  In this review article, we discuss recent studies on drops and bubbles in
Hele-Shaw cells, focusing on how scaling laws exhibit crossovers from the
three-dimensional counterparts and focusing on topics in which viscosity plays
an important role. By virtue of progresses in analytical theory and high-speed
imaging, dynamics of drops and bubbles have actively been studied with the aid
of scaling arguments. However, compared with three dimensional problems,
studies on the corresponding problems in Hele-Shaw cells are still limited.
This review demonstrates that the effect of confinement in the Hele-Shaw cell
introduces new physics allowing different scaling regimes to appear. For this
purpose, we discuss various examples that are potentially important for
industrial applications handling drops and bubbles in confined spaces by
showing agreement between experiments and scaling theories. As a result, this
review provides a collection of problems in hydrodynamics that may be
analytically solved or that may be worth studying numerically in the near
future.
