Colloidal gels tuned by oscillatory shear

Abstract

We examine microstructural and mechanical changes which occur during oscillatory shear flow and reformation after flow cessation of an intermediate volume fraction colloidal gel using rheometry and Brownian Dynamics (BD) simulations. A model depletion colloid-polymer mixture is used, comprising of a hard sphere colloidal suspension with the addition of non-adsorbing linear polymer chains. Results reveal three distinct regimes depending on the strain amplitude of oscillatory shear. Large shear strain amplitudes fully break the structure which results into a more homogenous and stronger gel after flow cessation. Intermediate strain amplitudes densify the clusters and lead to highly heterogeneous and weak gels. Shearing the gel to even lower strain amplitudes creates a less heterogonous stronger solid. These three regimes of shearing are connected to the microscopic shear-induced structural heterogeneity. A comparison with steady shear flow reveals that the latter does not produce structural heterogeneities as large as oscillatory shear. Therefore oscillatory shear is a much more efficient way of tuning the mechanical properties of colloidal gels. Moreover, colloidal gels presheared at large strain amplitudes exhibit a distinct nonlinear response characterized largely by a single yielding process while in those presheared at lower rates a two step yield process is promoted due to the creation of highly heterogeneous structures.2

Plasticity in colloidal gel strands

“…This suggests that the applied deformation leads to accelerated aging and coarsening of the gel strand, driving it gradually into a more favorable state by forming more inter-particle bonds. 25,40 A similar strain-induced increase of the number of interparticle bonds has been seen in computer simulations 41 and in experiments 24 on colloidal gel networks. We note that the strain-induced increase in the number of bonds is more pronounced and continues for a longer period as the gel strands get thicker (see also ESI, † Fig.…”

“…[15][16][17] The main control parameters that determine the structure of a colloidal gel are the magnitude of the attraction strength between the particles, [18][19][20][21][22] the particle volume fraction, 18,21,22 and the shear history of the gel. [23][24][25]44 In the limit of very strong attraction and very low volume fraction, irreversible aggregation leads to the formation of dilute, diffusion-limited fractal gels. 18 In this regime, the mechanics and dynamics of the gel can be described using scaling approaches or by simulation models based on percolating networks of gel strands that ignore the internal structure of the strands.…”

Superposition rheology and anisotropy in rheological properties of sheared colloidal gels

“…The open symbols show the evolution of the storage moduli parallel to the previous flow direction, indicated with the subscript h. Such time evolution after cessation of flow of what are typically called "the" moduli was reported for many types of colloidal gels, with an increase of orders of magnitude in storage modulus [37,[53][54][55][56][57]. In comparison, the loss moduli are far less affected by flow.…”

Shear melting and recovery of crosslinkable cellulose nanocrystal–polymer gels

“…Either the gel accumulates damage under shear until it is irreversibly destroyed (gel collapse), or the gel recovers after flow cessation, and eventually re-forms a percolated network, given a sufficiently long period of rest. These two types of behavior allow to us to distinguish between colloidal gels that experience an irreversible yielding transition [16][17][18][19], and colloidal gels that can be rejuvenated due to the shear-reversible nature of the interparticle attractive interactions such as van der Waals or depletion interactions [20][21][22][23]. The re- * Corresponding author: ajhart@mit.edu covery step of the latter category of gels has been termed "rheological aging" and the kinetics of such phenomena are a complex function of the volume fraction, the nature of the interparticle interactions and the shear history [24][25][26][27][28][29].…”