Self-induced buckling in hollow microgels

TL;DR

The paper demonstrates that hollow microgels can undergo self-induced buckling purely from interparticle crowding, without external stress. Using large-scale, monomer-resolved molecular dynamics simulations across two crosslinker concentrations, it shows that crowding induces symmetry-breaking deformations ranging from single dents to multiple indentations, with the onset and severity depending on shell elasticity. A shape phase diagram built from vesicle-inspired metrics $(v, \Delta a)$ reveals distinct regimes: softer shells compress mostly isotropically, while stiffer shells exhibit buckling at intermediate packing fractions, accompanied by observable changes in reduced volume. The findings offer a synthetic, tunable platform to explore buckling instabilities relevant to materials design and biological-like shape regulation, and they lay groundwork for experimental verification and future studies under shear or rheological conditions.

Abstract

Hollow microgels are elastic polymer shells easily realisable in experiments. Recent works have shown the emergence of buckling events in single hollow microgels under the effect of an added osmotic pressure. Here, we perform large-scale simulations to show that these microgels at high enough packing fractions undergo spontaneous symmetry-breaking deformations ranging from single large dents to multiple indentations, even in the absence of any externally applied stress. This buckling phenomenology is thus self-induced, solely driven by interparticle crowding. We construct a phase diagram inspired by vesicle shape theories, mapping local curvature metrics as a function of the reduced volume, to quantify these findings, and we also propose ways to observe the occurrence of buckling in experiments. The present results thus rationalise the deformations occurring in suspensions of micro- and nano-scale elastic shells, offering a synthetic analogue to biological ones, allowing direct control on buckling instabilities for potential applications. Beyond materials design, these insights may also shed light on shape regulation in natural systems such as cells and vesicles, where similar deformations are observed.

Figures (8)

• Figure 1: Size of the microgel versus nominal packing fraction $\zeta$ comparing hollow microgels with $\delta_{\mathrm{rel}} = 0.275$ and $c=5\,\%$ and $c=10\,\%$ studied in this work with non-hollow ones with $c=5\,\%$ from Ref. DelMonte2024.
• Figure 2: Radius of gyration distribution $p(R_{\mathrm{g}})$ of hollow microgels with $c=5\,\%$ (a) and $c=10\,\%$ (b). The inset in (b) shows $p(R_{\mathrm{g}})$ at $\zeta = 2.08$ where two peaks are clearly visible, which can be individually fitted with two separate Gaussians (dashed lines).
• Figure 3: Asphericity distribution of hollow microgels with $c=5\,\%$ (symbols) and $c=10\,\%$ (solid lines) at selected corresponding packing fractions (highlighted by the same colour coding).
• Figure 4: Snapshots of the simulated system for the two studied microgels: $c=5\,\%$ (a) and $c=10\,\%$ (b) at a comparable packing fraction $\zeta\sim2.1$. The black circles highlight buckled examples with one big dent.
• Figure 5: Radial distribution functions $g(r)$ between the microgels' centers of mass for $c=5\,\%$ (a) and $c=10\,\%$ (b), at comparable packing fractions.