Elasticity-mediated Morphogenesis in Interfacial Colloidal Assemblies

Abstract

We study the self-assembly of colloidal microgel particles at a quasi-two-dimensional air-water interface of a drying droplet. Using bright-field microscopy, we demonstrate that increasing particle elasticity drives interfacial organization from repulsion-stabilized crystallization to attraction-dominated gelation, via diverse metastable structures including clusters, voids and anisotropic aggregates. Molecular dynamics simulations using an effective potential that captures the interplay between hydrophobic, capillary, steric and dipolar interactions, reproduce the overall phenomenology of the observed colloidal morphogenesis. Our findings establish particle elasticity as a key parameter governing non-equilibrium structural organization of colloids at an interface.

Figures (4)

• Figure 1: (a) SEM images of dried microgels at different crosslinker (MBA) densities. (b) AFM height profiles of dried microgels of two different crosslinker densities. (c) $\alpha$ vs crosslinker density. (d) Side-view micrograph of a sessile microgel droplet on a glass substrate. (e) Schematic illustration of the droplet and adsorbed microgel particles (purple spheres). (f) Zoomed-in view of a single adsorbed microgel at the air-water interface. The effective diameter $\sigma_{\text{eff}}$ of a microgel is labeled as the sum of the diameter of the highly crosslinked core ($\sigma_{c}$) and twice the thickness ($d$) of the deformable corona.
• Figure 2: Reconstructed bright-field micrographs showing the spatiotemporal self-assembly of microgel cores for three particle elasticity values (top to bottom panels). Top panel (a-f): soft microgels ($\alpha$ = 2.57). Middle panel (g-l): microgels of intermediate elasticity ($\alpha$ = 1.95). Bottom panel (m-r): stiff microgels ($\alpha$ = 1.51). The local area fraction ($\phi$, denoted by $\phi$(s)) increases from left to right as the triple phase contact line of the droplet is approached. Within each panel, $\phi$ also increases from top to bottom (earlier and later time data, denoted by $\phi$(t)) due to evaporative water loss. Microgels are color-coded by the local hexagonal bond orientational order parameter, $|\psi_6|$. Scale bar at bottom right is 10 $\mu$m.
• Figure 3: Effective pair potentials, $U(r)$ (in units of $k_BT$), as a function of interparticle separation, $r$, for (a) $\alpha$ = 2.57; soft microgels, (b) $\alpha$ = 1.95; microgels of intermediate elasticity and (c) $\alpha$ = 1.51; stiff microgels. Snapshots obtained from MD simulations showing distinct microgel assemblies with increasing particle number densities (top to bottom), for (d-f): soft microgels, (g-i): microgels of intermediate elasticity and (j-l): stiff microgels.
• Figure 4: Pair correlation functions, $g(r)$, obtained from (a) microscopy images and (b) MD simulations of microgels with distinct elasticities. (c) Nearest neighbor distance (NND) as a function of microgel elasticity from experiments and simulations. (d) Evolution of $\langle |\psi_6| \rangle$ for different microgel elasticities computed for the highest $\phi$ regions in Figs. \ref{['fig:2']}, \ref{['fig:3']}. Light and dark colored bars denote early and late times respectively. Highest $\phi$ regions (Figs. \ref{['fig:2']}(f,l,r) and Figs. \ref{['fig:3']}(f,i,l)) were used to calculate $g(r)$ and NND. Figs. \ref{['fig:2']}(e-f,k-l,q-r) were analyzed to calculate $\langle |\psi_6| \rangle$ at early and late times.