디지털 홀로그래픽 3차원 미세 입자추적 기법을 이용한 마이크로 스케일 유동 내 입자의 횡방향 이동현상 연구

Abstract

The inertial migrations of particles and red blood cells in micro-scale flows were investigated experimentally using the digital holographic micro-PTV (DHM-PTV) system. To enhance the measurement accuracy along the light propagation direction, the autofocus function which quantifies the image sharpness was employed to detect the position of optimally focused reconstruction image. This method was found to be effective for accurate measurement of positional information, especially for large size particles. The developed DHM-PTV technique was applied to measure 3D motion of red blood cells (RBCs) in a micro-tube flow to check the feasibility of the system. The 3D coordinates of RBC were consecutively tracked and the resulting trajectories were obtained. The adoption of a high-speed camera in DHM-PTV system showed strong potential in analyzing the temporal evolution of RBCs motion in 3D. The DHM-PTV was used to investigate the inertial migration of neutrally buoyant spherical particles suspended in micro-scale pipe flows. The Segr？-Silberberg annulus formed by tubular pinch effect was apparently observed as the Reynolds number (Re) and the particle size increased. The equilibrium position, where the inward and the outward lift forces are balanced, drifted toward the wall with increasing Re. In the low Re range, the thickness of the particle-free layer near the wall was increased as Re increased. The degree of inertial migration was quantified using the PDF distribution of the particles. The number of particles in the PDF peak was increased as the reduced tube length L3 increased. When the value of L3 was around 0.4, the particles near the wall were fully migrated to the peak position due to the strong wall effect. When the value of L3 exceeds 3, over 80% of whole particles were migrated to the equilibrium peak position by the enhanced outward inertial migration force. The inertial migration of neutrally buoyant spherical particles in a square microchannel was experimentally investigated using the DHM-PTV technique. As the channel Reynolds number RC and the channel entry length L/H increased, the particles were laterally focused at a certain lateral equilibrium position named pseudo Segr？-Silberberg annulus. When RC was further increased, the particles were focused along cross-lateral direction into four attractors located at the center of each microchannel side. By quantifying the degree of inertial migration using the probability density functions of particle distributions, the cross-lateral focusing was found to linearly depend on the shear gradient of the flow and the particle diameter. From the obtained data, an inertial focusing number FC which can be used to determine the occurrence of inertial particle focusing was suggested. This parameter also can be used to estimate the design parameters required for inertial microfluidic devices.The effect of cell deformability on the inertial migration of RBCs was investigated using the DHM-PTV technique. Similar to the spherical solid particles, the equilibrium position of the unfixed (deformable) RBCs drifted toward the wall as Re increased. As reported previously, the unfixed RBCs were focused at the position nearer to the center axis than the fixed (rigid) RBCs and spherical particles. At low Re condition, the fixed RBCs exhibited similar equilibrium position with the spherical particles. However, as the Re increased, an upper bound was observed in the equilibrium position and the position maintained a constant value. The value of PDF peak was smaller for the fixed RBCs than the unfixed RBCs. This seems to be caused by vigorous tumbling motions of the fixed RBCs, inducing more significant flow disturbances.