Ultrasonic reflectometry

Af/f is small whenever rq,2 is close to one. Conversely, since the QCM only works well when the normalized frequency shift Af/ff is small, it makes sense to assume 1. Equation 39 shows that quartz crystals are acoustic re-flectometers. The results of QCM measurements can therefore be easily compared to data obtained with other forms of ultrasonic reflectometry [57,58]. It is well known from optical techniques such as elUpsometry [59] or surface plasmon resonance (SPR) spectroscopy [60] that a film thickness can be inferred from a measurement of the reflectivity. The same applies to acoustics. 

Nematic liquid crystals (LCs) are a classical example of complex fluids. If we trust the small-load approximation as well as the matured theory of nema-todynamics [76], we must be able to predict the frequency shift induced by nematic LCs. The theory of nematic LCs in contact with the QCM has been worked out in detail by people who did not know about the QCM as a tool to probe these phenomena. These authors performed ultrasonic reflectometry. As we know from Sect. 5, the results of these studies can be transported to the QCM in a straightforward way by just using Eq. 39. 

Evans et al. (2005) recently reported significant improvements in the scanning acoustic microscopy technique as well as in the extension of ultrasonic reflectometry to the... 

Figure 10.6. (A) Experimental set-up used for membrane compaction studies of a high-pressure separation system by ultrasonic time-domain reflectometry (B) Scheme of the separation cell showing the externally mounted transducer and the primary reflections identified as a, b and c, which correspond to the top plate-feed solution interface, feed solution-top membrane surface interface and bottom membrane surface-support plate interface, respectively (C) Change of the arrival time which translates into changes in membrane thickness during compaction. (Reproduced with permission of Elsevier, Ref [63].)...

There are relatively few reports of compaction of asymmetric gas separation membranes in the scientific literature. Reinsch et al. used ultrasonic time-domain reflectometry to look at the very first hour of compaction with a commercial CA membrane [35]. The 13.2% collapse of the membrane thickness in the first seconds probably comes from compression of the membrane sub-structure and/or the fabric support. The roughly 10% flux decline over this flrst hour was at a much slower rate, which may be an indication of the true compaction process. 

Membrane Characterization by Ultrasonic Time-Domain Reflectometry... 

Acoustic wave transmission and reflection have been used extensively throughout the twentieth century for nondestmctive testing. In the latter part of the twentieth century ultrasound techniques were developed for the noninvasive imaging of fetuses and internal organs in the health-care industry. In 1995 the first application of ultrasonic time-domain reflectometry (UTDR) for characterizing membrane processes was reported (Bond et al., 1995). This chapter will provide an overview on the developments in applying UTDR for membrane and membrane process characterization. 

MEMBRANE CHAFtACTERIZATION BY ULTRASONIC TIME-DOMAIN REFLECTOMETRY... 

Kools, W. F. C., Konagurthu, S., Greenberg, A. R., Bond, L. J., Rrantz, W.B., van den Boomgaard, T., and Strathmann, H. (1998). Use of ultrasonic time-domain reflectometry for real-time measurement of thickness changes during evaporative casting of polymeric films. J. Appl. Polym. Sci. 69, 2013. 

Li, J. X., and Sanderson, R. D. (2002). In situ measurement of particle deposition and its removal in microfiltration by ultrasonic time-domain reflectometry. Desalination 146, 169. 

Mairal, A. P., Greenberg, A. R., and Rrantz, W. B. (2000). Investigation of membrane fouling and cleaning using ultrasonic time-domain reflectometry. Desalination 130, 45. 

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