Abstract

This paper describes the use of Laguerre-Gaussian beams for the optical manipulation and fusion of single femtoliter- volume aqueous droplets, a technique developed to facilitate new applications in droplet microfluidics. Individual droplets were generated by emulsifying water in acetophenone with SPAN 80 surfactant. Such water droplets have a lower refractive index than the surrounding oil and therefore cannot be trapped using a conventional Gaussian beam optical tweezers - they are repelled from areas of high laser intensity. We demonstrate the ability of a Laguerre-Gaussian beam with an optical vortex (a dark core) to position trapped droplets precisely while excluding surrounding aqueous droplets from entering the trap, thereby preventing unwanted cross contamination by other nearby droplets. Additionally, the limitation of optical vortex traps for inducing droplet fusion is illustrated, whereby two droplets cannot be easily coagulated due to the optical repulsion of one droplet from a trap holding another droplet. We demonstrate a solution to this problem by modulation of the spatial intensity profile of the optical vortex beam. Spatial modulation was achieved by translating the etched computergenerated hologram (CGH) with respect to the input Gaussian beam (λ = 1064nm), thereby shifting the location of the embedded phase singularity within the optical vortex beam. We present both simulated and experimentally measured intensity profiles of the vortex beam caused by translation of the CGH. We further describe the use of this technique to achieve controlled fusion of two aqueous droplets. Fusion is illustrated in figure 1.

© 2007 IEEE