  Responsive poly-N-isopropylacrylamide-based microgels are commonly used as
model colloids with soft repulsive interactions. It has been shown that the
microgel-microgel interaction in solution can be easily adjusted by varying the
environmental parameters, e.g., temperature, pH, or salt concentration.
Furthermore, microgels readily adsorb to liquid-gas and liquid-liquid
interfaces forming responsive foams and emulsions that can be broken on-demand.
In this work, we explore the interactions between microgel monolayers at the
air-water interface and a hard colloid in the water. Force-distance curves
between the monolayer and a silica particle were measured with the Monolayer
Particle Interaction Apparatus. The measurements were conducted at different
temperatures and lateral compression, i.e., different surface pressures. The
force-distance approach curves display long-range repulsive forces below the
volume phase transition temperature of the microgels. Temperature and lateral
compression reduce the stiffness of the monolayer. The adhesion increases with
temperature and decreases with a lateral compression of the monolayer. When
compressed laterally, the interactions between the microgels are hardly
affected by temperature, as the directly adsorbed microgel fractions are nearly
insensitive to temperature. In contrast, our findings show that the
temperature-dependent swelling of the microgel fractions in the aqueous phase
strongly influences the interaction with the probe. The microgel monolayer
changes from a soft to a hard repulsive interface.
