  Deformable colloids and macromolecules adsorb at interfaces, as they decrease
the interfacial energy between the two media. The deformability, or softness,
of these particles plays a pivotal role in the properties of the interface. In
this study, we employ a comprehensive \emph{in situ} approach, combining
neutron reflectometry with molecular dynamics simulations, to thoroughly
examine the profound influence of softness on the structure of microgel
Langmuir monolayers under compression. Lateral compression of both hard and
soft microgel particle monolayers induces substantial structural alterations,
leading to an amplified protrusion of the microgels into the aqueous phase.
However, a critical distinction emerges: hard microgels are pushed away from
the interface, in stark contrast to the soft ones, which remain steadfastly
anchored to it. Concurrently, on the air-exposed side of the monolayer, lateral
compression induces a flattening of the surface of the hard monolayer. This
phenomenon is not observed for the soft particles as the monolayer is already
extremely flat even in the absence of compression. These findings significantly
advance our understanding of the pivotal role of softness on both the
equilibrium phase behavior of the monolayer and its effect when soft colloids
are used as stabilizers of responsive interfaces and emulsions.
