Microstructural Rearrangements and their Rheological Implications in a Model Thixotropic Elastoviscoplastic Fluid

Abstract

We identify the sequence of microstructural changes that characterize the evolution of an attractive particulate gel under flow and discuss their implications on macroscopic rheology. Dissipative particle dynamics is used to monitor shear-driven evolution of a fabric tensor constructed from the ensemble spatial configuration of individual attractive constituents within the gel. By decomposing this tensor into isotropic and nonisotropic components we show that the average coordination number correlates directly with the flow curve of the shear stress versus shear rate, consistent with theoretical predictions for attractive systems. We show that the evolution in nonisotropic local particle rearrangements are primarily responsible for stress overshoots (strain-hardening) at the inception of steady shear flow and also lead, at larger times and longer scales, to microstructural localization phenomena such as shear banding flow-induced structure formation in the vorticity direction. DOI: 10.1103/PhysRevLett.118.048003 Thixotropic elastoviscoplastic (TEVP) materials are a broad class of structured fluids that include (but are not limited to) most colloidal gels [1], nano emulsions [2], crude oils [3,4], and many biological systems such as blood clots and actin networks [5,6]. As a result of their complex underlying microstructure, TEVPs exhibit a wide range of rich and complex thermo-mechanical properties: Below a critical stress, the microstructural network formed by individual particles remains intact and resists large deformations by external forces. At this stage the macroscopic response of the material is similar to that of a viscoelastic solid. By progressively increasing the applied load, the material reaches its "yield stress" and starts to flow [7]. At this point the particle network that is responsible for solidlike response of the macroscopic sample undergoes plastic rearrangements over an increasingly wide range of length scales [8]. Upon complete yielding of this network, plastic flow results ultimately in a viscouslike response; however, as a result of constant formation and breakage events, the particle-level microstructure continues to evolve giving rise to thixotropic behavior. The many-body nature of the problem means that local forces exerted on a single particle change its energy landscape, which consequently defines its subsequent association or dissociation rate to neighboring particles [9,10]. When combined with multibody hydrodynamic effects in these fluids [11], the resulting microstructure-flow relationship becomes complex and may show long time scale transient behavior and multiple steady states [1,12]. This leads to a wide range of timedependent responses that can also be observed, including microphase separation [13], vorticity aligned structure formation [14][15][16], local rigid plug formation and shear banding [17], plus shear-induced rejuvenation of the particle network [18].Although the general form of the flow curve (relating the shear stress to shear rate) and some transient pheno...

Rheological investigation of gels formed by competing interactions: A numerical study

“…Also similarly to previous works 31,34,35 , we find that the maximum anisotropy is reached at the yielding point. After this point, the initial gel structure is lost due to the bonds breakage, thereby decreasing the amount of stored stress and consequent anisotropy.…”

Mean-Field Scenario for the Athermal Creep Dynamics of Yield-Stress Fluids

“…Stress overshoots in shear start up experiments are commonly observed in both dense hard sphere systems [40], but also in network forming colloidal gels [42]. In the latter case, the stress overshoot is closely related with the change of structural quantities, while in the former case, which is of our interests, the stress overshoot depends on the relaxation level (age) of the system [14,40,41].…”

Time-rate-transformation framework for targeted assembly of short-range attractive colloidal suspensions

“…Since flow strength can be characterized by the shear rate, or in dimensionless form by the Mason number, and the accumulated strain provides a convenient dimensionless measure of the time over which transport processes take place, we refer to this framework as a interacting particles. In this scheme, colloidal particles interact through Morse potential, as well as random, dissipative and lubrication forces, excluding traditional conservative forces in order to avoid discontinuities in particle interaction potentials (For details of simulation method refer to Supplementary Material and [12,18]). Having the exact expression for the interaction potentials, one can accordingly calculate the effective range of attraction using the Noro-Frankel expression [43]; however, it has been shown that the stickiness parameter and the second virial component of all interaction potentials become virtually identical at short distances where the separation between two particles is smaller than 0.1-0.3a [43,44].…”

“…The residual stress ( r  ), but they also have distinctly different micro-and meso-scale structures than the gels formed under quiescent conditions. Analysis of the fractal dimension of the gels using a 13 | P a g e box counting algorithm [12] shows that at intermediate shearing times, 10 Using these criteria elucidated above, we can construct a phase map of the different regimes and the corresponding microstructures ( Fig.4) using our direct numerical simulations of each regime.…”

“…Evolution in the ensemble-averaged coordination number (scaled by its value at quiescent conditions), for a range of attraction strengths, in attractive colloidal systems as a function of applied Mason number.Having the long-time rheology-microstructure relationship of these attractive colloids under flow at hand, we prescribe the following protocol for the Time-Rate-Transformation (TRT): all simulations are initiated with a random distribution of 25000 colloidal particles at 10 Mn  to ensure complete rejuvenation of the structure[12], and then all simulations are brought to arrest, different quench times, q t with a linear decrease in shear rate over time of the form:…”