Size Dependence of Laser-Photophoretic Efficiency of Polystyrene Microparticles in Water
Size Dependence of Laser-Photophoretic Efficiency of Polystyrene Microparticles in Water
复制标题
水中聚苯乙烯微粒激光光泳效率的尺寸依赖性
作者：
H. Monjushiro;A. Hirai;H. Watarai
液体中微米级微粒的分离和表征是当前分析化学、胶体化学、环境化学和生物技术中的重要课题。为了这些目的，已经使用了几种外场来迁移微粒、胶体和生物大分子。例如，重力场或离心场用于场流分离，1非均匀电场用于介电泳，2-4磁场用于磁电泳。5最近，我们提出了一种颗粒的激光电泳，6这是一种通过使用激光辐射压力的散射力来迁移和表征液体中微粒的新技术。激光束的辐射压力也已用于在逆流整体流动条件下对乳胶进行尺寸分离。[7]具有与周围介质不同的折射率的粒子被激光施加辐射力。当介质中的微粒被激光束照射时，观察到两种不同的现象。如果光束被一个数值孔径很大的透镜紧紧聚焦，粒子就会被梯度力困在焦点处，这被称为“光镊”。而如果光束稍微聚焦或不聚焦，则粒子通过辐射的散射力向前移动，这被称为“电泳”。虽然激光电泳有望成为表征和分离液体或气体介质中微粒的一种新技术，但微粒电泳速度受何种因素支配还不清楚。在本文中，我们报告了聚苯乙烯微粒（0.4-1.4-µm半径）在水中与连续Nd：YAG激光（1064 nm）照射的电泳行为。实验上阐明了电泳速度对颗粒尺寸的依赖性，并将结果与基于简单射线光学理论和Mie散射理论的理论计算结果进行了比较。
Separation and characterization of micrometer-sized particles in liquids are important subjects in current analytical chemistry, colloidal chemistry, environmental chemistry, and biological technology. For these purposes, several kinds of external fields have been used to migrate microparticles, colloids, and biological macromolecules. For example, a gravitational field or a centrifugal field was utilized in a field flow fractionation, 1 a nonuniform electric field in dielectrophoresis, 2-4 and a magnetic field in magnetophoresis. 5 Recently, we have proposed a laserphotophoresis of particles, 6 which is a new technique to migrate and to characterize microparticles in liquids, by using the scattering force of laser-radiation pressure. The radiation pressure of a laser beam has been also employed for the size-separation of latexes under counter current bulk flow conditions. 7A particle that possesses a refractive index different from that of the surrounding medium is exerted a radiation force by a laser light. When a microparticle in a medium is irradiated by a laser beam, two different phenomena are observed. If the beam is tightly focused by a lens with a large numerical aperture, the particle is trapped at the focal point by the gradient force, which is known as “optical tweezers”. While if the beam is slightly or not focused, the particle is moved forward by the scattering force of the radiation, which is termed “photophoresis”. Although laser-photophoresis is expected to have high potential as a new technique for characterization and separation of microparticles in liquid or gas media, it is not well-known yet by what kind of factor the photophoretic velocity of microparticle is dominated. In this paper, we report the photophoretic behavior of polystyrene microparticles (0.4-1.4-µm radius) in water with irradiation by a CW Nd: YAG laser (1064 nm). The dependence of the photophoretic velocity on the particle size was elucidated experimentally and the results were compared to those obtained by the theoretical calculations on the basis of the simple ray-optics theory and the Mie scattering theory.