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

Abstract

This paper describes the shrinkage and re-expansion of individual femtoliter-volume aqueous droplets that were suspended in an organic medium and held in an optical vortex trap. To elucidate the mechanism behind this phenomenon, we constructed a heat and mass transfer model and carried out experimental verifications of our model. From these studies, we conclude that an evaporation mechanism sufficiently describes the shrinkage of aqueous droplets held in a vortex trap, while a mechanism based on the supersaturation of the organic phase by water that surrounds the droplet adequately explains the re-expansion of the shrunk droplet. The proposed mechanisms correlate well with experimental observations using different organic media, when H 2 O was replaced with D 2 O, and when an optical tweezer was used to induce droplet shrinkage rather than an optical vortex trap. For H 2 O droplets, the temperature rise within the droplet during shrinkage was on the order of 1K or less, owing to the rapid thermal conduction of heat away from the droplet at the microscale and the sharp increase in solubility for water by the organic phase with slight elevations in temperature. Because most chemical species confined to droplets can be made impenetrable to the aqueous/organic interface, a change in the volume of aqueous droplets translates into a change in concentration of the dissolved species within the droplets. Therefore, this phenomenon should find use in the study of fundamental chemical processes that are sensitive to concentration, such as macromolecular crowding and protein nucleation and crystallization.

Optically assembled droplet interface bilayer (OptiDIB) networks from cell-sized microdroplets

“…It was also found that the droplets remained intact for over 24 hours provided that no droplet shrinkage was observed. Interestingly it was noted that this shrinkage phenomenon, which has been reported previously for similarly sized microdroplets in oil, [22][23][24][25][26] was not routinely observed in our experiments. We speculate that this may be due to the inevitable variability of the water to oil ratio of the final microdroplet dispersion manually dispensed into the well.…”

Chemistry and Biology in Femtoliter and Picoliter Volume Droplets

“…One remarkable characteristic of vortex trapping of aqueous droplets, we found, is the ability to tune dynamically the concentration of contained molecules 21,36,37 Figure 8. shows our experimental findings, in which the optical vortex trap slowly “peeled off” layers of water molecules at the interface of the aqueous and immiscible fluids, thus concentrating the encapsulated molecules that were retained within the droplet.…”

Droplets for Ultrasmall-Volume Analysis

“…Once formed, droplets can be manipulated with a wide range of physical mechanisms, such as pressure, dielectrophoresis, 47 magnetic fields, 48 optical fields, 29,49 and thermal gradients. 50,51 Some methods can be used to manipulate a fast-flowing stream of droplets in a high-throughput format, whereas others are best suited for controlling individual droplets.…”

“…12,14,49 In Figure 3d, a droplet is held by an optical vortex trap, 12,13,49,53 shrinks in volume, and upon release from the trap or when the power of the trap is lowered, becomes larger again. An optical vortex trap is formed using a class of laser beam called the Laguerre–Gaussian beam, which is characterized by a helical phase distribution across its wave front that results in the formation of a stably propagating dark core.…”