Adsorption of microgels at an oil–water interface: correlation between packing and 2D elasticity

Abstract

The aim of this paper is to determine how microgels adsorb at a model oil-water interface and how they adapt their conformation to compression, which gives rise to surface elasticity depending on the microgel packing. The structure of the film is determined by the Langmuir films approach (forced compression) and compared to spontaneous adsorption using the pendant drop method. The behaviour of microgels differs significantly from that of non-deformable particles but resembles that of linear polymers or proteins. We also correlate the properties of microgels spontaneously adsorbed at model interfaces to their forced adsorption during emulsification. Finally we propose a route to easily control a posteriori the microgel packing at the surface of droplets and the flow properties of emulsions stabilised by the microgels.

Isostructural solid–solid phase transition in monolayers of soft core–shell particles at fluid interfaces: structure and mechanics

“…The isotherms identify several regimes previously discussed in the literature 42,47 , but here we report a direct insight into the monolayer microstructure and show interesting features of the transitions between the different regions. Representative AFM images of the monolayer in the various regimes are reported next to the isotherm.…”

“…Additionally, measurements of compression isotherms showed a counterintuitive response in the presence of bulk charges in the microgels, with uncharged particles interacting via the interface at larger inter-particle separations compared to charged systems 43 . Furthermore, dilatational elasticity showed an unexpected non-monotonic behavior of the elastic modulus with compression of interfacial microgel monolayers, suggesting that the presence of coreshell interactions couples non-trivially to the interface structure and mechanical properties 42,44,45 . Finally, to date, measuring reliably the shear rheology of microgel-laden interfaces as a function of interface microstructure and compression remains an elusive task, but one of high importance to determine the response of such systems to the mechanical deformations frequently present during processing.…”

“…removing the oil), the microgels in practice look like "fried eggs" with a less densely cross-linked shell surrounding a more cross-linked, and thus stiffer, core. This morphology has prompted the description of the interactions between the particles at the fluid interface as soft repulsive core-shell interactions, where, depending on the inter-particle distance and surface pressure, different microstructures of the interface have been hypothesized 37,42 . In particular, the existence of both shell-shell and core-core contacts has been assumed, but to date, a systematic visualization of these structures and a thorough study on the transitions between them is still lacking.…”

Adsorption of soft particles at fluid interfaces

“…Finally, an important question is how particles behave when they are no longer dilute, but packed (e.g. hexagonally) at interfaces [8,19,54]. Packed particles lose their spherical symmetry and do not fully stretch out on the interface -this is expected to significantly affect their adsorption energy, and thus emulsion behaviour [55].…”

“…1: pNIPAM particles at an oil-water interface, imaged using cryo-SEM. The microgels were synthesized by free-radical precipitation polymerization with a 5 % cross-linking density [19] following standard routes [20][21][22]. The cryo-SEM sample was prepared creating an interface between a 0.36 w/v % aqueous suspension of the microgels and purified n-decane in a custom copper holder, letting the microgels spontaneously adsorb at the interface over 10 minutes.…”