  In these lecture notes, we discuss the physics of a two-dimensional binary
mixture of Bose gases at zero temperature, close to the point where the two
fluids tend to demix. We are interested in the case where one of the two fluids
(the bath) fills the whole space, while the other one (the minority component)
contains a finite number of atoms. We discuss under which condition the
minority component can form a stable, localized wave packet, which we relate to
the celebrated "Townes soliton". We discuss the formation of this soliton and
the transition towards a droplet regime that occurs when the number of atoms in
the minority component is increased. Our investigation is based on a
macroscopic approach based on coupled Gross-Pitaevskii equations, and it is
complemented by a microscopic analysis in terms of bath-mediated interactions
between the particles of the minority component.
