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Emanuela Zaccarelli

Publications and source records attributed to Emanuela Zaccarelli.

At least 37 records · Page 2Linked to original sources
arXiv2024

Gellan-Based Hydrogels and Microgels for culturage heritage: a rheological perspective

Gellan gum has gained significant attention due to its versatility in multiple applications in the form of hydrogels and microgels. A thorough understanding of the rheological behaviour of these systems is crucial both for fundamental research and to optimize the manufacturing needs. To this aim, here we extensively characterize the rheological behaviour of gellan based hydrogels and microgels recently used for efficient paper cleaning for restoration interventions. In particular, we study their viscoelastic properties, also during hydrogel and microgel formation, assessing the role of temperature, gellan concentration, and, importantly, the presence of different cations, which plays a crucial role in the gelation process. We find the interesting result that, in the conditions where they are efficient for cleaning, gellan hydrogels exhibit a double yielding behavior. In addition, we provide a detailed description of gellan microgels preparation, ensuring high control and reproducibility of the samples. Altogether our study sheds light on the mechanical stability, network structure, and overall functionality of the gellan-based gels, providing valuable insights into optimizing conditions for desired applications in paper cleaning.

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arXiv2024

Hyper-Auxeticity and the Volume Phase Transition of Polymer Gels

Thermoresponsive hydrogels exhibit reversible deswelling at the Volume Phase Transition (VPT), associated to a minimum of the Poisson's ratio $ν$. Recent numerical investigations uncovered the occurrence of a Hyper-Auxetic Transition (HAT) ($ν=-1$) for low-crosslinked hydrogels at low temperature, accompanied by a critical-like behavior. Here, we perform extensive numerical simulations to unveil the relation between these two transitions. We find that the HAT occurs at different temperatures $T$ up to a maximum value of the crosslinker concentration $c$, thus being clearly distinct from the VPT, taking place at fixed $T$ and for all $c$. Our results provide novel fundamental insights on the interplay between network collapse and mechanical properties of these fascinating materials.

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arXiv2024

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 \emph{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 steadfastly 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 pivotal 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.

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arXiv2024

Auxetic polymer networks: The role of crosslinking, density and disorder

Low-connectivity polymer networks were recently found to behave auxetically when subjected to small tensions, that is, their Poisson's ratio $ν$ becomes negative. In addition, for specific state points, numerical simulations revealed that diamond-like networks reach the limit of mechanical stability, exhibiting values of $ν= -1$, a condition that we define hyper-auxeticity. This behavior is interesting per se for its consequences in material science, but also appealing for fundamental physics because the mechanical instability is accompanied by evidences of criticality. In this work, we deepen our understanding of this phenomenon by performing a large set of equilibrium and stress-strain simulations in combination with a phenomenological elasticity theory. The two approaches are found to be in good agreement, confirming the above results. We also extend our investigations to disordered polymer networks and find that the hyper-auxetic behavior also holds in this case, still manifesting a similar critical-like behavior as in the diamond one. Finally, we highlight the role of the number density, that is found to be a relevant control parameter determining the elastic properties of the disordered system. The validity of the results in disordered conditions paves the way to an experimental investigation of this phenomenon in real systems, such as hydrogels.

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arXiv2024

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 non-monotonically with $ζ_{2D}$ at the interface, confirming long-sought predictions of reentrant behavior for Hertzian-like systems.

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arXiv2023

Combining scattering experiments and colloid theory to characterize charge effects in concentrated antibody solutions

Charges and their contribution to protein-protein interactions are essential for the key structural and dynamic properties of monoclonal antibody (mAb) solutions. In fact, they influence the apparent molecular weight, the static structure factor, the collective diffusion coefficient or the relative viscosity and their concentration dependence. Further, charges play an important role in the colloidal stability of mAbs. There exist standard experimental tools to characterise mAb net charges such as the measurement of the electrophoretic mobility, the second virial coefficient, or the diffusion interaction parameter. However, the resulting values are difficult to be directly related to the actual overall net charge of the antibody and to theoretical predictions based on its known molecular structure. Here, we report the results of a systematic investigation of the solution properties of a charged IgG1 mAb as a function of concentration and ionic strength using a combination of electrophoretic measurements, static and dynamic light scattering, small-angle x-ray scattering (SAXS), and tracer particle-based microrheology. We analyse and interpret the experimental results using established colloid theory and coarse-grained computer simulations. We discuss the potential and limits of colloidal models for the description of interaction effects of charged mAbs, in particular pointing out the importance of incorporating shape and charge anisotropy when attempting to predict structural and dynamic solution properties at high concentrations.

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arXiv2023

Observation of empty liquids and equilibrium gels in a colloidal clay

The relevance of anisotropic interactions in colloidal systems has recently emerged in the context of the rational design of new soft materials. Patchy colloids of different shapes, patterns and functionalities are considered the new building blocks of a bottom-up approach toward the realization of selfassembled bulk materials with predefined properties. The ability to tune the interaction anisotropy will make it possible to recreate molecular structures at the nano- and microscales (a case with tremendous technological applications), as well as to generate new unconventional phases, both ordered and disordered. Recent theoretical studies suggest that the phase diagram of patchy colloids can be significantly altered by limiting the particle coordination number (that is, valence). New concepts such as empty liquids-liquid states with vanishing density-and equilibrium gels arrested networks of bonded particles, which do not require an underlying phase separation to form-have been formulated. Yet no experimental evidence of these predictions has been provided. Here we report the first observation of empty liquids and equilibrium gels in a complex colloidal clay, and support the experimental findings with numerical simulations.

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arXiv2023

Structural and microscopic relaxations in a colloidal glass

The aging dynamics of a colloidal glass has been studied by multiangle Dynamic Light Scattering, Neutron Spin Echo, X-ray Photon Correlation Spectroscopy and Molecular Dynamics simulations. The two relaxation processes, microscopic (fast) and structural (slow), have been investigated in an unprecedentedly wide range of time and length scales covering both ergodic and nonergodic regimes. The microscopic relaxation time remains diffusive at all length scales across the glass transition scaling with wavevector Q as Q^{-2}. The length-scale dependence of structural relaxation time changes from diffusive, characterized by a Q^{-2}-dependence in the early stages of aging, to Q^{-1}- dependence in the full aging regime which marks a discontinuous hopping dynamics. Both regimes are associated with a stretched behaviour of the correlation functions. We expect these findings to provide a general description of both relaxations across the glass transition.

cond-mat.soft↗
arXiv2023

Toward a unified description of the electrostatic assembly of microgels and nanoparticles

The combination of soft responsive particles, such as microgels, with nanoparticles (NPs) yields highly versatile complexes of great potential for applications, from ad-hoc plasmonic sensors to controlled protocols for loading and release. However, the assembly process between these microscale networks and the co-dispersed nano-objects has not been investigated so far at the microscopic level, preempting the possibility of designing such hybrid complexes a priori. In this work, we combine state-of-the-art numerical simulations with experiments, to elucidate the fundamental mechanisms taking place when microgels-NPs assembly is controlled by electrostatic interactions. We find a general behavior where, by increasing the number of interacting NPs, the microgel deswells up to a minimum size, after which a plateau behavior occurs. This occurs either when NPs are mainly adsorbed to the microgel corona via the folding of the more external chains, or when NPs penetrate inside the microgel, thereby inducing a collective reorganization of the polymer network. By varying microgel properties, such as fraction of crosslinkers or charge, as well as NPs size and charge, we further show that the microgel deswelling curves can be rescaled onto a single master curve, for both experiments and simulations, demonstrating that the process is entirely controlled by the charge of the whole microgel-NPs complex. Our results thus have a direct relevance in fundamental materials science and offer novel tools to tailor the nanofabrication of hybrid devices of technological interest.

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arXiv2023

Using cluster theory to calculate the experimental structure factors of antibody solutions

Monoclonal antibody solutions are set to become a major therapeutic tool in the years to come, capable of targeting various diseases by clever designing their antigen binding site. However, the formulation of stable solutions suitable for patient self-administration typically presents challenges, as a result of the increase in viscosity that often occurs at high concentrations. Here, we establish a link between the microscopic molecular details and the resulting properties of an antibody solution through the characterization of clusters, which arise in the presence of self-associating antibodies. In particular, we find that experimental small-angle X-ray scattering data can be interpreted by means of analytical models previously exploited for the study of polymeric and colloidal objects, based on the presence of such clusters. The latter are determined by theoretical calculations and supported by computer simulations of a coarse-grained minimal model, in which antibodies are treated as Y-shaped colloidal molecules and attractive domains are designed as patches. Using the theoretically-predicted cluster size distributions, we are able to describe the experimental structure factors over a wide range of concentration and salt conditions. We thus provide microscopic evidence for the well-established fact that the concentration-dependent increase in viscosity is originated by the presence of clusters. Our findings bring new insights on the self-assembly of monoclonal antibodies, which can be exploited for guiding the formulation of stable and effective antibody solutions.

cond-mat.soft↗
arXiv2023

Structure and elasticity of model disordered, polydisperse and defect-free polymer networks

The elasticity of disordered and polydisperse polymer networks is a fundamental problem of soft matter physics that is still open. Here, we self-assemble polymer networks via simulations of a mixture of bivalent and tri- or tetravalent patchy particles, which result in an exponential strand length distribution analogous to that of experimental randomly crosslinked systems. After assembly, the network connectivity and topology are frozen and the resulting system is characterized. We find that the fractal structure of the network depends on the number density at which the assembly has been carried out, but that systems with the same mean valence and same assembly density have the same structural properties. Moreover, we compute the long-time limit of the mean-squared displacement, also known as the (squared) localization length, of the crosslinks and of the middle monomers of the strands, showing that the dynamics of long strands is well described by the tube model. Finally, we find a relation connecting these two localization lengths at high density, and connect the crosslink localization length to the shear modulus of the system.

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arXiv2023

Probing temperature-responsivity of microgels and its interplay with a solid surface by superresolution microscopy and numerical simulations

Superresolution microscopy has become a powerful tool to investigate the internal structure of complex colloidal and polymeric systems, such as microgels, at the nanometer scale. The ability to monitor microgels response to temperature changes in situ opens new and exciting opportunities to design and precisely control their behaviour for various applications. 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 dSTORM microscopy and advanced coarse-grained molecular dynamics simulations to investigate, for the first time, how individual microgels anchored on hydrophilic and hydrophobic surfaces undergo their volume phase transition in 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 in 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 remarkable 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, being these either macromolecules with peculiar internal structure or nanocomplexes where molecules of interest can be encapsulated in the microgel network and controllably released with temperature.

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arXiv2022

Molecular origin of the two-step mechanism of gellan aggregation

Among hydrocolloids, gellan is one of the most used anionic polysaccharides, because of its capability of forming mechanically stable gels at relatively low concentrations. Despite its long-standing use and importance, the gellan aggregation mechanism is still not presently understood at the microscopic level due to the lack of atomistic information. Here we will fill this gap by reporting molecular dynamics simulations of gellan chains at different polymer and salt contents, being able to unveil the occurrence of the two steps in the process, in agreement with existing hypotheses. At first, the formation of double helices takes place, followed by the aggregation into super-structures. For both steps, the role of bivalent cations appears to be crucial, as also shown by rheology and atomic force microscopy measurements: they not only facilitate the junction of the chains into double helices, but also promote through bridging their arrangement into larger aggregates. On the other hand, monovalent cations have a much more reduced role, making it possible to form double helices only at very high salt content and not actively participating in the formation of gels. Our simulations thus offer the first complete microscopic overview of gellan aggregation and will be important for future use of gellan-based systems for a wide variety of applications, ranging from food science to art restoration.

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arXiv2022

The role of charge content in the two-step deswelling of Poly(N-isopropylacrylamide)-based microgels

Poly(N-isopropylacrylamide)-based microgels are soft colloids undergoing a Volume Phase Transition (VPT) close to ambient temperature. Although widely employed for fundamental research and application purposes, the modifications of the microgel internal structure occurring at the VPT are not yet completely understood, especially concerning the role of electrostatics. Here we study in detail, both experimentally and numerically, the effect of the addition of acrylic acid (AAc) co-monomer on the microgel deswelling process. By combining viscosimetry, light scattering and electrophoresis, we show that the progressive addition of AAc increases the microgel mass and suppresses the occurrence of the VPT, progressively shifting the microgel collapse to higher temperatures. Most importantly, it also highly enhances the two-step deswelling of these submicron-sized networks, so that the inner core collapses at temperatures always lower than those marking the transition of the outer corona. These results indicate that a net increase of the charge density mismatch between the bulk and the surface of the microgels takes place. Numerical simulations fully confirm this scenario and clarify the impact of the charge distribution on the two-step deswelling, with mobile counterions efficiently screening the charges within the inner core, while leaving more monomers ionized on the surface. Our work unambiguously shows how electrostatic interactions influence the behavior of thermosensitive microgels in aqueous environment close to the VPT.

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arXiv2022

In-situ study of the impact of temperature and architecture on the interfacial structure of microgels

The structural characterization of microgels at interfaces is fundamental to understand both their 2D phase behavior and their role as stabilizers that enable emulsions to be broken on demand. However, this characterization is usually limited by available experimental techniques, which do not allow a direct investigation at interfaces. To overcome this difficulty, here we employ neutron reflectometry, which allows us to probe the structure and responsiveness of the microgels in-situ at the air-water interface. We investigate two types of microgels with different cross-link density, thus having different softness and deformability, both below and above their volume phase transition temperature, combining experiments with computer simulations of realistic in silico synthesized microgels. We find that temperature only affects the portion of microgels in water, while the strongest effect of the microgels softness is observed in their ability to protrude into the air. In particular, standard microgels have an apparent contact angle of few degrees, while ultra-low cross-linked microgels form a flat polymeric layer with zero contact angle. Altogether, this study provides an in-depth microscopic description of how different microgel architectures affect their arrangements at interfaces, and will be the foundation for a better understanding of their phase behavior and assembly. This manuscript has been accepted for publication in Nature Communications (open access). The final version of the manuscript including the Supplementary Information will be available in the future.

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arXiv2022

Critical active dynamics is captured by a colored-noise driven field theory

We numerically investigate the correlation function, the response and the breakdown of the Fluctuation-Dissipation Theorem (FDT) in active particles close to the motility-induced critical point. We find a strong FDT violation in the short time and wavelength regime, where the response function has a larger amplitude than the fluctuation spectrum. Conversely, at larger spatiotemporal scales, the FDT is restored and the critical slowing-down is compatible with the Ising universality class. Building on these results, we develop a novel field-theoretical description employing a space-time correlated noise which qualitatively captures the numerical results already at the Gaussian level. By performing a one-loop renormalization group analysis we show that the correlated noise does not change the critical exponents with respect to the equilibrium. Our results demonstrate that a correlated noise field is a fundamental ingredient to capture the features of critical active matter at the coarse-grained level.

cond-mat.soft↗
arXiv2022

Soft colloids for complex interfacial assemblies

The design of complex materials and the formation of specific patterns often arise from the properties of the individual building blocks. In this respect, colloidal systems offer a unique opportunity because nowadays they can be synthesized in the laboratory with many different shapes and features. Hence, an appropriate choice of the particle characteristics makes it possible to generate macroscopic structures with desired properties. The versatility of colloids can also be explored in two dimensions, using liquid-liquid or air-liquid interfaces as privileged substrates where they can adsorb and self-assemble. Besides being innovative model systems for fundamental studies, the great interest of the scientific community is also technological and applicative, since colloidal-scale surface patterns are very promising for example in photonics or biosensing. In a recent study published in PNAS, Menath and coworkers combine these elements and exploit core-shell colloids, consisting of a silica core and a soft, non-crosslinked polymer shell, to make an important step forward in controlling the assembly of complex structures at an interface.

cond-mat.soft↗
arXiv2022

Two-step deswelling in the Volume Phase Transition of thermoresponsive microgels

Thermoresponsive microgels are one of the most investigated class of soft colloids, thanks to their ability to undergo a Volume Phase Transition (VPT) close to ambient temperature. However, this fundamental phenomenon still lacks a detailed microscopic understanding, particularly regarding the presence and the role of charges in the deswelling process. Here we fill this gap by combining experiments and simulations to show that the microgel collapse does not happen in a homogeneous fashion, but through a two-step mechanism, entirely attributable to electrostatic effects. The signature of this phenomenon is the emergence of a minimum in the ratio between gyration and hydrodynamic radii at the VPT. Thanks to simulations of several microgels with different cross-linker concentrations, charge contents and charge distributions, we build a unifying master-curve able to predict the two-step deswelling. Our results have direct relevance on fundamental soft condensed matter science and on microgel applications ranging from materials to biomedical technologies.

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