title: Isostructural solid-solid phase transition in monolayers of soft core-shell particles at fluid interfaces: structure and mechanics.
abstract: We have studied the complete two-dimensional phase diagram of a core-shell microgel-laden fluid interface by synchronizing its compression with the deposition of the interfacial monolayer. Applying a new protocol, different positions on the substrate correspond to different values of the monolayer surface pressure and specific area. Analyzing the microstructure of the deposited monolayers, we discovered an isostructural solid-solid phase transition between two crystalline phases with the same hexagonal symmetry, but with two different lattice constants. The two phases corresponded to shell-shell and core-core inter-particle contacts, respectively; with increasing surface pressure the former mechanically failed enabling the particle cores to come into contact. In the phase-transition region, clusters of particles in core-core contacts nucleate, melting the surrounding shell-shell crystal, until the whole monolayer moves into the second phase. We furthermore measured the interfacial rheology of the monolayers as a function of the surface pressure using an interfacial microdisk rheometer. The interfaces always showed a strong elastic response, with a dip in the shear elastic modulus in correspondence with the melting of the shell-shell phase, followed by a steep increase upon the formation of a percolating network of the core-core contacts. These results demonstrate that the core-shell nature of the particles leads to a rich mechanical and structural behavior that can be externally tuned by compressing the interface, indicating new routes for applications, e.g. in surface patterning or emulsion stabilization.
authors.affiliation: Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland. lucio.isa@mat.ethz.ch.
authors.forename: Marcel
authors.initials: M
authors.lastname: Rey
authors.affiliation: Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland. lucio.isa@mat.ethz.ch and Biocolloid and Fluid Physics Group, Applied Physics Department, Faculty of Sciences, University of Granada, 18071 Granada, Spain.
authors.forename: Miguel Ángel
authors.initials: MÁ
authors.lastname: Fernández-Rodríguez
authors.affiliation: Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland. lucio.isa@mat.ethz.ch and Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California 93106-5080, USA.
authors.forename: Mathias
authors.initials: M
authors.lastname: Steinacher
authors.affiliation: Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland. lucio.isa@mat.ethz.ch.
authors.forename: Laura
authors.initials: L
authors.lastname: Scheidegger
authors.affiliation: Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany.
authors.forename: Karen
authors.initials: K
authors.lastname: Geisel
authors.affiliation: Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany.
authors.forename: Walter
authors.initials: W
authors.lastname: Richtering
authors.affiliation: Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California 93106-5080, USA.
authors.forename: Todd M
authors.initials: TM
authors.lastname: Squires
authors.affiliation: Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland. lucio.isa@mat.ethz.ch.
authors.forename: Lucio
authors.initials: L
authors.lastname: Isa