APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016;
Baltimore, Maryland
Session K37: Soft Matter at Interfaces (Particles)
8:00 AM–11:00 AM,
Wednesday, March 16, 2016
Room: 340
Sponsoring
Unit:
GSOFT
Chair: Michael Rubenstein, Univ. North Carolina
Abstract ID: BAPS.2016.MAR.K37.1
Abstract: K37.00001 : Soft particles at fluid interfaces: wetting, structure, and rheology.*
8:00 AM–8:36 AM
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Abstract
Author:
Lucio Isa
(ETH Zurich)
Most of our current knowledge concerning the behavior of colloidal particles
at fluid interfaces is limited to model spherical, hard and uniform objects.
Introducing additional complexity, in terms of shape, composition or surface
chemistry or by introducing particle softness, opens up a vast range of
possibilities to address new fundamental and applied questions in soft
matter systems at fluid interfaces.
In this talk I will focus on the role of particle softness, taking the case
of core-shell microgels as a paradigmatic example. Microgels are highly
swollen and cross-linked hydrogel particles that, in parallel with their
practical applications, e.g. for emulsion stabilization and surface
patterning, are increasingly used as model systems to capture fundamental
properties of bulk materials. Most microgel particles develop a core-shell
morphology during synthesis, with a more cross-linked core surrounded by a
corona of loosely linked and dangling polymer chains. I will first discuss
the difference between the wetting of a hard spherical colloid and a
core-shell microgel at an oil-water interface, pinpointing the interplay
between adsorption at the interface and particle deformation. I will then
move on to discuss the interplay between particle morphology and the
microstructure and rheological properties of the interface. In particular, I
will demonstrate that synchronizing the compression of a core-shell
microgel-laden fluid interface with the deposition of the interfacial
monolayer makes it possible to transfer the 2D phase diagram of the
particles onto a solid substrate, where different positions correspond to
different values of the surface pressure and the specific area. Using atomic
force microscopy, we analyzed the microstructure of the monolayer and
discovered a phase transition between two crystalline phases with the same
hexagonal symmetry, but with two different lattice constants. The two phases
correspond to shell-shell or core-core inter-particle contacts,
respectively, where with increasing surface pressure the former mechanically
fail 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 extended our analysis
to measure the interfacial rheology of the monolayers as a function of the
surface pressure using an interfacial microdisk rheometer; the interfaces
always show a strong elastic response, with a dip in the elastic modulus in
correspondence of the melting of the shell-shell phase, followed by a steep
increase upon formation of a percolating network of the core-core contacts.
The presented results highlight the complex interplay between the wetting
and deformation of individual soft particles at fluid interfaces and the
overall interface microstructure and mechanics. They show strong connections
to fundamental studies on phase transitions in two-dimensional systems and
pave the way for novel nanoscale surface patterning routes.
*The author acknowledges financial support from the Swiss National Science Foundation Grant PP00P2-144646/1
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2016.MAR.K37.1