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ELSEVIER AS Dukhin 8, PI Goetz A, TH Wines B COLLOIDS AND Colloids and Surfaces SURFACES A ELSEVIER A: Physicochemicaland Engineering AspectsJ73 (2000) 127-158 www.elsevier.nl{Jocate/colsurfa A.S. Dukhin 8, P.I. Goetz a, T.H. Wines b, P. Somasundaran b,* a Dispersion TechnologyInc., 3 Hillside Ave~, Mt Kisco, NY 10549, USA b Columbia University, Rm. 911, .500 W. 12OthStreet, 1140Amsterdam Avenue, New York, NY 10027, USA Received 28 September 1999; ~ 13 March 2000 Abstract Two new ultrasound based techniques (acoustics and electroacoustics)otTer a unique opportunity to characterize concentrated dispersion, emulsions and microemulsions in their natural state, without dilution. Elimination of the dilution protocol is crucial for an adequate characterization of liquid dispersions, especially structured. Dilution changes the thermodynamic equilibrium in these systems and atTects their reological properties. Changes in equilibrium conditions can lead to variation of the particle size and can also affect surface chemistry. In this paper, a short review of the theoretical basis of the ultrasound techniques is given. Emphasis is placed on the theoretical models which are supposed to be valid in concentrated systems.These theories have been developed recently on the basis of 'cell model concept' for bOth acoustics and electroacoustics. This approach opens the way to implement particle-particle interaction into the theoretical model. Experiment proves that these theories are adequate in concentrated systems up to 45% vol. Second part of the paper is dedicated to the applications of acoustics and electroacoustics.The list of applications includes: ceramics, mixed dispersed systems, chemical-polishing materials, emulsions, food emulsions, microemulsions and latecies. @ 2000 Elsevier Science B.V. All rights reserved. Keywordr: Acoustic spectroscopy; Electroacoustic spectroscopy; Ultrasound techniques . Introduction capabilities for being successful. The first hard- ware for measuring acoustic properties of liquids The widespread acceptanceand commercializa- was developed more than 50 years ago at MIT [I] tion of acoustic spectroscopy has been slow to by Pellam and Galt. The first acoustic theory for develop. This technique has been overlooked by heterogeneoussystems was created by Sewell 90 many in academia and industry in the past, but years ago [2]. The general principles of the acous- has recently been showing increased levels of ac- tic theory were formulated 45 years ago by Ep- ceptance.This powerful method of characterizing Stein and Carhart [3]. There is a long list of concentrated heterogeneous systems has all the applications and experiments using acoustic spec- troscopy, see reviews [4,5]. Despite all of these . Corresponding author. Tel.: + 1-212-8542926;fax: -j developments, however, acoustic spectroscopy is 212-8548362. rarely mentioned in modem handbooks on colloid E-mail address:[email protected] (P. Somasundaran). science[6,7]. 0927-7757/001$- see front matter 0 2000 Elsevier Science B. V. All rights reserved. PO: 80927-7757(00)00593-8 128 A.S. Dukhin et a/.jCol/oids and Surfaces A: Physicochem.Eng. Aspects 173 (2tXXJ)127~lSB Acoustics is able to provide reliable particle size degree of complexity in fitting experimental re- information for concentrated dispersions without sults to theoretical models based on various any dilution. There are examples when acoustics acoustic loss mechanisms. The advent of high yields size information at volume fractions above speedcomputers and the refinement of these theo- 400/0.This in-situ characterization of concentrated retical models has made the inherent complexity systems makes the acoustic method very useful of this analysis of little consequence.In compari- and unique in this capability compared to alter- son, many other particle sizing techniquessuch as nate methods including light scattering where di- photon correlation spectroscopyalso rely on simi- " lution is required. Acoustics is also able to deal lar levels of complexity in analyzing experimental with low dispersed phase volume fractions and in rcsults. some systems can characterize down to below Acoustics has a related field that is usually 0.1% vol. This flexibility for concentration range referred to as 'electroacoustics' [8]. Electroacous- provides an overlap with classic methods for di- tics can provide particle size distribution as well lute systems. In the overlap range, acoustics size as ~-potential. This relatively new technique is characterization has been found to have excellent more complex than acoustics because an addi- agreement with these other techniques. tional electric field is involved. As a result, both Acoustics is not only a particle sizing technique, hardware and theory become more complicated. but also provides information about the mi- There are even two different versions of electroa- crostructure of the dispersedsystem. The acoustic coustics depending on what field is used as a spectrometer can be considered as a micro- driving force. Electrokinetic sonic amplitude rheometer. In acoustics, stressesare applied in the (ESA) involves the generation of sound energy caused by the driving force of an applied electric same way as regular rheometers, but over a very short distances on the micron scale. In this way, field. Colloid vibration current (CVI) is the phe- nomenon where sound energy is applied to a the microstructure of the dispersed system can be system and a resultant electric field or current is sensed.Currently, this feature of the acoustics is created by the vibration of the colloid electric only beginning to be exploited, but it is certainly double layers. very promising. Coming back to acoustics, it's lack of wide- Many people have perceivedacoustics to have a spread acceptancemay be related to the fact that high degree of complexity. The operating princi- it yields too much, sometimes overwhelming in- ples are in fact quite straightforward. The acoustic formation. Instead of dealing with interpretation spectrometer generates sound pulses that pass of the acoustic spectra it is often easier to dilute through a sample system and are then measured the system of interest and apply light based tech- by a receiver. The passage through the sample niques. It was often naively assumed that the system causes the sound energy to change in dilution had not affected the dispersion character- intensity and phase.The acoustic instrument mea- istics. Lately, many researchersare coming to the sures the sound energy losses (attenuation) and realization that dispersed systemsneed to be ana- the sound speed.The sound attenuates due'to the lyzed in their natural concentrated form, and that interaction with the particles and liquid in the dilution destroys a lot of useful and important sample system, Acoustic spectrometers generally properties. operate with sound in the frequency range of The authors are optimistic about the future of , 1-100 MHz. This is a much higher sound fre- acoustics in colloid science. It is amazing what quency than the upper limit of the hearing which this technique can do especially in combination is only 0.02 MHz. with electroacoustics for characterizing eleCtric While the operating principles are relatively surface properties. It is hoped that this review will simple, the analysis of the attenuation data to allow one to taste the power and opportunities obtain particle size distributions does involve a related to these sound based techniques. A.S. Dukhin et al. / Colloid\' and Surfaces A: Physicochem.Eng. Aspects 173 (2O(XJ)127-158 129 2. Theoretical background field, and consequently to alternating electric cur- rent. As a result, a part of the acoustic energy is There are six known mechanismsof the ultra- transformed into electric energy and then irre- sound interaction with a dispersed system: (1) versibly to heat. viscous (av;J; (2) thermal (ath); (3) scattering (asc); Only the first four loss mechanisms (viscous, (4) intrinsic (ainJ; (5) structural (a.tr); and (6) thermal, scattering and intrinsic) make a signifi- electrokinetic (aeJJ. cant contribution to the overall attenuation spec- (1). The viscous losses of the acoustic energy tra in most cases.Structural lossesare significant occur due to the shear waves generated by the only in structured systems that require a quite particle oscillating in the acoustic pressure field. different theoretical framework. These four mech- These shear waves appear becauseof the differ- anisms form the basis for acoustic spectroscopy. ence in the densities of the particles and medium. Total attenuation measured with acoustic spec- This density contrast causes the particle motion trometer usually equals to the sum of these four with respect to the medium. As a result, the liquid partial attenuations: layers in the particle vicinity slide relative to each (X (Xvis + (Xlh + (xsc + (XiDI (1) other. This sliding non-stationary motion of the = liquid near the particle is referred to as the 'shear The contribution of electrokinetic lossesto the wave'. Viscous lossesare dominant for small rigid total sound attenuation is almost always negligi- particles with sizes below 3 ~m, such as oxides, bly small [9] and will be neglected.This opens an pigments, paints, ceramics, cement, graphite, etc. opportunity to separate acoustic spectroscopy (2). The reason for the thermal losses is the from electroacoustic spectroscopy beCauseacous- temperature gradients generated near the particle tic attenuation spectra is independent of the elec- surface. These temperature gradients are due to tric properties
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