ULTRASONIC MANIPULATION OF PARTICLES IN MICROGRAVITY

The manipulation of yeast cells and latex particles of diameters 1.3, 12 and 20 mu m in aqueous suspension in 1 and 3 MHz ultrasonic standin g waves has been examined and compared in microgravity (0 g), 1 g and 1.8 g. The experiments were carried out during the 23rd ESA parabolic flight campaign. The suspended particles concentrated to form bands at half wavelength separation in the axial direction of a vertical tubul ar sample holder with a Bessel pressure amplitude distribution profile . At 1 g small (1.3 mu m latex) particles formed bands but these broke up within a few seconds. In contrast throughout 0 g bands of these pa rticles formed and remained stable. The transition from 0 to 1.8 g dur ing flight induced streaming which broke up the bands of 1.3 mu m late x. Bands formed with yeast cells were more stable at 1 g but, during t ransitions from 0 to 1.8 g, some bands broke up. Bands of the larger ( 12 and 20 mu m) particles were stable at 0, 1 and 1.8 g and during all transitions between the fields. Thermal gradient convective flow rath er than acoustic streaming was identified as the main source of flow i n the sample holder at 1 g. The absence of thermal streaming at 0 g al lowed manipulation of smaller particles in that situation. A frequency ramping technique appropriate for removal of particles from suspensio n in 1 g had a better performance in 0 g.