Sheared Suspensions Research Articles

Structural origin of relaxation in dense colloidal suspensions

Amorphous solids relax via slow molecular rearrangement induced by thermal fluctuations or applied stress. Microscopic structural signatures predicting these structural relaxations have been long searched for but have so far only been found in dynamic quantities such as vibrational quasi-localized soft modes or with structurally trained neural networks. A physically meaningful structural quantity remains elusive. Here, we introduce a structural order parameter derived from the mean-field caging potential experienced by the particles due to their neighbors, which reliably predicts the occurrence of structural relaxations. The structural parameter, derived from density functional theory, provides a measure of susceptibility to particle rearrangements that can effectively identify weak or defect-like regions in disordered systems. Using experiments on dense colloidal suspensions, we demonstrate a strong correlation between this order parameter and the structural relaxations of the amorphous solid. In quiescent suspensions, this correlation increases with density, when particle rearrangements become rarer and more localized. In sheared suspensions, the order parameter reliably pinpoints shear transformations; the applied shear weakens the caging potential due to shear-induced structural distortions, causing the proliferation of plastic deformation at structurally weak regions. Our work paves the way to a structural understanding of the relaxation of a wide range of amorphous solids, from suspensions to metallic glasses.

A phenomenological model for chains and bands in dipolar suspensions

We introduce a phenomenological model for the dipolar interaction of polarizable particles under an external field, where the relative radial and rotational components of a particle pair interaction can be tuned. We show that the relative strengths of these two components govern the microstructure and dynamics of a suspension of such particles. Notably, dominant radial interactions give rise to the formation of zigzag band patterns, which were previously only thought to occur in systems where hydrodynamic interactions dominate. Through this phenomenological model, we show that dipolar interactions can be used to access an array of patterns in suspensions of polarizable particles, from chains to bands, which would dramatically affect suspension shear rheology, for instance.

Jamming Memory into Acoustically Trained Dense Suspensions under Shear

Systems driven far from equilibrium often retain structural memories of their processing history. This memory has, in some cases, been shown to dramatically alter the material response. For example, work hardening in crystalline metals can alter the hardness, yield strength, and tensile strength to prevent catastrophic failure. Whether memory of processing history can be similarly exploited in flowing systems, where significantly larger changes in structure should be possible, remains poorly understood. Here, we demonstrate a promising route to embedding such useful memories. We build on work showing that exposing a sheared dense suspension to acoustic perturbations of different power allows for dramatically tuning the sheared suspension viscosity and underlying structure. We find that, for sufficiently dense suspensions, upon removing the acoustic perturbations, the suspension shear jams with shear stress contributions from the maximum compressive and maximum extensive axes that reflect or “remember” the acoustic training. Because the contributions from these two orthogonal axes to the total shear stress are antagonistic, it is possible to tune the resulting suspension response in surprising ways. For example, we show that differently trained sheared suspensions exhibit (1) different susceptibility to the same acoustic perturbation, (2) orders of magnitude changes in their instantaneous viscosities upon shear reversal, and (3) even a shear stress that increases in magnitude upon shear cessation. We work through these examples to explain the underlying mechanisms governing each behavior. Then, to illustrate the power of this approach for controlling suspension properties, we demonstrate that flowing states well below the shear jamming threshold can be shear jammed via acoustic training. Collectively, our work paves the way for using acoustically induced memory in dense suspensions to generate rapidly and widely tunable materials. Published by the American Physical Society 2024

Exact Results for Non-Newtonian Transport Properties in Sheared Granular Suspensions: Inelastic Maxwell Models and BGK-Type Kinetic Model.

The Boltzmann kinetic equation for dilute granular suspensions under simple (or uniform) shear flow (USF) is considered to determine the non-Newtonian transport properties of the system. In contrast to previous attempts based on a coarse-grained description, our suspension model accounts for the real collisions between grains and particles of the surrounding molecular gas. The latter is modeled as a bath (or thermostat) of elastic hard spheres at a given temperature. Two independent but complementary approaches are followed to reach exact expressions for the rheological properties. First, the Boltzmann equation for the so-called inelastic Maxwell models (IMM) is considered. The fact that the collision rate of IMM is independent of the relative velocity of the colliding spheres allows us to exactly compute the collisional moments of the Boltzmann operator without the knowledge of the distribution function. Thanks to this property, the transport properties of the sheared granular suspension can be exactly determined. As a second approach, a Bhatnagar-Gross-Krook (BGK)-type kinetic model adapted to granular suspensions is solved to compute the velocity moments and the velocity distribution function of the system. The theoretical results (which are given in terms of the coefficient of restitution, the reduced shear rate, the reduced background temperature, and the diameter and mass ratios) show, in general, a good agreement with the approximate analytical results derived for inelastic hard spheres (IHS) by means of Grad's moment method and with computer simulations performed in the Brownian limiting case (m/mg→∞, where mg and m are the masses of the particles of the molecular and granular gases, respectively). In addition, as expected, the IMM and BGK results show that the temperature and non-Newtonian viscosity exhibit an S shape in a plane of stress-strain rate (discontinuous shear thickening, DST). The DST effect becomes more pronounced as the mass ratio m/mg increases.

Interparticle friction in sheared dense suspensions: Comparison of the viscous and frictional rheology descriptions

In the literature, two different frameworks exist for describing the rheology of solid/liquid suspensions: (1) the “viscous” framework in terms of the relative suspension viscosity, ηr, as a function of the reduced solid volume fraction, ϕ/ϕm, with ϕm the maximum flowable packing fraction, and (2) the “frictional” framework in terms of a macroscopic friction coefficient, μ, as a function of the viscous number, Iv, defined as the ratio of the viscous shear to the wall-normal particle stress. Our goal is to compare the two different frameworks, focusing on the effect of friction between particles. We have conducted a particle-resolved direct numerical simulation study of a dense non-Brownian suspension of neutrally buoyant spheres in slow plane Couette flow. We varied the bulk solid volume fraction from ϕb=0.1 to 0.6 and considered three different Coulomb friction coefficients: μc=0, 0.2, and 0.39. We find that ηr scales well with ϕ/ϕm, with ϕm obtained from fitting the Maron–Pierce correlation. We also find that μ scales well with Iv. Furthermore, we find a monotonic relation between ϕ/ϕm and Iv, which depends only weakly on μc. Since ηr=μ/Iv, we thus find that the two frameworks are largely equivalent and that both account implicitly for Coulomb friction. However, we find that the normal particle stress differences, N1 and N2, when normalized with the total shear stress and plotted against either ϕ/ϕm or Iv, remain explicitly dependent on μc in a manner that is not yet fully understood.

Stress-activated friction in sheared suspensions probed with piezoelectric nanoparticles.

A hallmark of concentrated suspensions is non-Newtonian behavior, whereby the viscosity increases dramatically once a characteristic shear rate or stress is exceeded. Such strong shear thickening is thought to originate from a network of frictional particle-particle contact forces, which forms under sufficiently large stress, evolves dynamically, and adapts to changing loads. While there is much evidence from simulations for the emergence of this network during shear thickening, experimental confirmation has been difficult. Here, we use suspensions of piezoelectric nanoparticles and exploit the strong local stress focusing within the network to activate charge generation. This charging can then be detected in the measured ac conductance and serve as a signature of frictional contact formation. The direct link between stress-activated frictional particle interactions and piezoelectric suspension response is further demonstrated by tracking the emergence of structural memory in the contact network under oscillatory shear and by showing how stress-activated friction can drive mechano-transduction of chemical reactions with nonlinear reaction kinetics. Taken together, this makes the ac conductance of piezoelectric suspensions a sensitive in-situ reporter of the micromechanics associated with frictional interactions.

Modulating particle-particle interaction in phyllosilicate serpentine aqueous suspensions using sodium citrate

The depletion of high-grade nickel sulfide ores has led to an increased interest in low-grade ultramafic ores, which are widely available but challenging to process due to their high content of phyllosilicate serpentine. The anisotropic surface charge and shape of serpentine particles lead to attractive interactions, which increase the viscosity of ore slurry and associated operational and equipment costs. To overcome this challenge, we investigate the application of an environmentally benign reagent, sodium citrate, to modify the particle-particle interactions in serpentine aqueous suspensions. Our results show that at alkaline pH (10), sodium citrate selectively adsorbs on the brucite plane of serpentine, making the entire particle overall negatively charged, which weakens the interparticle attraction and reduces the suspension's shear viscosity, yield stress, and storage modulus. We confirmed these results using colloidal probe atomic force microscopy (AFM) technique, which showed a significant weakening of attraction between silica and brucite surfaces upon the addition of citrate. This work enhances the understanding of surface interaction mechanisms of phyllosilicate serpentine particles in aqueous media with the addition of sodium citrate and provides useful implications for the application of green reagents in mineral operations involving phyllosilicate gangues. Our findings suggest that sodium citrate has great potential as an effective and sustainable reagent in the processing of abundantly available low grade ultramafic nickel ores.

Shear influence on colloidal cluster growth: a SANS and USANS study.

This study examines the time evolution of silica/water clusters where the formation of a gel network from unitary silica particles is interrupted by a simple Couette shear field. The aim is to enable the general understanding of this simple system by examining the microscopic basis for the changes in viscosity by providing structural inputs from small-angle scattering for a simple theoretical model. The experimental system is an 8.3 nm particle silica solution (Ludox) where the gelation has been initiated by lowering the pH in a Couette cell providing a constant shear rate of 250 s-1. A unified small-angle neutron scattering (SANS) and ultra-small-angle neutron scattering (USANS) procedure is described to measure the scattered intensity in a wavevector range of 3 × 10-4 ≤ q (nm-1) ≤ 3.1 × 10-1, probing structural changes over a broad range of length scales from the nanometre to the micrometre. Scattering data provide a new means of better understanding the behaviour of colloidal clusters when subjected to an external applied shear over a continuous time sequence after gel initiation; a fit of the time-dependent scattered intensity leads to an estimation of the cluster's effective volume fraction and size as a function of time. A reductionist theoretical basis is described to predict the time-dependent viscosity behaviour of the sheared colloidal suspension gel-initiated cluster growth from the volume fraction of the clusters.

Emergence of transient reverse fingers during radial displacement of a shear-thickening fluid

A highly sheared dense aqueous suspension of granular cornstarch particles displays rich nonlinear rheology. We had previously demonstrated the growth and onset of interfacial instabilities when shear-thinning cornstarch suspensions were displaced by a Newtonian fluid, and had suggested methods to maximise displacement efficiency [Palak, R. Sathayanath, S. K. Kalpathy and R. Bandyopadhyay, Colloids Surf. A Physicochem. Eng. Asp., 629 (2021) 127405]. In the present work, we explore the miscible displacement of a dense aqueous cornstarch suspension in its discontinuous shear-thickening regime in a quasi-two-dimensional radial Hele-Shaw cell. We systematically study the growth kinetics of the inner interface between water and the cornstarch suspension, and also of the outer interface between the suspension and air. In addition to the growth of interfacial instabilities at the inner interface, we observe a transient withdrawal of the suspension and the formation of fingering instabilities at the outer interface. We demonstrate that these `reverse fingering' instabilities are extremely sensitive to the injection flow rate of water, the gap of the Hele-Shaw cell and the concentration of the displaced cornstarch suspension, {and emerge irrespective of immiscibility between the fluid pair. We believe that as the cornstarch suspension dilates due to the high shear rate imposed by the displacing fluid, the outer suspension-air interface responds with a restoring force, resulting in the penetration of air into the suspension and the formation of reverse fingers. We note that the growth of reverse fingers significantly reduces the displacement efficiency of the suspension. Finally, we demonstrate a correlation in the growth of inner and outer interfacial patterns by computing the velocity with which stresses propagate in the confined dense suspension. Our findings are useful in understanding the flow of granular materials through constrained geometries and can be extended to study stress propagation in shear-thickening materials due to a sudden imposition of high shear rate, such as in impact behaviour.

Shear thinning and brittle failure in crystal-bearing magmas arise from local non-Newtonian effects in the melt

Crystal-bearing magmas are often assumed to be intrinsically non-Newtonian, but the mechanism underlying suspension shear thinning remains unconstrained. Here, we test the hypothesis that shear thinning in these suspensions arises solely from unrelaxed shear thinning in the melt phase or from viscous heating or both. We compile existing data for the rheology of high-temperature crystal-bearing silicate magmas across a wide range of conditions. In order to test this, we define a ‘lever’ function L(ϕ) which scales the strain rate in the melt phase between crystals relative to the bulk strain rate of the suspension as a function of crystal volume fraction ϕ. We show that, in general, the parameterisation of the amplification of strain rates via the L(ϕ) factor coupled with a shear-thinning law for the melt phase fully accounts for the non-Newtonian behaviour of the suspension. Next, we use the Brinkman number Br to demonstrate that some existing data derive from experiments that were subject to viscous heating, resulting in a further manifestation of apparent shear thinning. Taken together, our results provide a theoretical framework that predicts microphysical regimes for shear thinning. This implies that crystal-bearing magmas are Newtonian except where the scaled conditions for viscous heating are met, or when the strain rates in the melt phase approach the inverse of the structural relaxation timescale. This shear thinning due to non-Newtonian behaviour of the melt between crystals occurs very close to the brittle threshold, yielding an extension of the brittle failure criterion to crystal-bearing magmas, with direct implications for volcanic eruption dynamics.

Dichotomous behaviors of stress and dielectric relaxations in dense suspensions of swollen thermoreversible hydrogel microparticles.

While the mechanical disruption of microscopic structures in complex fluids by large shear flows has been studied extensively, the effects of applied strains on the dielectric properties of macromolecular aggregates have received far less attention. Simultaneous rheology and dielectric experiments can be employed to study the dynamics of sheared colloidal suspensions over spatiotemporal scales spanning several decades. Using a precision impedance analyzer, we study the dielectric behavior of strongly sheared aqueous suspensions of thermoreversible hydrogel poly(N-isopropylacrylamide) (PNIPAM) particles at different temperatures. We also perform stress relaxation experiments to uncover the influence of large deformations on the bulk mechanical moduli of these suspensions. All the sheared PNIPAM suspensions exhibit distinct dielectric relaxation processes in the low and high frequency regimes. At a temperature below the lower consolute solution temperature (LCST), the complex permittivities of highly dense PNIPAM suspensions decrease with increase in applied oscillatory strain amplitudes. Simultaneously, we note a counter-intuitive slowdown of the dielectric relaxation dynamics. Contrary to our rheo-dielectric findings, our bulk rheology experiments, performed under identical conditions, reveal shear-thinning dynamics with increasing strain amplitudes. We propose the shear-induced rupture of fragile clusters of swollen PNIPAM particles to explain our observations. Our work illustrates that rheo-dielectric studies have enormous potential for providing deep insights into the length scale-dependent dynamical properties of complex systems such as dense suspensions and soft glasses.

Aggregation of amphiphilic nanocubes in equilibrium and under shear.

We investigate the self-assembly of amphiphilic nanocubes into finite-sized aggregates in equilibrium and under shear, using molecular dynamics (MD) simulations and kinetic Monte Carlo (KMC) calculations. These patchy nanoparticles combine both interaction and shape anisotropy, making them valuable models for studying folded proteins and DNA-functionalized nanoparticles. The nanocubes can self-assemble into various finite-sized aggregates ranging from rods to self-avoiding random walks, depending on the number and placement of the hydrophobic faces. Our study focuses on suspensions containing multi- and one-patch cubes, with their ratio systematically varied. When the binding energy is comparable to the thermal energy, the aggregates consist of only few cubes that spontaneously associate/dissociate. However, highly stable aggregates emerge when the binding energy exceeds the thermal energy. Generally, the mean aggregation number of the self-assembled clusters increases with the number of hydrophobic faces and decreases with increasing fraction of one-patch cubes. In sheared suspensions, the more frequent collisions between nanocube clusters lead to faster aggregation dynamics but also to smaller terminal steady-state mean cluster sizes. The results from the MD and KMC simulations are in excellent agreement for all investigated two-patch cases, whereas the three-patch cubes form systematically smaller clusters in the MD simulations compared to the KMC calculations due to finite-size effects and slow aggregation kinetics. By analyzing the rate kernels, we are able to identify the primary mechanisms responsible for (shear-induced) cluster growth and breakup. This understanding allows us to tune nanoparticle and process parameters to achieve desired cluster sizes and shapes.

A constitutive model for sheared dense fiber suspensions

We propose a constitutive model to predict the viscosity of fiber suspensions, which undergoes shear thinning, at various volume fractions, aspect ratios, and shear stresses/rates. We calibrate the model using the data from direct numerical simulation and prove the accuracy by predicting experimental measurements from the literature. We use a friction coefficient decreasing with the normal load between the fibers to quantitatively reproduce the experimentally observed shear thinning in fiber suspensions. In this model, the effective normal contact force, which is directly proportional to the bulk shear stress, determines the effective friction coefficient. A rise in the shear stress reduces the effective friction coefficient in the suspension. As a result, the jamming volume fraction increases with the shear stress, resulting in a shear thinning in the suspension viscosity. Moreover, we extend the model to quantify the effects of fiber volume fraction and aspect ratio in the suspension. We calibrate this model using the data from numerical simulations for the rate-controlled shear flow. Once calibrated, we show that the model can be used to predict the relative viscosity for different volume fractions, shear stresses, and aspect ratios. The model predictions are in excellent agreement with the available experimental measurements from the literature. The findings of this study can potentially be used to tune the fiber size and volume fraction for designing the suspension rheology in various applications.

Motility-induced shear thickening in dense colloidal suspensions.

Phase transitions and collective dynamics of active colloidal suspensions are fascinating topics in soft matter physics, particularly for out-of-equilibrium systems, which can lead to rich rheological behaviours in the presence of steady shear flow. Here the role of self-propulsion in the rheological response of a dense colloidal suspension is investigated by using particle-resolved Brownian dynamics simulations. First, the combined effect of activity and shear in the solid on the disordering transition of the suspension is analyzed. While both self-propulsion and shear destroy order and melt the system if critical values are exceeded, self-propulsion largely lowers the stress barrier needed to be overcome during the transition. We further explore the rheological response of the active sheared system once a steady state is reached. While passive suspensions show a solid-like behaviour, turning on particle motility fluidises the system. At low self-propulsion, the active suspension behaves in the steady state as a shear-thinning fluid. Increasing the self-propulsion changes the behaviour of the liquid from shear-thinning to shear-thickening. We attribute this to clustering in the sheared suspensions induced by motility. This new phenomenon of motility-induced shear thickening (MIST) can be used to tailor the rheological response of colloidal suspensions.

High-speed videomicroscopy of sheared carbonyl iron suspensions

The postyield rheological regime is investigated in sheared magnetic field-responsive composites (i.e. carbonyl iron based magnetorheological fluids). When subjected to uniaxial DC fields, high-speed videomicroscopy techniques and dedicated image analysis tools demonstrate that dispersed magnetic microparticles self-assemble to form concentric layered patterns above a particular shear rate (). This critical shear rate for layer formation is dictated by a critical Mason number 1 that is associated to the destruction of the last doublet in the chain-like aggregates. The number of layers, mean width, percentage of occupation and mean period are found to be very weakly dependent on the shear rate in start-up shearing flow tests. Experimental data for the mean period are in good agreement with an energy minimization theory.

Order and density fluctuations near the boundary in sheared dense suspensions

We introduce a novel approach to reveal ordering fluctuations in sheared dense suspensions, using line scanning in a combined rheometer and laser scanning confocal microscope. We validate the technique with a moderately dense suspension, observing modest shear-induced ordering and a nearly linear flow profile. At high concentration (ϕ = 0.55) and applied stress just below shear thickening, we report ordering fluctuations with high temporal resolution, and directly measure a decrease in order with distance from the suspension’s bottom boundary as well as a direct correlation between order and particle concentration. Higher applied stress produces shear thickening with large fluctuations in boundary stress which we find are accompanied by dramatic fluctuations in suspension flow speeds. The peak flow rates are independent of distance from the suspension boundary, indicating that they likely arise from transient jamming that creates solid-like aggregates of particles moving together, but only briefly because the high speed fluctuations are interspersed with regions flowing much more slowly, suggesting that shear thickening suspensions possess complex internal structural dynamics, even in relatively simple geometries.

Transient flows and migration in granular suspensions: key role of Reynolds-like dilatancy

We investigate the transient dynamics of a sheared suspension of neutrally buoyant particles in pressure-imposed rheology configuration, subject to a sudden change in shear rate or external pressure. Discrete element method simulations show that, depending on the flow parameters (particle and system size, initial volume fraction), the early stress response of the suspension may differ strongly from the prediction of the suspension balance model based on the steady-state rheology. We show that a two-phase model incorporating the Reynolds-like dilatancy law of Pailha & Pouliquen (J. Fluid Mech., vol. 633, 2009, pp. 115–135), which prescribes the dilation rate of the suspension over a strain scale $\gamma _0$ , captures quantitatively the suspension dilation/compaction over the whole range of parameters investigated. Together with the Darcy flow induced by the pore pressure gradient during dilation or compaction, this Reynolds-like dilatancy implies that the early stress response of the suspension is non-local, with a non-local length scale $\ell$ that scales with the particle size and diverges algebraically at jamming. In regions affected by $\ell$ , the stress level is fixed, not by the steady-state rheology, but by the Darcy fluid pressure gradient resulting from the dilation/compaction rate. Our results extend the validity of the Reynolds-like dilatancy flow rule, initially proposed for jammed suspensions, to flowing suspension below the critical volume fraction at which the suspension jams, thereby providing a unified framework to describe dilation and shear-induced migration. They pave the way for understanding more complex unsteady flows of dense suspensions, such as impacts, transient avalanches or the impulsive response of shear-thickening suspensions.

Stress overshoot, hysteresis, and the Bauschinger effect in sheared dense colloidal suspensions.

The mechanical nonlinear response of dense Brownian suspensions of polymer gel particles is studied experimentally and by means of numerical simulations. It is shown that the response to the application of a constant shear rate depends on the previous history of the suspension. When the flow starts from a suspension at rest, it exhibits an elastic response followed by a stress overshoot and then a plastic flow regime. Conversely, after flow reversal, the stress overshoot does not occur, and the apparent elastic modulus is reduced while numerical simulations reveal that the anisotropy of the local microstructure is delayed relative to the macroscopic stress.

Ordered domains in sheared dense suspensions: The link to viscosity and the disruptive effect of friction

Monodisperse suspensions of Brownian colloidal spheres crystallize at high densities, and ordering under shear has been observed at densities below the crystallization threshold. We perform large-scale simulations of a model suspension containing over 105 particles to quantitatively study the ordering under shear and to investigate its link to the rheological properties of the suspension. We find that at high rates, for Pe>1, the shear flow induces an ordering transition that significantly decreases the measured viscosity. This ordering is analyzed in terms of the development of layering and planar order, and we determine that particles are packed into hexagonal crystal layers (with numerous defects) that slide past each other. By computing local ψ6 and ψ4 order parameters, we determine that the defects correspond to chains of particles in a squarelike lattice. We compute the individual particle contributions to the stress tensor and discover that the largest contributors to the shear stress are primarily located in these lower density, defect regions. The defect structure enables the formation of compressed chains of particles to resist the shear, but these chains are transient and short-lived. The inclusion of a contact friction force allows the stress-bearing structures to grow into a system-spanning network, thereby disrupting the order and drastically increasing the suspension viscosity.

Dynamics of particle-laden turbulent Couette flow: Turbulence modulation by inertial particles

In particle-laden turbulent flows, it is established that the turbulence in the carrier fluid phase gets affected by the dispersed particle phase for volume fractions above 10−4. Hence, reverse coupling or two-way coupling becomes relevant in that volume fraction regime. Due to their greater inertia, larger particles change either the mean flow or the intensity of fluid-phase fluctuations. In a recent study [Muramulla et al., “Disruption of turbulence due to particle loading in a dilute gas–particle suspension,” J. Fluid Mech. 889, A28 (2020)], a discontinuous decrease of turbulence intensity is observed in a vertical particle-laden turbulent channel flow for a critical volume fraction O(10−3) for particles with varying Stokes numbers (St) in the range of 1−420 based on the fluid-integral time scales. The collapse of turbulent intensity is found out to be the result of a “catastrophic reduction of turbulent energy production rate.” Mechanistically, a turbulent Couette flow differs from a pressure-driven channel flow in many ways, such as fluid-phase mean-velocity profile and turbulent coherent structures. In the particle-laden Couette flow, particles are treated as neutrally buoyant. Therefore, it is worth investigating the mechanism of turbulence modulation by inertial particles in the particle-laden turbulent Couette flow. In this article, the turbulence modulation in the fluid phase in the presence of inertial particles is investigated using two-way coupled direct numerical simulations of a particle-laden sheared turbulent suspension. The particle volume fraction (ϕ) is varied from 1.75×10−4 to 1.05×10−3 and the Reynolds number based on the half-channel width (δ) and the wall velocity (U) (Reδ) is 750. The particles are of high inertia with St∼367 based on a fluid integral timescale represented by δ/U. A discontinuous decrease in turbulence intensity and Reynolds stress is observed beyond a critical volume fraction ϕcr∼7.875×10−4. The drastic reduction of shear production of turbulence leads to the collapse of fluid-phase turbulence. The stepwise particle injection and stepwise removal study confirm the role of critical volume loading in the discontinuous transition. Additionally, the effect of the nature of particle–particle and particle–wall collisions has been investigated. It is observed that the inelastic collisions increase the ϕcr marginally although the nature of turbulence modulation remains similar. The explicit role of the inter-particle collisions has also been investigated by switching off the particle–particle collisions. In this case, ϕcr increases more than in the case of an inelastic collision. The turbulence modulation carries the signatures of transition from sheared turbulence to particle-driven fluid fluctuation at higher volume loading.