Bio-mimetic flow

 

Optical separation of ellopsoidal particles in a uniform flow

 

-  The behavior of an ellipsoidal particle subjected to a vertical optical force by a loosely focused laser beam in a uniform flow was studied numerically.


- The optical force and optically induced torque on the ellipsoidal particle varied according to the aspect ratio and initial inclination angle.


-  The ellipsoidal particles with different aspect ratios could be effectively separated if the rotation along the spanwise direction was suppressed.


 

Schematic diagram showing an ellipsoidal particle subjected to an optical force induced by a loosely focused laser beam in a uniform flow

(a) An oblate spheroidal particle with major and minor semi-axes of a and b, respectively; (b) paths of a ray on an ellipsoidal particle with the inclination angle 3 in the x-y plane (rotation about the z- axis); (c) an ellipsoidal particle with the inclination angle 1 in the y-z plane (rotation about the x- axis).

Separation of ellipsoidal particles with the initial inclination angle of 3=/2


For the ellipsoidal particle aligned with the laser beam axis (3=/2), the migration distance decreased as the aspect ratio decreased, and the ellipsoidal particles were found to be separable according to the aspect ratio in a COPS system.


The migration distance remained constant for a given S number, even if its components differed, as in the case of a rigid spherical particle.



Trajectories of the ellipsoidal particles with an initial inclination angle of 3=/2 and different aspect ratios.



(a) Inclination angle of ellipsoidal particles with an initial inclination angle of 3=/2 as a function of the streamwise position; the instantaneous positions and the distributions of the optical force on the surfaces of the ellipsoidal particles with an initial inclination angle of 3=/2 near the laser beam axis: (b) b/a=0.9; (c) b/a=0.5; (d) b/a=0.3.


The migration distances of ellipsoidal particles with an initial inclination angle of 3=/2 as a function of the S number for either a fixed fluid velocity or a fixed laser beam power. The analytical result was included to compare with numerical results for the spherical particle.



-  Effects of the initial inclination angle and the S number on the migration distance


The migration distance depended on the initial inclination angle, even for a given ellipsoidal particle shape. The effect of the rotation was increased as the S number increased.

The migration distance of the ellipsoids initially rotated about the x- axis with 3=0 was not significantly varied because the optical force induced by the inclination acted along the z- axis. To achieve the effective separation of ellipsoidal particles according to the aspect ratio, the rotation about the axis along the spanwise direction should be suppressed.

The Sc number was introduced by including the effect of aspect ratio, which could be used to predict the migration distance for ellipsoidal particles.


The migration distances of ellipsoidal particles with different aspect ratios as a function of the initial inclination angle 3: (a) S=4; (b) S=6; (c) S=8; (d) S=10.