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The influences of ultrasonic on embedding nanoparticles into porous fabric materials
Applied Acoustics 69 (2008) 763–769 www.elsevier.com/locate/apacoust
The influences of ultrasonic on embedding nanoparticles into porous fabric materials Jianwei Ma
a,b,*
, Shaojuan Chen b, Chengkun Liu b, Wenna Xu b, Shanyuan Wang
b
a
a College of Textile, Donghua University, Shanghai 200051, People’s Republic of China College of Textile and Apparel, Qingdao University, Qingdao 266071, People’s Republic of China
Received 12 January 2006; received in revised form 27 January 2007; accepted 28 May 2007 Available online 25 September 2007
Abstract The feasibility of embedding 40 nm ZnO particles into porous materials by using ultrasonic and its influence factors were studied in this paper. Through investigations, it is proved that the higher the concentration of nanoparticle suspension is, the higher the weight of embedding percentage (EPW) is. However, the increasing trend of EPW will minish quickly when the concentration exceeds 1%. In addition, the longer the process time of ultrasonic is, the higher EPW is. EPW is always higher when the distance between film and ultrasonic transducer is k/2 and 3k/2, and lower when the distance is 1 k and 2 k. For 40 nm ZnO particles, EPW of the film effected by ultrasonic with the frequency of 42 kHz reaches the maximum. 2007 Elsevier Ltd. All rights reserved. Keywords: Nanomaterials; Porous materials; Ultrasonic; Dispersion
1. Introduction Ultrasonic has been widely used in chemistry, dyeing, finishing and cleaning industries because of its obvious advantages in particle treatment such as dispersion and agglomeration effects, etc., [1–6]. During recent years, treatment of nanomaterials using ultrasonic has been a research hot and many research findings have been achieved in this field [7–9]. Meanwhile, the applications of nanomaterials have received considerable attention on textile finishing and some valuable functional textiles such as anti-bacterial and anti-ultraviolet products etc have been led to the market [10–12]. Fundamental researches have been carried out on the effects of ultrasonic on fine particles [13–17], the results showed that the scattering effect on fine particles mainly * Corresponding author. Address: College of Textile, Donghua University, Shanghai 200051, People’s Republic of China. Fax: +86 053285950557. E-mail address: [email protected] (J. Ma).
0003-682X/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apacoust.2007.05.012
came from cavitation effect closely related to the ultrasonic frequency, power and viscidity of dispersions. In the process of ultrasonic treatment, fine particles in dispersion liquid are affected by ultrasonic radiation force, acoustic streaming, Stokes drag force, valid buoyancy force and gravity. In these factors, ultrasonic radiation force and acoustic streaming are related with ultrasonic power, frequency as well as the size of fine particles. Greater ultrasonic power, lower frequency or greater size of fine particles indicates higher intensity of the radiation force and acoustic streaming [18–22]. The Stokes drag force is related to viscidity of dispersing solution, diameters and moving speed of fine particles. In this paper, it is found that nanoparticles can be dispersed temporarily and fully in water using ultrasonic and then embedded into the porous materials under the effect of acoustic field. We also study the effects of ultrasonic frequency and particle size on EPW. However, there has been no report available on these effects in ultrasonicrelated literature up to now.
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Fig. 1. The relationship between the ultrasonic pressure and radiation force of particles.
In the ultrasonic field, nanoparticles in the suspension are mainly affected by acoustic radiation force (Fa), stocks drag force (Fd) and buoyancy force (Fb). (1) Acoustic radiation force It keeps the same direction as the transmitting direction of traveling wave, so the acoustic radiation force in plane traveling wave field always be positive. But the direction of that in plane standing wave field always points to sound pressure nodes. The relationship between ultrasonic pressure and radiation force of particles is shown in Fig. 1. There is no radiation force in both pressure nodes and anti-nodes. Ultrasonic radiation force’s cycle (T) is equal to half of that of sound pressure. The direction of radiation force changes per T/2 (or per one fourth of sound pressure’s cycle), which keeps it pointing to sound pressure nodes. Provided that the particles are in a condition which is a continuous and ideal nonstick medium and the whole force process is adiabatic. Moreover, the ambient temperature is room temperature. Rigid particles will experience the acoustic radiation force (F1) which can be calculated as follows in the plane traveling field. F 1 ¼ R3 jEF ;
ð1Þ
where R is the particle radius, j is the wavenumber of the acoustic field, E is the energy density of the acoustic field, F is the acoustic contrast factor1 for the particle–fluid system. Suspended particles will scatter that sound field. The interaction of the particles with the scattered sound field generated from neighboring particles gives rise to the secondary 1 It is a parameter describing the difference of density between fluid and p a r t ic l e s . , w h i2ch c a n b e e x p r e s s e d a s f o l l o w e q u a t i o n : 5q 2q qc F ¼ 13 2qpp þql l q l cl2 , where, q and c stand for density and acoustic p p speed, and the subscript p and l denote particle and liquid respectively.
acoustic radiation force [23,24]. This force can lead to gravitation between two particles or between particles and porous materials. Although it is difficult to predict an equivalent secondary acoustic force between the suspended particles and film, we expect it to be of similar order of magnitude with the value between particles and porous medium comprised by glass bead which was calculated in [25]. (2) Stocks drag force In the fluid with the density of qf and the viscosity of lf, when the orbicular particle with the diameter of dp moves at the speed of t, it will experience the resistance (Fd) from fluid. When Re < 2 (Stocks area), the following expression can be gained: F d ¼ 3p lf d p t:
ð2Þ
This formula shows that Stocks drag force is proportional to the density and viscosity of the fluid, the relative speed and diameter of the particle. (3) Valid buoyancy force Buoyancy force indicates the summation of gravity and buoyancy. Its value can be calculated with the formula below: p F b ¼ ðqf qP Þg d 2P ; ð3Þ 6 where qp is the density of particles. From the expression, we can know that the value of valid buoyancy force is proportional to the diameter of particles and also increases with the decrease of the density of particles. Particles positioned within the porous medium also experience a hydrodynamic drag force due to the ongoing flow of the medium. The hydrodynamic drag of particles
J. Ma et al. / Applied Acoustics 69 (2008) 763–769
moving near surfaces of pore wall has been theoretically and experimentally studied (see, e.g., Ref. [26]). Thus, it is reasonable to expect the. primary and secondary acoustic radiation force can exceed the hydrodynamic drag force of particles. This leads to three possible explanations of the origins of the particle trapping phenomena within porous material. 1.1. Trapping by primary acoustic radiation and blockage by the porous material If porous material does not strongly interfere with the propagation of the primary sound field, then individual particle may experience the primary acoustic radiation force. For particles with positive acoustic contrast factor (most solids suspending in water), this force causes the particles to collect at pressure nodes of the field. The role of porous material may only be to prevent particles from being entrained by the fluid flow. As the flow field tries to entrain particles, their motion (along the flow direction) is blocked by solids comprising porous materials. The primary acoustic radiation force will block any motion of particles around the obstruction and return them to a nodal position. 1.2. Formation of particle flocs by second forces The secondary acoustic radiation force acting between the suspended particles can lead to apparent flocculation between them. Upon cessation of the acoustic field, these flocs would be easily dispersed due to the action of fluid drag. If these flocs are larger than the necks between the elements of porous material, they will be trapped within it. Alternatively, the primary acoustic radiation force could act to constrict the particle flocs at nodal position of the resonant field. This force could prevent flocs from being entrained by fluid from the film.
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Ultrasonic plane wave can propagate in porous material by coupling with the liquid in the suspending liquid (see Fig. 2). When the suspending liquid travels through a sample, particles are trapped in the porous material due to the action of the acoustic radiation force. For aqueous suspensions, sound impedances of water (1.5 · 106 kg/m2 s) and porous material (3.0 · 106 kg/m2 s for polypropylene) are a little different from each other. Thus, it is generally believed that there would be internal reflection and dispersion of acoustic field as it passes through the porous material. In our experiments, the wavelength of the primary acoustic field is comparable to the dimension of the elements within the porous material. So we expect that the sound field in the fluid filled porous material will have three-dimensional property and will be difficult to describe exactly. However, consideration of the acoustic radiation force acting on particles subjected to a simple acoustic field can provide insight into the mechanism by which particles may be collected within the porous material. In the standing wave system, ultrasonic radiation force will push nanoparticles from acoustic pressure node to anti-node. If particles collect into definite weight which is large enough to overcome the summation of Stocks drag force and valid buoyancy force, they will deposit. However, when particles which have gotten together do not experience very strong action of ultrasonic, just be subject to the disturbance of ultrasonic, these particles will not be dispersed. Nevertheless, if the strength of ultrasonic is great enough, these assembled particles will be scattered into suspension again.
1.3. Deposition of particles on the surface of porous material by secondary acoustic radiation force Secondary acoustic radiation force between the particles and medium may give rise to the direct deposition of particles within the porous material. Under the strong action of this force, particles would be fast embedded onto the inner surface of the porous material and the flow field may not be able to displace them. The force may cause other particles to attach onto the anchored particles in turn. The exact mechanism of particle retention in the porous material is still a matter of investigation for the time being. Since the theory was founded by Biot in 1956 [27], the propagation of elastic waves in a fluid-saturated porous material has been studied widely. But as known to us, the analysis of the acoustic radiation force experienced by particles in porous material has not been mentioned before.
Fig. 2. Schematic design of the treatment unit of ultrasonic equipment: (1) Sample Schematic design of the treatment unit of ultrasonic equipment: (1) Sample holder, (2) finishing trough (with the bottom size of 12 cm · 12 cm), (3) transducer, (4) sample, and (5) dispersion.
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2. Materials and methods 2.1. Test materials ZnO nanoparticles (40 nm, the particle size distribution of it is shown in Fig. 3) purchased from Shanghai Bona Co. Ltd., PR China were used as test samples. PP films whose weight per square meter, thickness, pore-size distribution (tested by bubble-point methods), water flow, porosity and bubble pressure are 8 g/m2, 150 lm, 150 nm, 10 ml/ min, 73% and 4.80 kg/cm2, respectively were supplied by Shanghai Xinya Filter Co. Ltd., PR China. The PP film was a melt-spun PP fabric which was cut into equal-sized circular pieces with the diameter of 10 cm. Ultrasonic equipment including four independent treatment units (12 cm · 13 cm · 14.5 cm), a signal generator (A001, Qingdao University, China) and a signal amplifier (B001, Qingdao University, China) was provided as the acoustic field. As shown in Fig. 1, each treatment unit was equipped with one transducer at bottom. There were four transducers with different frequency (28, 42, 51 or 59 kHz,1Bl6035d, Ilsan Suntek Corporation, Korea), but with the same input electric power of 60 W. 2.2. Test methods Fabric weight (g/m2) of PP was measured before and after ultrasonic treatment using a physical balance. To keep the sample at the same status, fabric samples were dried for 2 h at 80 C and then allowed to cool for 24 h in the desiccator before and after the treatment. Five samples for each experimental condition were tested in order to obtain the mean weight, which was used to calculate EPW with the following equation: EPW ¼ ðG1 G0 Þ=G0 100%;
ð4Þ
where G1 = measured weight after ultrasonic treatment and G0 = measured weight before ultrasonic treatment. Each treatment unit was poured with 1000 mL distilled water, 5 g ZnO nanoparticles, with the pH of the dispersing solution adjusted to 7 by using glacial acetic acid and acidity meter. The ultrasonic equipment was turned on and the pre-dispersing time before the addition of a fabric sample lasted 20 min. Then the test sample was fixed on one end
of the sample holder which was placed into one treatment unit with the different distance between the sample and the transducer for one frequency. The sample was treated for 20 min, and then washed with distilled water 2 times followed by distilled water wash under the acoustic field for another 3 times until the rinse water was limpidly observed with the naked eye. 3. Results and discussion 3.1. Influences of the concentration of nanoparticle dispersion to EPW This experiment was conducted with the frequency of 40 kHz, the distance between film and sound source of 3k/2 and the process time of 20 min. The results were shown in Fig. 4. Fig. 4 shows that EPW increases with the increase of the concentration of nanoparticle dispersion. Moreover, after 1%, speed of the increase of EPW is apparently slow. 3.2. Influences of the process time to EPW This experiment was conducted with the frequency of 40 kHz, the distance between film and sound source of 3k/2, the concentration of 0.5%. The results were shown in Fig. 5. Fig. 5 shows that EPW increases with the process time.
3 2.5
EPW (%)
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2 1.5 1 0.5 0 0
0.5
1
1.5
Concentration (%) Fig. 4. Influences of the concentration of nanodispersion to EPW.
EPW (%)
4 3 2 1 0 0
5
10
15
20
Time (min) Fig. 3. The particle size distribution of nano ZnO.
Fig. 5. Influences of the process time to EPW.
25
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3.3. Influences of the distance between the film and sound source to EPW
EPW (%)
5 4 3 2 1 0 0
5
10
15
Distance (cm) Fig. 6. Influences of the distance between the film and sound source to EPW when the frequency is 28 kHz.
1.2
EPW (%)
This experiment was carried out with the frequency of 28 kHz, 42 kHz and 51 kHz, the concentration of dispersion of 0.5%, the process time of 20 min. The positions of acoustic pressure node and anti-node change with the frequency or wavelength of ultrasonic. Thus, samples were set at the positions with the distance between the film and sound source of k/2, 3k/4, 1 k, 3k/2 and 2 k, respectively for the three frequencies. Selected particles and test procedure were the same as above. For the frequency of 28 kHz, influences of the distance between the film and sound source to EPW are shown in Fig. 6. The form of the curve is fluctuant. When the distances are 1 k and 2 k, EPW values are both small. However, when k/2 and 3k/2, they are both large. In addition, the minimum value is 0.89% and the maximum one is 4.36% gained from the figure. For the frequency of 42 kHz, the curve of the influence rule is also fluctuant (see Fig. 7). When the distance is 3k/2, EPW value is the largest. The minimum value is 2.55%. For the frequency of 51 kHz, the influence of the distance between the film and sound source to EPW has an N-word form (see Fig. 8). For the sample with the distance of 1 k or 2 k, EPW value is small. However, EPW value is large for the sample with the distance of k/2 or 3k/2. In addition, the minimum value is 0.25% and the maximum one is 0.95% known from Fig. 6. By comparing Figs. 5–7, we find that EPW value (5.81%) is the largest for the 40 nm ZnO particles when the frequency is 42 kHz and the distance between the sam-
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1 0.8 0.6 0.4 0.2 0
0
2
4
6
8
Distance (cm) Fig. 8. Influences of the distance between the film and sound source to EPW when the frequency is 51 kHz.
ple and sound source is 3k/2. However, when the frequency is 51 kHz and the distance between the sample and sound source is 2 k, EPW value (0.25%) is the smallest. The maximum value is more than 20 times larger than the minimum one. It is very important for the functional finish with nanomaterials in the future. 3.4. Influences of the ultrasonic frequency to EPW To observe the influences of the ultrasonic frequency to EPW 28 kHz, 42 kHz, 51 kHz and 59 kHz were selected in this experiment. The distance between samples and ultrasonic transducer was 3k/2 and process time was 20 min. The results are shown in Fig. 9. Fig. 9 shows that EPW has a trend of decreasing with the increase of the ultrasonic frequency. Up to 42 kHz, EPW begins to have an opposite trend. This may relate to cavitation threshold which will increase with the increase of ultrasonic frequency. Fig. 10 shows that nanomaterials were surely pushed into the PP film. In addition, we found that almost all the SEMs of the testing PP films showed the same phenomenon – many nanoparticles collects around the entrance of pores in the PP films. Through the observation, it may be due to the extrusion coming from the ambient fibers surrounding the pores. Although the test samples embedded with the fine particles were subject to washing and ultrasonic cleaning after each experiment, most of fine particles would still remain intrafilm. Therefore, inside the textile materials ultrasonic energy can not reach the cavitation threshold, instead slight vibration in the pores may enhance the collision chances between the particles having existed in the films and lead
8
EPW (%)
EPW (%)
8 6 4 2 0
0
2
4
6
8
Distance (cm) Fig. 7. Influences of distance between the film and sound source to EPW when the frequency is 42 kHz.
6 4 2 0 0
20
40
60
80
Frequency (kHz) Fig. 9. Influences of the frequency of ultrasonic to EPW.
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Fig. 10. SEM photos of the internal film finished with nanomaterials by ultrasonic.
to agglomeration onto the wall of holes, which may ensure the stability and durability of fine particles having been embedded into the porous materials. 4. Conclusions The research results show that fine particles can be deeply embedded into the PP films with the effect of ultrasonic. EPW has relations to four main factors as follows: 1. The higher the concentration of nanoparticles dispersion is, the larger EPW is. When the concentration increases up to 1%, the increasing speed of EPW begins to become apparently slow. 2. The longer the process time is, the greater EPW is. 3. The film has a larger EPW value when the distance between the film and ultrasonic transducer is k/2 or 3k/2. However, when the distance is 1 k or 2 k, EPW is smaller. 4. For 40 nm ZnO particles, EPW is the largest when the ultrasonic frequency is 42 kHz.
So we can deduce that if only appropriate parameters such as frequency and power etc are selected, the ultrasonic energy in treatment solution will disperse nanomaterials outside of porous materials whereas agglomerate inside. By doing this, we can enrich the application of nanomaterials in functional textile finishing. It is necessary to be pointed out that the influences of related parameters in acoustic field such as frequency or power to EPW are diverse for different nanomaterials and different textiles. The rules and mechanism are to be further studied in the future. Acknowledgement This work was supported by Science Foundation of Shandong Province, PR China, under Contract Y2001F04. References [1] Liu Y, Yang H, Sakanishi A. Ultrasound: Mechanical gene transfer into plant cells by sonoporation. Biotechnol Adv 2006;24(January/February):1–16.
J. Ma et al. / Applied Acoustics 69 (2008) 763–769 [2] Shao HP, Lee, Hyosook et al. Synthesis of nano-size magnetite coated with chitosan for MRI contrast agent by sonochemistry. In: Proc of INTERMAG ASIA 2005: Digests of the IEEE Int Magnetics Conf; 2005; 231. [3] Thakore KA, Smith CB, Clapp TG. Application of ultrasound to textile wit processing. ADR 1990;79(10):30–8. p. 44. [4] Saligram AN, Shukla SR, Mathur M. Dyeing of polyester fibers using ultrasound. JSDC 1993;109(July/August):263–6. [5] Jiang Y, Gan YJ. The application of acoustic chemistry in textile dyeing processor. Printing and Dyeing 1998;24(11):52–4 [in Chinese]. [6] Chen SZ. The present situation and its application developing trend of high power ultrasonic technique in China. Tech Acoust 2002;21(1):46–9 [in Chinese]. [7] Song XL et al. Dispersion behaviors of CeO_2 nanoparticles in water suspension. Chin J Rare Met 2005;29(2):167–72 [in Chinese]. [8] Jia XL, Tan W. The development of nano-powder dispersion technique. Non-Met Mines 2003;26(4):1–4 [in Chinese]. [9] Li ZH, Li FQ. Elimination methods and mechanism of agglomeration of ultrafine powders. J Salt Lake Res 2005;13(1):31–6 [in Chinese]. [10] Wang YH, Meng JG. The sterilization mechanism and producing method of nano-anti-bacterial Fabrics. Knit Ind 2005(6):56–8 [in Chinese]. [11] Ma ZH, Xu HY. Research on the application of the nanometer material anti-UV finishing. Textile Auxiliar 2005;22(4):11–4 [in Chinese]. [12] Mehert Reiner et al. UV-curable acrylate nanocomposites for functional coatings: properties and applications. VDI Berichte 2005: 45–53. [13] Sun DH, Shi B. The driving force of phonochemical reaction. Leather Sci Eng 2001;11(3):18–22 [in Chinese].
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[14] Wang PH. A study and analysis on influencing factor of the ultrasonic cavitation. J Hebei Inst Technol 2003;25(4):154–61 [in Chinese]. [15] Gao QS. Further exploration of liquid cavitation mechanism. J Hohai Univ 1999;27(5):63–7 [in Chinese]. [16] Huang JT. The principle and application of cavitation and cavitation erosion. Qinghua University Publishing House: Beijing; 1991. pp. 25–9. [in Chinese]. [17] Uchida, Takeyoshi, Hamano, et al. Basic study on effects of acoustic intensity and sound pressure on surface modification and dispersion of diamond particles. In: Proc IEEE Ultrasonics Symp (Montreal); 2004, p. III.1635–8. [18] Wu J, Li BH. Particles suspension in a standing wave tube and its analysis (in Chinese). J Exp Mech 2001;6(2):180–7 [in Chinese]. [19] King LB. On the acoustic radiation pressure on sphere. London Pro R Soc 1934;A147:211–40. [20] Rudnick J. Measurements of the acoustic radiation pressure on a sphere in standing wave field. J Acoust Soc Am 1951;23:633–4. [21] Westervelt PJ. The theory of steady force caused by sound waves. J Acoust Soc Am 1951;23:312–5. [22] Simon A, Penpenic L, Gondrexon N, Taha S, Dorange G. Ultrason Sonochem 2000;7:183–6. [23] Weiser MHA, Apfel RE, Neppiras EA. Acustica 1984;56:114. [24] Apfel RE. Am J Phys 1988;56:726. [25] Gupta Sanjay, Feke Donald L. Acoustically driven collection of suspended particles within porous media. Ultrasonics 1997;35: 131–9. [26] Schlichting H. Boundary Layer Theory. New York: McGraw-Hill; 1968. p. 154. [27] Biot MA. J Acoust Soc Am 1956;28(2):68.
The influences of ultrasonic on embedding nanoparticles into porous fabric materials Jianwei Ma
a,b,*
, Shaojuan Chen b, Chengkun Liu b, Wenna Xu b, Shanyuan Wang
b
a
a College of Textile, Donghua University, Shanghai 200051, People’s Republic of China College of Textile and Apparel, Qingdao University, Qingdao 266071, People’s Republic of China
Received 12 January 2006; received in revised form 27 January 2007; accepted 28 May 2007 Available online 25 September 2007
Abstract The feasibility of embedding 40 nm ZnO particles into porous materials by using ultrasonic and its influence factors were studied in this paper. Through investigations, it is proved that the higher the concentration of nanoparticle suspension is, the higher the weight of embedding percentage (EPW) is. However, the increasing trend of EPW will minish quickly when the concentration exceeds 1%. In addition, the longer the process time of ultrasonic is, the higher EPW is. EPW is always higher when the distance between film and ultrasonic transducer is k/2 and 3k/2, and lower when the distance is 1 k and 2 k. For 40 nm ZnO particles, EPW of the film effected by ultrasonic with the frequency of 42 kHz reaches the maximum. 2007 Elsevier Ltd. All rights reserved. Keywords: Nanomaterials; Porous materials; Ultrasonic; Dispersion
1. Introduction Ultrasonic has been widely used in chemistry, dyeing, finishing and cleaning industries because of its obvious advantages in particle treatment such as dispersion and agglomeration effects, etc., [1–6]. During recent years, treatment of nanomaterials using ultrasonic has been a research hot and many research findings have been achieved in this field [7–9]. Meanwhile, the applications of nanomaterials have received considerable attention on textile finishing and some valuable functional textiles such as anti-bacterial and anti-ultraviolet products etc have been led to the market [10–12]. Fundamental researches have been carried out on the effects of ultrasonic on fine particles [13–17], the results showed that the scattering effect on fine particles mainly * Corresponding author. Address: College of Textile, Donghua University, Shanghai 200051, People’s Republic of China. Fax: +86 053285950557. E-mail address: [email protected] (J. Ma).
0003-682X/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apacoust.2007.05.012
came from cavitation effect closely related to the ultrasonic frequency, power and viscidity of dispersions. In the process of ultrasonic treatment, fine particles in dispersion liquid are affected by ultrasonic radiation force, acoustic streaming, Stokes drag force, valid buoyancy force and gravity. In these factors, ultrasonic radiation force and acoustic streaming are related with ultrasonic power, frequency as well as the size of fine particles. Greater ultrasonic power, lower frequency or greater size of fine particles indicates higher intensity of the radiation force and acoustic streaming [18–22]. The Stokes drag force is related to viscidity of dispersing solution, diameters and moving speed of fine particles. In this paper, it is found that nanoparticles can be dispersed temporarily and fully in water using ultrasonic and then embedded into the porous materials under the effect of acoustic field. We also study the effects of ultrasonic frequency and particle size on EPW. However, there has been no report available on these effects in ultrasonicrelated literature up to now.
764
J. Ma et al. / Applied Acoustics 69 (2008) 763–769
Fig. 1. The relationship between the ultrasonic pressure and radiation force of particles.
In the ultrasonic field, nanoparticles in the suspension are mainly affected by acoustic radiation force (Fa), stocks drag force (Fd) and buoyancy force (Fb). (1) Acoustic radiation force It keeps the same direction as the transmitting direction of traveling wave, so the acoustic radiation force in plane traveling wave field always be positive. But the direction of that in plane standing wave field always points to sound pressure nodes. The relationship between ultrasonic pressure and radiation force of particles is shown in Fig. 1. There is no radiation force in both pressure nodes and anti-nodes. Ultrasonic radiation force’s cycle (T) is equal to half of that of sound pressure. The direction of radiation force changes per T/2 (or per one fourth of sound pressure’s cycle), which keeps it pointing to sound pressure nodes. Provided that the particles are in a condition which is a continuous and ideal nonstick medium and the whole force process is adiabatic. Moreover, the ambient temperature is room temperature. Rigid particles will experience the acoustic radiation force (F1) which can be calculated as follows in the plane traveling field. F 1 ¼ R3 jEF ;
ð1Þ
where R is the particle radius, j is the wavenumber of the acoustic field, E is the energy density of the acoustic field, F is the acoustic contrast factor1 for the particle–fluid system. Suspended particles will scatter that sound field. The interaction of the particles with the scattered sound field generated from neighboring particles gives rise to the secondary 1 It is a parameter describing the difference of density between fluid and p a r t ic l e s . , w h i2ch c a n b e e x p r e s s e d a s f o l l o w e q u a t i o n : 5q 2q qc F ¼ 13 2qpp þql l q l cl2 , where, q and c stand for density and acoustic p p speed, and the subscript p and l denote particle and liquid respectively.
acoustic radiation force [23,24]. This force can lead to gravitation between two particles or between particles and porous materials. Although it is difficult to predict an equivalent secondary acoustic force between the suspended particles and film, we expect it to be of similar order of magnitude with the value between particles and porous medium comprised by glass bead which was calculated in [25]. (2) Stocks drag force In the fluid with the density of qf and the viscosity of lf, when the orbicular particle with the diameter of dp moves at the speed of t, it will experience the resistance (Fd) from fluid. When Re < 2 (Stocks area), the following expression can be gained: F d ¼ 3p lf d p t:
ð2Þ
This formula shows that Stocks drag force is proportional to the density and viscosity of the fluid, the relative speed and diameter of the particle. (3) Valid buoyancy force Buoyancy force indicates the summation of gravity and buoyancy. Its value can be calculated with the formula below: p F b ¼ ðqf qP Þg d 2P ; ð3Þ 6 where qp is the density of particles. From the expression, we can know that the value of valid buoyancy force is proportional to the diameter of particles and also increases with the decrease of the density of particles. Particles positioned within the porous medium also experience a hydrodynamic drag force due to the ongoing flow of the medium. The hydrodynamic drag of particles
J. Ma et al. / Applied Acoustics 69 (2008) 763–769
moving near surfaces of pore wall has been theoretically and experimentally studied (see, e.g., Ref. [26]). Thus, it is reasonable to expect the. primary and secondary acoustic radiation force can exceed the hydrodynamic drag force of particles. This leads to three possible explanations of the origins of the particle trapping phenomena within porous material. 1.1. Trapping by primary acoustic radiation and blockage by the porous material If porous material does not strongly interfere with the propagation of the primary sound field, then individual particle may experience the primary acoustic radiation force. For particles with positive acoustic contrast factor (most solids suspending in water), this force causes the particles to collect at pressure nodes of the field. The role of porous material may only be to prevent particles from being entrained by the fluid flow. As the flow field tries to entrain particles, their motion (along the flow direction) is blocked by solids comprising porous materials. The primary acoustic radiation force will block any motion of particles around the obstruction and return them to a nodal position. 1.2. Formation of particle flocs by second forces The secondary acoustic radiation force acting between the suspended particles can lead to apparent flocculation between them. Upon cessation of the acoustic field, these flocs would be easily dispersed due to the action of fluid drag. If these flocs are larger than the necks between the elements of porous material, they will be trapped within it. Alternatively, the primary acoustic radiation force could act to constrict the particle flocs at nodal position of the resonant field. This force could prevent flocs from being entrained by fluid from the film.
765
Ultrasonic plane wave can propagate in porous material by coupling with the liquid in the suspending liquid (see Fig. 2). When the suspending liquid travels through a sample, particles are trapped in the porous material due to the action of the acoustic radiation force. For aqueous suspensions, sound impedances of water (1.5 · 106 kg/m2 s) and porous material (3.0 · 106 kg/m2 s for polypropylene) are a little different from each other. Thus, it is generally believed that there would be internal reflection and dispersion of acoustic field as it passes through the porous material. In our experiments, the wavelength of the primary acoustic field is comparable to the dimension of the elements within the porous material. So we expect that the sound field in the fluid filled porous material will have three-dimensional property and will be difficult to describe exactly. However, consideration of the acoustic radiation force acting on particles subjected to a simple acoustic field can provide insight into the mechanism by which particles may be collected within the porous material. In the standing wave system, ultrasonic radiation force will push nanoparticles from acoustic pressure node to anti-node. If particles collect into definite weight which is large enough to overcome the summation of Stocks drag force and valid buoyancy force, they will deposit. However, when particles which have gotten together do not experience very strong action of ultrasonic, just be subject to the disturbance of ultrasonic, these particles will not be dispersed. Nevertheless, if the strength of ultrasonic is great enough, these assembled particles will be scattered into suspension again.
1.3. Deposition of particles on the surface of porous material by secondary acoustic radiation force Secondary acoustic radiation force between the particles and medium may give rise to the direct deposition of particles within the porous material. Under the strong action of this force, particles would be fast embedded onto the inner surface of the porous material and the flow field may not be able to displace them. The force may cause other particles to attach onto the anchored particles in turn. The exact mechanism of particle retention in the porous material is still a matter of investigation for the time being. Since the theory was founded by Biot in 1956 [27], the propagation of elastic waves in a fluid-saturated porous material has been studied widely. But as known to us, the analysis of the acoustic radiation force experienced by particles in porous material has not been mentioned before.
Fig. 2. Schematic design of the treatment unit of ultrasonic equipment: (1) Sample Schematic design of the treatment unit of ultrasonic equipment: (1) Sample holder, (2) finishing trough (with the bottom size of 12 cm · 12 cm), (3) transducer, (4) sample, and (5) dispersion.
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2. Materials and methods 2.1. Test materials ZnO nanoparticles (40 nm, the particle size distribution of it is shown in Fig. 3) purchased from Shanghai Bona Co. Ltd., PR China were used as test samples. PP films whose weight per square meter, thickness, pore-size distribution (tested by bubble-point methods), water flow, porosity and bubble pressure are 8 g/m2, 150 lm, 150 nm, 10 ml/ min, 73% and 4.80 kg/cm2, respectively were supplied by Shanghai Xinya Filter Co. Ltd., PR China. The PP film was a melt-spun PP fabric which was cut into equal-sized circular pieces with the diameter of 10 cm. Ultrasonic equipment including four independent treatment units (12 cm · 13 cm · 14.5 cm), a signal generator (A001, Qingdao University, China) and a signal amplifier (B001, Qingdao University, China) was provided as the acoustic field. As shown in Fig. 1, each treatment unit was equipped with one transducer at bottom. There were four transducers with different frequency (28, 42, 51 or 59 kHz,1Bl6035d, Ilsan Suntek Corporation, Korea), but with the same input electric power of 60 W. 2.2. Test methods Fabric weight (g/m2) of PP was measured before and after ultrasonic treatment using a physical balance. To keep the sample at the same status, fabric samples were dried for 2 h at 80 C and then allowed to cool for 24 h in the desiccator before and after the treatment. Five samples for each experimental condition were tested in order to obtain the mean weight, which was used to calculate EPW with the following equation: EPW ¼ ðG1 G0 Þ=G0 100%;
ð4Þ
where G1 = measured weight after ultrasonic treatment and G0 = measured weight before ultrasonic treatment. Each treatment unit was poured with 1000 mL distilled water, 5 g ZnO nanoparticles, with the pH of the dispersing solution adjusted to 7 by using glacial acetic acid and acidity meter. The ultrasonic equipment was turned on and the pre-dispersing time before the addition of a fabric sample lasted 20 min. Then the test sample was fixed on one end
of the sample holder which was placed into one treatment unit with the different distance between the sample and the transducer for one frequency. The sample was treated for 20 min, and then washed with distilled water 2 times followed by distilled water wash under the acoustic field for another 3 times until the rinse water was limpidly observed with the naked eye. 3. Results and discussion 3.1. Influences of the concentration of nanoparticle dispersion to EPW This experiment was conducted with the frequency of 40 kHz, the distance between film and sound source of 3k/2 and the process time of 20 min. The results were shown in Fig. 4. Fig. 4 shows that EPW increases with the increase of the concentration of nanoparticle dispersion. Moreover, after 1%, speed of the increase of EPW is apparently slow. 3.2. Influences of the process time to EPW This experiment was conducted with the frequency of 40 kHz, the distance between film and sound source of 3k/2, the concentration of 0.5%. The results were shown in Fig. 5. Fig. 5 shows that EPW increases with the process time.
3 2.5
EPW (%)
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2 1.5 1 0.5 0 0
0.5
1
1.5
Concentration (%) Fig. 4. Influences of the concentration of nanodispersion to EPW.
EPW (%)
4 3 2 1 0 0
5
10
15
20
Time (min) Fig. 3. The particle size distribution of nano ZnO.
Fig. 5. Influences of the process time to EPW.
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3.3. Influences of the distance between the film and sound source to EPW
EPW (%)
5 4 3 2 1 0 0
5
10
15
Distance (cm) Fig. 6. Influences of the distance between the film and sound source to EPW when the frequency is 28 kHz.
1.2
EPW (%)
This experiment was carried out with the frequency of 28 kHz, 42 kHz and 51 kHz, the concentration of dispersion of 0.5%, the process time of 20 min. The positions of acoustic pressure node and anti-node change with the frequency or wavelength of ultrasonic. Thus, samples were set at the positions with the distance between the film and sound source of k/2, 3k/4, 1 k, 3k/2 and 2 k, respectively for the three frequencies. Selected particles and test procedure were the same as above. For the frequency of 28 kHz, influences of the distance between the film and sound source to EPW are shown in Fig. 6. The form of the curve is fluctuant. When the distances are 1 k and 2 k, EPW values are both small. However, when k/2 and 3k/2, they are both large. In addition, the minimum value is 0.89% and the maximum one is 4.36% gained from the figure. For the frequency of 42 kHz, the curve of the influence rule is also fluctuant (see Fig. 7). When the distance is 3k/2, EPW value is the largest. The minimum value is 2.55%. For the frequency of 51 kHz, the influence of the distance between the film and sound source to EPW has an N-word form (see Fig. 8). For the sample with the distance of 1 k or 2 k, EPW value is small. However, EPW value is large for the sample with the distance of k/2 or 3k/2. In addition, the minimum value is 0.25% and the maximum one is 0.95% known from Fig. 6. By comparing Figs. 5–7, we find that EPW value (5.81%) is the largest for the 40 nm ZnO particles when the frequency is 42 kHz and the distance between the sam-
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1 0.8 0.6 0.4 0.2 0
0
2
4
6
8
Distance (cm) Fig. 8. Influences of the distance between the film and sound source to EPW when the frequency is 51 kHz.
ple and sound source is 3k/2. However, when the frequency is 51 kHz and the distance between the sample and sound source is 2 k, EPW value (0.25%) is the smallest. The maximum value is more than 20 times larger than the minimum one. It is very important for the functional finish with nanomaterials in the future. 3.4. Influences of the ultrasonic frequency to EPW To observe the influences of the ultrasonic frequency to EPW 28 kHz, 42 kHz, 51 kHz and 59 kHz were selected in this experiment. The distance between samples and ultrasonic transducer was 3k/2 and process time was 20 min. The results are shown in Fig. 9. Fig. 9 shows that EPW has a trend of decreasing with the increase of the ultrasonic frequency. Up to 42 kHz, EPW begins to have an opposite trend. This may relate to cavitation threshold which will increase with the increase of ultrasonic frequency. Fig. 10 shows that nanomaterials were surely pushed into the PP film. In addition, we found that almost all the SEMs of the testing PP films showed the same phenomenon – many nanoparticles collects around the entrance of pores in the PP films. Through the observation, it may be due to the extrusion coming from the ambient fibers surrounding the pores. Although the test samples embedded with the fine particles were subject to washing and ultrasonic cleaning after each experiment, most of fine particles would still remain intrafilm. Therefore, inside the textile materials ultrasonic energy can not reach the cavitation threshold, instead slight vibration in the pores may enhance the collision chances between the particles having existed in the films and lead
8
EPW (%)
EPW (%)
8 6 4 2 0
0
2
4
6
8
Distance (cm) Fig. 7. Influences of distance between the film and sound source to EPW when the frequency is 42 kHz.
6 4 2 0 0
20
40
60
80
Frequency (kHz) Fig. 9. Influences of the frequency of ultrasonic to EPW.
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Fig. 10. SEM photos of the internal film finished with nanomaterials by ultrasonic.
to agglomeration onto the wall of holes, which may ensure the stability and durability of fine particles having been embedded into the porous materials. 4. Conclusions The research results show that fine particles can be deeply embedded into the PP films with the effect of ultrasonic. EPW has relations to four main factors as follows: 1. The higher the concentration of nanoparticles dispersion is, the larger EPW is. When the concentration increases up to 1%, the increasing speed of EPW begins to become apparently slow. 2. The longer the process time is, the greater EPW is. 3. The film has a larger EPW value when the distance between the film and ultrasonic transducer is k/2 or 3k/2. However, when the distance is 1 k or 2 k, EPW is smaller. 4. For 40 nm ZnO particles, EPW is the largest when the ultrasonic frequency is 42 kHz.
So we can deduce that if only appropriate parameters such as frequency and power etc are selected, the ultrasonic energy in treatment solution will disperse nanomaterials outside of porous materials whereas agglomerate inside. By doing this, we can enrich the application of nanomaterials in functional textile finishing. It is necessary to be pointed out that the influences of related parameters in acoustic field such as frequency or power to EPW are diverse for different nanomaterials and different textiles. The rules and mechanism are to be further studied in the future. Acknowledgement This work was supported by Science Foundation of Shandong Province, PR China, under Contract Y2001F04. References [1] Liu Y, Yang H, Sakanishi A. Ultrasound: Mechanical gene transfer into plant cells by sonoporation. Biotechnol Adv 2006;24(January/February):1–16.
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