Abstract
Water in oil emulsions have a wide range of applications from chemical technology to microfluidics, where the stability of water droplets is of paramount importance. Here, using an accessible and easily reproducible experimental setup we describe and characterize the dissolution of water in oil, which renders nanoliter-sized droplets unstable, resulting in their shrinkage and disappearance in a time scale of hours. This process has applicability in creating miniature reactors for crystallization. We test multiple oils and their combinations with surfactants exhibiting widely different rates of dissolution. We derived simple analytical equations to determine the product of the diffusion coefficient and the relative saturation density of water in oil from the measured dissolution data. By measuring the moisture content of mineral and silicone oils with Karl Fischer titration before and after saturating them with water, we calculated the diffusion coefficient of water in these two oils.
Highlights
Water in oil (w/o) emulsions consist of a continuous oil phase with dispersed water droplets
By measuring the saturation density of the oils with Karl Fischer titration, we determined the diffusion coefficient of water in mineral and silicone oils. When both the diffusion coefficient and the saturation density are known, our method can predict the dissolution rate, which is useful in designing experiments with sessile droplets under cover oils
The hydrophobic Petri dish containing the w/o emulsion of thousands of droplets was placed onto an inverted microscope and let to sediment for 5 min
Summary
Water in oil (w/o) emulsions consist of a continuous oil phase with dispersed water droplets. Such arrangements are common both in industrial and laboratory settings [1,2]. The droplet-based approach has several advantages, mainly that the volume of the droplets matches the desirable size range for single-cell manipulations, but it minimizes the amount of reagents needed. Such setups have been successfully commercialized and they proved to be a revolutionary tool in single-cell analysis [5] enabling the development of lab-on-a-chip devices [6] that are capable of integrating entire bioassay workflows on handheld microfluidic chips. These applications center around the printing of aqueous droplets under oils for protein engineering [8], genome amplification by PCR [9] and transpriptomics [10]
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