Magnetic field dependence of ultrasonic response times in ferrofluids

A continuous wave method has been used to investigate the influence of a magnetic field upon the ultrasonic response times in a water-base ferrofluid. Values of response times as long as several seconds were observed. Measurements were made of response times versus field angle at 2.262 MHz and at 830 KHz. It was found that the response time is a strong function of the field angle; also it was unusually long at some angles of rotation. This long reponse time was interpreted as an indication of the formation of larger agglomerate or needle-shaped structures due to the external magnetic field. It was concluded that ultrasonic measurements can he a powerful tool for studying the dynamics of ferrofluids under the action of an external magnetic field.