Imprinting Macroscopic Fracture during Gelation: A Mechanism for Tuning Colloidal Gels

TL;DR

This study demonstrates how nonlinear oscillatory shear during gelation can permanently imprint fracture patterns in colloidal gels, significantly altering their viscoelastic properties and microstructure.

Contribution

It introduces a mechanism where nonlinear shear influences gelation, enabling control over gel properties through shear-induced imprinting of fracture patterns.

Findings

Large-amplitude oscillatory shear alters gel stiffness irreversibly.

Gels formed under shear show increased ductility and fracture patterns.

Additional fractional element needed to model damage in rheological spectra.

Abstract

Colloidal gels form through the sol-gel transition of attractive particle suspensions, where local aggregation leads to a space-spanning network with solid-like properties. Their microstructure and mechanical properties are highly sensitive to external perturbations, which can substantially alter the pathway of network formation. Here, we investigate how nonlinear oscillatory shear affects the sol-gel transition of colloidal silica suspensions. Using large-amplitude oscillatory shear (LAOS), we vary both the strain amplitude and the duration of oscillatory forcing, varying between one and two times the gelation time. We find that sufficiently large strain amplitudes, or prolonged exposure to oscillations in the nonlinear regime, alter irreversibly the gel properties: the storage modulus $G'$ decreases while its frequency dependence remains unchanged. In contrast, the loss modulus $G''$, which decreases monotonically with frequency under quiescent gelation, exhibits an upturn at high frequencies when the gel is formed under strong oscillatory shear. The viscoelastic spectra of gels formed under quiescent conditions are well captured by a fractional Maxwell model, while gels formed under LAOS require an additional fractional element to account for damage-induced dissipation. Rheo-imaging experiments corroborate this interpretation by revealing the growth of cracks in gels formed under LAOS. We further show that these gels display a progressively more ductile nonlinear response for prolonged exposure to LAOS during gelation. These results demonstrate that the interplay between non-linear shear and gelation can permanently imprint a macroscopic fracture pattern into colloidal gels, offering a route to tune their viscoelastic properties.