Scaling relationships between viscosity and diffusivity in shear-thickening suspensions.

Dense suspensions often exhibit a dramatic response to large external deformation. The recent body of work has related this behavior to transition from an unconstrained lubricated state to a constrained frictional state. Here, we use numerical simulations to study the flow behavior and shear-induced diffusion of frictional non-Brownian spheres in two dimensions under simple shear flow. We first show that both viscosity η and diffusivity D/ of the particles increase under characteristic shear stress, which is associated with lubrication to frictional transition. Subsequently, we propose a one-to-one relationship between viscosity and diffusivity using the length scale ξ associated with the size of collective motions (rigid clusters) of the particles. We demonstrate that η and D/ are controlled by ξ in two distinct flow regimes, i.e. in the frictionless and frictional states, where the one-to-one relationship is described as a crossover from D/ ∼ η (frictionless) to η1/3 (frictional). We also confirm that the proposed power laws are insensitive to the interparticle friction and system size.

Leveraging the Polymer Glass Transition to Access Thermally Switchable Shear Jamming Suspensions.

Suspensions of polymeric nano- and microparticles are fascinating stress-responsive material systems that, depending on their composition, can display a diverse range of flow properties under shear, such as drastic thinning, thickening, and even jamming (reversible solidification driven by shear). However, investigations to date have almost exclusively focused on nonresponsive particles, which do not allow in situ tuning of the flow properties. Polymeric materials possess rich phase transitions that can be directly tuned by their chemical structures, which has enabled researchers to engineer versatile adaptive materials that can respond to targeted external stimuli. Reported herein are suspensions of (readily prepared) micrometer-sized polymeric particles with accessible glass transition temperatures (T g) designed to thermally control their non-Newtonian rheology. The underlying mechanical stiffness and interparticle friction between particles change dramatically near T g. Capitalizing on these properties, it is shown that, in contrast to conventional systems, a dramatic and nonmonotonic change in shear thickening occurs as the suspensions transition through the particles' T g. This straightforward strategy enables the in situ turning on (or off) of the system's ability to shear jam by varying the temperature relative to T g and lays the groundwork for other types of stimuli-responsive jamming systems through polymer chemistry.

Structure and Dynamics of Force Clusters and Networks in Shear Thickening Suspensions.

Dense suspensions can exhibit shear thickening in response to large deformation. A consensus has emerged over the past few years on the formation of force networks, that span the entire system size, that lead to increased resistance to motion. Nonetheless, the characteristics of these networks are to a large extent poorly understood. Here, force networks formed in continuous and discontinuous shear thickening dense suspensions (CST and DST, respectively) are studied. We first show the evolution of the network formation and its topological heterogeneities as the applied stress increases. Subsequently, we identify force communities and coarse grain the suspension into a cluster network, and show that cluster-level dynamics are responsible for stark differences between the CST and DST behavior. Our results suggest that the force clusters formed in the DST regime are considerably more constrained in their motion, while CST clusters are loosely connected to their surrounding clusters.

Shear stress dependence of force networks in 3D dense suspensions.

The geometric organization and force networks of 3D dense suspensions that exhibit both shear thinning and thickening have been examined as a function of varying strength of interparticle attractive interactions using lubrication flow discrete element simulations. Significant rearrangement of the geometric topology does not occur at either the local or global scale as these systems transition across the shear thinning and shear thickening regimes. In contrast, massive rearrangements in the balance of attractive, lubrication, and contact forces are observed with interesting behavior of network growth and competition. In agreement with prior work, in shear thinning regions the attractive force is dominant, however as the shear thickening region is approached there is growth of lubrication forces. Lubrication forces oppose the attraction forces, but as viscosity continues to increase under increasing shear stress, the lubrication forces are dominated by contact forces that also resist attraction. Contact forces are the dominant interactions during shear thickening and are an order of magnitude higher than their values in the shear-thinning regime. At high attractive interaction strength, contact networks can form even under shear thinning conditions, however high shear stress is still required before contact networks become the driving mechanism of shear thickening. Analysis of the contact force network during shear thickening generally indicates a uniformly spreading network that rapidly forms across empty domains; however the growth patterns exhibit structure that is significantly dependent upon the strength of interparticle interactions, indicating subtle variations in the mechanism of shear thickening.

Shear Thickening and Jamming of Dense Suspensions: The "Roll" of Friction.

Particle-based simulations of discontinuous shear thickening (DST) and shear jamming (SJ) suspensions are used to study the role of stress-activated constraints, with an emphasis on resistance to gearlike rolling. Rolling friction decreases the volume fraction required for DST and SJ, in quantitative agreement with real-life suspensions with adhesive surface chemistries and "rough" particle shapes. It sets a distinct structure of the frictional force network compared to only sliding friction, and from a dynamical perspective leads to an increase in the velocity correlation length, in part responsible for the increased viscosity. The physics of rolling friction is thus a key element in achieving a comprehensive understanding of strongly shear-thickening materials.

Structure and dynamics of hydrodynamically interacting finite-size Brownian particles in a spherical cavity: Spheres and cylinders.

The structure and dynamics of confined suspensions of particles of arbitrary shape are of interest in multiple disciplines from biology to engineering. Theoretical studies are often limited by the complexity of long-range particle-particle and particle-wall forces, including many-body fluctuating hydrodynamic interactions. Here, we report a computational study on the diffusion of spherical and cylindrical particles confined in a spherical cavity. We rely on an immersed-boundary general geometry Ewald-like method to capture lubrication and long-range hydrodynamics and include appropriate non-slip conditions at the confining walls. A Chebyshev polynomial approximation is used to satisfy the fluctuation-dissipation theorem for the Brownian suspension. We explore how lubrication, long-range hydrodynamics, particle volume fraction, and shape affect the equilibrium structure and the diffusion of the particles. It is found that once the particle volume fraction is greater than 10%, the particles start to form layered aggregates that greatly influence particle dynamics. Hydrodynamic interactions strongly influence the particle diffusion by inducing spatially dependent short-time diffusion coefficients, stronger wall effects on the particle diffusion toward the walls, and a sub-diffusive regime-caused by crowding-in the long-time particle mobility. The level of asymmetry of the cylindrical particles considered here is enough to induce an orientational order in the layered structure, decreasing the diffusion rate and facilitating a transition to the crowded mobility regime at low particle concentrations. Our results offer fundamental insights into the diffusion and distribution of globular and fibrillar proteins inside cells.

From Yielding to Shear Jamming in a Cohesive Frictional Suspension.

Simulations are used to study the steady shear rheology of dense suspensions of frictional particles exhibiting discontinuous shear thickening and shear jamming, in which finite-range cohesive interactions result in a yield stress. We develop a constitutive model that combines yielding behavior and shear thinning at low stress with the frictional shear thickening at high stresses, in good agreement with the simulation results. This work shows that there is a distinct difference between solids below the yield stress and in the shear-jammed state, as the two occur at widely separated stress levels, with an intermediate region of stress in which the material is flowable.

Microscopic Origin of Frictional Rheology in Dense Suspensions: Correlations in Force Space.

We develop a statistical framework for the rheology of dense, non-Brownian suspensions, based on correlations in a space representing forces, which is dual to position space. Working with the ensemble of steady state configurations obtained from simulations of suspensions in two dimensions, we find that the anisotropy of the pair correlation function in force space changes with confining shear stress (σ_{xy}) and packing fraction (ϕ). Using these microscopic correlations, we build a statistical theory for the macroscopic friction coefficient: the anisotropy of the stress tensor, µ=σ_{xy}/P. We find that µ decreases (i) as ϕ is increased and (ii) as σ_{xy} is increased. Using a new constitutive relation between µ and viscosity for dense suspensions that generalizes the rate-independent one, we show that our theory predicts a discontinuous shear thickening flow diagram that is in good agreement with numerical simulations, and the qualitative features of µ that lead to the generic flow diagram of a discontinuous shear thickening fluid observed in experiments.

Current status of Mycobacterium avium subspecies paratuberculosis infection in animals & humans in India: What needs to be done?

Mycobacterium avium subspecies paratuberculosis (MAP) has emerged as a major health problem for domestic livestock and human beings. Reduced per animal productivity of domestic livestock seriously impacts the economics of dairy farming globally. High to very high bioload of MAP in domestic livestock and also in the human population has been reported from north India. Presence of live MAP bacilli in commercial supplies of raw and pasteurized milk and milk products indicates its public health significance. MAP is not inactivated during pasteurization, therefore, entering into human food chain daily. Recovery of MAP from patients with inflammatory bowel disease or Crohn's disease and animal healthcare workers suffering with chronic gastrointestinal problems indicate a close association of MAP with a number of chronic and other diseases affecting human health. Higher bioload of MAP in the animals increases the risk of exposure to the human population with MAP. This review summarizes the current status of MAP infection in animals as well as in human beings and also highlights the prospects of effective management and control of disease in animals to reduce the risk of exposure to human population.

Application of IS1311 locus 2 PCR-REA assay for the specific detection of 'Bison type' Mycobacterium avium subspecies paratuberculosis isolates of Indian origin.

BACKGROUND & OBJECTIVES: Of the three major genotypes of Mycobacterium avium subspecies paratuberculosis (MAP), 'Bison type' is most prevalent genotype in the domestic livestock species of the country, and has also been recovered from patients suffering from Crohn's disease. Recently, a new assay based on IS1311 locus 2 PCR- restriction endonuclease analysis (REA) was designed to distinguish between 'Indian Bison type' and non-Indian genotypes. The present study investigated discriminatory potential of this new assay while screening of a panel of MAP isolates of diverse genotypes and from different geographical regions. METHODS: A total of 53 mycobacterial isolates (41 MAP and 12 mycobacterium other than MAP), three MAP genomic DNA and 36 MAP positive faecal DNA samples from different livestock species (cattle, buffaloes, goat, sheep and bison) and geographical regions (India, Canada, USA, Spain and Portugal) were included in the study. The extracted DNA samples (n=92) were analyzed for the presence of MAP specific sequences (IS900, ISMav 2 and HspX) using PCR. DNA samples were further subjected to genotype differentiation using IS1311 PCR-REA and IS1311 L2 PCR-REA methods. RESULTS: All the DNA samples (except DNA from non-MAP mycobacterial isolates) were positive for all the three MAP specific sequences based PCRs. IS1311 PCR-REA showed that MAP DNA samples of Indian origin belonged to 'Bison type'. Whereas, of the total 19 non-Indian MAP DNA samples, 2, 15 and 2 were genotyped as 'Bison type', 'Cattle type' and 'Sheep type', respectively. IS1311 L2 PCR-REA method showed different restriction profiles of 'Bison type' genotype as compared to non-Indian DNA samples. INTERPRETATION & CONCLUSIONS: IS1311 L2 PCR-REA method successfully discriminated 'Indian Bison type' from other non-Indian genotypes and showed potential to be future epidemiological tool and for genotyping of MAP isolates.

Effect of cohesion on shear banding in quasistatic granular materials.

Cohesive powders have widely different bulk behavior due to their peculiar interactions. We use discrete element simulations to investigate the effect of contact cohesion on the steady state flow of dense powders in a slowly sheared split-bottom Couette cell, which imposes a wide stable shear band. The intensity of cohesive forces can be quantified by the granular Bond number (Bo), namely the ratio between maximum attractive force and average force due to external compression. We find that the shear banding phenomenon is almost independent of cohesion for Bond numbers Bo<1, however for Bo≥1 cohesive forces start to play an important role, as both width and center position of the band increase. Inside the shear band, the mean normal contact force is independent of cohesion and depends only on the confining stress. In contrast, when the behavior is analyzed focusing on the eigendirections of the local strain rate tensor, a dependence on cohesion shows up. Forces carried by contacts along the compressive and tensile directions are symmetric about the mean force (larger and smaller respectively), while the force along the third, neutral direction follows the mean force. This anisotropy of the force network increases with cohesion, just like the heterogeneity in all (compressive, tensile and neutral) directions.