Spontaneous deswelling of microgels controlled by counterion clouds

Gasser, Urs; Scotti, Andrea; Fernández‐Nieves, Alberto

doi:10.1103/physreve.99.042602

Cited by 24 publications

(45 citation statements)

References 39 publications

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“…Note that this is true for ionic microgels (with charges distributed along all the polymeric networks) but also for neutral microgels since they possess charges in the particle periphery due to the polar initiator used in the precipitation polymerization synthesis [53]. This is particularly important for microgel suspensions since microgels are compressible and can shrink in response to an unbalanced osmotic pressure inside and outside the particles [29,45,59,60,78].…”

Section: Discussionmentioning

confidence: 99%

“…The observed linearity in the probed ζ range can be understood from the fact that, in contrast to ionic microgels, pNIPAM microgels have a neutral network and thus there is no Donnan potential; the ions can then locate both inside and outside the particles [29,30,59]. Increasing the particle number density proportionally increases the number of free ions per particle, and since these can explore the inside and outside of the microgels, π should increase linearly with ζ .…”

Section: B Osmotic Pressure Of Ionic Microgel Suspensionsmentioning

confidence: 99%

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Osmotic pressure of suspensions comprised of charged microgels

et al. 2021

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We determine the osmotic pressure of microgel suspensions using membrane osmometry and dialysis, for microgels with different softnesses. Our measurements reveal that the osmotic pressure of solutions of both ionic and neutral microgels is determined by the free ions that leave the microgel periphery to maximize their entropy and not by the translational degrees of freedom of the microgels themselves. Furthermore, up to a given concentration it is energetically favorable for the microgels to maintain a constant volume without appreciable deswelling. The concentration where deswelling starts weakly depends on the crosslinker concentration, which affects the microgel dimension; we explain this by considering the dependence of the osmotic pressure and the microgel bulk modulus on the particle size.

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Section: Discussionmentioning

confidence: 99%

Section: B Osmotic Pressure Of Ionic Microgel Suspensionsmentioning

confidence: 99%

Osmotic pressure of suspensions comprised of charged microgels

et al. 2021

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“…We expect to assist in a transition from a swollen state where charges are distributed throughout the microgel's hydrated volume to a state where charges are mainly located in the periphery of the particle upon the microgel collapse 27 . In both cases, the presence of a so-called "fuzzy surface" of loosely cross-linked polymer chains also renders the location of the Stern layer and of the counterion cloud experimentally poorly defined 36 . Finally, the different swelling state of the microgel below and above the VPTT leads to a swelling-and crosslinking-density-profile-dependent electrophoretic mobility, as described by Ohshima's theory 37 , such that simple zeta potential measurements cannot be used to extract the surface conductivity without detailed knowledge of the microgel conformation and charge distribution 38 .…”

Section: Fabrication Of Reconfigurable Colloidal Clustersmentioning

confidence: 99%

Reconfigurable artificial microswimmers with internal feedback

et al. 2021

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Self-propelling microparticles are often proposed as synthetic models for biological microswimmers, yet they lack the internally regulated adaptation of their biological counterparts. Conversely, adaptation can be encoded in larger-scale soft-robotic devices but remains elusive to transfer to the colloidal scale. Here, we create responsive microswimmers, powered by electro-hydrodynamic flows, which can adapt their motility via internal reconfiguration. Using sequential capillary assembly, we fabricate deterministic colloidal clusters comprising soft thermo-responsive microgels and light-absorbing particles. Light absorption induces preferential local heating and triggers the volume phase transition of the microgels, leading to an adaptation of the clusters’ motility, which is orthogonal to their propulsion scheme. We rationalize this response via the coupling between self-propulsion and variations of particle shape and dielectric properties upon heating. Harnessing such coupling allows for strategies to achieve local dynamical control with simple illumination patterns, revealing exciting opportunities for developing tactic active materials.

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“…From this, they conclude that in bimodal or polydisperse mixtures, larger particles selectively shrink. As a consequence, uniform shrinkage may suppress or delay the liquid-to-solid transition and facilitate crystallization 115 . If and under which conditions such deswelling may occur is not entirely clear and has been debated 25,92,116 .…”

Section: Characterization Of Microgels With Superresolution Microscopymentioning

confidence: 99%

Pathways and challenges towards a complete characterization of microgels

Scheffold

2020

Nat Commun

103

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Due to their controlled size, sensitivity to external stimuli, and ease-of-use, microgel colloids are unique building blocks for soft materials made by crosslinking polymers on the micrometer scale. Despite the plethora of work published, many questions about their internal structure, interactions, and phase behavior are still open. The reasons for this lack of understanding are the challenges arising from the small size of the microgel particles, complex pairwise interactions, and their solvent permeability. Here we describe pathways toward a complete understanding of microgel colloids based on recent experimental advances in nanoscale characterization, such as super-resolution microscopy, scattering methods, and modeling. T he term "microgel" usually refers to a cross-linked polymer network of colloidal size that is swollen in a good solvent and has a diameter between~100 nm and several micrometers 1-3 (Fig. 1). Colloidal hydrogels are a common subtype where the continuous phase is water. Often, but not always, these networks exhibit conformational changes in response to changes in their environment, such as the solvent composition or temperature 1,4,5. The sensitivity to external stimuli, such as temperature, pH, or solvent conditions, is an essential feature of many microgels, in particular for the most widely studied type of microgels, based on pNIPAM (poly-N-isopropylacrylamide) and its derivatives 1,6 (Fig. 1b). Here, we consider homogeneous suspensions of these particles at different densities from the dilute to the highly packed regime, as well as microgels adsorbed at surfaces. It is also possible to synthesize larger, millimeter-sized, gel particles 7 , or to study two-dimensional assemblies at an interface 8,9. Additional functions, for example, for chemical transformation or sensing applications 10,11 , can be added using core-shell particles or by decorating the microgels with inorganic nanoparticles 12,13. Microgels are already used in several applications as viscosity modifiers and lubricants 14 , for CO 2 capturing 15 or 3D bioprinting 16 , as biocompatible additives and delivery vehicles 17 , sensors, or stimuli-responsive color-changing systems 18,19. Despite their popularity and widespread application, the internal structure of microgels is not well-understood. During the microgel synthesis, the degrees of freedom for the polymer strands are intentionally reduced by forming covalent cross-links between polymer chains. The chemical crosslinking itself is a nonequilibrium process, and it does not necessarily progress uniformly, which may lead to spatial fluctuations, gradients, and heterogeneities in the microgel particle architecture 20. For a complete understanding of a microgel colloid as a material building block or a carrier, it is imperative to know the spatial distribution of polymer and cross-link densities on the nanoscale. Initially, many studies assumed that microgels could be described simply as spherical, homogeneous gel particles, as discussed in ref. 1. Based on this assumption, t...

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Spontaneous deswelling of microgels controlled by counterion clouds

Cited by 24 publications

References 39 publications

Osmotic pressure of suspensions comprised of charged microgels

Osmotic pressure of suspensions comprised of charged microgels

Reconfigurable artificial microswimmers with internal feedback

Pathways and challenges towards a complete characterization of microgels

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