Abstract
We investigate holographic optical tweezers manipulating micro-beads at a suspended air-liquid interface. Axial confinement of the particles in the two-dimensional interface is maintained by the interplay between surface tension and gravity. Therefore, optical trapping of the micro-beads is possible even with a long distance air objective. Efficient micro-circulation of the liquid can be induced by fast rotating beads, driven by the orbital angular momentum transfer of incident Laguerre-Gaussian (doughnut) laser modes. Our setup allows various ways of creating a tailored dynamic flow of particles and liquid within the surface. We demonstrate examples of surface manipulations like efficient vortex pumps and mixers, interactive particle flow steering by arrays of vortex pumps, the feasibility of achieving a "clocked" traffic of micro beads, and size-selective guiding of beads along optical "conveyor belts".
Highlights
Diffractive optical tweezers [1] have been demonstrated for various applications like flexible trapping and manipulation of microscopic particles [2, 3, 4, 5], sorting of particles in microfluidic flows [6, 7], size-selective particle trapping [8], or pumping and guiding particles by angular momentum transferring beams [9, 10, 11]
The present article intends to give an overview on optical manipulation methods of a gas-liquid surface which are realized by holographic methods
Some of the presented results have been already reported previously by other groups, the main difference of the present experiments are the application to interfaces, with the advantage of using air-objectives which may have a rather low numerical aperture for trapping of the particles in the two-dimensional surface
Summary
Diffractive optical tweezers [1] have been demonstrated for various applications like flexible trapping and manipulation of microscopic particles [2, 3, 4, 5], sorting of particles in microfluidic flows [6, 7], size-selective particle trapping [8], or pumping and guiding particles by angular momentum transferring beams [9, 10, 11]. For the purpose of three-dimensional trapping one has to use microscope objectives with a high numerical aperture, required mainly for particle confinement in the axial direction. Since the objectives are immersed in a liquid it is not possible to manipulate particles at a gas-liquid interface. We present a diffractive optical tweezers setup acting at a well-defined microscopic gasliquid surface. A well-defined micro-surface with a diameter of about 200 microns is formed at the bottom hole, which can be observed from below by an inverted microscope with an air objective. In order to manipulate particles at the inverted surface with laser tweezers, we use a holographically steered optical tweezers setup which is described
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