Applied Physics

Optical Sifting

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Science  14 Apr 2006:
Vol. 312, Issue 5771, pp. 163
DOI: 10.1126/science.312.5771.163a

The separation and sorting of micrometer-scale particles by size, shape, optical properties, or some combination thereof is necessary in a broad range of applications, from fundamental lab-on-chip studies to the filtering of colloids for materials synthesis. The available techniques tend to rely on the precisely controlled microfluidic flow of particles through a separator.

Ricárdez-Vargas et al. present a simple alternative method, based on reconfigurable patterns of light, that eliminates the need for a microfluidic system. Two interfering laser beams form a periodic potential energy landscape, resembling a washboard, in the liquid layer suspending the polydisperse sample of particles. The spatial periodicity of the fringes is varied to accommodate particles of different sizes. By modulating one of the laser beams with a sawtooth signal that directs an interferometer mirror, the authors effectively vibrate the potential landscape from side to side. This jiggling motion is sufficient to separate particles that are 1 to 5 μm in diameter by size: the larger ones are driven in one direction and the smaller ones in the opposite direction. Moreover, particles of similar size but different optical refractive index (such as latex and silica) can be separated by varying the intensity of the light. — ISO

Appl. Phys. Lett. 88, 121116 (2006).

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