Optical separation of elastic capsules in a viscous flow

Abstract

The lateral migration of elastic capsules or particles under an optical force in a uniform flow was stud-ied to show the separation characteristics according to the elastic properties and shape in the cross-type optical separator. The particle or capsule was moved through the fluid flow using a laser beam with a Gaussian distribution focused along the direction normal to the flow device surface. To simulate such a system, a penalty immersed boundary method was adopted to enable fluid-membrane coupling, and a dynamic ray tracing method was applied to the optical force calculation. As a preliminary study for the separation of particles or capsules, the optical force on a non-spherical particle subjected to a loosely focused laser beam was calculated. Ellipsoidal particles with different aspect ratios, inclination angles, and positions were modeled, and the effects of these parameters on the optical force were examined. The vertical component of the optical force parallel to the laser beam axis decreased as the aspect ratio decreased, whereas the ellipsoidal particle with a small aspect ratio and a large inclination angle experienced a large vertical optical force. The spherical capsule was always pulled to the laser beam axis. On the other hand, the ellipsoidal particles were pulled toward or repelled away from the laser beam axis, depending on the inclination angle, and they experienced a torque near the focal point. The behavior of the ellipsoidal particles in a viscous fluid was examined by analyzing a dynamic simulation. As the ellipsoidal particles levitated along the direction of the laser beam propagation, they moved horizontally with rotation. Except for the ellipsoidal particle with a small aspect ratio and a zero inclination angle near the focal point, the ellipsoidal particles rotated until the major axis aligned with the laser beam axis.Following the study of the ellipsoidal particles in a stationary fluid, the behavior of an ellipsoidal par-ticle su...