Effect of Internal Architecture on the Assembly of Soft Particles at Fluid Interfaces

Abstract

Monolayers of soft colloidal particles confined at fluid interfaces are at the core of a broad range of technological processes, from the stabilization of responsive foams and emulsions to advanced lithographic techniques. However, establishing a fundamental relation between their internal architecture, which is controlled during synthesis, and their structural and mechanical properties upon interfacial confinement remains an elusive task. To address this open issue, which defines the monolayer’s properties, we synthesize core–shell microgels, whose soft core can be chemically degraded in a controlled fashion. This strategy allows us to obtain a series of particles ranging from analogues of standard batch-synthesized microgels to completely hollow ones after total core removal. Combined experimental and numerical results show that our hollow particles have a thin and deformable shell, leading to a temperature-responsive collapse of the internal cavity and a complete flattening after adsorption at a fluid interface. Mechanical characterization shows that a critical degree of core removal is required to obtain soft disk-like particles at an oil–water interface, which present a distinct response to compression. At low packing fractions, the mechanical response of the monolayer is dominated by the outer polymer chains forming a corona surrounding the particles within the interfacial plane, regardless of the presence of a core. By contrast, at high compression, the absence of a core enables the particles to deform in the direction orthogonal to the interface and to be continuously compressed without altering the monolayer structure. These findings show how fine, single-particle architectural control during synthesis can be engineered to determine the interfacial behavior of microgels, enabling one to link particle conformation with the resulting material properties.

Soft Colloidal Particles at Fluid Interfaces

“…Vialetto et al [ 21 ] demonstrated that the control of the microgel structure during the synthesis process is a very interesting strategy for modulating the properties of the monolayers at fluid interfaces, providing a link between the particle conformation and the material properties. At small values of the interfacial density, it emerges that a mechanical response governed by the polymer corona formed within the interfacial plane by the most external chains, with the specific characteristics of the internal core playing a negligible role.…”

“…Solvated microgel networks can be compressed upon adsorption to fluid interfaces, leading to the deformation of the particle shape, to adapt their conformation to the interface. These morphological changes also control the dynamic behavior and interfacial packing of microgels at the interface, which, in turn, dominate the interfacial tension and rheological properties of the microgel-laden interfaces [ 19 ], playing a very important role in emulsion and foam stabilization [ 20 , 21 ]. In particular, the stretching of soft particles observed when assembled at fluid interfaces provides a better stability of the interface than the adsorption of traditional hard colloids.…”

“…Given the above considerations, despite the broad interest in the adsorption of microgels to fluid/fluid interfaces, the current knowledge about the effect of the specific characteristics of the microgels on their interfacial organization and mechanical properties is scarce, even though it emerges as a critical point on the design of reconfigurable interfaces for specific applications [ 21 ]. This review provides an updated perspective of the current knowledge about the assembly of microgel layers at fluid/fluid interfaces, and how different physico-chemical aspects can be exploited to obtain reconfigurable soft interfaces for different technological purposes, paying special interest to their uses for emulsion stabilization.…”

Interfacial assembly of anisotropic core-shell and hollow microgels

“…The interfacial phase behavior of such spherical microgels has been studied intensively showing the effect of size and hard cores to the spreading of spherical microgels at the interface and their response to an increase in surface pressure within the Langmuir-Pockels trough. [14][15][16][17][18][19][20][21][22][23][24][25][26] Only few studies investigate the interfacial behavior of particles with an anisotropic shape.…”

Soft colloids for complex interfacial assemblies

“…The major challenge now lies in synthesizing a system that actually meets these conditions. Although the characteristics of the shell may recall systems such as ultra-low-crosslinked [17] or hollow microgels [18], these do not represent suitable systems to reproduce this behavior due to the lack of a true internal core. Menath et al [1] propose to use a hard core-soft shell system with clearly separated length-scales, in order to satisfy the first requirement posed by Jagla.…”