Colloids and Surfaces A: Physicochemical and Engineering Aspects
Microgravity laser-photophoresis of high density microparticles in water
Introduction
Separation and characterization of micrometer-sized particles in liquids are important subjects in current analytical chemistry, colloidal chemistry, environmental chemistry, and biological technology [1]. For these purposes, several kinds of external fields have been used to migrate microparticles and biological macromolecules. For example, a gravitational field or a centrifugal field was utilized in field-flow fractionation [2] and centrifugation [3], a nonuniform electric field in dielectrophoresis [4], [5], [6], and a magnetic field in magnetophoresis [7], [8], [9].
Laser-photophoresis is a new migration technique for microparticles in liquids using the scattering force of radiation pressure. We have studied photophoretic migration of a single organic droplet in water and polystyrene microparticles in water previously and we have confirmed that the laser-photophoresis is promising as a new method for separation and characterization of microparticles in liquid [10], [11]. Although the scattering force of the radiation pressure is weaker than the gravitational force in the case of high density particles, it is sufficient for migrating the particles if the density is compensated for that of the medium.
A characteristic feature of this method is that an individual microparticle can be moved by a non-contacting force. By using laser photophoresis, we can separate microparticles due to the difference in their refractive indices and sizes. Some studies on photophoretic behavior of microparticles in air or liquids have been reported [12], [13], but the photophoretic efficiency, the fraction of the light power converted to the force on the particle, has been hardly discussed. In order to predict the photophoretic velocity of microparticles, the accumulation of the data of photophoretic efficiency for various kinds of materials with different optical properties is essential. However, the measurement of the photophoretic velocity for various kinds of microparticles with different densities is not so easy.
In normal gravity condition, the density of particle and the buoyancy due to the medium affect the motion of particle. For example, when the density of the particle is different from that of the medium, the particle has to be sedimented to the bottom or go up to the top of the cell. Furthermore, when the particle absorbs light and generates heat on the laser irradiation, the convection of the medium around the particle also affects the motion of the particle. These effects make the accurate photophoretic velocity measurement difficult.
Therefore, in order to make the precise determination of photophoretic velocity of the particle, it is necessary to perform experiments under microgravity where the sedimentation, the buoyancy and the convection are all negligible. In this study, laser-photophoresis of high-density solid particles in water are examined under microgravity condition to observe the precise photophoretic behavior of the particles.
Section snippets
Experimental
The microgravity condition was achieved by using a free drop experiment. The free drop experiments were performed at the Micro-Gravity Laboratory of Japan (MGLAB, Toki, Gifu, Japan). In this facility, high-quality microgravity environment of 10−5 g can be obtained for about 4.5 s by dropping the capsule for 100 m within the vacuum of an underground shaft.
The photophoretic velocity of a solid particle in water was measured by the use of an apparatus shown in Fig. 1. Sample particles were
Theoretical section
In this report, the radiation force in the forward direction is termed the photophoretic force [11], [17], [18]. Assuming the Gaussian profile of the irradiated laser beam, the photophoretic force, FP, which acts on a small spherical particle at the center of the beam, is represented by the relationship:where P is the incident laser power; n, the refractive index of the suspending medium; r, the radius of the particle; c, the velocity of light in a vacuum; ω, the radius of the laser
Results and discussion
All the solid micro-particles in water examined under microgravity conditions migrated under the influence of the irradiation of the laser beam in the direction of the light. Under normal gravity condition, all the micro-particles went down to the bottom of the cell and could not be moved by the photophoretic force except carbon micro-particles. In the case of the carbon micro-particles, the photophoretic migration was observed even under normal gravity condition, because the carbon
Conclusion
The photophoretic efficiencies of high-density solid particles in water have been determined experimentally for the first time by an experiment under microgravity conditions. It was found that the photophoretic efficiencies of the photo-absorbing carbon particle and the photo-reflecting metal particles were much larger than those of transparent particles. It was proved that the Mie scattering theory was useful for the prediction of the photophoretic efficiency of various kinds of particles in
Acknowledgements
This study was supported in part by “Ground Research Announcement for Space Utilization” promoted by NASDA and Japan Space Forum.
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