4 research outputs found
Soft particles at liquid interfaces: From molecular particle architecture to collective phase behavior
Soft particles such as microgels and core-shell particles can undergo
significant and anisotropic deformations when adsorbed to a liquid interface.
This, in turn, leads to a complex phase behavior upon compression. Here we
develop a multiscale framework to rationally link the molecular particle
architecture to the resulting interfacial morphology and, ultimately, to the
collective interfacial phase behavior, enabling us to identify the key
single-particle properties underlying two-dimensional continuous,
heterostructural, and isostructural solid-solid transitions. Our approach
resolves existing discrepancies between experiments and simulations and thus
provides a unifying framework to describe phase transitions in interfacial
soft-particle systems. We establish proof-of-principle for our rational
approach by synthesizing three different poly(N-isopropylacrylamide)
soft-particle architectures, each of which corresponds to a different targeted
phase behavior. In parallel, we introduce a versatile and highly efficient
coarse-grained simulation method that adequately captures the qualitative key
features of each soft-particle system; the novel ingredient in our simulation
model is the use of auxiliary degrees of freedom to explicitly account for the
swelling and collapse of the particles as a function of surface pressure.
Notably, these combined efforts allow us to establish the first experimental
demonstration of a heterostructural transition to a chain phase in a
single-component system, as well as the first accurate in silico account of the
two-dimensional isostructural transition. Overall, our multiscale framework
provides a bridge between physicochemical soft-particle characteristics at the
molecular- and nanoscale and the collective self-assembly phenomenology at the
macroscale, paving the way towards novel materials with on-demand interfacial
behavior
Interface-induced hysteretic volume phase transition of microgels: simulation and experiment
Thermo-responsive microgel particles can exhibit a drastic volume shrinkage
upon increasing the solvent temperature. Recently we found that the spreading
of poly(N-isopropylacrylamide)(PNiPAm) microgels at a liquid interface under
the influence of surface tension hinders the temperature-induced volume phase
transition. In addition, we observed a hysteresis behavior upon temperature
cycling, i.e. a different evolution in microgel size and shape depending on
whether the microgel was initially adsorbed to the interface in expanded or
collapsed state. Here, we model the volume phase transition of such microgels
at an air/water interface by monomer-resolved Brownian dynamics simulations and
compare the observed behavior with experiments. We reproduce the experimentally
observed hysteresis in the microgel dimensions upon temperature variation. Our
simulations did not observe any hysteresis for microgels dispersed in the bulk
liquid, suggesting that it results from the distinct interfacial morphology of
the microgel adsorbed at the liquid interface. An initially collapsed microgel
brought to the interface and subjected to subsequent swelling and collapsing
(resp. cooling and heating) will end up in a larger size than it had in the
original collapsed state. Further temperature cycling, however, only shows a
much reduced hysteresis, in agreement with our experimental observations. We
attribute the hysteretic behavior to a kinetically trapped initial collapsed
configuration, which relaxes upon expanding in the swollen state. We find a
similar behavior for linear PNiPAm chains adsorbed to an interface. Our
combined experimental - simulation investigation provides new insights into the
volume phase transition of PNiPAm materials adsorbed to liquid interfaces
Versatile strategy for homogeneous drying patterns of dispersed particles
After spilling coffee, a tell-tale stain is left by the drying droplet. This universal phenomenon, known as the coffee ring effect, is observed independent of the dispersed material. However, for many technological processes such as coating techniques and ink-jet printing a uniform particle deposition is required and the coffee ring effect is a major drawback. Here, we present a simple and versatile strategy to achieve homogeneous drying patterns using surface-modified particle dispersions. High-molecular weight surface-active polymers that physisorb onto the particle surfaces provide enhanced steric stabilization and prevent accumulation and pinning at the droplet edge. In addition, in the absence of free polymer in the dispersion, the surface modification strongly enhances the particle adsorption to the air/liquid interface, where they experience a thermal Marangoni backflow towards the apex of the drop, leading to uniform particle deposition after drying. The method is independent of particle shape and applicable to a variety of commercial pigment particles and different dispersion media, demonstrating the practicality of this work for everyday processes
Interfacial self-assembly of SiO<sub>2</sub>–PNIPAM core–shell particles with varied crosslinking density
Spherical particles confined to liquid interfaces generally self-assemble into hexagonal patterns. It was theoretically predicted by Jagla two decades ago that such particles interacting via a softrepulsive potential are able to form complex, anisotropic assembly phases. Depending on the shape and range of the potential, the predicted minimum energy configurations include chains, rhomboidand square phases. We recently demonstrated that deformable core-shell particles consisting of a hard silica core and a soft poly(N-isopropylacrylamide) shell adsorbed at an air/water interface canform chain phases if the crosslinker is primarily incorporated around the silica core. Here, we systematically investigate the interfacial self-assembly behavior of such SiO2-PNIPAM core-shell particles as a function of crosslinker content and core size. We observe chain networks predominantly at low crosslinking densities and smaller core sizes, whereas higher crosslinking densities lead to the formation of rhomboid packing. We correlate these results with the interfacial morphologies of the different particle systems, where the ability to expand at the interface and form a thin corona at the periphery depends on the degree of crosslinking close to the core. We perform minimum energy calculations based on Jagla-type pair potentials with different shapes of the soft repulsive shoulder. We compare the theoretical phase diagram with experimental findings to infer to which extend the interfacial interactions of the experimental system may be captured by Jagla pairwise interaction potentials