  Anisotropic, submicrometer sized particles are versatile systems providing
interesting features in creating ordering in 2-dimensional systems. Combining
hard ellipsoids with a soft shell further enhances the opportunities to trigger
and control order and alignment. In this work, we report a rich 2D phase
behavior and show how softness affects the ordering of anisotropic particles at
fluid oil-water interfaces. Three different core-shell systems were synthesized
such that they have the same elliptical hematite-silica core but differ with
respect to thickness and stiffness of the soft microgel shell. Compression
isotherms, the shape of individual core-shell microgel as well as their 2D
order at a decane-water interface are investigated by means of the
Langmuir-Blodgett technique combined with ex-situ Atomic Force Microscopy (AFM)
imaging, as well as by Disspiative Particle Dynamics (DPD) simulations. We show
how softness, size and anisotropy of the microgel shell affect the side-to-side
vs. tip-to-tip ordering of anisotropic hybrid microgels as well as the
alignment with respect to the direction of compression in the Langmuir trough.
A large and soft microgel shell leads to an ordered structure with a tip-to-tip
alignment directed perpendicular to the direction of compression. In contrast,
a thin and harder microgel shell leads to side-to-side ordering orientated
parallel to the compression direction. In addition, the thin and harder
microgel shell induces clustering of the microgels in the dilute state
indicating the presence of strong capillary interactions. Our findings
highlight the relevance of softness for the complex ordering of anisotropic
hybrid microgels at interfaces.
