We perform direct numerical simulations of elastoviscoplastic (EVP) duct flows at particle volume fractions up to φ=15%. Unlike Newtonian suspensions, which exhibit pronounced drag increase with particle loading, EVP suspensions show only modest drag growth in dilute and semi-dilute conditions and achieve significant drag reduction relative to their Newtonian counterparts beyond a threshold φ that increases with the Bingham number. This behaviour results from two coupled mechanisms: viscoelasticity drives particles away from the walls towards the duct core, and the unyielded plug traps them with negligible slip, thereby minimizing their stress contribution. As a consequence, the mean velocity profile remains largely independent of solid volume fraction, with viscous and elastic stresses nearly unchanged. In addition, we observe pronounced shear-thinning in viscoelastic and EVP suspensions, in contrast to earlier predictions. These findings demonstrate that accurate drag prediction requires explicit modelling of the local solid fraction in EVP particle-laden flows.

Elastoviscoplastic (EVP) fluids, characterized by the coexistence of elastic, viscous, and yield-stress properties, play a central role in diverse applications, including drug delivery, 3D printing, and hydraulic fracturing. These fluids often transport non-spherical particles whose migration dynamics strongly influence flow behaviour.In this work, we employ interface-resolved direct numerical simulations to investigate the migration and orientation dynamics of finite-size spheroidal particles suspended in EVP duct flows across a wide range of governing parameters. Our results show that the equilibrium position and orientation of the particles are influenced significantly by both their aspect ratio and the carrier fluid rheology. In Saramito fluids, spheroidal particles migrate towards the duct centre and align along the duct diagonals in the presence of inertia. At sufficiently high elasticity, they penetrate the central plug and reach the duct core, irrespective of their initial position or shape. At lower elasticities, where larger plug regions persist, interactions with the plug alter the angular dynamics of the particles, leading to unsteady, quasi-periodic tumbling and spinning motions.In contrast, in Saramito-Giesekus fluids, the interplay between inertial forces, shear-thinning plastic viscosity and yield stress drives particles towards the duct corners, aligning them perpendicular to the duct diagonals. In semi-dilute suspensions, flattened particles maintain a greater distance from the walls, whereas their spherical counterparts tend to cluster directly at the corners. These findings reveal complex migration and orientation behaviours unique to EVP media and suggest new opportunities for geometry-based particle separation in microfluidics applications.

The transport of particles in elastoviscoplastic (EVP) fluids is of significant interest across various industrial and scientific domains. However, the physical mechanisms underlying the various particle distribution patterns observed in experimental studies remain inadequately understood in the current literature. To bridge this gap, we perform interface-resolved direct numerical simulations to study the collective dynamics of spherical particles suspended in a pressure-driven EVP duct flow. In particular, we investigate the effects of solid volume fraction, yield stress, inertia, elasticity, shear-Thinning viscosity, and secondary flows on particle migration and formation of plug regions in the suspending fluid. Various cross-streamline migration patterns are observed depending on the rheological parameters of the carrier fluid. In EVP fluids with constant plastic viscosity, particles aggregate into a large cluster at the duct centre. Conversely, EVP fluids with shear-Thinning plastic viscosity induce particle migration towards the duct walls, leading to formation of particle trains at the corners. Notably, we observe significant secondary flows (compared to the mean velocity) in shear-Thinning EVP suspensions, arising from the interplay of elasticity, shear-Thinning viscosity and particle presence, which further enhances corner-ward particle migration. We elucidate the physical mechanism by which yield stress augments the first normal stress difference, thereby significantly amplifying elastic effects. Furthermore, through a comprehensive analysis of various EVP suspensions, we identify critical thresholds for elasticity and yield stress necessary to achieve particle focusing at the duct corners.

We present interface-resolved direct numerical simulations of turbulent channel flow of elastoviscoplastic (EVP) fluids laden with viscous droplets. The simulations are conducted at Re_bulk = 2800 for differnet Weissenberg numbers (Wi) in viscoelastic FENE-P fluids and different Bingham numbers (Bn) in elastoviscoplastic Saramito FENE-P fluids, in order to examine the influence of elasticity and yield stress of the carrier fluid on multiphase turbulent flow dynamics. The drag-reducing effects associated with fluid elasticity observed in classical viscoelastic turbulence, as well as in prior studies of single-phase elastoviscoplastic channel flows, are found to persist in the present study. Introducing viscous droplets into the carrier fluid does not produce significant variations in overall drag relative to the single-phase cases, with a clear absence of near-wall droplet layers as the droplets preferentially migrate towards the channel centre. The normal stress gradient generated by the mean flow drives this centre-ward migration, leading to a depletion of droplets in the near-wall region. Both elasticity and yield stress are found to have a strong influence on the droplet size distribution. Increasing either Wi or Bn tends to promote the formation of larger droplets by suppressing breakup through the stabilisation of interfacial instabilities.

Conventional immersed boundary methods (IBM) enforce no-slip and no-penetration conditions but introduce a sharp velocity transition across the fluid–solid interface, yielding non-smooth velocity fields and poor stress convergence, especially in viscoelastic flows where accurate velocity gradients are essential. In this work, we employ a multidimensional extrapolation technique, where twoadditional partial differential equations are solved to smoothly extend the fluidvelocity into the solid region. This approach ensures that velocity discontinuitiesare eliminated. We use a volume penalization IBM to model static boundaries.The advantage of volume penalization, besides its simplicity, is that no actual flow occurs inside the solid region, and introducing fictitious velocities into the solids does not violate the divergence-free condition of the velocity field.The method is validated for Newtonian and viscoelastic benchmark problems, including pipe flow, Taylor--Couette flow, and flow past a cylinder. In allcases, we observed smooth and accurate velocity fields, as well as convergent viscoelastic stress at the boundaries.

The combination of flow elasticity and inertia has emerged as a viable tool for focusing and manipulating particles using microfluidics. Although there is considerable interest in the field of elasto-inertial microfluidics owing to its potential applications, research on particle focusing has been mostly limited to low Reynolds numbers (Re<1), and particle migration toward equilibrium positions has not been extensively examined. In this work, we thoroughly studied particle focusing on the dynamic range of flow rates and particle migration using straight microchannels with a single inlet high aspect ratio. We initially explored several parameters that had an impact on particle focusing, such as the particle size, channel dimensions, concentration of viscoelastic fluid, and flow rate. Our experimental work covered a wide range of dimensionless numbers (0.05 < Reynolds number < 85, 1.5 < Weissenberg number < 3800, 5 < Elasticity number < 470) using 3, 5, 7, and 10 µm particles. Our results showed that the particle size played a dominant role, and by tuning the parameters, particle focusing could be achieved at Reynolds numbers ranging from 0.2 (1 µL/min) to 85 (250 µL/min). Furthermore, we numerically and experimentally studied particle migration and reported differential particle migration for high-resolution separations of 5 µm, 7 µm and 10 µm particles in a sheathless flow at a throughput of 150 µL/min. Our work elucidates the complex particle transport in elasto-inertial flows and has great potential for the development of high-throughput and high-resolution particle separation for biomedical and environmental applications. (Figure presented.)

Nanoscale biological particles, such as lipoproteins (10–80 nm) or extracellular vesicles (30–200 nm), play pivotal roles in health and disease, including conditions like cardiovascular disorders and cancer. Their effective analysis is crucial for applications in diagnostics, quality control, and nanomedicine development. While elasto-inertial focusing offers a powerful method to manipulate particles without external fields, achieving consistent focusing of nanoparticles (<500 nm) has remained a challenge. In this study, elasto-inertial focusing of nanoparticles as small as 25 nm is experimentally demonstrated using straight high-aspect-ratio microchannels in a sheathless flow. Systematic investigations reveal the influence of channel width, particle size, viscoelastic concentration, and flow rate on focusing behavior. Additionally, through numerical simulations and experimental validation, insights are provided into particle migration dynamics and viscoelastic forces governing nanoparticle focusing. Finally, biological particles, including liposomes (90–140 nm), extracellular vesicles (100 nm), and lipoproteins (10–25 nm) is successfully focused, under optimized conditions, showcasing potential applications in medical diagnostics and targeted drug delivery. These findings mark a significant advancement toward size-based high-resolution particle separation, with implications for biomedicine and environmental sciences.