics than the control. Critically the measurements at 7 minutes where the sample was ∼35 °C, shows a significant improvement in the rate of crystallisation (μmax) and a reduction in the induction time (λ). This shows that in the absence of observable crystals, HIU has a positive effect on the kinetics.30–32 This presumably indicates that the bubble population is able to contribute to primary nucleation of the sample. This is supported by the lifetime measurements shown in Fig. 2 which showed that detectable bubbles last for at least 300 s (and smaller bubbles will be longer lasting in comparison). This agrees with the buoyancy and dissolution effects previously discussed. Hence the bubble population generated at 7 mins will last into the time period when crystallization occurs. This supports the primary nucleation effect within the sample. However, as the time period before the application of HIU was increased, significant attenuation of the hydrophone signal was observed (see Table 2) and a greater acceleration in the crystallisation kinetics was seen (see Table 3). This observation suggests that HIU has a significant secondary nucleation effect whereby crystals are fragmented by the conditions present (either shear effects around oscillating bubbles present or the large pressure shocks generated by the cluster collapses, supported by the results shown in Table 1).
Conditions [temp/°C] | a G (%) | μ max (% min−1) | λ (min) |
---|---|---|---|
woHIU | 7.3 ± 0.5a | 0.29 ± 0.04a | 29 ± 1a |
wHIU 7 min [35] | 7.6 ± 0.1b | 0.49 ± 0.03b | 21.5 ± 0.3b |
wHIU 10 min [30] | 7.7 ± 0.1b | 0.52 ± 0.06b | 18.8 ± 0.3c |
woHIU 15 min [30] | 7.7 ± 0.3b | 0.63 ± 0.02c | 14.2 ± 0.6d |
woHIU 18 min [30] | 7.6 ± 0.4b | 0.76 ± 0.08d | 14.5 ± 0.9d |
In order to probe these conditions further, polarised light imaging of samples after different sets of conditions at the point of HIU application and after 60 minutes was undertaken. Fig. 7 shows the images obtained. These images show that no observable crystals are present at 7 min (Tm) in agreement with the acoustic attenuation data shown in Fig. 6. Fig. 7 also shows the changes in the microstructure of the sample after 60 mins was similar to the woHIU control. However, the enhanced kinetics observed at this point suggest that the primary (bubble induced) crystallisation rate does not have a marked effect on the resultant microstructure. This observation suggests that primary nucleation occurs in both the treated and untreated samples in a similar way as expected. However, as the crystal population increased (see Fig. 7, panel ‘A’, TSC, FC and XC), crystals are observed prior to the application of ultrasound. The size and number of crystals is also seen to increase as the cooling period was increased. The application of ultrasound (see Fig. 7 panel ‘B’ for the structure after 60 minutes) shows that these crystals are broken up significantly with the average crystal size reducing visibly while the packing density seems to be greater for those samples held for longer at 30 °C before the application of ultrasound (10 s, 75 W in all cases) to the system. Hence, the more delayed the application of ultrasound (and the generation of a significant crystal population) yields significant secondary30 nucleation effects further enhancing the crystallisation kinetics (see Table 3) and altering the microstructure (see Fig. 7).
It could be envisaged that this change in microstructure in response to HIU (and enhanced secondary nucleation combined with primary nucleation effects) can alter the material properties of the resultant material. Fig. 8 shows how the material characteristics are affected by these effects. First, the elastic modulus is the same for the control (wo) and 7 minute experiment. However, where secondary effects are expected (when there is a significant crystal population when the sample was exposed to ultrasound), the elastic modulus increases significantly and continues to increase as the pre-HIU crystal population increases. Second, there is an increase in the hardness of all samples when exposed to HIU which increases further as the cooling time was extended. This presumably indicates that any increase in the kinetics (both primary and secondary) affects this parameter.
The evidence presented suggests that both primary and secondary effects are triggered by the application of HIU to the APS samples with the exact combination dictated by the crystal population within the sample at the point of HIU application. To test this assertion further, a set of experiments were performed where APS crystals were added to the media (by the addition of a small quantity of APS allowed to crystallise in the absence of HIU for 25 minutes) and the effect of this addition was investigated. Table 4 shows the effect of these additions. The addition of crystals with stirring or HIU display a greater rate than HIU alone. This suggests that the crystal population and the mechanical agitation of this media plays a significant role in the enhancement of the crystallisation kinetics.33,34 The material characteristics of the samples also showed some variation. However, the HIU treated samples showed the greatest change in agreement with the similarity of the PLM images of these samples (see SI Fig. S5 and S6, ESI† respectively).
Conditions | a G (%) | μ max (% min−1) | λ (min) |
---|---|---|---|
woHIU | 7.3 ± 0.5a | 0.29 ± 0.04a | 29 ± 1a |
10 cm3 crystal addition, 200 rpm | 7.8 ± 0.1b | 0.77 ± 0.03b | 18.4 ± 0.3b |
10 cm3 crystal addition, wHIU | 7.6 ± 0.2b | 0.72 ± 0.05b | 14.3 ± 0.4c |
wHIU | 7.7 ± 0.4b | 0.63 ± 0.08c | 14.2 ± 0.6c |
Turning to the effect of stirring the sample (and the induced shear, which has been shown to be effective at accelerating crystallisation of IESBO34), a set of experiments were performed to probe this effect. Table 5 shows the results of this investigation. These results indicated that mild stirring (200 rpm) was not effective in enhancing the crystallisation process while higher rates (600 rpm) showed a significant increase.
Conditions | a G (%) | μ max (% min−1) | λ (min) |
---|---|---|---|
woHIU | 7.3 ± 0.5a | 0.29 ± 0.04a | 29 ± 1a |
200 rpm | 8.1 ± 0.2b | 0.33 ± 0.04a | 29 ± 2a |
600 rpm | 7.9 ± 0.3b | 0.42 ± 0.06b | 23 ± 2b |
wHIU | 7.7 ± 0.3b | 0.63 ± 0.02c | 14.2 ± 0.6c |
This suggests that mechanical agitation, if of sufficient intensity, is effective at accelerating the crystallisation process. Interestingly, the velocity of the stirrer tips for 600 rpm can be estimated at ∼0.6 m s−1. This is slightly lower than the streamer velocity produced by the cluster.16 However, the rates of crystallisation are far lower than for the HIU case suggesting that it is not only the flow but the other physical effects of the cluster (shocks, local shear etc.) which need to be considered.
The results presented here all suggest that the effect of ultrasound on fat crystallisation is multifaceted. The cavitation environment can generate primary nucleation sites through the production of a significant long lived bubble population. In turn the action of the cluster, which produces a strong and regular shock like emission, further accelerate the crystallisation kinetics through, presumably, secondary nucleation effects. At this point some regard to the structure and nature of the APS sample should be noted.
It has been shown that crystal fragmentation35–37 can be generated in a variety of systems and this process is related to the mechanical properties of the sample. For example, Kim et al. showed that salt crystals could be fragmented in an ultrasonic field with the rate of fragmentation related to the Vickers hardness and Youngs modulus of the material employed.36 If we consider APS in comparison, it is interesting to note that the crystals38,39 (or spherulites) are really conglomerates of smaller TAG nanocrystals held together by weak van der Waals forces.39 Correspondingly, the Youngs modulus38 of a single spherulite is nearly 105 smaller38 at ∼1 MPa compared to a material like NaF (77.5 GPa), for example. Hence, we can expect that such APS spherulites will be fragmented easily in comparison by the forces generated by the HIU treatment of the sample. This agrees with the observation in the data provided above. However, further insight is possible. The bifurcated streamer (BiS) system which can be generated in these oil systems was shown18 to be highly active in the acceleration of APS crystallisation, in excess of what would be expected on a power basis alone (compared to other clusters). The reasons for this are now, perhaps, more apparent.
The BiS system possess two of the key components that have been shown to enhance APS crystallisation. First, it generates a set of small gas bubbles which populate the fluid. Thus, this dual streamer could be expected to enhance primary nucleation events in excess of a single cluster stream. Second, the strong pulse like shock generated by the cluster collapse (the strongest globally measurable emission in the system) is more frequent in comparison to all other clusters. Indeed, the BiS's anatomy consists of two clusters oscillating at an f/2 period but 180° out of phase with one another. This essentially results in a shock emission into the sample at f, the ultrasonic source, (and hence a weak emission). The fluid, bubble population and crystals are hence exposed to a pressure shock every cycle of the source which is more often than any other cluster environment measured for these oil clusters. The shear forces around the clusters themselves may also play a role, as yet to be quantified. However, the BiS event has two clusters so may also be at an advantage here. Finally, the bubbles within the media will also be an active part of the system with their motion, presumably classed as non-inertial cavitation, in the associated sound field will produce local shear40 through their motion and the associated microstreaming.7 Hence, these events will contribute to the secondary nucleation effects. Fig. 9 brings the observed components together and highlights the proposed important factors and the characteristic of each environment.
Finally, the contribution of the relative shear of the differing components, whether that is the fluid driven in the streamer, the motions around the cluster itself or the motion of gas bubbles in response to the streamer or the shock emission into the bulk, all need to be assessed and quantified. This remains a fascinating question in relation to the relative effects of the physical processes within the crystallising media.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d1cp05701d |
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