A non-invasive ultrasonic method for viscosity measurements

A method for noninvasive measurement of viscosity, based on acoustic streaming, is described. The induced streaming velocity, which is measured by an ultrasonic Doppler technique, is used as a measure of the viscosity of the liquid. Experiments were performed on a glycerin/water mixture with a transmitted sound frequency of 10.4 MHz. The viscosity was varied from 40 mPa-s to 330 mPa-s by changing the temperature of the experimental liquid. Transmitted sound intensity was varied up to 640 mW/cm², yielding streaming velocities up to 14 mm/s. The streaming velocity is reported to be proportional to the sound intensity and inversely proportional to the viscosity of the liquid. Experiments on milk turning sour are also reported. Since ultrasound propagates through most materials for enclosing liquids, the method is suited for the assessment of the viscosity of liquids in sealed containers