Session Y29: Dense Suspension Rheology Beyond Spheres: Fibres, Rods, Rocks, Chains and Everything in Between

Dense suspension rheology beyond flow curves

Dense suspensions often undergo dramatic shear thickening, where the apparent viscosity can increase by an order of magnitude or more upon increasing stress or shear rate.  Rheological measurements of average flow curves (suspension viscosity as a function of applied stress or strain rate)  can be quantitatively described by mean field models attributing shear thickening to  a  transition from primarily hydrodynamic interactions at low stress to primarily frictional interactions at high stress, or more generally to a stress-induced increase in  constraints on relative particle motion. Our measurements of surface stresses with high spatial and temporal resolution, however, show a complexity that is not evident in flow curves and is not captured in mean field models [1-5].  We have observed similar behavior in Brownian monodisperse colloids [1,2], non-Brownian irregular particles (Cornstarch) [3], and colloidal rods [4].  We observe substantial concentration fluctuations associated with stress fluctuations [3-5],  including directly observable fluid migration[3]. In sheared colloidal rod suspensions, we find that  stress fluctuations are associated  with fluctuations in rod orientation and a dynamic order-disorder transition [4].  These results highlight the importance of moving beyond measurements of average properties and mean field models in order to develop a mechanistic understanding of shear thickening.

1. Rathee, V., Blair, D. L. & Urbach, J. S. Localized stress fluctuations drive shear thickening in dense suspensions. PNAS 114, 8740–8745 (2017).

2. Rathee, V., Blair, D. L. & Urbach, J. S. Localized transient jamming in discontinuous shear thickening. J Rheol 64, 299–308 (2020).

3. Rathee, V., Miller, J., Blair, D. L. & Urbach, J. S. Structure of propagating high-stress fronts in a shear-thickening suspension. Proc National Acad Sci 119, e2203795119 (2022).

4. Rathee, V. et al. Role of particle orientational order during shear thickening in suspensions of colloidal rods. Phys Rev E 101, 040601 (2020).

5. Miller, J. M., Blair, D. L. & Urbach, J. S. Order and density fluctuations near the boundary in sheared dense suspensions. Frontiers in Physics 10, 991540 (2022).   

Modelling of Curved Rods in Dense Suspensions goes Full Circle

Dense suspensions are a broad class of out-of-equilibrium systems that often display interesting macroscopic behaviour under flow. The rich physics observed for suspensions of spheres can be expanded to particles of more complex geometries, resulting in novel flow phenomena. Real-world compositions that diversify beyond uniaxial rods are ubiquitous in nature and engineering processes. This work applies concepts from dry granular physics to model the rheology of suspensions of curved rods. Specifically, we investigate the interplay between rod curvature (exploring an angular range of 0-2π) and packing efficiency. In our sheared suspensions, the stress response provides insight into the emergent force networks and is used to identify the shape-dependent jamming transition. Notably, the jamming volume fraction is a non-monotonic function of curvature, which we believe to be a consequence of arising orientational order attributed to the different packing abilities of rods, arches and disks.   

Colloids with a Twist: Entanglement's Effect on Diffusion of Helical Filaments

Connecting the thermal motion of a dense ensemble of microscopic particles to the system's macroscopic properties remains a challenge. We examine this relationship using a model colloidal system built from bacterial flagella, a biological colloid whose overall shape can be engineered and controlled. Point mutations of the flagellar constituent protein result in shapes ranging from a rigid straight rod to a helix with a well-defined pitch and wavelength. The 3D motion of individual filaments in a dense suspension is characterized using confocal microscopy. Helicity changes particle dynamics: A dense suspension of helices exhibits entanglements, which constrain the motion of individual helices. This results in a corkscrew diffusive motion that is absent for straight rods. These entangled helices have a significantly higher viscosity than a system consisting of straight rods. Utilizing a diffusion-informed model provides a framework to understand how viscosity can emerge from the confined motion of individual particles. Together, these results show how small changes in a colloid's shape can lead to a multi-scale change in a system's behavior.   

Rheological Properties of Aspherical Particle Networks

Materials such as carbon aerogels, silk and graphene composites can exhibit high-load-bearing capabilities at ultra-low density are networks of aspherical particles. We simulate 2D and 3D packings of aspherical particles with JKR-type interparticle interactions by means of DEM-based tools. The 2D and 3D percolation threshold is quantified as a function of the particle aspect ratio and the interparticle interaction. The various packings are subjected to compression and shear, and the elastic properties and relaxation spectrum are investigated under varying particle parameters.   

Micromechanical origin of elastoplasticity of randomly packed fibers

Disordered packings of unbonded, semiflexible fibers represent a class of materials spanning contexts and scales. From twig-based bird nests to unwoven textiles, bulk mechanics of disparate systems emerge from the bending of constituent slender elements about impermanent contacts. In experimental and computational packings of wooden sticks, we identify prominent features of their response to cyclic oedometric compression: non-linear stiffness, transient plasticity, and eventually repeatable velocity-independent hysteresis. We trace these features to their micromechanic origins, identified in characteristic appearance, disappearance, and displacement of internal contacts.   

Arezoo Ardekani

Rheology of concentrated suspension of spherical particles and fibers

 

Controlling the flow of concentrated particle suspensions is a crucial piece of an unresolved puzzle in many applications, e.g., biofuel production, solar cells, electronic cooling, fiber-reinforced thermoplastics. Unravelling the underlying physics and governing mechanisms that determine such materials' behavior is thus crucial. Understanding the flow behavior, quantifying the particle microstructure, and exploring ways to manipulate it in suspensions are essential to increase the efficiency and throughput of processes involving such dense suspensions of particles. Accurate predictions of rheology provide control over the flow of these materials. We developed experimentally validated and physics-based computational approach to study the flow of dense suspensions near jamming limits and quantitatively determined the role of inter-particle short range interactions on the rheology, the jamming fraction, and the microstructure of dense suspensions.   

The role of friction in dense, sheared rod-like suspensions: a simulation study

The complex interplay between nematic order, frictional contacts, and flow in dense suspensions of rigid rods has yet to be fully elucidated. Here, we present simulations of suspensions of non-Brownian hard sphere chains under shear, with Coulomb contact friction and lubrication between particles. We measure rod-to-rod contacts at varying nematic order, volume fraction, and aspect ratio; and propose a modified form of Philipse's random contact equation. We discuss the distribution of tangential frictional forces, particularly in the regimes of discontinuous and continuous shear thickening. We also make connection to the Wyart & Cates model for spheres by modelling the viscosity as a function of a jamming fraction and nematic order.   

Knotting up a Network: Controlling Particle Shape to Tune Macroscopic Rheology

The macroscopic rheological behavior of a dense suspension is intrinsically linked to the microscopic interactions of its constituent particles. We construct a novel colloidal system using bacterial flagella, a rigid biological filament, that allows control over these interactions from the colloid's shape. Flagella are polymorphic, capable of transforming between a number of discrete shapes in response to external chemical conditions, temperature, and stress. For instance, after mild heating in a particular biocompatible solvent, straight flagella filaments twist into helices with uniform pitch and diameter. This morphological transformation is directly imaged using fluorescent microscopy. In a dense suspension, the abrupt change in shape results in intricate collective motion as each coiling particle rapidly entangles with its neighbors. Macroscopically, these entanglements manifest as a dramatic increase in the system's bulk viscosity, which we characterize using a rheometer. Many such shape changes are reversed upon restoration of the initial sample conditions, offering a unique way to switch the system's rheological properties between markedly different states. Our findings pave the way for a new category of complex fluids whose viscous properties can be toggled by manipulating their constituent particles' shape.   

Shear thickening and jamming of bidisperse non-Brownian suspensions: Beyond Wyarts-Cates Model

We study the rheology of bidisperse non-Brownian suspensions using particle-based simulation, mapping the viscosity as a function of the size ratio of the species, their relative abundance, and the overall solid content. The variation of the viscosity with applied stress exhibits paradigmatic shear thickening phenomenology irrespective of composition, though the stress-dependent limiting solids fraction governing the viscosity and its divergence point are non-monotonic in the mixing ratio. Contact force data demonstrate an exchange in dominant stress contribution from large-large to small-small particle contacts as the mixing ratio of the species evolves. Combining a prior model for shear thickening with one for composition-dependent jamming, we obtain a full description of the rheology of bidisperse non-Brownian suspensions capable of predicting effects such as the viscosity reduction observed upon adding small particle fines to a suspension of large particles.   

van der Waals Interaction between Rods: Analytical Description and Numerical Implementation

An analytical description based on Hamaker theory has been derived for the van der Waals interaction between thin rods that can be approximated as lines of Lennard-Jones beads and between a rod and a Lennard-Jones bead. The description is implemented in LAMMPS, a widely used molecular simulator, to allow large-scale molecular dynamics simulations where rod-shaped objects are involved. The package is systematically tested and used to simulate the behavior of rods in a solvent modelled as a Lennard-Jones liquid. The simulation results reveal the anisotropic nature of the solvent-mediated rod-rod interaction in addition to the intrinsic anisotropic rod-rod interaction. Such interaction strongly affects the behavior of rods in the solvent under various flow conditions such as the alignment of rods in a shear flow. Evidence will also be presented on the isotropic-nematic transition in dense suspension of rods. The robust and flexible package will allow efficient computational studies of a wide range of rod-like systems, including liquid crystals, colloids, and filamentous materials such as carbon nanotubes and biological filaments.