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The study of diffusion through gels has a variety of applications in food science and the immobilization of microorganisms, as well as in improving the understanding of gel properties. Conventional methods of measuring diffusion coefficients in gels include the use of diaphragm cells, gel beads, nuclear magnetic resonance techniques, and radioactive tracers. We have developed a new technique which involves use of ultrasonic waves in a gel-coated silicon-based device that is in contact with a liquid. This device, a flexural plate-wave (FPW) sensor, is a recently developed member of a class of what are termed "acoustic" sensors. All these sensors function by measuring the change in the velocity or attenuation of an acoustic wave that is produced by the measuring of interest. The modes of ultrasonic wave propagation differ among acoustic sensors; the FPW device we describe has a high sensitivity in comparison to other acoustic sensors and is fabricated by high-volume repeatable processes like those used to make integrated circuits. Our experiments are apparently the first in which an acoustic sensor has been used to measure diffusion of liquids in gels, although acoustic sensors employing the so-called quartz crystal microbalance (QCM) and surface acoustic waves have been used to monitor the process of gelation and to measure diffusion of gases in thin films.
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