Soft microgels are widely used as deformable building blocks for two-dimensional assemblies, yet solid substrates are often treated as passive supports after interfacial deposition. Here, we show that substrate wettability mechanically preconditions soft microgels before drying. Using in-liquid force-volume atomic force microscopy, we find that the same microgels adopt markedly different hydrated shapes and stiffness profiles depending on substrate contact angle: hydrophobic substrates induce spreading, flattening, and internal stiffening, whereas hydrophilic substrates preserve taller, softer microgels with smaller contact areas. These single-microgel states can explain how Langmuir-Blodgett-deposited monolayers respond during drying. On hydrophilic substrates, the observed assemblies are consistent with soft and weakly immobilized microgels rearranging under immersion-capillary forces, producing distinct corona-corona and core-core contact states and an apparent isostructural transition. On hydrophobic substrates, the flattened and stiffened microgels are more strongly immobilized, likely suppressing capillary-driven rearrangements and largely preserving the transferred interfacial assembly structure. These findings establish substrate-controlled microgel mechanics as the missing link between interfacial self-assembly and the final structures observed after transfer and drying.

Radionuclides including Cs+ pose severe threats to the environment, while their efficient and economic separation from polluted water is challenging. Froth flotation is demonstrated as an economic and scalable water treatment technique, but needs the ions concentrated first in particles by a separate step (adsorption or flocculation), making the whole process complex. Also, the prevalent usage of collectors in flotation can cause secondary pollution. Herein, a smart flotation with foams spontaneously stabilized by Cs+ adsorbents was developed to purify wastewater in one step. The adsorbent, a thermo-responsive poly(N-isopropylacrylamide-co-1-vinylimidazole)(PNV) /potassium copper ferrocyanide (KCuHCF) nanocomposite microgel, was prepared by in situ growth of KCuHCF nanoparticles in PNV via Cu2+ chelation between imidazole and cyano groups, with a loading amount of KCuHCF estimated to be 24.26 wt%. The composite microgel PNV20C100K0.1 shows excellent adsorption capability towards Cs+ with a qmax ~ 167.8 mg/g and good selectivity in the presence of competitive ions including Na+, K+, Mg2+ and Sr2+ with extremely high concentrations (1 mol/L). The adsorption is stable in a wide pH range and exhibits temperature responsiveness. Moreover, the composite microgel has good interfacial activity to spontaneously adsorb at the air/water interface, exhibiting good foamability. The produced foams are quite stable thanks to the excellent interfacial viscoelasticity of the microgel-laden interface and are thermo-responsive to break rapidly across the volumetric phase transition temperature of the microgels. Acting as Cs+ adsorbent and thermo-responsive foam stabilizer, the composite microgel allows development of surfactant-free and smart flotation to selectively recover Cs+ from waste water at a high efficiency (> 80% in 25 min).

Ultrasound-induced degradation of soft polymeric colloids, like microgels, as well as a controlled drug release enabled by mechanoresponsive bonds, has recently attracted considerable attention. However, most examples in the literature focus primarily on the applications rather than examining the underlying mechanisms of the structural changes occurring in microgels due to cavitation─changes that are crucial for developing effective drug delivery systems. In this work, we provide a comprehensive view of how microgel structure governs the susceptibility to rupture and mass loss upon cavitation, investigating both conventional microgels containing mechanoresponsive disulfide bonds and more complex asymmetrically cross-linked core-shell microgels. By combining dynamic and static light scattering, small-angle X-ray scattering, and atomic force microscopy, we demonstrate that an interplay between mechanoresponsive cross-links and the swelling degree determines the microgels' susceptibility to ultrasound-induced damage. Our findings indicate that local stress from cavitation bubbles varies strongly within the microgel dispersion. The majority of microgels undergo gradual erosion at their periphery, resulting in smaller yet structurally intact particles over time, observable by light scattering and AFM. In contrast, microgels closer to a cavitation bubble can experience partial rupture or complete disintegration, producing smaller, more polydisperse fragments, which contribute substantially to the overall mass loss observed. In the core-shell microgels with different cross-linkers in the core and shell, degradation occurs nearly uniformly across both regions, instead of selectively targeting the weaker part. These observations highlight the complexity of the degradation dynamics as well as the similarity to processes seen in linear polymers and bulk hydrogels.

Abstract The effective charge of polyelectrolytes is a crucial parameter that determines their conformational and thermodynamic properties in solution. The Oosawa-Manning condensation model predicts that when the distance between charges becomes smaller than the Bjerrum length ( l B ), additional charges condense onto the backbone, rendering the effective charge independent of the chemical charge density. This implies that above the condensation threshold the effective charge is inversely proportional to l B . This prediction has remained largely untested due to the limited solubility of common polyelectrolytes in apolar media. Here we use a semiflexible polyelectrolyte, carboxymethyl cellulose with organic counterions, which extends its solubility across a wide range of solvents, enabling a test over a broad range of l B . The effective charge is found to be inversely proportional to l B in the low dielectric region and independent of l B when nm, in agreement with the theory.

Hydrogels with tailored porosity and microstructure are essential for biomedical applications such as drug delivery and tissue engineering, yet precise control over their internal architecture remains a challenge. A promising strategy relies on bicontinuous systems formed via spinodal decomposition of polymer blends, enabling the design of hydrogels with tunable and interconnected porosity. By selectively using one polymer as a sacrificial template, hydrogels with large interconnected pores can be developed, enhancing cell growth and migration, nutrient transport, and cellular waste removal. However, the inherent instability of bicontinuous systems, makes it difficult to arrest the microstructure at a defined stage, limiting reproducibility and precise control over pore architecture. Herein we report a straightforward strategy to regulate the phase separation process of GelMA-dextran aqueous two-phase systems (ATPS), enabling 3D printing of hydrogels with tunable porous microarchitectures. By introducing glucono delta-lactone (GDL) into the ATPS, a gradual decrease in pH is achieved, which delays and slows down the kinetics of phase separation. UV photocrosslinking at a selected time point arrests the evolving bicontinuous structure, offering precise control over the pore size and morphology. The results confirm fine-tuning of the phase separation dynamics and enhanced reproducibility. Notably, the GDL-mediated pH control stabilizes the mixture long enough to allow 3D printing, without interfering with the phase separation or the final microstructure. The printed hydrogels retain their interconnected morphology, with tunable channel sizes depending on the timing of crosslinking. This approach offers a robust and versatile route to structure hydrogels with controlled porosity and architecture. It opens new opportunities for the design of biofunctional materials with improved mass transport and mechanical properties, tailored to specific biomedical applications, and it is compatible with advanced fabrication methods like 3D printing.

Microgels are soft nanometer-sized polymer systems that show high potential as responsive functional coatings. Here, we report colloidal probe measurements with a silica particle and thermo-responsive PNIPAM-co-APMH microgels adsorbed to a solid substrate immersed in water at different surface concentrations and at different temperatures. We show that the increase in the microgel concentration allows for a higher deformation leading to an effective softer microgel monolayer. We attribute this to a lower lateral expansion of the microgel on the substrate leading to a higher protrusion of the soft microgel corona into the water phase. Further, it is shown that even a small number of charged functional groups significantly impacts the properties of the microgel coating. As the microgels collapse, the viscoelastic properties of the network change and the microgels become stiffer. Additionally, the surface charge density increases. Thus, adjusting the concentration of the microgels at the interface as well as the temperature allows controlling the viscoelastic properties of the monolayer as well as the steric and electrostatic interactions perpendicular to the interface.

Poly(N-isopropylacrylamide) (PNIPAM) microgels hold promise for various biomedical applications, yet the mechanisms governing microgel interactions with cells remain poorly understood. Recent studies have highlighted the influence of cross-linker content and microgel size on cellular uptake. In this study, we employed atomic force microscopy to systematically investigate the internal structure of both conventional and ultralow cross-linked (ULC) PNIPAM microgels physically adsorbed at the glass/water interface. By studying the correlation between the degree of microgel deformation on a rigid substrate and their stiffness, measured via force spectroscopy, we developed a characterization method that predicts microgel uptake ability in HEK293T cells. Notably, our findings extend to micron-sized ULC microgels, validating the proposed concept. Together, our approach enables the prediction of cellular internalization across a wide range of microgel types, potentially streamlining the screening of crucial microgel properties during early synthesis stages, prior to extensive cell interaction experiments.

Studying the properties of soft nanoparticles exposes students to emerging trends in materials science, fosters interdisciplinary knowledge, and prepares them for their own contributions in both academic and industrial settings. We developed a laboratory atomic force microscopy (AFM) experiment using poly-N -isopropylacrylamide (PNI- PAM) microgels and investigated single nanogels as well as monolayers by AFM and quantitative image analysis. The experiments show how soft nanogels are deformed at interfaces and the students learn to quantify the deformation by quantitative analysis of height and phase images. The deformation is related to a core-corona type of crosslinker distribution inside the microgel. Further experiments address the structure of microgel monolayers and demonstrate structural transitions from a hexagonal phase of microgels in corona-corona contact toward a different regime at higher interfacial concentrations in which microgels form a second hexagonal phase in core-core contact. A quantitative analysis of height images provides the distribution of nearest-neighbor distances. The students use dip-coating to prepare the samples and learn how to correlate AFM exper- iments in the dry state, i.e., at the solid/air interface after evaporation of the solvent, with properties of the microgel in bulk solution, and at the water/air interface.

Polyelectrolytes often display good solubility in water but not in organic solvents, a feature that limits their applications in nonaqueous media such as hand sanitizers. Here, we show that this limitation can be overcome by tuning the counterion-solvent affinity. To this end, the solubility and chain conformation of carboxymethylcellulose (CMC) salts with different organic counterions in a variety of solvents were studied by employing the Hansen solubility parameter (HSP) framework and small-angle X-ray scattering (SAXS), respectively. The solubility phase mapping demonstrates an increase in the soluble region in HSP space for the polyelectrolyte to encompass more solvents as the counterion side arm length increases or if the side arm is substituted with a large functional group, while substituting the central atom does not change the solubility, suggesting that the solubility is mainly influenced by the interaction between the peripheral atoms and the solvents. Scattering measurements revealed that for a given solvent, the nature of the counterion does not influence the conformation of chains in solution, as seen by the independence of the stretching parameter B on counterion type.

Arborescent (dendrigraft) polymers are high-molecular-weight dendritic macromolecules with a regular, multilevel branched topology and a high density of functional end groups in their periphery. Their well-defined architecture, devoid of cross-links or loops, imparts a particle-macromolecule duality that becomes particularly pronounced at interfaces. However, the underlying mechanisms governing their interfacial behavior remain largely unexplored. Here, we elucidate how the unique topology dictates the interfacial organization of water-soluble arborescent polymers. Using an iterative grafting-from approach via single-electron transfer living radical polymerization, we synthesized narrowly dispersed polymers with controlled branching and ultra-high molecular weight of 6.2 x 106 g mol-1. These polymers transition from spherical rigid particles in solution, to highly flexible, two-dimensional conformations upon interfacial adsorption. At solid interfaces, increasing segment density shifts surface morphologies from quasi-2D discs to fried-egg-like structures, as observed by atomic force microscopy and corroborated by dissipative particle dynamics simulations. At liquid-liquid interfaces, the absence of substrate constraints facilitates complete spreading into uniform 2D discs, driven by the energy gain due to polymer-segment adsorption. Furthermore, we uncover that macromolecular crowding and topological constraints inherent to the arborescent architecture dictate the response to compression of the adsorbed polymer layer, contrasting sharply with the behavior of conventional flexible linear or star polymers. The combination of high interfacial activity, spatially adaptable end groups, and extreme molecular flexibility will enable arborescent polymers to adapt to complex interfaces, acting as versatile platforms for multivalent and superselective interactions. These properties open new avenues for designing multivalent nanocarriers and adaptive interfacial materials with cooperative binding effects.

Correction for 'Harnessing the polymer-particle duality of ultra-soft nanogels to stabilise smart emulsions' by Alexander V. Petrunin et al., Phys. Chem. Chem. Phys., 2023, 25, 2810-2820, https://doi.org/10.1039/D2CP02700C.

Due to their unique properties, microgels have garnered large interest in recent times for various applications, including interfacial applications. In this work, we study the internal structure of microgels within microgel monolayers at solid-liquid interfaces using atomic force microscopy (AFM). To capture the rich phase behavior of microgels, the monolayers were deposited at different surface pressures, offering insights into their structural responses under varying compression conditions. The results of the measurements show that the confinement of microgels within a dense monolayer impacts their internal structure and leads to an increase in the contact stiffness, and therefore polymer density, from the third compression regime onward. Additionally, the compression of microgels at the solid-liquid interface leads to the emergence of an attractive interaction between microgels and the sharp AFM tip, resulting in jumps-to-contact in the recorded force-distance curves. The occurrence of jumps-to-contact at high lateral compressions suggests a structural transition within the microgel monolayer, leading to an increase in van-der-Waals and/or electrostatic interactions between microgel and AFM tip.

We study the conformational, conductometric and rheological properties of semiflexible polyelectrolyte carboxymethyl cellulose in mixtures of water and three non-solvents (ethanol, isopropanol and acetone). Small angle x-ray scattering measurements of the correlation length reveal that the local conformation of the carboxymethyl chain is unchanged by the presence of a non-solvent, even for solutions not far from the phase boundary. Rheological measurements confirm the invariance of the correlation length upon non-solvent addition. Conductivity measurements show that as the non-solvent content is increased, the fraction of condensed counterions increases, presumably due to the lowering of the dielectric constant of the solvent media. These results therefore show that under room temperature and pressure, the conformation of polyelectrolyte chains is independent of the effective charge fraction of the backbone. We suggest this occurs because the bare Kuhn length (≃10nm) is much larger than electrostatic blob size (≃1−2nm).

Microgels are confined to fluid interfaces in numerous applications, yet many aspects of the microgel-covered interface remain unclear. In this work, we use interfacial shear and dilatational rheology to study the effects of electrostatics on the mechanical characteristics of polyelectrolyte microgel monolayers at oil-water interfaces as a function of the microgel interfacial concentration. We find a clear correlation between the influence of charges on the mechanical properties of the monolayers and the influence of charges on their two-dimensional phase behavior. At lower microgel interfacial concentrations, the moduli of uncharged monolayers are larger than those of charged monolayers. Consistent with our previous findings on the phase behavior, here, the mechanical response of the interfacial layer is controlled by in-plane interactions of the microgels within the interface. At higher microgel interfacial concentrations, the moduli of charged monolayers are larger than those of uncharged monolayers. The mechanical response becomes dominated by out-of-plane interactions between the fractions of the adsorbed microgels further from the interface. Evidently, electrostatic interactions do not contribute directly to the mechanical response of the interfacial layer, that is, through charge repulsion, but indirectly through the difference in the swelling properties of uncharged and charged microgels. These results advance our understanding of how the charge-dependent microstructure of the interfacial layer affects its mechanical properties, which is not only important from a fundamental point of view but is also relevant to applications where polyelectrolyte microgels are used as responsive emulsion stabilizers.

Responsive hollow microgels are a fascinating class of soft model systems at the crossover between polymer capsules and microgels. The presence of the cavity makes them promising materials for encapsulation and controlled release applications but also confers them an additional softness that is reflected by their peculiar behaviour in bulk and at interfaces. Their responsivity to external stimuli, such as temperature, pH, and ionic strength, can be designed from their synthesis conditions and the choice of functional moieties. So far most studies have focused on "small" hollow microgels that were mostly studied with scattering or atomic force microscopy techniques. In our previous study, we have shown that large fluorescent hollow poly(N-isopropylacrylamide) (PNIPAM) microgels could be synthesized using micrometer-sized silica particles as sacrificial templates allowing their investigation in situ via confocal microscopy. In this work, we extend this approach to charged large hollow microgels based on poly(N-isopropylacrylamide-co-itaconic acid) (P(NIPAM-co-IA)). Hereby, we compare the structure and responsivity of "neutral" (PNIPAM) and "charged" (P(NIPAM-co-IA)) hollow microgel systems synthesized under similar conditions with the same sacrificial template using confocal and atomic force microscopy and light scattering techniques. In particular, we could demonstrate the extremely soft character of the swollen charged hollow microgels and their responsivity to pH, ionic strength, and temperature. To conclude this study, the buckling behavior of the different capsules was investigated illustrating the potential of such systems to change its conformation by varying the osmotic pressure and pH conditions.