Gelation of particles with short-range attraction

Tag : solvent red 8

Nanoscale or colloidal particles are important in many realms ofscience and technology. They can dramatically change the propertiesof materials, imparting solid-like behaviour to a wide variety ofcomplex fluids 1, 2 . This behaviour arises when particles aggregate to form mesoscopicclusters and networks. The essential component leading toaggregation is an interparticle attraction, which can be generatedby many physical and chemical mechanisms. In the limit ofirreversible aggregation, infinitely strong interparticle bondslead to diffusion-limited cluster aggregation 3 (DLCA). This is understood as a purely kinetic phenomenon that canform solid-like gels at arbitrarily low particle volume fraction 4, 5 . Far more important technologically are systems with weakerattractions, where gel formation requires higher volume fractions.Numerous scenarios for gelation have been proposed, including DLCA 6 , kinetic or dynamic arrest 4, 7, 8, 9, 10 , phase separation 5, 6, 11, 12, 13, 14, 15, 16 , percolation 4, 12, 17, 18 and jamming 8 . No consensus has emerged and, despite its ubiquity andsignificance, gelation is far from understood—even thelocation of the gelation phase boundary is not agreed on 5 . Here we report experiments showing that gelation of sphericalparticles with isotropic, short-range attractions is initiated byspinodal decomposition; this thermodynamic instability triggers theformation of density fluctuations, leading to spanning clustersthat dynamically arrest to create a gel. This simple picture ofgelation does not depend on microscopic system-specific details,and should thus apply broadly to any particle system withshort-range attractions. Our results suggest thatgelation—often considered a purely kinetic phenomenon 4, 8, 9, 10 —is in fact a direct consequence of equilibriumliquid–gas phase separation 5, 13, 14, 15 . Without exception, we observe gelation in all of our samplespredicted by theory and simulation to phase-separate; this suggeststhat it is phase separation, not percolation 12 , that corresponds to gelation in models for attractive spheres.