In situ imaging of the three-dimensional shape of soft responsive particles at fluid interfaces by atomic force microscopy

Effect of particle stiffness and surface properties on the non-linear viscoelasticity of dense microgel suspensions

HYPOTHESIS

Particle surface chemistry and internal softness are two fundamental parameters in governing the mechanical properties of dense colloidal suspensions, dictating structure and flow, therefore of interest from materials fabrication to processing.

EXPERIMENTS

Here, we modulate softness by tuning the crosslinker content of poly(N-isopropylacrylamide) microgels, and we adjust their surface properties by co-polymerization with polyethylene glycol chains, controlling adhesion, friction and fuzziness. We investigate the distinct effects of these parameters on the entire mechanical response from restructuring to complete fluidization of jammed samples at varying packing fractions under large-amplitude oscillatory shear experiments, and we complement rheological data with colloidal-probe atomic force microscopy to unravel variations in the particles' surface properties.

FINDINGS

Our results indicate that surface properties play a fundamental role at smaller packings; decreasing adhesion and friction at contact causes the samples to yield and fluidify in a lower deformation range. Instead, increasing softness or fuzziness has a similar effect at ultra-high densities, making suspensions able to better adapt to the applied shear and reach complete fluidization over a larger deformation range. These findings shed new light on the single-particle parameters governing the mechanical response of dense suspensions subjected to deformation, offering synthetic approaches to design materials with tailored mechanical properties.

Modulating the conformation of microgels by complexation with inorganic nanoparticles

HYPOTHESIS

The complexation of microgels with rigid nanoparticles is an effective way to impart novel properties and functions to the resulting hybrid particles for applications such as in optics, catalysis, or for the stabilization of foams/emulsions. The nanoparticles affect the conformation of the polymer network, both in bulk aqueous environments and when the microgels are adsorbed at a fluid interface, in a non-trivial manner by modulating the microgel size, stiffness and apparent contact angle.

EXPERIMENTS

Here, we provide a detailed investigation, using light scattering, in-situ atomic force microscopy and nano-indentation experiments, of the interaction between poly(N-isopropylacrylamide) microgels and hydrophobized silica nanoparticles after mixing in aqueous suspension to shed light on the network reorganization upon nanoparticle incorporation.

FINDINGS

The addition of nanoparticles decreases the microgels' bulk swelling and thermal response. When adsorbed at an oil-water interface, a higher ratio of nanoparticles influences the microgel's stiffness as well as their hydrophobic/hydrophilic character by increasing their effective contact angle, consequently modulating the monolayer response upon interfacial compression. Overall, these results provide fundamental understanding on the complex conformation of hybrid microgels in different environments and give inspiration to design new materials where the combination of a soft polymer network and nanoparticles might result in additional functionalities.

Relationship between π-A isotherms and single microgel/microgel array structures revealed via the direct visualization of microgels at the air/water interface

The structures of single microgels and microgel arrays formed at the air/water interface were visualized directly, and their structures correlated with π-A isotherms in order to understand the compression behavior of soft and deformable microgels at this interface. Large microgels (ca. 4 μm) were synthesized so that these can be clearly visualized at the air/water interface, even under high compression, and a series of microgel compression experiments were directly evaluated using a Langmuir trough equipped with a fluorescence microscope. The experiments revealed that upon compressing the microgel arrays at the interface voids disappeared and colloidal crystallinity increased. However, the colloidal crystallinity decreased when the microgel arrays were strongly compressed. In addition, when the structures were observed at higher magnification, it became clear that the single microgel structures, when visualized from above, changed from circular to polygonal upon compressing the microgel array. The results of this study can be expected to improve the understanding of the compression behavior of microgel arrays adsorbed at the air/water interface and will thus be useful for the creation of new functional microgel stabilizers with potential applications in e.g., bubbles and emulsions.

Peculiarities of Emulsions Stabilized by Stimuli-Responsive Interpenetrating Polymeric Network Microgels

Emulsions have become a crucial product form in various industries in modern times. Expanding the class of substances used to stabilize emulsions can improve their stability or introduce new properties. Particularly, the use of stimuli-responsive microgels makes it possible to create "smart" emulsions whose stability can be controlled by changing any of the specified stimuli. Thus, finding new ways to stabilize emulsions may broaden their application. In this work, for the first time, we applied microgels based on interpenetrating polymeric networks (IPNs) of poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAA) as stabilizing agents for "oil-in-water" emulsions. We have demonstrated that emulsions stabilized by such soft particles can remain colloidally stable for an extended period, even after being heated up to 40 °C, which is above the lower critical solution temperature (LCST) of PNIPAM. On the contrary, the emulsions stabilized by PNIPAM homopolymer microgels were broken upon heating. To understand the stabilization mechanism of the emulsions, mesoscopic computer simulations were performed to study the IPN microgels at the liquid-liquid interface. The simulations demonstrated that when the first subnetwork (PNIPAM) collapses, the particle adopts a flattened core-shell morphology with a highly swollen PAA-rich shell and a collapsed PNIPAM-rich core. Unlike its PNIPAM homopolymer counterpart, the IPN microgel maintains its three-dimensional shape, which provides stability to the microgel-based emulsions over a wide range of temperatures. Our combined findings could be useful in developing new approaches to emulsions' storage, biphasic catalysis, and lubrication of mechanisms in various operating and climatic conditions.

Softness matters: effects of compression on the behavior of adsorbed microgels at interfaces

Deformable colloids and macromolecules adsorb at interfaces as they decrease the interfacial energy between the two media. The deformability, or softness, of these particles plays a pivotal role in the properties of the interface. In this study, we employ a comprehensive in situ approach, combining neutron reflectometry with molecular dynamics simulations, to thoroughly examine the profound influence of softness on the structure of microgel Langmuir monolayers under compression. Lateral compression of both hard and soft microgel particle monolayers induces substantial structural alterations, leading to an amplified protrusion of the microgels into the aqueous phase. However, a critical distinction emerges: hard microgels are pushed away from the interface, in stark contrast to the soft ones, which remain firmly anchored to it. Concurrently, on the air-exposed side of the monolayer, lateral compression induces a flattening of the surface of the hard monolayer. This phenomenon is not observed for the soft particles as the monolayer is already extremely flat even in the absence of compression. These findings significantly advance our understanding of the key role of softness on both the equilibrium phase behavior of the monolayer and its effect when soft colloids are used as stabilizers of responsive interfaces and emulsions.

Ultrasound-Induced Adsorption of Acousto-Responsive Microgels at Water-Oil Interface

Ultrasonic mixing is a well-established method to disperse and mix substances. However, the effects of ultrasound on dispersed soft particles as well as on their adsorption kinetics at interfaces remain unexplored. Ultrasound not only accelerates the movement of particles via acoustic streaming, but recent research indicates that it can also manipulate the interaction of soft particles with the surrounding liquid. In this study, it evaluates the adsorption kinetics of microgel at the water-oil interface under the influence of ultrasound. It quantifies how acoustic streaming accelerates the reduction of interfacial tension. It uses high-frequency and low-amplitude ultrasound, which has no destructive effects. Furthermore, it discusses the ultrasound-induced shrinking and thus interfacial rearrangement of the microgels, which plays a secondary but non-negligible role on interfacial tension reduction. It shows that the decrease in interfacial tension due to the acoustic streaming is stronger for microgels with higher cross-linker density. Moreover, it shows that ultrasound can induce a reversible decrease in interfacial tension due to the shrinkage of microgels at the interface. The presented results may lead to a better understanding in any field where ultrasonic waves meet soft particles, e.g., controlled destabilization in foams and emulsions or systems for drug release.

Interfacial Fluid Rheology of Soft Particles

In situ interfacial rheology and numerical simulations are used to investigate microgel monolayers in a wide range of packing fractions, ζ_{2D}. The heterogeneous particle compressibility determines two flow regimes characterized by distinct master curves. To mimic the microgel architecture and reproduce experiments, an interaction potential combining a soft shoulder with the Hertzian model is introduced. In contrast to bulk conditions, the elastic moduli vary nonmonotonically with ζ_{2D} at the interface, confirming long-sought predictions of reentrant behavior for Hertzian-like systems.

The compression of deformed microgels at an air/water interface

The compression of deformed hydrogel microspheres (microgels) at air/water interfaces was investigated using a Langmuir-Blodgett trough with simultaneous in situ visualization of the process using a fluorescent microscope. The relationship between the structure of the microgel arrays and the compression behavior was clarified using microgels with different degrees of crosslinking.

Interactions between interfaces dictate stimuli-responsive emulsion behaviour

Stimuli-responsive emulsions offer a dual advantage, combining long-term storage with controlled release triggered by external cues such as pH or temperature changes. This study establishes that thermo-responsive emulsion behaviour is primarily determined by interactions between, rather than within, interfaces. Consequently, the stability of these emulsions is intricately tied to the nature of the stabilizing microgel particles - whether they are more polymeric or colloidal, and the morphology they assume at the liquid interface. The colloidal properties of the microgels provide the foundation for the long-term stability of Pickering emulsions. However, limited deformability can lead to non-responsive emulsions. Conversely, the polymeric properties of the microgels enable them to spread and flatten at the liquid interface, enabling stimuli-responsive behaviour. Furthermore, microgels shared between two emulsion droplets in flocculated emulsions facilitate stimuli-responsiveness, regardless of their internal architecture. This underscores the pivotal role of microgel morphology and the forces they exert on liquid interfaces in the control and design of stimuli-responsive emulsions and interfaces.

Exploring the 3D Conformation of Hard-Core Soft-Shell Particles Adsorbed at a Fluid Interface

The encapsulation of a rigid core within a soft polymeric shell allows obtaining composite colloidal particles that retain functional properties, e.g., optical or mechanical. At the same time, it favors their adsorption at fluid interfaces with a tunable interaction potential to realize tailored two-dimensional (2D) materials. Although they have already been employed for 2D assembly, the conformation of single particles, which is essential to define the monolayer properties, has been largely inferred via indirect or ex situ techniques. Here, by means of in situ atomic force microscopy experiments, the authors uncover the interfacial morphology of hard-core soft-shell microgels, integrating the data with numerical simulations to elucidate the role of the core properties, of the shell thicknesses, and that of the grafting density. They identify that the hard core can influence the conformation of the polymer shells. In particular, for the case of small shell thickness, low grafting density, or poor core affinity for water, the core protrudes more into the organic phase, and the authors observe a decrease in-plane stretching of the network at the interface. By rationalizing their general wetting behavior, such composite particles can be designed to exhibit specific inter-particle interactions of importance both for the stabilization of interfaces and for the fabrication of 2D materials with tailored functional properties.

Compression and Ordering of Hollow Microgels in Monolayers Formed at Liquid-Liquid Interfaces

Monolayers of polymer microgels with a spherical cavity adsorbed at the liquid-liquid interface were studied using mesoscopic computer simulations. One liquid, named water, was always considered as a good solvent, while the microgel solubility in the second liquid, named oil, was varied. The symmetric and asymmetric cases of vanishing and the strong differences in solubility between the network particles and the liquids were considered. The simulations provided us with an insight into the shape and volume changes of the microgels upon compression, making it possible to relate the response of the individual network with the collective order and structure of the monolayer. Similar to regular microgels, the compression of the monolayer of hollow particles led to a decrease in lateral sizes accompanied by shape transformation from a flattened to a nearly spherical shape. However, the presence of a cavity filled with solvent caused some unique differences in the behavior of the system. The adsorption pathway of hollow microgels at the liquid interface predefines: (a) the position of the particles with respect to the interface and (b) the structure of the monolayer. A striking discovery is that in the symmetric case of similar solubility of the microgel in both liquids, it is possible to produce a monolayer in which one part of the network faces the aqueous phase and the other part faces the oil phase. The polymer concentration profiles plotted along the normal to the interface reveal a redistribution of polymeric mass of the microgels relative to the interface, distinguishing between the microgels whose cavities are filled with water and oil, respectively. Moreover, the ratio between the microgels faced in water and oil does not change upon compression and predetermines the response and order of the monolayer.

Surface Wettability-Switchable Janus Fiber Fragments Stabilize Pickering Emulsions for Effective Oil/Water Separation

Pickering emulsions indicate stronger resistance against droplet coalescence than the surfactant-stabilized emulsions. To resemble the surfactant amphiphilicity, Janus fiber fragments (JFs) were herein prepared through side-by-side electrospinning of poly(styrene-maleic anhydride) (PSMA) derivatives and cryosection of the aligned fibers, followed by conjugation of hydrophobic cetylamine (C₁₆) and hydrophilic poly(N-isopropylacrylamide) (PNIPAm) ligands on the separate sides. Orthogonal analysis table L₂₅(5⁶) was designed to examine the effect of process parameters on the emulsification efficiency and stability index of Pickering emulsions. The emulsification efficiency is dominated by the JF concentration and length, while the emulsion stability could be prolonged through adjusting the JF concentration and hydrophilic graft density. JF-stabilized emulsions exhibit a much higher stability index (96.4%) than that of Janus microparticle counterparts (37.7%). Though there is no apparent effect on the surface wettability, JFs with PNIPAm grafts of about 2200 Da achieve the most stable Pickering emulsions. Superparamagnetic Fe₃O₄ nanoparticles are inoculated into JFs to collect emulsion droplets under a magnetic field, and the emulsions could be demulsified at an elevated temperature to harvest oil. Meanwhile, the recovered JF emulsifiers could be repeatedly used without loss of the emulsification efficiency. Thus, this study demonstrates surface-switchable JFs to be effective stabilizers of Pickering emulsions and readily recycled for oil harvesting from wastewater.

Probing Temperature Responsivity of Microgels and Its Interplay with a Solid Surface by Super-Resolution Microscopy and Numerical Simulations

Super-resolution microscopy has become a powerful tool to investigate the internal structure of complex colloidal and polymeric systems, such as microgels, at the nanometer scale. An interesting feature of this method is the possibility of monitoring microgel response to temperature changes in situ. However, when performing advanced microscopy experiments, interactions between the particle and the environment can be important. Often microgels are deposited on a substrate, since they have to remain still for several minutes during the experiment. This study uses direct stochastic optical reconstruction microscopy (dSTORM) and advanced coarse-grained molecular dynamics simulations to investigate how individual microgels anchored on hydrophilic and hydrophobic surfaces undergo their volume phase transition with temperature. We find that, in the presence of a hydrophilic substrate, the structure of the microgel is unperturbed and the resulting density profiles quantitatively agree with simulations performed under bulk conditions. Instead, when a hydrophobic surface is used, the microgel spreads at the interface and an interesting competition between the two hydrophobic strengths,monomer-monomer vs monomer-surface,comes into play at high temperatures. The robust agreement between experiments and simulations makes the present study a fundamental step to establish this high-resolution monitoring technique as a platform for investigating more complex systems, these being either macromolecules with peculiar internal structure or nanocomplexes where molecules of interest can be encapsulated in the microgel network and controllably released with temperature.