Use of the ultrasonic slow wave to identify pore closure caused by deposition of a polymeric surface layer

The critical wave number of an ultrasonic slow wave can be directly related to the permeability of the porous material. In particular, this technique can be used to identify pore closure in a porous medium such as a micro- or ultrafiltration membrane. A 10.5 mm-thick porous alumina sample was tested using four broadband transducers with central frequencies from 1 MHz to 20 MHz. Experimental results demonstrate the sensitivity of this method to the change of permeability caused by closure of the pores. This technique could be used for the detection of membrane fouling, especially for monitoring the development of early-stage biofouling. Early-stage biofouling results in a decrease in the permeability of a porous material, although the deposited layer is, in general, not detectable using ultrasonic methods. A feasibility study was carried out in a laboratory-scale flow cell, which was developed to simulate the measurement of pore closure under realistic operating conditions. A 5 MHz broadband transducer and waveguide were used for this testing.