Conical Refraction Bottle Beams for Entrapment of Absorbing Droplets

Michael Esseling,Cornelia Denz,Robert Meissner,Jens Schnelle,Christina Alpmann

doi:10.1038/s41598-018-23399-y

Abstract

Conical refraction (CR) optical bottle beams for photophoretic trapping of airborne absorbing droplets are introduced and experimentally demonstrated. CR describes the circular split-up of unpolarised light propagating along an optical axis in a biaxial crystal. The diverging and converging cones lend themselves to the construction of optical bottle beams with flexible entry points. The interaction of single inkjet droplets with an open or partly open bottle beam is shown implementing high-speed video microscopy in a dual-view configuration. Perpendicular image planes are visualized on a single camera chip to characterize the integral three-dimensional movement dynamics of droplets. We demonstrate how a partly opened optical bottle transversely confines liquid objects. Furthermore we observe and analyse transverse oscillations of absorbing droplets as they hit the inner walls and simultaneously measure both transverse and axial velocity components.

Highlights

The versatility of optical techniques has boosted ever-increasing interest in their exploitation for micromechanical purposes
Photophoresis, which can be described as light-induced thermophoresis, describes the migration of microscopic particles in a temperature-gradient that is generated by inhomogeneous heating of the particle surface by light[14], an effect that obviously comes into play the more absorbing a particle is
The light is focused onto the surface of a KTP crystal by a 4x microscope objective (MO) (Nikon, NA 0.1)

Summary

Introduction

The versatility of optical techniques has boosted ever-increasing interest in their exploitation for micromechanical purposes. Some indirect techniques were proposed, in which light does not directly interact with the objects under investigation, but its influence is mediated by thermal or electric media[6,7,8,9] These secondary techniques, such as photophoresis or dielectrophoresis, contribute to the tool-set of optical confinement, a method to handle non-transparent particles, which is impossible in direct optical tweezers due to excessive scattering forces. Among these techniques, photophoresis has been demonstrated as a versatile mechanism to handle highly-absorbing airborne particles. We investigate different protocols to fill this trap and present a sophisticated dual-view observation technique to locate the droplet position in three dimensions

Methods

Results

Conclusion