non-Brownian Particles Research Articles

Discrepancies in dynamic yield stress measurements of cement pastes

The dynamic yield stress associated with the flow cessation of cement pastes is measured using a rheometer equipped with various shear geometries such as vane, helical, sandblasted co-axial cylinders, and serrated parallel plates, as well as with the mini-cone spread test. Discrepancies in yield stress values are observed for cement pastes at various volume fractions, with one to two orders of magnitude difference between vane, helical and mini-cone spread measurements on the one hand, and co-axial cylinder and parallel plate measurements on the other hand. To understand this discrepancy, the flow profile of a cement paste in the parallel-plate geometry is investigated with a high-speed camera, revealing the rapid formation of an un-sheared band near the static bottom plate. The width of this band depends upon the rotational velocity of the top plate, and upon the shear time. Recalculation of shear stress shows that the reduced sheared gap alone cannot explain the low measured yield stress. Further exploration suggests the formation of zones with lower particle content, possibly linked to cement particle sedimentation. Here, we argue that the complex nature of cement pastes, composed of negatively buoyant non-Brownian particles with attractive interactions due to highly charged nano-size hydration products, accounts for their complex rheological behavior.Graphical abstract

Scaling variation in the pinch-off of colloid-polymer mixtures

Hypothesis: The scaling laws of drop pinch-off are known to be affected by drop compositions including dissolved polymers and non-Brownian particles. When the size of the particles is comparable to the characteristic length scale of the polymer network, these particles may interact strongly with the polymer environment, leading to new types of scaling behaviors not reported before.Experiments: Using high-speed imaging, we experimentally studied the time evolution of the neck diameter hmin of drops composed of silica nanoparticles dispersed in PEO solution when extruded from a nozzle.Findings: After initial Newtonian necking with hmin ∼ t2/3, the subsequent stage may exhibit scaling variation, characterized by either exponential or power-law decay, depending on the nanoparticle volume fraction ϕ. The exponential decay hmin ∼ e-t/τ signifies the coil-stretch transition in typical viscoelastic suspensions. We conducted an analysis of the power-law scenario hmin ∼ tα at high ϕ, categorizing the entire process into three distinct regimes based on the exponents α. The dependences of critical thicknesses at transition points and exponents on polymer concentration offer initial insights into the potential transition from heterogeneous to homogeneous thinning in the mixture. This novel scaling variation bears implications for accurately predicting and controlling droplet fragmentation in industrial applications.

On the use of sinusoidal vibrations for disaggregating clusters of non-settling inertial particles immersed in yield-stress fluids

The effect of sinusoidal vibration is numerically investigated on the dynamical behavior of a cluster of 50 identical non-Brownian circular solid particles randomly distributed in a circular envelope. The cluster is immersed in a finite vessel filled with an inelastic viscoplastic fluid obeying the Casson model. The solid particles are modeled using the improved smoothed-profile method (iSPM) whereas the flow of the continuous phase is modeled using the lattice Boltzmann method (LBM). An in-house LBM-iSPM code, modified for Casson fluid, is used to study the effect of sinusoidal vibration on disaggregating the cluster. We have deliberately ignored the gravitational term in the equations of motion so that the sole effect of vibration on the cluster response can better be investigated. Numerical results suggest that vibration can disaggregate the cluster with its efficiency depending on the fluid's yield stress. For any given yield stress, vibration can disperse the cluster provided the frequency and/or amplitude of the forced oscillation are larger than a threshold. The secondary flow formed in the channel during the transient phase is shown to be the main cause of the cluster's fluid-mediated dispersion. It is shown that the system reaches equilibrium when the secondary flow is vanished through dissipation. Vibration is predicted to become more effective in disaggregating clusters the larger the size of the particles or the smaller their number density.

Permanent fluidic magnets for liquid bioelectronics.

Brownian motion allows microscopically dispersed nanoparticles to be stable in ferrofluids, as well as causes magnetization relaxation and prohibits permanent magnetism. Here we decoupled the particle Brownian motion from colloidal stability to achieve a permanent fluidic magnet with high magnetization, flowability and reconfigurability. The key to create such permanent fluidic magnets is to maintain a stable magnetic colloidal fluid by using non-Brownian magnetic particles to self-assemble a three-dimensional oriented and ramified magnetic network structure in the carrier fluid. This structure has high coercivity and permanent magnetization, with long-term magnetization stability. We establish a scaling theory model to decipher the permanent fluid magnet formation criteria and formulate a general assembly guideline. Further, we develop injectable and retrievable permanent-fluidic-magnet-based liquid bioelectronics for highly sensitive, self-powered wireless cardiovascular monitoring. Overall, our findings highlight the potential of permanent fluidic magnets as an ultrasoft material for liquid devices and systems, from bioelectronics to robotics.

Shear thickening and hysteresis in dense suspensions: The effect of particle shape

The flow of dense suspension of non-Brownian particles has been considered by various studies affected by their significance in a variety of industries and natural phenomena. In this study, we investigate the effect of polyhedron morphology on shear thickening, shear jamming, and hysteresis characteristics of non-Brownian suspension of acrylate particles. Particles with the same chemical nature and three different shapes of spherical (aspect ratio Γ = 1), elliptical paraboloid (Γ ≈ 1), and boat-shaped (Γ ≈ 3) are fabricated via photopolymerization-based methods. Studied suspensions show the shear-thinning behavior at low shear stresses and shear thickening behavior at the higher range of shear stress. Also, the strength of observed shear thickening is enhanced for the suspensions of polyhedron particles, which can be attributed to the heightened degree of interparticle frictional contacts. Furthermore, it is found that angularity not only shifts the predicted frictionless and frictional jamming packing fractions to lower values but also expands the shear jamming packing fraction range. Finally, a history-dependent hysteresis is observed in all samples due to the different particle spatial structures forming in ascending and descending flow modes. The observed hysteresis loops strongly depend on the volume fraction and diminish near the jamming packing fraction due to the restricted mobility space of particles. In addition, the tumbling of elongated particles also can decrease the hysteresis loop by enhancing viscosity in the ascending flow mode, where the structures are not fully developed.

Dense turbulent suspensions at a liquid interface

The nexus between turbulence, particle interaction and interfacial tension is virtually unexplored, despite being highly relevant to a wealth of industrial and environmental settings. Here we investigate it by conducting experiments on non-Brownian spherical particles at the interface of turbulent liquid layers. The latter are electromagnetically stirred in a quasi-two-dimensional apparatus, while the particles are individually tracked. By systematically varying interfacial conditions, turbulence intensity, particle size and concentration from dilute to dense, we map the system behaviour over a wide parameter space. We reveal how the dynamics is governed by the balance of drag, capillarity and lubrication. Based on their scaling, we propose a phase diagram comprising three distinct regimes, characterized by widely different levels of clustering and fluctuating energy of the particles. This is quantitatively confirmed by the experimental results.

Granular aqueous suspensions with controlled interparticular friction and adhesion.

We present a simple route to obtain large quantities of suspensions of non-Brownian particles with stimuli-responsive surface properties to study the relation between their flow and interparticle interactions. We perform an alkaline hydrolysis reaction on poly(methyl methacrylate) (PMMA) particles to obtain poly(sodium methacrylate) (PMAA-Na) particles. We characterize the quasi-static macroscopic frictional response of their aqueous suspensions using a rotating drum. The suspensions are frictionless when the particles are dispersed in pure water. We relate this state to the presence of electrosteric repulsion between the charged surfaces of the ionized PMAA-Na particles in water. Then we add monovalent and multivalent ions (Na+, Ca2+, La3+) and we observe that the suspensions become frictional whatever the valency. For divalent and trivalent ions, the quasi-static avalanche angle θc at large ionic strength is greater than that of frictional PMMA particles in water, suggesting the presence of adhesion. Finally, a decrease in the pH of the suspending solution leads to a transition between a frictionless plateau and a frictional one. We perform atomic force microscopy (AFM) to relate our macroscopic observations to the surface features of the particles. In particular, we show that the increase in friction in the presence of multivalent ions or under acidic conditions is driven by a nanoscopic phase separation and the bundling of polyelectrolyte chains at the surface of the particle. Our results highlight the importance of surface interactions in the rheology of granular suspensions. Our particles provide a simple, yet flexible platform to study frictional suspension flows.

The role of rolling resistance in the rheology of wizarding quidditch ball suspensions

To elucidate the effect of particle shape on the rheology of a dense, viscous suspension of frictional, non-Brownian particles, experimental measurements are presented for suspensions of polystyrene particles with different shapes in the same solvent. The first suspension is made of spheres whereas the particles which compose the second suspension are globular but with flattened faces. We present results from steady shear and shear-reversal rheological experiments for the two suspensions over a wide range of stresses in the viscous regime. Notably, we show that the rheology of the two suspensions is characterised by a shear-thinning behaviour, which is stronger in the case of the suspension of globular particles. Since the shear-reversal experiments indicate an absence of adhesive particle interactions, we attribute the shear thinning to a sliding friction coefficient which varies with stress as has been observed previously for systems similar to the first suspension. We observe that the viscosity of the two suspensions is similar at high shear stress where small sliding friction facilitates particle relative motion due to sliding. At lower shear stress, however, the sliding friction is expected to increase and the particle relative motion would be associated with rolling. The globular particles attain a higher viscosity at low shear stress than the spherical particles. We attribute this difference to a shape-induced resistance to particle rolling that is enhanced by the flattened faces. Image analysis is employed to identify features of the particle geometry that contribute to the resistance to rolling. It is shown that the apparent rolling friction coefficients inferred from the rheology are intermediate between the apparent dynamic and static rolling friction coefficients predicted on the basis of the image analysis. All three rolling resistance estimates are larger for the globular particles with flat faces than for the spherical particles and we argue that this difference yields the stronger shear thinning of the globular particle suspension.

Deposition and alignment of fiber suspensions by dip coating

Hypothesis: The dip coating of suspensions made of monodisperse non-Brownian spherical particles dispersed in a Newtonian fluid leads to different coating regimes depending on the ratio of the particle diameter to the thickness of the film entrained on the substrate. In particular, dilute particles dispersed in the liquid are entrained only above a threshold value of film thickness. In the case of anisotropic particles, in particular fibers, the smallest characteristic dimension will control the entrainment of the particle. Furthermore, it is possible to control the orientation of the anisotropic particles depending on the substrate geometry. In the thick film regime, the Landau-Levich-Derjaguin model remains valid if one account for the change in viscosity.Experiment: To test the hypotheses, we performed dip-coating experiments with dilute suspensions of non-Brownian fibers with different length-to-diameter aspect ratios. We characterize the number of fibers entrained on the surface of the substrate as a function of the withdrawal velocity, allowing us to estimate a threshold capillary number below which all the particles remain in the liquid bath. Besides, we measure the angular distribution of the entrained fibers for two different substrate geometries: flat plates and cylindrical rods. We then measure the film thickness for more concentrated fiber suspensions.Findings: The entrainment of the fibers on a flat plate and a cylindrical rod is primarily controlled by the smaller characteristic length of the fibers: their diameter. At first order, the entrainment threshold scales similarly to that of spherical particles. The length of the fibers only appears to have a minor influence on the entrainment threshold. No preferential alignment is observed for non-Brownian fibers on a flat plate, except for very thin films, whereas the fibers tend to align themselves along the axis of a cylindrical rod for a large enough ratio of the fiber length to the radius of the cylindrical rod. The Landau-Levich-Derjaguin law is recovered for more concentrated suspension by introducing an effective capillary number accounting for the change in viscosity.

Rheology of non-Brownian particle suspensions in viscoelastic solutions. Part 1: Effect of the polymer concentration

We study the effect of varying polymer concentration, measured by the dimensionless polymer viscosity partition function β, on the steady shear rheology of rigid particle suspensions using direct numerical simulation of the Oldroyd-B model. We compare the bulk rheology using immersed boundary simulations at Φ=2.5% and 5% to body-fitted single-particle simulations and find that the per-particle viscosity and first normal stress difference coefficient are always shear-thickening at all values of β considered. However, as β decreases, the polymer stress transforms the flow field near each particle from closed concentric streamlines to helical streamlines that advect stretched polymers away from the particle surface. At low β, the polymer stress is diffuse, where the distribution of the particle induced fluid stress (PIFS) caused by the stretched polymers is spread out in the simulation domain rather than concentrated near the particle surface. Therefore in multiparticle simulations, the polymer stress can be significantly affected by particle-particle interactions. The stress generated by a given particle is disrupted by the presence of particles in its vicinity, leading to a significantly lower PIFS than that of the single-particle simulation. In addition, at increased volume fractions and low values of β, the polymer stress distribution on the particle surface shifts so as to increase the magnitude of the polymer stress moments, resulting in a shear-thickening stresslet contribution to the viscosity that is not seen in single particle or high β simulations. This result indicates that for suspensions in highly viscoelastic suspending fluids that are characterized by a low β parameter, hydrodynamic interactions are significant even at modest particle concentrations and fully resolved multiparticle simulations are necessary to understand the rheological behavior.

Rheology of non-Brownian particle suspensions in viscoelastic solutions. Part II: Effect of a shear thinning suspending fluid

The shear rheology of particle suspensions in shear-thinning polymeric fluids is studied experimentally using parallel plate measurements and numerically using fully resolved, 3D finite volume simulations with the Giesekus fluid model. We show in our experiments that the steady shear viscosity and first normal stress difference coefficient of the suspension evolve from shear-thickening to substantially shear-thinning as the degree of shear-thinning of the suspending fluid increases. Moreover, in highly shear-thinning fluids, the suspension exhibits greater shear-thinning of the viscosity than the suspending fluid itself. Our dilute body-fitted simulations show that in the absence of hydrodynamic interactions, shear-thinning can arise from the particle-induced fluid stress (PIFS), which ceases to grow with increasing shear rate at low values of β (solvent viscosity ratio) and finite values of α (the Giesekus drag coefficient). In a Giesekus suspending fluid, the polymers surrounding the suspended particle are unable to stretch sufficiently at high Weissenberg numbers (Wi) and the reduced polymer stress results in a lower PIFS. When coupled with the shear-thinning stresslet, this effect creates an overall shear-thinning of the viscosity. We then explore the effects of particle-particle interactions on the suspension rheology using immersed boundary simulations. We show that multiparticle simulations are necessary to obtain the shear-thinning behavior of the per-particle viscosity of suspensions in shear-thinning fluids at moderate values of β. Particle-particle interactions lead to a substantial decrease in the PIFS and an enhancement of the shear-thinning of the stresslet compared to the single particle simulations. This combination leads to the shear-thinning of the per-particle viscosity seen in experiments. We also find that very low values of β and finite values of α have opposing effects on the per-particle viscosity that can lead to a nonmonotonic per-particle viscosity versus shear rate in a highly shear-thinning fluid. Overall, the addition of rigid particles to highly shear-thinning fluids, such as joint synovial fluid, leads to increased viscosity and also increased shear-thinning at high shear rates.

Particle Rafts and Armored Droplets

Particles floating at interfaces are commonly observed in nature, as well as in industrial processes. When the particles are non-Brownian particles, large deformations of the interface are created that induce long-ranged capillary interactions and lead to the formation of particle rafts with unique characteristics. In this review we discuss recent efforts in investigating particle raft formation and the role of the rafts’ own weight during dynamic clustering. Under specific conditions, these rafts can ultimately collapse and sink. When subjected to external or internal forces, the raft undergoes large deformations that test the mechanical characteristics of this interfacial composite material. It can behave as a continuous elastic sheet under compression, although its discrete nature can also trigger its fragmentation via interparticle interactions. Finally, armored droplets, drops covered by a protective shell of particles, can lose their integrity when submitted to dynamic deformations, resulting in the ejection of particles or the fracturing of the armor. Open questions to understand the properties of this material are highlighted and future research to understand the fundamental physics of particle rafts, the customization of the cluster formation, or the disassembly of this collective material is suggested.

Effect of molecular weight on polymer solution facilitated transfer of non-Brownian particles

Solution driven particle transfer on solid substrates has many industrial applications but can be difficult to control and characterize. In this study, poly(methyl methacrylate) (PMMA) is considered as the binder polymer, and the effect of the molecular weight (MW) on particle transfer through entrainment is investigated. The transfer phenomenon of polydisperse spherical rigid micro-particles from the PMMA solution has been analyzed and two hypotheses are tested here: (i) there exists a threshold polymer content below which only the smallest particles will be transferred from the bulk; (ii) at the same viscosity of different molecular weight PMMA solution, the solid transfer phenomenon will be similar. By changing the mass and viscosity using three PMMA samples with different MWs in the 1,3-dioxolane solvent, we observe low, intermediate, and high viscous regimes of the PMMA-dioxolane solution system. The low, intermediate, and high viscous regimes can be controlled with the MW and can entrain distinct particle size distributions onto surfaces. This behavior is thoroughly analyzed and relates to the film formation. These results confirmed our first hypothesis that we could use polymer content to control the particle size distribution and that a threshold polymer content exists. However, at similar viscosities, higher molecular weight polymers yielded lower average particle sizes and narrower distributions, which is counter to our second hypothesis. These findings can provide a better understanding of the binder behavior and its influence on the particle transfer process which opens-up the possibility of particle sorting process from a polydisperse suspension mixture and high-yield material transfer for porous structure coating and joining processes. Understanding the particle transfer process can also enable reverse engineering of the liquid carrier system in a complex mixture, which can be beneficial for industrial applications.

Clustering and collision of Brownian particles in homogeneous and isotropic turbulence

The collision rate between primary nanoparticles in the turbulent flow is of significance for accurately predicting the growth rate of agglomerates during the flame synthesis process. In this work, the clustering and the collision of Brownian particles in the free-molecular regime in homogeneous isotropic turbulence are investigated using the direct numerical simulation and the Langevin dynamics. It is found that the Brownian motion can distribute the particles more uniformly, leading to the formation of a plateau on the curve of the radial distribution function (RDF). This anti-clustering effect is significant only at a small separation distance of particle pairs and cannot be observed when the separation is larger than a critical value. The radial relative velocity (RRV) is significantly enhanced, especially at the small separation distance, when the Brownian motion is taken into account. A velocity superposition analysis shows that the statistics of RRV of Brownian particles can be obtained by adding a random variable onto the RRV of non-Brownian particles with the same Stokes number. The increased radial relative velocity counteracts the anti-clustering effect of the Brownian motion and leads to the increase of the geometric collision kernel. Finally, the collision kernel of Brownian particles is formulated as a function of asymptotic values of RDF and RRV at a vanishing separation. The proposed collision kernel enables us to estimate the collision rate between nanoparticles that are much smaller than the Kolmogorov length scale.

Turbulent channel flow of suspensions of neutrally buoyant particles over porous media

This study discusses turbulent suspension flows of non-Brownian, non-colloidal, neutrally buoyant and rigid spherical particles in a Newtonian fluid over porous media with particles too large to penetrate and move through the porous layer. We consider suspension flows with the solid volume fraction ${{\varPhi _b}}$ ranging from 0 to 0.2, and different wall permeabilities, while porosity is constant at 0.6. Direct numerical simulations with an immersed boundary method are employed to resolve the particles and flow phase, with the volume-averaged Navier–Stokes equations modelling the flow within the porous layer. The results show that in the presence of particles in the free-flow region, the mean velocity and the concentration profiles are altered with increasing porous layer permeability because of the variations in the slip velocity and wall-normal fluctuations at the suspension-porous interface. Furthermore, we show that variations in the stress condition at the interface significantly affect the particle near-wall dynamics and migration toward the channel core, thereby inducing large modulations of the overall flow drag. At the highest volume fraction investigated here, ${{\varPhi _b}}= 0.2$ , the velocity fluctuations and the Reynolds shear stress are found to decrease, and the overall drag increases due to the increase in the particle-induced stresses.

Migration of non-Brownian particles localized inside sheared droplets

• Particles inside sheared droplets migrate along the vorticity direction. • The migration direction depends on the viscosity ratio. • The migration kinetics depends on Ca, interfacial tension and particle concentration. • VOF simulations provide velocity and shear rate distribution in a particle-free droplet. • Shear-induced migration plays a major role. The migration of non-Brownian particles inside a droplet subjected to shear flow is investigated. The viscosity ratio, particle concentration, particle to droplet size ratio and capillary number are systematically varied. It was observed that particles migrate in the vorticity direction either towards or away from the equatorial plane. The main factor determining the migration direction is the viscosity ratio. The particle concentration and applied capillary number have no influence on the equilibrium distribution and only affect the time necessary to reach this distribution. In order to identify the flow profile, both velocity and shear rate magnitudes to which the particles inside the droplet are subjected, are numerically simulated using the Volume-of-Fluid method in a particle-free droplet/matrix system. The numerical predictions of the flow are used to evaluate the presence of possible driving mechanisms for cross-streamline particle motion, such as particle inertia and shear rate gradients. Depending on the conditions, the simulations reveal significant differences in the streamline profiles and shear rate gradients within the droplet. The observed steady state particle distributions can partially be explained by shear-induced migration towards zones of low shear rates and/or low streamline curvature. However, in complex flows, such as inside a deformed droplet, different migration mechanisms can occur simultaneously. Hence, confinement inside the droplet and potential secondary flows may also play a role.

Collision efficiency of non-Brownian spheres in a simple shear flow – the role of non-continuum hydrodynamic interactions

We study the collisions in a gaseous medium of a dilute bidisperse suspension of non-Brownian spherical particles sedimenting along the flow axis of a simple shear flow. Continuum lubrication forces prevent particles from coming into contact in a finite time, thus collisions can occur only due to attractive interactions such as the van der Waals force. However, in a low-pressure medium, the lubrication forces are weaker than their continuum counterparts and allow particle pairs to collide, even without any attractive forces. The Knudsen number, defined as the ratio of the mean free path of the medium to the mean radius of the interacting spheres, captures the significance of non-continuum interactions. We use uniformly valid hydrodynamic mobility functions, reflecting non-continuum lubrication at small separations and full continuum hydrodynamic interactions at moderate to larger separations. Due to the nature of the pair trajectory topology, the collision efficiency vanishes at a critical Knudsen number when simple shear flow alone drives the dynamics. Thus we perform collision calculations where particles experience the combined effects of van der Waals attraction and non-continuum hydrodynamics; van der Waals interactions enable collisions below the critical Knudsen number. Next, we calculate the collision efficiency for coupled differential sedimentation and simple shear driven motion in the presence of van der Waals interaction and non-continuum hydrodynamics. Finally, we explore the role of small particle inertia on relative trajectories and collision efficiencies in a non-continuum gas subject to a simple shear flow, ignoring the van der Waals force and gravity.

Slurries of complex fluids: Rheology, microstructure and fluid mechanics

<h2>Abstract</h2> Slurries of complex fluids are suspensions of non-Brownian particles in complex fluids (e.g. shear thinning, shear thickening, yield stress and viscoelastic fluids). These materials exist all around us. Examples include the following: food products, hygiene products, mine tailings, concrete, fracturing fluids, shock absorbing materials, drilling muds and muds in mudslides, lava, and numerous others. As expected from their ubiquity, non-Newtonian slurries contribute significantly to the economy both positively and negatively. Therefore, even small increases in efficiency when processing slurries of complex fluids could make significant positive economic and environmental impacts. Obviously, a thorough understanding of the rheology and fluid mechanics of these materials in natural and industrial settings is essential to improving the efficiency of production. However, this is extremely challenging due to the complex rheology of the suspending fluids, the interaction of fluid and particle phases, and multiple-body and short-range interactions of particles. My presentation will introduce an array of experimental and modelling techniques that my research team uses to investigate rheological properties and fluid dynamical behaviour of the slurries of complex fluids. The goal is to establish a continuum framework and refine it through a series of microstructure investigations. I will discuss how our recent results can be used to address and resolve mixing and pumping problems in concrete industries and to maximizing petroleum reservoir production using hydraulic fracturing processes. Finally, open questions will be disclosed, which must be answered to build a firm foundation for a long-term contribution to the area of complex suspensions.

X-ray computerized tomography observation of Lycopodium paste incorporating memory of shaking.

In a uniform layer consisting of a mixture of granular material and liquid, it is known that desiccation cracks exhibit various anisotropic patterns that depend on the nature of the shaking that the layer experienced before drying. The existence of this effect implies that information regarding the direction of shaking is retained as a kind of memory in the arrangements of granular particles. In this work we make measurements in paste composed of Lycopodium powder using microfocus x-ray computerized tomography (μCT) in order to investigate the three-dimensional arrangements of particles. We find shaking-induced anisotropic arrangements of neighboring particles and density fluctuations forming interstices mainly in the lower part of the layer. We compare the observed properties of these arrangements with numerical results obtained in the study of a model of non-Brownian particles under shear deformation. In the experimental system, we also observe crack tips in the μCT images and confirm that these cracks grow along interstices in the direction perpendicular to the initial shaking.

Elastic reinforcement and yielding of starch-filled lipid gels

Many foods involve complex suspensions of assorted particles in a Newtonian liquid or viscoelastic medium. In this work, we study the case of suspensions of non-Brownian non-interacting rigid particles: starch, embedded in a soft solid: a colloidal lipid gel. We relate the macroscopic properties of the suspensions to the mechanics of the colloidal gel and the particle volume fraction. As particle volume fraction increases, the suspension gradually stiffens and becomes brittle as the system approaches its maximum packing fraction. The latter is independently determined from a geometric theory of random close packing for polydisperse hard spheres based on the log normal distribution of starch particles dispersed in oil. The elastic modulus, yield stress and yield strain are interrelated through simple scaling laws from a micromechanical homogenization analysis of hard spheres isotropically-distributed in yield stress fluids. • The rheology of non-Brownian non-interacting starch particles filling a viscoelastic colloidal lipid gel was investigated. • As starch volume fraction increases the suspension stiffens, turns brittle as it approaches its maximum packing fraction. • The maximum packing fraction is determined from a geometric theory of random close packing for polydisperse hard spheres. • Linear and nonlinear rheology are interrelated through simple scaling laws from a homogenization micromechanical analysis.