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Pneumatic foam generation in the presence of a high-intensity ultrasound field
Ultrasonics Sonochemistry 12 (2005) 385–393 www.elsevier.com/locate/ultsonch
Pneumatic foam generation in the presence of a high-intensity ultrasound field K.S. Lim, M. Barigou
*
Department of Chemical Engineering, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 16 February 2004; accepted 4 May 2004 Available online 15 July 2004
Abstract Designer foams find applications in a wide range of industries. Foam quality is mostly determined by its complex cellular structure which defines its texture, rheology and stability. In addition to formulation design, the formation process is crucial to the development of a foam with an optimum structure. There is, therefore, a need for techniques that can assist in the generation of controlled foam structures. The work described in this paper demonstrates the potential of using high-intensity ultrasound to control foam structure during production. Foam generated in the presence of ultrasound usually exhibits a narrower bubble size distribution, i.e. a more uniform texture. Such enhanced homogeneity in texture is desirable to reduce the presence of aesthetically unattractive large cavities, and to reduce the destabilising effects of foam coarsening. In addition, a smaller mean bubble size and a slower rate of foam collapse usually result when ultrasound is applied. The work shows the effects on foams stabilised with different surfactants. 2004 Elsevier B.V. All rights reserved. Keywords: Foam; Ultrasound; Foam stability; Foam structure; Cell size distribution
1. Introduction Gas–liquid foams are colloidal in nature, consisting of a large gas volume fraction dispersed within a continuous liquid in the form of bubbles of a size ranging typically from 10 lm to several millimetres. The continuous liquid phase exists within a network of thin liquid films which exhibit complex hydrodynamics, and Plateau borders which denote the regions of intersection of these thin films. Foams derive their stability by offering resistance to external and internal stresses that can cause foam destruction. In most practical systems the foam stabilising component is either a soluble surfactant, a finely divided solid, or both. The adsorption of surface
*
Corresponding author. Tel.: +44 0121 414 5277/3344; fax: +44 0121 414 5324. E-mail address: [email protected] (M. Barigou). 1350-4177/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2004.05.001
active material to bubble surfaces acts to provide the foam stability although foams remain manifestly unstable. Foams can be found in many processes and can occur naturally or by design. In many situations foams are unwanted; for example, expensive antifoam agents are usually required to limit the foam volume in bioreactors. Unlimited foam formation and insufficient foam collapse can have disastrous effects in an aerated system such as a fermentation process and is one of the main operating problems in commercial fermentations. A number of methods, chemical and mechanical, are used to control these unwanted foams and the search for more efficient and cost-effective methods continues. Previous research has demonstrated the effectiveness of high-intensity ultrasound in destabilising unwanted foams, both static and dynamic [1,2]. The ability to suppress dynamic foams is particularly suited to processes that require continuous defoaming such as bioreactors.
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Nomenclature Cs Cv d10 d32 di H H0 K M
surfactant concentration coefficient of variation number mean bubble diameter Sauter mean bubble diameter equivalent circular diameter of individual bubble i foam height initial foam height kurtosis median
The technique, being non-invasive, has a crucial advantage over other chemical and mechanical methods in processes where contamination cannot be tolerated. The use of ultrasound to positively influence the structure of foams during their generation does not seem to have been studied before, however. The production of designer foamed products is a major activity across the whole process industries including food, cosmetics, personal care and pharmaceuticals, which often derive a wide range of different textures from basically similar starting components. The direct or indirect use of gas in developing foam structure is a principal element of the process, and bubble mechanics are essentially the same across all the sectors concerned. Despite their widespread use, however, a number of fundamental issues relating to foams remain poorly understood including the production and control of bubbles, how they are entrained, broken up, dispersed, stabilised, measured, how they contribute to product structure, appearance, rheology, and how they can be fully exploited to produce novel textures and product differentiation. Foam structures, for example, in food products such as ice cream, mousses and confectionery offer many benefits including improved product volumes and, thus, a reduction in product density, improvements in the rheology and textural quality and, thus, taste and mouthfeel of the product, increased surface area, alteration of digestibility and shelf-life due to increased porosity, and modulated flavour intensity. In addition to the association with high quality, foams also offer attractive aesthetics as well as great possibilities for novel structures and textures. This is not specific to food foams but is equally valid for many other foam-based products such as cosmetics and personal care products. One way of controlling these features is through formulation design. However, the foam generation process itself is crucial in achieving foam products with enhanced quality and stability, and it would thus be of great industrial benefit to develop processing methods that can assist in the generation of foams with controlled structures.
n Q S r
number of bubbles in a sample gas flow rate skewness standard deviation
Abbreviations CTAB cetyl trimethyl ammonium bromide SDS sodium dodecyl sulphate US ultrasound
This paper reports the results of a preliminary study which aims to demonstrate the potential of using highintensity ultrasound to control foam structure during pneumatic generation.
2. Materials and methods 2.1. Experimental setup Foam was generated pneumatically in a glass column of 43.5 mm diameter and 750.0 mm height, immersed in an ultrasound bath to a depth of 120 mm, as shown in Fig. 1. The bath (Grant ultrasonic bath model XB 22) had dimensions of 505 mm length, 300 mm width, and
Fig. 1. Foam generation in an ultrasound bath: (1) rotameter; (2) control valve; (3) ultrasound bath; (4) sparger; (5) surfactant solution; (6) foam; (7) camera; (8) water.
K.S. Lim, M. Barigou / Ultrasonics Sonochemistry 12 (2005) 385–393
150 mm depth with a capacity of 22 L. It generates ultrasonic waves at a fixed frequency of 38 kHz, and the rootmean-squared ultrasonic power generated is 238 W with the peak power being equivalent to 475 W. The column was positioned at the point of maximum ultrasound intensity determined by mapping out the ultrasound field inside the bath using a sheet of aluminium foil, as described by Mason and Peters [3]. A sintered glass disc of porosity 40–100 lm was used to sparge nitrogen gas at a rate within the range Q = 0.13–0.50 L min1 in a surfactant solution and, thus, generate the foam. Three different types of surfactants were separately used as foam stabilisers: sodium dodecyl sulphate (SDS) which is anionic, cetyl trimethyl ammonium bromide (CTAB) which is cationic, and triton X100 which is non-ionic. All surfactants were dissolved in distilled water at different concentrations, as shown in Table 1. Surfactant concentration, Cs, is also expressed in cmc, where cmc is the critical micelle concentration of the particular surfactant in pure water, corresponding to the minimum static surface tension of the mixture. The solutions were all of Newtonian rheological character and the values of their viscosity and equilibrium surface tension are given in Table 1. The foam column was always thoroughly washed between experiments involving a change in surfactant type or concentration. Since ultrasound produces microbubbles in liquids via the process of cavitation, it was important to check whether cavitation contributed to the foaming process, i.e. whether any cavitation bubbles were causing additional generation of foam to that produced by the nitrogen gas injected. A test was conducted whereby ultrasound was applied to the surfactant solution in the column with the nitrogen gas supply switched off. The test was carried out repeatedly over extended periods of time exceeding those used for the normal foaming experiments described above, but no foaming was observed. Therefore, it could be safely assumed that, under the range of conditions investigated, any ultrasound effects on the pneumatically generated foams could not
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have arisen, wholly or partly, from possible foaming caused by cavitation. 2.2. Foam bubble size characterisation Bubble size distribution was determined by photographing the foam through the transparent column wall using a digital camera. Photographs were taken using fibre-optics back-lighting, at a position approximately 20 mm above the liquid surface inside the column. As foams are inherently unstable, this measure was adopted to ensure that only freshly generated foam was photographed, thus, capturing any ultrasound effects on foam structure at the earliest stage possible before any foam ageing effects (drainage, coarsening) set in. The digital images were analysed on Q-Win-Pro Leica software. Due to the transparent nature of the foam, the image contrast was not sufficient to enable automatic identification of most bubble contours by the image analysis software. Furthermore, the software was unable to automatically resolve individual objects with a common boundary such as bubbles in a foam. Thus, the bubbles at the wall, which had sharp contours and were clearly visible to the naked eye, were traced manually. Some bubbles in the bulk of the foam were visible through the bubbles at the wall, but they were faint and out of focus and did not interfere with the foreground measurements. All the bubbles on a given image were measured by using several copies of the same picture and tracing alternate bubbles. The images were then scanned and processed on the image analyser to give the equivalent circular diameters of the bubbles, which were then collated into one distribution. 2.3. Foam collapse measurement The effect of using ultrasound on the stability of the foams generated was assessed by measuring their collapse and comparing it to that of foams obtained from the same foaming system in the absence of ultrasound. In this experiment, a foam bed of height H0 = 25.0 cm
Table 1 Foaming systems used Surfactant
Surfactant concentration, Cs
Static surface tension (mN m1)
Viscosity (mPa s)
wt.%
g L1
cmc
SDS
0.12 0.24
1.18 2.36
0.5 1.0
49.6 37.3
1.15 1.09
Triton X-100
0.017 0.033 0.066
0.16 0.33 0.66
0.5 1.0 2.0
32.7 29.8 29.7
1.05 1.10 1.25
CTAB
0.017
0.16
0.5
41.2
1.10
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was generated; the gas flow and ultrasound source (when used) were then immediately switched off, and foam collapse was measured by monitoring the decline in foam height with time. The duration of the experiment depended on the nature of the foam and its stability, and varied from many minutes to several hours. 3. Results and discussion
"
# X d i d 10 4 nðn þ 1Þ K¼ ðn 1Þðn 2Þðn 3Þ r
3.1. Descriptive statistics In analysing the foam pictures to characterize the bubble size distribution, a number of statistical parameters were used and are defined as follows, where di is the equivalent circular diameter of an individual bubble i, and n is the number of bubbles in the sample: (i) Number mean bubble diameter: n P di i¼1 ð1Þ d 10 ¼ n (ii) Sauter mean bubble diameter: n P d 3i i¼1 d 32 ¼ P n d 2i
ð2Þ
i¼1
While the d10 gives the average bubble size, d32 is considered to be a better representative of the mean bubble diameter as it is directly related to the ratio of gas holdup and interfacial area in the foam. (iii) Standard deviation: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uP un u ðd i d 10 Þ2 t r ¼ i¼1 n1
This parameter gives a measure of the asymmetry of the distribution. The direction of skewness is to the tail; the larger the number, the longer the tail. If S is positive, skewness is to the right and the tail on the right side of the distribution will be longer. If S is negative, skewness is to the left and the tail on the left side will be longer. A normal distribution has S = 0. (vii) Kurtosis:
ð3Þ
2
3ðn 1Þ ðn 2Þðn 3Þ
ð6Þ
This is a measure of the combined weight of the tails in relation to the rest of the distribution. As the tails of a distribution become heavier, K will increase, and as the tails become lighter, K will decrease. A histogram with a normal distribution has K = 0. If the distribution is peaked (tall and slender), K will be greater than 0, and is said to be leptokurtic. If the distribution is flat, K will be less than zero, and is said to be platykurtic. To determine the minimum number of bubbles required for statistical analysis, samples of 30, 50, 100, 150, 200, 400, 600 and 800 were analysed. The results plotted in Fig. 2 show that the main parameters of the bubble size distribution, i.e. d10, d32 and r remain unaffected when a sample of 200 bubbles or more is used. The bubble size distribution curves obtained from samples of 200 bubbles or more also clearly coincide, as shown in Fig. 3. Some statistical bias can be expected from smaller samples, however. Therefore, a sample size of 200 bubbles was deemed sufficient to avoid statistical bias when determining foam bubble size distribution in a given experiment.
(iv) Coefficient of variation: r d 10
ð4Þ
Whereas r gives a measure of how different the values are from each other and from the mean (d10), the coefficient of variation is a measure of the spread of the distribution relative to its mean, i.e. a measure of the significance of r in relation to the mean. The larger the Cv, the more significant the r, relative to the mean. (v) Median: this is the middle number, M, in the data set when the data points are arranged from low to high. It is worth recalling that for a normal distribution, the median and the mean coincide, i.e. M = d10. (vi) Skewness: X d i d 10 3 n S¼ ð5Þ ðn 1Þðn 2Þ r
0.6
Mean Bubble Diameter (mm)
Cv ¼
0.5 0.4
0.3 0.2 0.1 0.0 0
100
200
300
400
500
600
700
800
900
Number of Bubbles Fig. 2. Effect of sample size on foam bubble parameters: h d10; · d32; n r.
K.S. Lim, M. Barigou / Ultrasonics Sonochemistry 12 (2005) 385–393
389
0.8
0.5
0.7 0.6
Number Fraction
Number Fraction
0.4
0.3
0.2
0.1
0.5 0.4 0.3 0.2 0.1
0.0 0.0
0.2
0.4
0.6
0.8
1.0
Bubble Diameter, d (mm)
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Bubble Diameter, d (mm)
Fig. 3. Bubble size distributions showing insignificant bias for samples of 200 bubbles or more: 200 bubbles; j 400 bubbles; m 600 bubbles; s 800 bubbles.
3.2. Ionic surfactants: SDS and CTAB solutions A significant reduction in bubble size, as reflected in both d10 (22%) and d32 (24%), was observed when the anionic surfactant SDS was used at a concentration of 0.5 cmc and Q = 0.13 L min1, while the coefficient of variation Cv decreased from 0.22 to 0.14, thus resulting in a finer and more uniform foam texture as exemplified by the foam images shown in Fig. 4, and their corresponding bubble size distribution curves depicted in Fig. 5. It should be pointed out that the generation of foam with a uniform bubble size is very difficult under normal circumstances. Such enhanced homogeneity in texture is highly desirable to reduce the presence of aesthetically undesirable large cavities, and to enhance foam stability by reducing the destabilising effects of coarsening or Ostwald ripening, i.e. the growth of large gas bubbles at the expense of smaller ones due to gas diffusion driven by the higher Laplace pressure of the gas in the smaller cells.
Fig. 5. Effect of ultrasound on foam bubble size distribution: SDS 0.5 cmc; Q = 0.13 L min1; m without ultrasound; h with ultrasound.
The statistics used to describe and compare the cell size distributions are summarized in Table 2. The bubble size distribution of foam generated in the absence of ultrasound is positively skewed, having a long tail towards the larger diameters (Fig. 5). Bearing in mind that a normal distribution has a skewness of zero, high skewness values above zero indicate the presence of some large bubbles in the foam resulting in the median M being greater than the number mean bubble diameter d10. Ultrasound reduced skewness considerably, thus eliminating most of these large bubbles (Table 2). Kurtosis values are positive for foams generated with or without the assistance of ultrasound, so the distributions are always leptokurtic, but the value in the case of ultrasound is considerably smaller indicating a distribution which is much closer to the normal distribution. In fact, the absolute value of Kurtosis is the classical measure of non-gaussianity, i.e. the degree of departure from the bell-shaped normal distribution. The results obtained indicate a taller narrower bubble size distribution with much thinner tails and a sharper peak (Fig. 5).
Fig. 4. Foam images: SDS 0.5 cmc; Q = 0.13 L min1.
0.70 1.35 1.95 2.21 0.62 1.77 0.87 1.47 1.19 1.47 1.22 1.11 0.70 0.40 0.36 0.70 0.46 0.44 0.31 0.41 0.17 0.32 0.43 0.17 0.23 0.20 0.06 0.24 0.24 0.08 0.93 0.77 0.48
1.1
0.76 0.56 0.45
0.75 0.49 0.37
0.91 0.67 0.39
1.29 11.99 0.80 2.54 0.33 0.43 0.14 0.25 0.05 0.11 0.51 0.44
0.34
0.35
4.37 3.27 1.95 11.94 0.82 0.77 0.72 2.14 1.72 1.63 1.26 2.85 0.32 0.37 0.31 0.31 0.38 0.36 0.40 0.34 0.14 0.14 0.17 0.24 0.22 0.33 0.28 0.34 0.05 0.05 0.05 0.08 0.09 0.13 0.12 0.12 0.45 0.48 0.51 0.48 0.41 0.39 0.43 0.32
0.32 0.38 0.31 0.37
0.34 0.40 0.33 0.37
Without US Without US With US Without US
With US
Without US
With US
Without US
With US
Without US
With US
Without US
With US
K S M Cv r d32 (mm) d10 (mm)
When the concentration of SDS was increased to 1.0 cmc, though the effect of ultrasound on d10 was negligible, there were significant reductions in both d32 and Cv. It is worth pointing out that although d32 is a better representative of the mean bubble diameter, as it is directly related to the interfacial area and gas holdup in the foam, it still does not capture all of the information about the foam texture, and a detailed and complete picture of the foam structure can only be obtained by considering the whole bubble size distribution and its descriptive statistics, as summarized in Table 2. Similar to the 0.5 cmc SDS systems, the foam generated in the presence of ultrasound has a much smaller skewness value. Thus, the overall effect of ultrasound in producing a more homogeneous foam structure, much less skewed and more clustered around the mean, remains significant. The foam collapse experiments also revealed some positive ultrasound effects. In general, a typical foam collapse curve consists of three stages [4]: (i) an initial plateau region where the foam height remains constant over a period ranging from a few minutes to many minutes depending on the nature of the foam; during this phase most of the liquid contained in the foam drains but no foam breakage occurs; (ii) at the end of this period foam films have thinned sufficiently to become unstable and foam breakage begins in the driest region at the top; foam collapses at a fairly fast rate and foam height decreases almost linearly, but not quite, whilst the remaining liquid drains; (iii) in the third part of the curve foam collapse proceeds, again in an approximately linear manner, but very slowly with almost no drainage taking place. Depending on the nature of the foam, not all three regions are always observed; for example, in highly unstable or very dry foams, the initial plateau may not be present as the foam starts to collapse immediately on formation. Foams produced under sonication appear to collapse more slowly and have a longer lifetime than native foam, as shown in Figs. 6 and 7 for two different SDS
1.0
0.8
H /H o
0.13 0.13 0.13
0.13
0.7
0.13 0.13 0.24 0.50
Q (L min1)
0.9
0.6 0.5
0.5 1.0 2.0
0.5
0.3
0.5 1.0 0.5 0.5
Cs (cmc)
0.4
0.2 0.1
Triton X-100
CTAB
SDS
0.0
Surfactant
Table 2 Statistical parameters for foam bubble size distributions
3.28 1.17 1.46 7.64
K.S. Lim, M. Barigou / Ultrasonics Sonochemistry 12 (2005) 385–393
With US
390
0
20
40
60
80
100
120
140
160
Time (min) Fig. 6. Foam collapse curves: SDS 0.5 cmc; Q = 0.13 L min1; H0=25.0 cm; m without ultrasound; h with ultrasound.
K.S. Lim, M. Barigou / Ultrasonics Sonochemistry 12 (2005) 385–393 1.1
391
0.6
1.0
0.5
Number Fraction
0.9 0.8
H /H o
0.7 0.6 0.5 0.4 0.3
0.4 0.3 0.2 0.1
0.2 0.1
0.0
0.0 0
50
100
150
200
250
300
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Bubble Diameter, d (mm)
Time (min)
Fig. 7. Foam collapse curves: SDS 1.0 cmc; Q = 0.13 L min1; H0 = 25.0 cm; m without ultrasound; h with ultrasound.
Fig. 9. Effect of ultrasound on foam bubble size distribution: SDS 0.5 cmc; Q = 0.24 L min1; m without ultrasound; h with ultrasound.
concentrations. Thus, ultrasound seems to have the added benefit of enhancing foam stability. To investigate the effect of the pneumatic rate of foam formation, the gas flowrate, Q, was increased to 0.24 and 0.50 L min1 for SDS solutions of 0.5 cmc concentration. Similar results were obtained whereby significant reductions in the mean bubble size (d10 and d32), skewness value and coefficient of variation were obtained. The foams generated when ultrasound was applied exhibited a finer and more homogeneous texture, as demonstrated by the images in Fig. 8, and their corresponding bubble size distribution curves in Fig. 9. There is a clear narrowing of the foam bubble size distribution when ultrasound is applied reflecting the enhanced homogeneity observed in the images of Fig. 8, and a reduction in the mean bubble size reflecting the finer texture of the foam. Consequently, the ultrasound effects remain consistent at different rates of foam generation. Foams stabilized with the cationic surfactant CTAB exhibited similar results under sonication. The effects are clearly demonstrated by the images in Fig. 10 and
the bubble size distribution curves in Fig. 11. Sonication obviously shifted the bubble size distribution curve of foam towards the lower end of the spectrum and eliminated most of the larger bubbles, thus, reducing skewness substantially (Table 2). There is also an order of magnitude reduction in kurtosis, thus bringing the bubble size distribution much closer to a normal distribution. Hence, ultrasound appears to be just as efficient with cationic surfactant foams as it is with anionic ones. 3.3. Non-ionic surfactant: Triton X-100 The non-ionic surfactant Triton X-100 was used at concentrations of 0.5, 1.0 and 2.0 cmc at a gas flowrate of 0.13 L min1. The statistical parameters of the foams generated are summarized in Table 2. The effect of ultrasound on d10 was negligible at the low surfactant concentration of 0.5 cmc, whilst significant reductions of 22% and 18% were obtained at concentrations of 1.0 and 2.0 cmc, respectively. Similarly, there are significant reductions in d32 of 24% and 19%, respectively, at these two concentrations, but little effect at 0.5 cmc.
Fig. 8. Foam images: SDS 0.5 cmc; Q = 0.24 L min1.
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K.S. Lim, M. Barigou / Ultrasonics Sonochemistry 12 (2005) 385–393
Fig. 10. Foam images: CTAB 0.5 cmc; Q = 0.13 L min1. 0.8
0.5
0.4
0.6
Number Fraction
Number Fraction
0.7
0.5 0.4 0.3 0.2
0.3
0.2
0.1 0.1 0
0.0 0
0.2
0.4
0.6
0.8
1
0
0.2
Bubble Diameter, d (mm)
Fig. 11. Effect of ultrasound on foam bubble size distribution: CTAB 0.5 cmc; Q=0.13 L min1; m without ultrasound; h with ultrasound.
Despite the reductions observed in the mean bubble size, however, the results showed that the application of ultrasound did not bring about a significant improvement in the quality of the foam texture, as shown in Fig. 12. Despite a slight narrowing in the bubble size distribution and a sharper peak, all the other statistics retained more or less similar values (Table 2). Therefore, it can be concluded that although the uniformity of foam texture was significantly enhanced by the sonication process under all conditions in the case of ionic surfactants (anionic and cationic), this was not the case for the non-ionic surfactant Triton X-100. The major reasons behind this irregularity are difficult to explain a` priori, but must be related to surfactant molecular behaviour at the film interfaces. However, given that the range of experiments in this study was rather limited, one should not exclude the possibility that under different conditions (e.g. higher frequency or power) ultrasound might have more pronounced effects on Triton foams, especially considering that some reduction in bubble size has actually been achieved. Overall, a close examination of the results shown in Table 2 seems to suggest that the degree of ultrasound
0.4
0.6
0.8
1
1.2
1.4
1.6
Bubble Diameter, d (mm)
Fig. 12. Effect of ultrasound on foam bubble size distribution: Triton X-100 1.0 cmc, Q = 0.13 L min1; m without ultrasound; h with ultrasound.
influence on the mean bubble size of foam may be a function of the bubble size that can be generated in the absence of an ultrasound field, i.e. that there is probably an optimum bubble size at which the bubble size reduction effects of ultrasound will be maximum. In the absence of ultrasound, foam bubble size is controlled by the porosity of the foam generator, gas flowrate, surfactant type and concentration. Although the available data set is rather limited, and therefore firm conclusions cannot be readily drawn, a significant reduction in foam cell size (approximately 20–30%) was generally obtained only when the original mean bubble size of the foam (d10) was greater than about 0.4 mm. On the other hand, significant reductions in d32, the better representative of the mean foam bubble size, were nearly always achieved, however. This point needs more detailed investigation.
4. Conclusions High-intensity ultrasound affected the generation of foam in various ways depending on the formulation of
K.S. Lim, M. Barigou / Ultrasonics Sonochemistry 12 (2005) 385–393
the foam. Under most conditions investigated, highintensity ultrasound was able to reduce foam mean bubble size and enhance foam texture homogeneity, which is highly desirable to reduce the presence of aesthetically unattractive large cavities and to enhance foam stability by reducing the destabilising effects of Ostwald ripening. Foam collapse curves showed that foams generated in the presence of an ultrasound field collapsed more slowly than native foams. The effects were consistent at different foam generation rates, and were similar for the anionic SDS and cationic CTAB surfactant systems studied. However, the effects were not as significant in the case of the non-ionic surfactant Triton X-100. The work presented has served to demonstrate the positive effects that power ultrasound can have on the generation of foams with controlled structures. However, more detailed work is necessary to fully demonstrate the potential of the technique. In particular, the results presented here were all obtained at one ultrasound frequency. It is plausible that higher frequencies may have a much greater impact on foam structure, and these need to be investigated up to the MHz range and probably beyond. It would also be important to fully investigate the effect of increasing ultrasound amplitude on both foam homogeneity and bubble size,
393
as it is not obvious that a higher ultrasound power will always generate a reduction in bubble size, or better foam homogeneity. It is quite conceivable that ultrasound may provide a novel elegant technique for controlling foam texture by affecting the initial bubble formation and/or by affecting bubble coalescence, and hence offer improved product quality, enhanced foam stability and product shelf-life. If fully proven, the results should eventually advance the scientific ability within industry to produce foambased products with improved structures and stability.
References [1] N. Sandor, H.N. Stein, Foam destruction by ultrasonic vibrations, Journal of Colloid and Interface Science 161 (1993) 265–267. [2] M.D. Morey, N.S. Deshpande, M. Barigou, Foam destabilization by mechanical and ultrasonic vibrations, Journal of Colloid and Interface Science 219 (1999) 90–98. [3] T.J. Mason, D. Peters, Practical sonochemistsry: uses and applications of ultrasound, second ed., Horwood, Chichester, 2002. [4] N.S. Deshpande, M. Barigou, Foam formation, drainage and collapse in the presence of antifoams, in: S. Perucci (Ed.), Fourth Italian Conference on Chemical and Process Engineering––Selected Papers, AIDIC-Italian Association of Chemical Engineering Conference Series, vol. 4, Milano, Italy, 1999, pp. 295–301.
Pneumatic foam generation in the presence of a high-intensity ultrasound field K.S. Lim, M. Barigou
*
Department of Chemical Engineering, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 16 February 2004; accepted 4 May 2004 Available online 15 July 2004
Abstract Designer foams find applications in a wide range of industries. Foam quality is mostly determined by its complex cellular structure which defines its texture, rheology and stability. In addition to formulation design, the formation process is crucial to the development of a foam with an optimum structure. There is, therefore, a need for techniques that can assist in the generation of controlled foam structures. The work described in this paper demonstrates the potential of using high-intensity ultrasound to control foam structure during production. Foam generated in the presence of ultrasound usually exhibits a narrower bubble size distribution, i.e. a more uniform texture. Such enhanced homogeneity in texture is desirable to reduce the presence of aesthetically unattractive large cavities, and to reduce the destabilising effects of foam coarsening. In addition, a smaller mean bubble size and a slower rate of foam collapse usually result when ultrasound is applied. The work shows the effects on foams stabilised with different surfactants. 2004 Elsevier B.V. All rights reserved. Keywords: Foam; Ultrasound; Foam stability; Foam structure; Cell size distribution
1. Introduction Gas–liquid foams are colloidal in nature, consisting of a large gas volume fraction dispersed within a continuous liquid in the form of bubbles of a size ranging typically from 10 lm to several millimetres. The continuous liquid phase exists within a network of thin liquid films which exhibit complex hydrodynamics, and Plateau borders which denote the regions of intersection of these thin films. Foams derive their stability by offering resistance to external and internal stresses that can cause foam destruction. In most practical systems the foam stabilising component is either a soluble surfactant, a finely divided solid, or both. The adsorption of surface
*
Corresponding author. Tel.: +44 0121 414 5277/3344; fax: +44 0121 414 5324. E-mail address: [email protected] (M. Barigou). 1350-4177/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2004.05.001
active material to bubble surfaces acts to provide the foam stability although foams remain manifestly unstable. Foams can be found in many processes and can occur naturally or by design. In many situations foams are unwanted; for example, expensive antifoam agents are usually required to limit the foam volume in bioreactors. Unlimited foam formation and insufficient foam collapse can have disastrous effects in an aerated system such as a fermentation process and is one of the main operating problems in commercial fermentations. A number of methods, chemical and mechanical, are used to control these unwanted foams and the search for more efficient and cost-effective methods continues. Previous research has demonstrated the effectiveness of high-intensity ultrasound in destabilising unwanted foams, both static and dynamic [1,2]. The ability to suppress dynamic foams is particularly suited to processes that require continuous defoaming such as bioreactors.
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K.S. Lim, M. Barigou / Ultrasonics Sonochemistry 12 (2005) 385–393
Nomenclature Cs Cv d10 d32 di H H0 K M
surfactant concentration coefficient of variation number mean bubble diameter Sauter mean bubble diameter equivalent circular diameter of individual bubble i foam height initial foam height kurtosis median
The technique, being non-invasive, has a crucial advantage over other chemical and mechanical methods in processes where contamination cannot be tolerated. The use of ultrasound to positively influence the structure of foams during their generation does not seem to have been studied before, however. The production of designer foamed products is a major activity across the whole process industries including food, cosmetics, personal care and pharmaceuticals, which often derive a wide range of different textures from basically similar starting components. The direct or indirect use of gas in developing foam structure is a principal element of the process, and bubble mechanics are essentially the same across all the sectors concerned. Despite their widespread use, however, a number of fundamental issues relating to foams remain poorly understood including the production and control of bubbles, how they are entrained, broken up, dispersed, stabilised, measured, how they contribute to product structure, appearance, rheology, and how they can be fully exploited to produce novel textures and product differentiation. Foam structures, for example, in food products such as ice cream, mousses and confectionery offer many benefits including improved product volumes and, thus, a reduction in product density, improvements in the rheology and textural quality and, thus, taste and mouthfeel of the product, increased surface area, alteration of digestibility and shelf-life due to increased porosity, and modulated flavour intensity. In addition to the association with high quality, foams also offer attractive aesthetics as well as great possibilities for novel structures and textures. This is not specific to food foams but is equally valid for many other foam-based products such as cosmetics and personal care products. One way of controlling these features is through formulation design. However, the foam generation process itself is crucial in achieving foam products with enhanced quality and stability, and it would thus be of great industrial benefit to develop processing methods that can assist in the generation of foams with controlled structures.
n Q S r
number of bubbles in a sample gas flow rate skewness standard deviation
Abbreviations CTAB cetyl trimethyl ammonium bromide SDS sodium dodecyl sulphate US ultrasound
This paper reports the results of a preliminary study which aims to demonstrate the potential of using highintensity ultrasound to control foam structure during pneumatic generation.
2. Materials and methods 2.1. Experimental setup Foam was generated pneumatically in a glass column of 43.5 mm diameter and 750.0 mm height, immersed in an ultrasound bath to a depth of 120 mm, as shown in Fig. 1. The bath (Grant ultrasonic bath model XB 22) had dimensions of 505 mm length, 300 mm width, and
Fig. 1. Foam generation in an ultrasound bath: (1) rotameter; (2) control valve; (3) ultrasound bath; (4) sparger; (5) surfactant solution; (6) foam; (7) camera; (8) water.
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150 mm depth with a capacity of 22 L. It generates ultrasonic waves at a fixed frequency of 38 kHz, and the rootmean-squared ultrasonic power generated is 238 W with the peak power being equivalent to 475 W. The column was positioned at the point of maximum ultrasound intensity determined by mapping out the ultrasound field inside the bath using a sheet of aluminium foil, as described by Mason and Peters [3]. A sintered glass disc of porosity 40–100 lm was used to sparge nitrogen gas at a rate within the range Q = 0.13–0.50 L min1 in a surfactant solution and, thus, generate the foam. Three different types of surfactants were separately used as foam stabilisers: sodium dodecyl sulphate (SDS) which is anionic, cetyl trimethyl ammonium bromide (CTAB) which is cationic, and triton X100 which is non-ionic. All surfactants were dissolved in distilled water at different concentrations, as shown in Table 1. Surfactant concentration, Cs, is also expressed in cmc, where cmc is the critical micelle concentration of the particular surfactant in pure water, corresponding to the minimum static surface tension of the mixture. The solutions were all of Newtonian rheological character and the values of their viscosity and equilibrium surface tension are given in Table 1. The foam column was always thoroughly washed between experiments involving a change in surfactant type or concentration. Since ultrasound produces microbubbles in liquids via the process of cavitation, it was important to check whether cavitation contributed to the foaming process, i.e. whether any cavitation bubbles were causing additional generation of foam to that produced by the nitrogen gas injected. A test was conducted whereby ultrasound was applied to the surfactant solution in the column with the nitrogen gas supply switched off. The test was carried out repeatedly over extended periods of time exceeding those used for the normal foaming experiments described above, but no foaming was observed. Therefore, it could be safely assumed that, under the range of conditions investigated, any ultrasound effects on the pneumatically generated foams could not
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have arisen, wholly or partly, from possible foaming caused by cavitation. 2.2. Foam bubble size characterisation Bubble size distribution was determined by photographing the foam through the transparent column wall using a digital camera. Photographs were taken using fibre-optics back-lighting, at a position approximately 20 mm above the liquid surface inside the column. As foams are inherently unstable, this measure was adopted to ensure that only freshly generated foam was photographed, thus, capturing any ultrasound effects on foam structure at the earliest stage possible before any foam ageing effects (drainage, coarsening) set in. The digital images were analysed on Q-Win-Pro Leica software. Due to the transparent nature of the foam, the image contrast was not sufficient to enable automatic identification of most bubble contours by the image analysis software. Furthermore, the software was unable to automatically resolve individual objects with a common boundary such as bubbles in a foam. Thus, the bubbles at the wall, which had sharp contours and were clearly visible to the naked eye, were traced manually. Some bubbles in the bulk of the foam were visible through the bubbles at the wall, but they were faint and out of focus and did not interfere with the foreground measurements. All the bubbles on a given image were measured by using several copies of the same picture and tracing alternate bubbles. The images were then scanned and processed on the image analyser to give the equivalent circular diameters of the bubbles, which were then collated into one distribution. 2.3. Foam collapse measurement The effect of using ultrasound on the stability of the foams generated was assessed by measuring their collapse and comparing it to that of foams obtained from the same foaming system in the absence of ultrasound. In this experiment, a foam bed of height H0 = 25.0 cm
Table 1 Foaming systems used Surfactant
Surfactant concentration, Cs
Static surface tension (mN m1)
Viscosity (mPa s)
wt.%
g L1
cmc
SDS
0.12 0.24
1.18 2.36
0.5 1.0
49.6 37.3
1.15 1.09
Triton X-100
0.017 0.033 0.066
0.16 0.33 0.66
0.5 1.0 2.0
32.7 29.8 29.7
1.05 1.10 1.25
CTAB
0.017
0.16
0.5
41.2
1.10
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was generated; the gas flow and ultrasound source (when used) were then immediately switched off, and foam collapse was measured by monitoring the decline in foam height with time. The duration of the experiment depended on the nature of the foam and its stability, and varied from many minutes to several hours. 3. Results and discussion
"
# X d i d 10 4 nðn þ 1Þ K¼ ðn 1Þðn 2Þðn 3Þ r
3.1. Descriptive statistics In analysing the foam pictures to characterize the bubble size distribution, a number of statistical parameters were used and are defined as follows, where di is the equivalent circular diameter of an individual bubble i, and n is the number of bubbles in the sample: (i) Number mean bubble diameter: n P di i¼1 ð1Þ d 10 ¼ n (ii) Sauter mean bubble diameter: n P d 3i i¼1 d 32 ¼ P n d 2i
ð2Þ
i¼1
While the d10 gives the average bubble size, d32 is considered to be a better representative of the mean bubble diameter as it is directly related to the ratio of gas holdup and interfacial area in the foam. (iii) Standard deviation: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uP un u ðd i d 10 Þ2 t r ¼ i¼1 n1
This parameter gives a measure of the asymmetry of the distribution. The direction of skewness is to the tail; the larger the number, the longer the tail. If S is positive, skewness is to the right and the tail on the right side of the distribution will be longer. If S is negative, skewness is to the left and the tail on the left side will be longer. A normal distribution has S = 0. (vii) Kurtosis:
ð3Þ
2
3ðn 1Þ ðn 2Þðn 3Þ
ð6Þ
This is a measure of the combined weight of the tails in relation to the rest of the distribution. As the tails of a distribution become heavier, K will increase, and as the tails become lighter, K will decrease. A histogram with a normal distribution has K = 0. If the distribution is peaked (tall and slender), K will be greater than 0, and is said to be leptokurtic. If the distribution is flat, K will be less than zero, and is said to be platykurtic. To determine the minimum number of bubbles required for statistical analysis, samples of 30, 50, 100, 150, 200, 400, 600 and 800 were analysed. The results plotted in Fig. 2 show that the main parameters of the bubble size distribution, i.e. d10, d32 and r remain unaffected when a sample of 200 bubbles or more is used. The bubble size distribution curves obtained from samples of 200 bubbles or more also clearly coincide, as shown in Fig. 3. Some statistical bias can be expected from smaller samples, however. Therefore, a sample size of 200 bubbles was deemed sufficient to avoid statistical bias when determining foam bubble size distribution in a given experiment.
(iv) Coefficient of variation: r d 10
ð4Þ
Whereas r gives a measure of how different the values are from each other and from the mean (d10), the coefficient of variation is a measure of the spread of the distribution relative to its mean, i.e. a measure of the significance of r in relation to the mean. The larger the Cv, the more significant the r, relative to the mean. (v) Median: this is the middle number, M, in the data set when the data points are arranged from low to high. It is worth recalling that for a normal distribution, the median and the mean coincide, i.e. M = d10. (vi) Skewness: X d i d 10 3 n S¼ ð5Þ ðn 1Þðn 2Þ r
0.6
Mean Bubble Diameter (mm)
Cv ¼
0.5 0.4
0.3 0.2 0.1 0.0 0
100
200
300
400
500
600
700
800
900
Number of Bubbles Fig. 2. Effect of sample size on foam bubble parameters: h d10; · d32; n r.
K.S. Lim, M. Barigou / Ultrasonics Sonochemistry 12 (2005) 385–393
389
0.8
0.5
0.7 0.6
Number Fraction
Number Fraction
0.4
0.3
0.2
0.1
0.5 0.4 0.3 0.2 0.1
0.0 0.0
0.2
0.4
0.6
0.8
1.0
Bubble Diameter, d (mm)
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Bubble Diameter, d (mm)
Fig. 3. Bubble size distributions showing insignificant bias for samples of 200 bubbles or more: 200 bubbles; j 400 bubbles; m 600 bubbles; s 800 bubbles.
3.2. Ionic surfactants: SDS and CTAB solutions A significant reduction in bubble size, as reflected in both d10 (22%) and d32 (24%), was observed when the anionic surfactant SDS was used at a concentration of 0.5 cmc and Q = 0.13 L min1, while the coefficient of variation Cv decreased from 0.22 to 0.14, thus resulting in a finer and more uniform foam texture as exemplified by the foam images shown in Fig. 4, and their corresponding bubble size distribution curves depicted in Fig. 5. It should be pointed out that the generation of foam with a uniform bubble size is very difficult under normal circumstances. Such enhanced homogeneity in texture is highly desirable to reduce the presence of aesthetically undesirable large cavities, and to enhance foam stability by reducing the destabilising effects of coarsening or Ostwald ripening, i.e. the growth of large gas bubbles at the expense of smaller ones due to gas diffusion driven by the higher Laplace pressure of the gas in the smaller cells.
Fig. 5. Effect of ultrasound on foam bubble size distribution: SDS 0.5 cmc; Q = 0.13 L min1; m without ultrasound; h with ultrasound.
The statistics used to describe and compare the cell size distributions are summarized in Table 2. The bubble size distribution of foam generated in the absence of ultrasound is positively skewed, having a long tail towards the larger diameters (Fig. 5). Bearing in mind that a normal distribution has a skewness of zero, high skewness values above zero indicate the presence of some large bubbles in the foam resulting in the median M being greater than the number mean bubble diameter d10. Ultrasound reduced skewness considerably, thus eliminating most of these large bubbles (Table 2). Kurtosis values are positive for foams generated with or without the assistance of ultrasound, so the distributions are always leptokurtic, but the value in the case of ultrasound is considerably smaller indicating a distribution which is much closer to the normal distribution. In fact, the absolute value of Kurtosis is the classical measure of non-gaussianity, i.e. the degree of departure from the bell-shaped normal distribution. The results obtained indicate a taller narrower bubble size distribution with much thinner tails and a sharper peak (Fig. 5).
Fig. 4. Foam images: SDS 0.5 cmc; Q = 0.13 L min1.
0.70 1.35 1.95 2.21 0.62 1.77 0.87 1.47 1.19 1.47 1.22 1.11 0.70 0.40 0.36 0.70 0.46 0.44 0.31 0.41 0.17 0.32 0.43 0.17 0.23 0.20 0.06 0.24 0.24 0.08 0.93 0.77 0.48
1.1
0.76 0.56 0.45
0.75 0.49 0.37
0.91 0.67 0.39
1.29 11.99 0.80 2.54 0.33 0.43 0.14 0.25 0.05 0.11 0.51 0.44
0.34
0.35
4.37 3.27 1.95 11.94 0.82 0.77 0.72 2.14 1.72 1.63 1.26 2.85 0.32 0.37 0.31 0.31 0.38 0.36 0.40 0.34 0.14 0.14 0.17 0.24 0.22 0.33 0.28 0.34 0.05 0.05 0.05 0.08 0.09 0.13 0.12 0.12 0.45 0.48 0.51 0.48 0.41 0.39 0.43 0.32
0.32 0.38 0.31 0.37
0.34 0.40 0.33 0.37
Without US Without US With US Without US
With US
Without US
With US
Without US
With US
Without US
With US
Without US
With US
K S M Cv r d32 (mm) d10 (mm)
When the concentration of SDS was increased to 1.0 cmc, though the effect of ultrasound on d10 was negligible, there were significant reductions in both d32 and Cv. It is worth pointing out that although d32 is a better representative of the mean bubble diameter, as it is directly related to the interfacial area and gas holdup in the foam, it still does not capture all of the information about the foam texture, and a detailed and complete picture of the foam structure can only be obtained by considering the whole bubble size distribution and its descriptive statistics, as summarized in Table 2. Similar to the 0.5 cmc SDS systems, the foam generated in the presence of ultrasound has a much smaller skewness value. Thus, the overall effect of ultrasound in producing a more homogeneous foam structure, much less skewed and more clustered around the mean, remains significant. The foam collapse experiments also revealed some positive ultrasound effects. In general, a typical foam collapse curve consists of three stages [4]: (i) an initial plateau region where the foam height remains constant over a period ranging from a few minutes to many minutes depending on the nature of the foam; during this phase most of the liquid contained in the foam drains but no foam breakage occurs; (ii) at the end of this period foam films have thinned sufficiently to become unstable and foam breakage begins in the driest region at the top; foam collapses at a fairly fast rate and foam height decreases almost linearly, but not quite, whilst the remaining liquid drains; (iii) in the third part of the curve foam collapse proceeds, again in an approximately linear manner, but very slowly with almost no drainage taking place. Depending on the nature of the foam, not all three regions are always observed; for example, in highly unstable or very dry foams, the initial plateau may not be present as the foam starts to collapse immediately on formation. Foams produced under sonication appear to collapse more slowly and have a longer lifetime than native foam, as shown in Figs. 6 and 7 for two different SDS
1.0
0.8
H /H o
0.13 0.13 0.13
0.13
0.7
0.13 0.13 0.24 0.50
Q (L min1)
0.9
0.6 0.5
0.5 1.0 2.0
0.5
0.3
0.5 1.0 0.5 0.5
Cs (cmc)
0.4
0.2 0.1
Triton X-100
CTAB
SDS
0.0
Surfactant
Table 2 Statistical parameters for foam bubble size distributions
3.28 1.17 1.46 7.64
K.S. Lim, M. Barigou / Ultrasonics Sonochemistry 12 (2005) 385–393
With US
390
0
20
40
60
80
100
120
140
160
Time (min) Fig. 6. Foam collapse curves: SDS 0.5 cmc; Q = 0.13 L min1; H0=25.0 cm; m without ultrasound; h with ultrasound.
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391
0.6
1.0
0.5
Number Fraction
0.9 0.8
H /H o
0.7 0.6 0.5 0.4 0.3
0.4 0.3 0.2 0.1
0.2 0.1
0.0
0.0 0
50
100
150
200
250
300
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Bubble Diameter, d (mm)
Time (min)
Fig. 7. Foam collapse curves: SDS 1.0 cmc; Q = 0.13 L min1; H0 = 25.0 cm; m without ultrasound; h with ultrasound.
Fig. 9. Effect of ultrasound on foam bubble size distribution: SDS 0.5 cmc; Q = 0.24 L min1; m without ultrasound; h with ultrasound.
concentrations. Thus, ultrasound seems to have the added benefit of enhancing foam stability. To investigate the effect of the pneumatic rate of foam formation, the gas flowrate, Q, was increased to 0.24 and 0.50 L min1 for SDS solutions of 0.5 cmc concentration. Similar results were obtained whereby significant reductions in the mean bubble size (d10 and d32), skewness value and coefficient of variation were obtained. The foams generated when ultrasound was applied exhibited a finer and more homogeneous texture, as demonstrated by the images in Fig. 8, and their corresponding bubble size distribution curves in Fig. 9. There is a clear narrowing of the foam bubble size distribution when ultrasound is applied reflecting the enhanced homogeneity observed in the images of Fig. 8, and a reduction in the mean bubble size reflecting the finer texture of the foam. Consequently, the ultrasound effects remain consistent at different rates of foam generation. Foams stabilized with the cationic surfactant CTAB exhibited similar results under sonication. The effects are clearly demonstrated by the images in Fig. 10 and
the bubble size distribution curves in Fig. 11. Sonication obviously shifted the bubble size distribution curve of foam towards the lower end of the spectrum and eliminated most of the larger bubbles, thus, reducing skewness substantially (Table 2). There is also an order of magnitude reduction in kurtosis, thus bringing the bubble size distribution much closer to a normal distribution. Hence, ultrasound appears to be just as efficient with cationic surfactant foams as it is with anionic ones. 3.3. Non-ionic surfactant: Triton X-100 The non-ionic surfactant Triton X-100 was used at concentrations of 0.5, 1.0 and 2.0 cmc at a gas flowrate of 0.13 L min1. The statistical parameters of the foams generated are summarized in Table 2. The effect of ultrasound on d10 was negligible at the low surfactant concentration of 0.5 cmc, whilst significant reductions of 22% and 18% were obtained at concentrations of 1.0 and 2.0 cmc, respectively. Similarly, there are significant reductions in d32 of 24% and 19%, respectively, at these two concentrations, but little effect at 0.5 cmc.
Fig. 8. Foam images: SDS 0.5 cmc; Q = 0.24 L min1.
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Fig. 10. Foam images: CTAB 0.5 cmc; Q = 0.13 L min1. 0.8
0.5
0.4
0.6
Number Fraction
Number Fraction
0.7
0.5 0.4 0.3 0.2
0.3
0.2
0.1 0.1 0
0.0 0
0.2
0.4
0.6
0.8
1
0
0.2
Bubble Diameter, d (mm)
Fig. 11. Effect of ultrasound on foam bubble size distribution: CTAB 0.5 cmc; Q=0.13 L min1; m without ultrasound; h with ultrasound.
Despite the reductions observed in the mean bubble size, however, the results showed that the application of ultrasound did not bring about a significant improvement in the quality of the foam texture, as shown in Fig. 12. Despite a slight narrowing in the bubble size distribution and a sharper peak, all the other statistics retained more or less similar values (Table 2). Therefore, it can be concluded that although the uniformity of foam texture was significantly enhanced by the sonication process under all conditions in the case of ionic surfactants (anionic and cationic), this was not the case for the non-ionic surfactant Triton X-100. The major reasons behind this irregularity are difficult to explain a` priori, but must be related to surfactant molecular behaviour at the film interfaces. However, given that the range of experiments in this study was rather limited, one should not exclude the possibility that under different conditions (e.g. higher frequency or power) ultrasound might have more pronounced effects on Triton foams, especially considering that some reduction in bubble size has actually been achieved. Overall, a close examination of the results shown in Table 2 seems to suggest that the degree of ultrasound
0.4
0.6
0.8
1
1.2
1.4
1.6
Bubble Diameter, d (mm)
Fig. 12. Effect of ultrasound on foam bubble size distribution: Triton X-100 1.0 cmc, Q = 0.13 L min1; m without ultrasound; h with ultrasound.
influence on the mean bubble size of foam may be a function of the bubble size that can be generated in the absence of an ultrasound field, i.e. that there is probably an optimum bubble size at which the bubble size reduction effects of ultrasound will be maximum. In the absence of ultrasound, foam bubble size is controlled by the porosity of the foam generator, gas flowrate, surfactant type and concentration. Although the available data set is rather limited, and therefore firm conclusions cannot be readily drawn, a significant reduction in foam cell size (approximately 20–30%) was generally obtained only when the original mean bubble size of the foam (d10) was greater than about 0.4 mm. On the other hand, significant reductions in d32, the better representative of the mean foam bubble size, were nearly always achieved, however. This point needs more detailed investigation.
4. Conclusions High-intensity ultrasound affected the generation of foam in various ways depending on the formulation of
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the foam. Under most conditions investigated, highintensity ultrasound was able to reduce foam mean bubble size and enhance foam texture homogeneity, which is highly desirable to reduce the presence of aesthetically unattractive large cavities and to enhance foam stability by reducing the destabilising effects of Ostwald ripening. Foam collapse curves showed that foams generated in the presence of an ultrasound field collapsed more slowly than native foams. The effects were consistent at different foam generation rates, and were similar for the anionic SDS and cationic CTAB surfactant systems studied. However, the effects were not as significant in the case of the non-ionic surfactant Triton X-100. The work presented has served to demonstrate the positive effects that power ultrasound can have on the generation of foams with controlled structures. However, more detailed work is necessary to fully demonstrate the potential of the technique. In particular, the results presented here were all obtained at one ultrasound frequency. It is plausible that higher frequencies may have a much greater impact on foam structure, and these need to be investigated up to the MHz range and probably beyond. It would also be important to fully investigate the effect of increasing ultrasound amplitude on both foam homogeneity and bubble size,
393
as it is not obvious that a higher ultrasound power will always generate a reduction in bubble size, or better foam homogeneity. It is quite conceivable that ultrasound may provide a novel elegant technique for controlling foam texture by affecting the initial bubble formation and/or by affecting bubble coalescence, and hence offer improved product quality, enhanced foam stability and product shelf-life. If fully proven, the results should eventually advance the scientific ability within industry to produce foambased products with improved structures and stability.
References [1] N. Sandor, H.N. Stein, Foam destruction by ultrasonic vibrations, Journal of Colloid and Interface Science 161 (1993) 265–267. [2] M.D. Morey, N.S. Deshpande, M. Barigou, Foam destabilization by mechanical and ultrasonic vibrations, Journal of Colloid and Interface Science 219 (1999) 90–98. [3] T.J. Mason, D. Peters, Practical sonochemistsry: uses and applications of ultrasound, second ed., Horwood, Chichester, 2002. [4] N.S. Deshpande, M. Barigou, Foam formation, drainage and collapse in the presence of antifoams, in: S. Perucci (Ed.), Fourth Italian Conference on Chemical and Process Engineering––Selected Papers, AIDIC-Italian Association of Chemical Engineering Conference Series, vol. 4, Milano, Italy, 1999, pp. 295–301.