Acoustophoresis of disk-shaped microparticles: A numerical and experimental study of acoustic radiation forces and torques.

Disk-shaped microparticles experience an acoustic radiation force and torque in an ultrasonic standing wave. Hence, they are translated by the acoustic field, an effect called acoustophoresis, and rotated. The torque effect is also known from the "Rayleigh disk" which is described in literature for sound intensity measurements. In this paper, inviscid numerical simulations of acoustic radiation forces and torques for disks with radius ≪ wavelength in water are developed in good agreement with former analytical solutions, and the dependence on disk geometry, density, and orientation is discussed. Experiments with alumina disks (diameter 7.5 µm), suspended in an aqueous liquid in a silicon microchannel, confirm the theoretical results qualitatively at the microscale and ultrasonic frequencies around 2 MHz. These results can potentially be applied for the synthesis of disk-reinforced composite materials. The insights are also relevant for the acoustic handling of various disk-shaped particles, such as red blood cells.

Microparticle production, neutrophil activation, and intravascular bubbles following open-water SCUBA diving.

The goal of this study was to evaluate annexin V-positive microparticles (MPs) and neutrophil activation in humans following decompression from open-water SCUBA diving with the hypothesis that changes are related to intravascular bubble formation. Sixteen male volunteer divers followed a uniform profile of four daily SCUBA dives to 18 m of sea water for 47 min. Blood was obtained prior to and at 80 min following the first and fourth dives to evaluate the impact of repetitive diving, and intravascular bubbles were quantified by trans-thoracic echocardiography carried out at 20-min intervals for 2 h after each dive. MPs increased by 3.4-fold after each dive, neutrophil activation occurred as assessed by surface expression of myeloperoxidase and the CD18 component of ß(2)-integrins, and there was an increased presence of the platelet-derived CD41 protein on the neutrophil surface indicating interactions with platelet membranes. Intravascular bubbles were detected in all divers. Surprisingly, significant inverse correlations were found among postdiving bubble scores and MPs, most consistently at 80 min or more after the dive on the fourth day. There were significant positive correlations between MPs and platelet-neutrophil interactions after the first dive and between platelet-neutrophil interactions and neutrophil activation documented as an elevation in ß(2)-integrin expression after the fourth dive. We conclude that MPs- and neutrophil-related events in humans are consistent with findings in an animal decompression model. Whether there are causal relationships among bubbles, MPs, platelet-neutrophil interactions, and neutrophil activation remains obscure and requires additional study.