  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.
