Vortex-trap-induced fusion of femtoliter-volume aqueous droplets

Abstract

This paper describes the use of an optical vortex trap for the transport and fusion of single femtoliter-volume aqueous droplets. Individual droplets were generated by emulsifying water in acetophenone with SPAN 80 surfactant. We demonstrate the ability of optical vortex traps 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, and a remedy is provided through modulation of the spatial intensity profile of the optical vortex beam. Spatial modulation was achieved by translating the computer-generated hologram (CGH) with respect to the input Gaussian beam, thereby shifting the location of the embedded phase singularity (dark core) 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 and facile fusion of two aqueous droplets.

Figure 1

(A) Schematic outlining the experimental setup used to generate dual optical vortex traps for the manipulation and fusion of aqueous droplets. The TEM₀₀ output of the CW Nd:YAG laser was sent through a computer generated hologram (CGH) to create a Laguerre-Gaussian (LG) beam, then spatially filtered through a pinhole (PH) before being split by a polarization beam splitter (BS) into two separate trapping beams. A dove prism (DP) was placed in the path of one of the beams to change the handedness of the beam for subsequent trapping. Both beams were then combined by a second polarization beamsplitting cube, prior to being sent into the back aperture of a high numerical aperture (NA = 1.3) objective. Abbreviations: CGH, computer generated hologram; BS, beamsplitter; M, mirror; DP, Dove prism; PH, pinhole; DC, dichroic mirror. (B) Schematic demonstrating the displacement of the CGH with respect to the laser beam, which caused a change in the spatial intensity profile of the optical vortex beam. Panels C-E demonstrate the optical trapping and placement of two aqueous droplets in acetophenone/SPAN 80 0.0025% w/w. The scale bar in (E) represents 10 μm.

Figure 2

(A-E) A series of images that depict the repulsion and exclusion of a free floating aqueous droplet (small droplet) from a vortex-trapped aqueous droplet (large droplet at the center of the image). The white arrows show the direction of movement of the small droplet that was being excluded from the vortex trap. (F) An overlay image that shows the trajectory of the small droplet as it was steered around the trapped droplet. (G-I) Schematics illustrating how an aqueous droplet that is outside of the vortex trap first impinges on the ring of laser light intensity that constitutes the vortex trap (G), then is stopped (H) and repelled (I) from the ring of light intensity. The scale bar in panel F represents 10 μm.

Figure 3

Schematic and images that illustrate the positioning and subsequent repulsion of two aqueous droplets in the dual vortex trap. (A-D) Two distant droplets were brought into close proximity; the schematic shows the positions of the two trapped droplets with respect to the positions of the rings of light intensity that form the dual vortex traps. (E, F) Repulsion that arose from the overlap of one of the droplets with the ring of light intensity that held the other droplet caused the loss of the droplet (white arrow indicates the direction of movement of the escaped droplet). The scale bar represents 10 μm.

Figure 4

Simulated and measured spatial intensity profiles for the optical vortex beam during the displacement of the hologram, which caused a lateral shift in embedded phase singularity or dark core. (A-D) shows the relative displacement of the laser beam (red spot) with respect to the dislocation in the hologram. (E-H) are simulations that show the resulting change in the intensity profile of the vortex beam. (I-P) are experimental measurements, where (I-L) are for the vortex beam that did not pass through the dove prism and (M-P) are for the beam that did pass through the dove prism.

Figure 5

Images showing vortex-trap induced fusion of two aqueous droplets in acetophenone (with 0.0025% w/w SPAN 80). The insets depict changes in the intensity profile of the vortex trap as the hologram was displaced; the images in the inset were obtained by recording the back-scattered laser light from the vortex trap off the interface between the coverslip and water. The scale bar represents 10 μm and applies to all panels.