Microscale Shear-Induced Gelation and Relaxation of Shake Gels

Colloidal suspensions can undergo gelation, transitioning from fluid-like dispersions to soft, solid-like networks under time-dependent or mechanical perturbation. Aqueous suspensions of laponite and polyethylene oxide (PEO) form "shake gels," in which charged laponite platelets are bridged by adsorbed PEO chains to create a reversible network that breaks under shear and reforms at rest, exhibiting fascinating viscoelastic behavior in response to shear. However, the microscale onset of gelation in shake gels remains poorly characterized. Here, we use nonlinear optical tweezers microrheology to investigate the early stages of gelation in a model shake gel comprising 1% Laponite and 0.3% PEO. We pull optically trapped microspheres through the suspension at varying strain rates and distances to measure the force resisting the strain and the subsequent relaxation. We identify conditions that allow for strain-induced gelation and examine the relaxation profile as the gel relaxes back to a viscoelastic fluid. We reveal nonlinear stiffening and stress overshoots as precursors to gelation, and a wide spectrum of relaxation profiles from rapid complete relaxation to sustained gel-like resistance. This work lays the foundation for probing the influence of composition and deformation rate on shake gel mechanics, with broader implications for understanding shear-dependent mechanical response of colloidal materials.