Glycerol is used in many applications of science and daily life as it is cheap and biologically non-invasive. In science, aqueous solutions of glycerol are commonly used for experimental investigations as their density can be adapted by changing the glycerol content in the solution. Although the density of aqueous glycerol solutions has been measured precisely since more than a century, current models show a deviation from measured data of up to 2%. In this work we present an analytical expression to accurately calculate the density of aqueous glycerol solutions. The presented empirical model is validated in the range between 15 and 30 • C and has a maximum deviation of less than 0.07% with respect to measured data. This improves the accuracy of current models by more than one order of magnitude. By knowing the temperature and glycerol content of the solution, its density can be simply calculated with the presented model. A Matlab function is provided in the supplementary material to allow a simple implementation in other scientific work.

In a microfluidic environment, the presence of bubbles is often detrimental to the functionality of the device, leading to clogging or cavitation, but microbubbles can also be an indispensable asset in other applications such as microstreaming. In either case, it is crucial to understand and control the growth or shrinkage of these bodies of air, in particular in common soft-lithography devices based on polydimethylsiloxane (PDMS), which is highly permeable to gases. In this work, we study the gas transport into and out of a bubble positioned in a microfluidic device, taking into account the direct gas exchange through PDMS as well as the transport of gas through the liquid in the device. Hydrostatic pressure regulation allows for the quantitative control of growth, shrinkage, or the attainment of a stable equilibrium bubble size. We find that the vapor pressure of the liquid plays an important role for the balance of gas transport, accounting for variability in experimental conditions and suggesting additional means of bubble size control in applications.

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No abstract