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 femtolitervolume 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.

Controlled Shrinkage and Re-expansion of a Single Aqueous Droplet inside an Optical Vortex Trap

“…Thus one can write, for a spherical particle: 16,17 (3) Where G = πa 2 , the area of particle perpendicular to the axis of beam propagation, V is the volume of the particle, and Q abs is a factor of absorption that accounts for the intrinsic absorption of the droplet and the interface refraction; given by: 16-18 (4) where N = (n 2 + ik 2 )/(n 1 + ik 1 ), a complex index of refraction of the particle (subscript 2) with respect to the surrounding (subscript 1); and x = 2πa / λ, sometimes known as a wave or diffraction parameter. Defining the intrinsic absorption of the droplet as α 2 = 4πk 2 /λ, one can re-write: (5) where f is a correction factor to the intrinsic absorption coefficient given by: (6) Using the physical parameters listed in Table I, we computed that for water droplet surrounded by acetophenone, f = 0.67 . For water droplet surrounded by decanol, f = 0.72, which agrees with the fact that because decanol's refractive index (1.437) is closer to the refractive index of water (1.33) than is for acetophenone (1.537), less power loss due to refraction is expected for a trapped water droplet in decanol.…”

“…With recent advances in optical vortex traps, which employ Laguerre-Gaussian (LG) beam profile, trapping low-index particles becomes possible. [3][4][5][6][7] LG profile is characterized by a dark focus in the center of the laser beam, which resembles a donut, and has found particular utility in trapping single femtoliter-volume aqueous droplets. [3][4][5][6][7] Aqueous droplets, when present in an immiscible phase that has a slight solubility for water, will shrink and concentrate their contents via passive diffusion and dissolution of water into the immiscible phase.…”

Optical manipulation of airborne particles: techniques and applications

“…3. For most trapping work, these arrays of spots are sufficient, but continuous patterns can be generated, 26 as can other beam types, such as Laguerre-Gaussian beams, 27 which are discussed below. The patterns can be produced using various algorithms, 28 which vary in their complexity depending on the amount of information that is known about the desired phase and whether spots or continuous patterns are required.…”

Magnetophoretic Evaluation of Interfacial Adsorption of Dysprosium(III) on a Single Microdroplet

“…This is the reason why a lot of techniques to investigate a single microparticle have been developed. [2][3][4][5][6][7] The migration analyses 8 can satisfy this requirement, since the microparticle can be caused to migrate by an extremely weak driving force. For example, only 10 -14 N is enough to move a particle of 1 μm radius at the velocity of 1 μm s -1 , which can be observed by a conventional optical microscope.…”