arXiv subjects

Jacopo Vialetto

Publications and source records attributed to Jacopo Vialetto.

10 recordsLinked to original sources

arXiv2026

Heterogeneous collapse in thermoresponsive copolymer microgels varying molar composition

Understanding the internal architecture of copolymer microgels is crucial for establishing how nanoscale polymer organization controls their stimuli-responsive behavior. Here we focus on thermoresponsive P(N-isopropylacrylamide-co-N-isopropyl-methacrylamide), P(NIPAM-co-NIPMAM), microgels with varying mole fraction of the components, synthesized via radical precipitation polymerization, and we demonstrate that changes in their volume phase transition and equilibrium swelling are governed by composition-dependent internal heterogeneity. Comparison between small-angle neutron scattering (SANS) with isotopic labeling and monomer-resolved simulations show a block-like monomer distribution of the two components. SANS analysis reveals a universal maximum in the polymer mesh correlation length near the transition, evidencing coexistence of collapsed NIPAM-rich and swollen domains. The correlation length increases with increasing NIPMAM content, with a maximum for 75 mol \% NIPMAM, implying sparse collapsed regions within the network and thus a large degree of heterogeneity induced by the presence of an increasingly large fraction of intercalated, non-collapsing PNIPMAM. The maximum heterogeneity correlates with the equilibrium swelling ratio, indicating that collapsed microgels retain a structural memory of the transition and present a less-compliant structure in response to temperature variations. Overall, these insights highlight a complex effect of the block-like monomer distribution on the responsive properties of copolymer microgels with different compositions, thus providing a design rule for tailoring responsive colloids for functional soft materials.

cond-mat.soft↗

arXiv2026

Resolving Light-Induced Structural Rearrangements in Responsive Microgels

Optically-responsive microgels offer a versatile platform for designing adaptive soft materials with coupled light and thermal responsiveness. Control over the crosslinking degree is particularly appealing as it can regulate not only particle size but also stiffness, thereby enabling remote tuning of key material functionalities. However, the internal structural changes that couple molecular photoresponsive mechanisms to mesoscopic properties remain poorly resolved. Here, we investigate different light-responsive microgels containing covalently incorporated coumarin moieties, which impart optical sensitivity through UV-induced cycloaddition, by combining dynamic light scattering, small-angle neutron scattering, and molecular dynamics simulations. We show that light irradiation alters not only particle size but also the internal polymer density distribution and subsequent thermal response. Before irradiation, the microgels exhibit a star-like architecture with a dense core and extended polymeric arms. After irradiation, the network evolves toward a markedly more compact structure. This transformation cannot be rationalized simply as an equivalent to an increase in crosslinking density during synthesis, as observed in the thermal response, revealing light as a powerful tool to regulate microgel architecture and multifunctional responsiveness.

cond-mat.soft↗

arXiv2025

Thermoresponsive copolymer microgels synthesized via single-step precipitation polymerization: random or block structure?

The inner structure of polymeric particles critically influences their phase behavior and functionality, governing their mechanical properties and their physical and chemical interactions. For thermoresponsive microgels, i.e. colloidal particles comprising a crosslinked polymer network that undergo a volume transition upon temperature changes, structural control is key to tailor the material responsivity and broaden the range of applications. In this work, we present a comprehensive investigation of the internal structure of poly(N-isopropylacrylamide-co-N-isopropylmethacrylamide), P(NIPAM-co-NIPMAM), copolymer microgels, combining small-angle neutron scattering (SANS), dynamic light scattering (DLS), and nuclear magnetic resonance (NMR) measurements with multi-scale simulations. By synthesizing different samples, we probe the microgels swelling behavior, revealing distinct signatures of the individual polymers. To elucidate their internal distribution, we perform monomer-resolved microgel simulations across different copolymer models. A direct comparison between experimental and numerical form factors under different, neutron-selective conditions provides evidence of a preferential organization into block structures rather than a random arrangement. These results are confirmed by 13C-NMR which reveals the clear presence of NIPAM blocks within a more random arrangement of the remaining monomers and by atomistic molecular dynamics simulations on copolymer chains, which also shed light on a possible origin in the dependence of the hydrogen bonding capability on the local environment. These findings provide a detailed microscopic picture of the inner architecture of P(NIPAM-co-NIPMAM) microgels, revealing an unexpected structural organization that may be generalized to other copolymer systems and could be promising to tailor microgel design and enhance control of material responsivity.

cond-mat.soft↗

arXiv2025

Star-like thermoresponsive microgels: a new class of soft nanocolloids

We provide experimental and numerical evidence of a new class of soft nanocolloids: star-like microgels with thermoresponsive character. This is achieved by using the standard precipitation polymerization synthesis of poly(N-isopropylacrylamide) (PNIPAM) microgels and replacing the usually employed crosslinking agent, N,N'-methylenebis(acrylamide) (BIS), with ethylene glycol dimethacrylate (EGDMA). The fast reactivity of EGDMA combined with its strong tendency to self-bind produces colloidal networks with a central, crosslinker-rich core, surrounded by a corona of long, crosslinker-free arms. These novel star-like microgels fully retain PNIPAM thermoresponsivity and undergo a volume phase transition at a temperature of 32°C that is very sharp as compared to standard PNIPAM-BIS microgels, independently of crosslinker content. Dynamic light scattering and small angle X-ray scattering experiments are compared to extensive simulation results, based on ideal star polymers as well as on state-of-the-art monomer-resolved simulations, offering a microscopic evidence of the star-like internal structure of PNIPAM-EGDMA microgels. This can be described by a novel model for the form factors combining star and microgel features. The present work thus bridges the fields of star polymers and microgels, providing the former with the ability to respond to temperature via a facile synthetic route that can be routinely employed, opening the way to exploit these soft particles for a variety of fundamental studies and applicative purposes.

cond-mat.soft↗

arXiv2024

Effect of particle stiffness and surface properties on the nonlinear viscoelasticity of dense microgel suspensions

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. 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 (PEG) 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. We find 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.

cond-mat.soft↗

arXiv2023

Tuning electrostatic interactions of colloidal particles at oil-water interfaces with organic salts

Monolayers of colloidal particles at oil-water interfaces readily crystalize owing to electrostatic repulsion, which is often mediated through the oil. However, little attempts exist to control it using oil-soluble electrolytes. We probe the interactions amongst charged hydrophobic micospheres confined at a water/hexadecane interface and show that repulsion can be continuously tuned over orders of magnitude upon introducing minor amounts of an organic salt into the oil. Our results show that charged groups at the particle/oil interface are subject to an associative discharging mechanism, analogous to the charge regulation kinetics observed for charged colloids in non-polar solvents.

cond-mat.soft↗

arXiv2022

The effect of curvature on the diffusion of colloidal bananas

Anisotropic colloidal particles exhibit complex dynamics which play a crucial role in their functionality, transport and phase behaviour. In this work, we investigate the two-dimensional diffusion of smoothly curved colloidal rods -- also known as colloidal bananas -- as a function of their opening angle, α. We measure the translational and rotational diffusion coefficients of the particles with opening angles ranging from 0° (straight rods) to nearly 360°(closed rings). In particular, we find that the anisotropic diffusion of the particles varies non-monotonically with their opening angle and that the axis of fastest diffusion switches from the long to the short axis of the particles when α>180°. We also find that the rotational diffusion coefficient of nearly closed rings is approximately an order of magnitude higher than that of straight rods of the same length. Finally, we show that the experimental results are consistent with Slender Body Theory, indicating that the dynamical behavior of the particles arises primarily from their local drag anisotropy. These results highlight the impact of curvature on the Brownian Motion of elongated colloidal particles, which must be taken into account when seeking to understand the behaviour of curved colloidal particles.

cond-mat.soft↗

arXiv2022

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

The reconfiguration of soft, deformable particles upon adsorption at the interface between two fluids underpins many aspects of their dynamics and interactions, ultimately controlling the macroscopic properties of particle monolayers of relevance for materials, such as particle-stabilized emulsions and foams, and processes, e. g. particle-based lithography. In spite of its importance, experimentally determining the three-dimensional shape of soft particles at fluid interfaces with high resolution remains an elusive task. In this work, we take poly(N-isopropylacrylamide) (pNIPAM) microgels as model soft particles and demonstrate that their conformation at the interface between an aqueous and an oil phase can be fully reconstructed by means of in-situ atomic force microscopy (AFM) imaging. We show that imaging the particle topography from both sides of the interface allows one to characterize the in-plane deformation of the particle under the action of interfacial tension and to visualize the occurrence of asymmetric swelling in the two fluids. Additionally, the technique enables investigating different fluid phases and particle architectures, as well as studying in situ the effect of temperature variations on particle conformation. We envisage that these results open up an exciting range of possibilities to provide microscopic insights between the single-particle behavior of soft objects at fluid interfaces and macroscopic material properties of relevance for applications and fundamental studies alike.

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arXiv2021

Influence of the interfacial tension on the microstructural and mechanical properties of microgels at fluid interfaces

Microgels are soft colloidal particles constituted by cross-linked polymer networks with a high potential for applications. In particular, after adsorption at a fluid interface, interfacial tension provides two-dimensional (2D) confinement for microgel monolayers and drives the reconfiguration of the particles, enabling their deployment in foam and emulsion stabilization and in surface patterning for lithography, sensing and optical materials. However, most studies focus on systems of fluids with a high interfacial tension, e.g. alkanes/ or air/water interfaces, which imparts similar properties to the assembled monolayers. Here, instead, we compare two organic fluid phases, hexane and methyl tert-butyl ether, which have markedly different interfacial tension ($γ$) values with water and thus tune the elasticity and deformation of adsorbed microgels. We rationalize how $γ$ controls the single-particle morphology, which consequently modulates the structural and mechanical response of the monolayers at varying interfacial compression. Specifically, when $γ$ is low, the microgels are less deformed within the interface plane and their polymer networks can rearrange more easily upon lateral compression, leading to softer monolayers. Selecting interfaces with different surface energy offers an additional control to customize the 2D assembly of soft particles, from the fine-tuning of particle size and interparticle spacing to the tailoring of mechanical properties.

cond-mat.soft↗

arXiv2021

The effect of internal architecture on the assembly of soft particles at fluid interfaces

Monolayers of soft colloidal particles confined at fluid interfaces have been attracting increasing interest for fundamental studies and applications alike. However, establishing the relation between their internal architecture, which is controlled during synthesis, and their structural and mechanical properties upon interfacial confinement, which define the monolayer's properties, remains an elusive task. Here, we propose a comprehensive study elucidating this relation for a system of microgels with tunable architecture. We synthesize core-shell microgels, whose soft core can be chemically degraded in a controlled fashion, yielding particles ranging from analogues of standard batch-synthesized to completely hollow microgels after total core removal. We characterize the internal structure of these particles, their swelling properties in bulk and their morphologies upon adsorption at an oil-water interface via a combination of numerical simulations and complementary experiments. In particular, we confirm that hollow microgels are mechanically stable in bulk aqueous conditions and that the progressive removal of the core leads to a significant flattening of the microgels, which become disk-like particles, at the interface. At low compression, the mechanical response of the monolayer is dominated by the presence of loosely crosslinked polymers forming a corona surrounding the particle 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. These findings shed new light on which structural features of soft particles determine their interfacial behaviour, enabling new design strategies for tailored materials.

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