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The formation of a line of equally spaced particles at the centerline of a microchannel, referred as "particle ordering," is desired in several microfluidic applications. Recent experiments and simulations highlighted the capability of viscoelastic fluids to form a row of particles characterized by a preferential spacing. When dealing with non-Newtonian fluids in microfluidics, the adherence condition of the liquid at the channel wall may be violated and the liquid can slip over the surface, possibly affecting the ordering efficiency. In this work, we investigate the effect of wall slip on the ordering of particles suspended in a viscoelastic liquid by numerical simulations. The dynamics of a triplet of particles in an infinite cylindrical channel is first addressed by solving the fluid and particle governing equations. The relative velocities computed for the three-particle system are used to predict the dynamics of a train of particles flowing in a long microchannel. The distributions of the interparticle spacing evaluated at different slip coefficients, linear particle concentrations, and distances from the channel inlet show that wall slip slows down the self-assembly mechanism. For strong slipping surfaces, no significant change of the initial microstructure is observed at low particle concentrations, whereas strings of particles in contact form at higher concentrations. The detrimental effect of wall slip on viscoelastic ordering suggests care when designing microdevices, especially in case of hydrophobic surfaces that may enhance the slipping phenomenon.

In this article, we present the mobilities of prolate ellipsoidal micrometric particles close to an air–water interface measured by dual wave reflection interference microscopy. Particle’s position and orientation with respect to the interface are simultaneously measured as a function of time. From the measured mean square displacement, five particle mobilities (3 translational and 2 rotational) and two translational–rotational cross-correlations are extracted. The fluid dynamics governing equations are solved by the finite element method to numerically evaluate the same mobilities, imposing either slip and no-slip boundary conditions to the flow at the air–water interface. The comparison between experiments and simulations reveals an agreement with no-slip boundary conditions prediction for the translation normal to the interface and the out-of-plane rotation, and with slip ones for parallel translations and in-plane rotation. We rationalize these evidences in the framework of surface incompressibility at the interface.

The settling dynamics of a rigid ellipsoid in an unbounded viscoelastic fluid under inertialess conditions is studied through direct numerical simulations. The governing equations are solved by the finite element method with an Arbitrary Lagrangian–Eulerian formulation to handle the particle motion. The viscoelastic fluid is modeled through the Giesekus constitutive equation. Simulations are carried out up to a value of Deborah number of 5. The settling of prolate and oblate spheroidal particles is first addressed. The sedimentation, lift, and angular velocities are computed as a function of the orientation angle and for aspect ratios from 1/8 to 8. Regardless of the particle shape, initial orientation, and Deborah number, the particle rotates to align its longest axis along the force direction. A high extensional stress region behind the particle is observed at high aspect ratios due to the large curvature of the tip, leading to a fast decay of the axial fluid velocity downstream and to the appearance of a negative wake. Similarly, a triaxial ellipsoid reaches a final orientation with major axis parallel to the falling direction. The particle shape affects the orientational dynamics, both in terms of the orbits followed by the orientation vectors and the time needed to reach the equilibrium orientation, and the steady-state settling velocity. The fastest sedimentation rate is observed for a prolate spheroid with aspect ratio of about 2 whereas the slowest one for a high aspect ratio oblate spheroid. Triaxial ellipsoids settle with rates in between these two limiting behaviors.

Strings of equally spaced particles, also called particle trains, have been employed in several applications, including flow cytometry and particle or cell encapsulation. Recently, the formation of particle trains in viscoelastic liquids has been demonstrated. However, only a few studies have focused on the topic, with several questions remaining unanswered. We here perform numerical simulations and experiments to elucidate the effect of the confinement ratio on the self-ordering dynamics of particles suspended in a viscoelastic liquid and flowing on the centerline of a microfluidic channel. For a fixed channel size, the particles self-order on shorter distances as the particle size increases due to the enhanced hydrodynamic interactions. At relatively low linear concentrations, the relative particle velocities scale with the fourth power of the confinement ratio when plotted as a function of the distance between the particle surfaces normalized by the channel diameter. As the linear concentration increases, the average interparticle spacing reduces and the scaling is lost, with an increasing probability to form strings of particles in contact. To reduce the number of aggregates, a microfluidic device made of an array of trapezoidal elements is fabricated and tested. The particle aggregates reduce down to 5% of the overall particle number, significantly enhancing the ordering efficiency. A good agreement between numerical simulations and experiments is found.

We report the design of a microcapillary rheometer (μCR) that allows to perform experiments rapidly and in a broad range of shear rates (i.e., from 0.1 to 1000 s−1), using small amounts of material (i.e., just few milligrams). Additionally, multiple measurement parallelization makes it suitable for High-Throughput Rheological Experimentation of polymer melts (HT-Rheo-E). The novel rheometer consists of a set of three cylindrical microcapillaries in which the fluid flows driven by a controlled pressure. A camera, placed at the capillary exit, records the fluid motion to measure its flow rate, from which the fluid viscosity can be determined. The optimization of the setup allowed for reliable and fast viscosity measurements using ca. 10 mg of material. The current work reports the design of the rheometer and validation measurements on several model fluids. The microfabricated μCR is of potential interest for applications in quality control and research where rapid and repeated measurements using limited milligrams of polymer are required, as well as for High-Throughput-Experimentation of complex fluids (e.g., biological systems).

Powder bed based additive manufacturing technologies allow for the production of more complex parts with overall better characteristics and are therefore of high industrial interest. However, due to an incomplete understanding of the multi-physic processes involved there are still some issues to be addressed. More specifically, in this work we will focus on the powder spreading stage. In powder bed-based technologies a thin layer of powder is deposited by a recoating device and is then processed by an energy source to obtain a layer of the final part. We use a numerical model based on the Discrete Element Method to reproduce the spreading process and propose an experimental analysis procedure to obtain data suitable for the validation of such numerical model. The experiments are carried out on a purposely designed device and both computations and experiment are referred to a single layer deposition. Being able to validate the numerical results can give way to a more confident use of DEM based models for the optimization of the spreading process and therefore improve the quality of printed parts. (Abstract)

The formation of equally-spaced structures (trains) of rigid, spherical particles suspended in a viscoelastic fluid flowing in a cylindrical microchannel is investigated by numerical simulations. Direct Numerical Simulations (DNS) have been employed to accurately compute the translational velocities of a system made by three particles aligned along a cylindrical channel for different interparticle distances. A shear-thinning, elastic fluid, modeled by the Giesekus costitutive equation, is considered. The DNS results are collected in a database used for simulating the dynamics of a multi-particle system. The evolution of the particle microstructure through the channel is presented in terms of interparticle distance distributions. The effects of the Deborah number (defined as the ratio between the fluid and flow characteristic times), the volume fraction, and the initial particle distribution on the train dynamics are investigated.

Here we describe the fabrication of smart biodegradable and biocompatible micro-needles for combining the drug delivery with advanced sensing properties that would be the base for advanced telemedicine tools.

The dynamics of a cell suspended in a Newtonian liquid subjected to a pressure-driven flow at non-negligible inertia in cylindrical and square cross section microfluidic channels is studied through three-dimensional arbitrary Lagrangian–Eulerian finite-element numerical simulations. The cell is modeled through the neo-Hookean hyper-elastic constitutive equation, which can describe biological particles undergoing moderate deformations. The cell-to-channel relative dimension is fixed to 0.2, whereas the Reynolds number Re, measuring the relative importance of liquid inertial and viscous forces, and the elastic capillary number [Formula: see text], measuring the relative importance of liquid viscous stress and solid elastic stress, are varied by several orders of magnitude. In a cylindrical tube, the cell migrates transversally to the flow direction until reaching a radial equilibrium position depending on Re and [Formula: see text]. Given Re, the softer the cell (i.e., the larger [Formula: see text]) the closer its equilibrium position to the tube axis, thus allowing for the separation of healthy and diseased cells which have similar dimensions but different mechanical properties. In a channel with a square cross section, a much more complex dynamics is found. Depending on Re and [Formula: see text], the cell can either migrate to the channel centerline, to the closest median of the channel cross section (thus, four equilibrium positions can be identified due to symmetry), to the closest diagonal (again, four equilibrium positions), or to an intermediate position in between the median and the diagonal (eight equilibrium positions).

Powder bed additive manufacturing allows for the production of fully customizable parts and is of great interest for industrial applications. However, the repeatability of the parts and the uniformity of the mechanical properties are still an issue. More specifically, the physical mechanism of the spreading process of the powders, which significantly affects the characteristics of the final part, is not completely understood. In powder bed fusion technologies, the spreading is performed by a device, typically a roller or a blade, that collects the powders from the feedstock and successively deposits them in a layer of several dozens of microns that is then processed with a laser beam. In this work, an experimental approach is developed and employed to study the powder spreading process and analyze in detail the motion of the powders from the accumulation zone to the deposition stage. The presented experiments are carried out on a home-made device that reproduces the spreading process and enables the measurement of the characteristics of the powder bed. Furthermore, the correlation with the process parameters, e.g., the speed of the spreading device, is also investigated. These results can be used to obtain useful insights on the optimal window for the process parameters.

The goal of the SensApp FET-Open project is to develop an innovative super-sensor that will be able to detect Alzheimer’s disease (AD) biomarkers (β-amyloid, Tau and pTAU) in peripheral blood. Considering that nowadays an accurate diagnosis of AD requires the highly invasive withdrawal and analysis of cerebrospinal fluid, SensApp will represent a breakthrough in the field of AD diagnosis thanks to the ability to detect the early stage of the disease by a simple blood collection. We call Droplet-Split-and-Stack (DSS) the new technology that will emerge from SensApp. The achievement of SensApp goal is enabled by the interdisciplinary cooperation between different research institutions and one company involved in the key fields of the project, Vrije Universiteit Brussels, VTT Technical Research Centre of Finland, University of Linz, Ginolis Ltd, IRCCS Centre “Bonino Pulejo”, under the coordination of CNR-Institute of Applied Sciences and Intelligent Systems. This communication will illustrate the progress of the activities.

Viscoelastic polymer solutions have been widely employed as suspending liquids for a myriad of microfluidic applications including particle and cell focusing, sorting, and encapsulation. It has been recently shown that viscoelastic solutions can drive the formation of equally spaced particles called "particle trains" as a result of the viscoelasticity-mediated hydrodynamic interactions between adjacent particles. Despite their potential impact on applications such as droplet encapsulation and flow cytometry, only limited experimental studies on viscoelastic ordering are currently available. In this work, we demonstrate that a viscoelastic shear-thinning aqueous xanthan gum solution drives the self-assembly of particle trains on the centerline of a serpentine microfluidic device with a nearly circular cross section. After focusing, the flowing particles change their mutual distance and organize in aligned structures characterized by a preferential spacing, quantified in terms of distributions of the interparticle distance. We observe the occurrence of multi-particle strings, mainly doublets and triplets, that interrupt the continuity of the particle train. To account for the fluctuations in the number of flowing particles in the experimental window, we introduce the concept of local particle concentration, observing that an increase of the local particle concentration leads to an increase of doublets and triplets. We also demonstrate that using only a single tube to connect the sample to the microfluidic device results in a drastic reduction of doublets/triplets, thus leading to a more uniform particle train. Our findings establish the foundation for optimized applications such as deterministic droplet encapsulation in viscoelastic liquids and optimized flow cytometry.

Hypothesis Thin liquid films are important in many scientific fields. In particular, films with both the surface layers exposed to a different fluid phase, known as freestanding films, are relevant in the ambit of foams and emulsions. Hence, there is a great interest in developing novel techniques allowing to form large and stable freestanding liquid films and to follow their dynamics. Experiments We develop a novel opto-mechanical tool allowing to perform and study the preparation and the capillary leveling flow of axisymmetric bare freestanding liquid films. The tool is composed by a customized motorized iris diaphragm and by an innovative joint imaging setup combining digital holography and white light color interferometry that enables real-time measurement of film thickness over a large field of view. The dynamics of films made of a model Newtonian fluid, i.e., high-viscosity silicone oil, is studied. Direct numerical simulations and a hydrodynamic model based on the lubrication theory are used to support the experimental results. Findings Iris opening induces the formation of large circular freestanding films with a stepped profile. Once iris opening is stopped, the films undergo a capillary leveling flow tending to flatten their profile. The leveling flow follows the theoretical scaling given by Ilton et al. [1]. We prove through numerical simulations that an equi-biaxial extensional flow occurs at the film center. Furthermore, we observe the formation and dynamics of dimples in bare freestanding films for the first time.

The use of nanomaterials, thanks to their peculiar properties and versatility, is becoming central in an increasing number of scientific and engineering applications. At the same time, the growing concern towards environmental issues drives the seeking of alternative strategies for a safer and more sustainable production of nanoparticles. Here we focus on a low-energy, magnetically-driven wet milling technique for the synthesis of metal nanoparticles starting from a bulky solid. The proposed approach is simple, economical, sustainable, and provides numerous advantages, including the minimization of the nanoparticles air dispersion and a greater control over the final product. This process is investigated by experiments and discrete element method simulations to reproduce the movement of the grinding beads and study the collision dynamics. The effect of several parameters is analyzed, including the stirring bar velocity, its inclination, and the grinding bead size, to quantify the actual frequency, energy, and angle of collisions. Experiments reveal a non-monotonous effect of the stirring velocity on the abrasion efficiency, whereas numerical simulations highlight the prevalent tangential nature of collisions, which is only weakly affected by the stirring velocity. On the other hand, the stirring velocity affects the collision frequency and relative kinetic energy, suggesting the existence of an optimal parameters combination. Although a small variation of the stirring bar length does not significantly affect the collision dynamics, the use of grinding beads of different dimensions offers several tuning opportunities.

One of the critical issues encountered when particle suspensions are made to flow in microfluidic devices is the adhesion of the suspended particles on the channel surfaces. This process, known as fouling, may lead to a progressive growth of clusters attached to the walls and, possibly, to a complete clogging of the microchannel. In this work, we employ Computational Fluid Dynamics combined with Discrete Element Method to study the initial growth of a cluster at the wall of a slit microchannel. We consider a suspension of ‘soft’ microparticles in a Newtonian liquid under laminar flow conditions, with a well-known simple model to describe particle-particle adhesion. The cluster growth dynamics is quantified in terms of morphology and projected area onto the slit wall. A comparison with some experimental data is carried out, showing that the model captures several qualitative experimental features of the fouling process, e.g., cluster morphology, possibility of break-up and resuspension, and linear growth rate of the cluster projected area onto the channel wall.

In this paper, we investigate the retraction of a circular viscoelastic liquid film with a hole initially present in its center by means of finite element numerical simulations. We study the whole retraction process, aiming at understanding the hole opening dynamics both when the hole does not feel any confinement and when it interacts with the solid wall bounding the film. The retraction behavior is also interpreted through a simple toy model, that highlights the physical mechanism underlying the process.We consider three different viscoelastic constitutive equations, namely, Oldroyd-B, Giesekus (Gsk), and Phan Thien-Tanner (PTT) models, and several system geometries, in terms of the film initial radius and thickness. For each given geometry, we investigate the effects of liquid inertia, elasticity, and flow-dependent viscosity on the dynamics of the hole opening. Depending on the relative strength of such parameters, qualitatively different features can appear in the retracting film shape and dynamics.When inertia is relevant, as far as the opening hole does not interact withthe wall bounding the film, the influence of liquid elasticity is very moderate,and the retraction dynamics tends to the one of Newtonian sheets; whenthe hole starts to interact with the solid wall, hole radius/opening velocityoscillations are detected. Such oscillations enhance at increasing elasticity.From the morphological point of view, the formation of a rim at the edge ofthe retracting film is observed. If inertial forces become less relevant withrespect to viscous forces, R-oscillations disappear, the hole opening velocitygoes through a maximum and then monotonically decays to zero, and norim forms during the film retraction. Geometrical changes have the effect ofenlarging or reducing the portion of the retraction dynamics not influencedby the presence of the solid wall with respect to the one governed by thehole-wall interactions.

Controlling the fate of particles and cells in microfluidic devices is critical in many biomedical applications, such as particle and cell alignment and separation. Recently, viscoelastic polymer solutions have been successfully used to promote transversal migration of particles and cells toward fixed positions in straight microchannels. When inertia is negligible, numerical simulations have shown that strongly shear-thinning polymer solutions (fluids with a shear viscosity that decreases with increasing flow rates) promote transversal migration of particles and cells toward the corners or toward the centerline in a straight microchannel with a square cross section, as a function of particle size, cell deformability, and channel height. However, no experimental evidence of such shifting in the positions for particles or cells suspended in strongly shear-thinning liquids has been presented so far. In this work, we demonstrate that particle positions over the channel cross section can be shifted "from the edge to the center" in a strongly shear-thinning liquid. We investigate the viscoelasticity-induced migration of both rigid particles and living cells (Jurkat cells and NIH 3T3 fibroblasts) in an aqueous 0.8 wt % hyaluronic acid solution. The combined effect of fluid elasticity, shear-thinning, geometric confinement, and cell deformability on the distribution of the particle/cell positions over the channel cross section is presented and discussed. In the same shear-thinning liquid, separation of 10 and 20 μm particles is also achieved in a straight microchannel with an abrupt expansion. Our results envisage further applications in viscoelasticity-based microfluidics, such as deformability-based cell separation and viscoelastic spacing of particles/cells.

Clogging is the mechanism that interrupts the flow in confined geometries due to the complete blockage of the channel cross section. It represents a critical issue in the processing of particle suspensions for both industrial and biological applications, and it is particularly relevant in microfluidics and membrane technology due to the high particle confinement and the difficult device cleaning. Although numerous experimental and numerical studies have been carried out to understand the mechanism governing this complex multiscale phenomenon, the picture is not yet clear and many questions still remain, especially at the particle level. In this regard, the numerical simulations may represent a useful investigation tool since they provide a direct insight to quantities not easily accessible from experiments. In this work, a detailed computational fluid dynamics-discrete element method simulation study on the clogging mechanism in a microchannel with planar contraction is carried out. Both constant flow rate and constant pressure drop conditions are investigated, highlighting the effect of flow conditions, particle volume fraction, cohesion forces, and contraction angle. The onset of clogging conditions is discussed.

The fast growth of microfluidic applications based on complex fluids is a result of the unique fluid dynamics of these systems, enabling the creation of devices for health care or biological and chemical analysis. Microchannels designed to focus, concentrate, or separate particles suspended in viscoelastic liquids are becoming common. The key fluid dynamical issue on which such devices work is viscoelasticity-induced lateral migration. This phenomenon was discovered in the 1960s in macroscopic channels and has received great attention within the microfluidic community in the past decade. This review presents the current understanding, both from experiments and theoretical analysis, of viscoelasticity-driven cross-flow migration. Examples of promising microfluidic applications show the unprecedented capabilities offered by such technology based on geometrically simple microchannels and rheologically complex liquids.

The aggregation/fragmentation dynamics of hard-gel microparticles suspended in a Newtonian liquid flowing through a straight channel is studied by numerical simulations. A one-way Discrete Element Method is employed to simulate the motion and the adhesion of the particles. The formation and fragmentation of aggregates, and their deposition at the channel walls are investigated by varying the Reynolds number and the strength of the adhesive force. A non-periodic channel is considered to simulate the start-up phase of aggregate formation till a ‘pseudo steady-state’ condition. Results in terms of microstructures, particle velocity profiles, and spatial and temporal evolution of aggregates in 2D and cylindrical (3D) channels are presented and discussed.

We address the effect of wall slip on the dynamics of a spherical particle suspended in an inertialess Newtonian or viscoelastic shear-thinning fluid under shear or Poiseuille flow. The study is performed through 3D direct finite element simulations by employing an Arbitrary Lagrangian-Eulerian method for the particle motion.In both shear and Poiseuille flows, wall slip reduces the difference between the particle translational velocity along the flow direction and the velocity of the unperturbed fluid, and slows down the particle rotational velocity. Remarkably, in a viscoelastic fluid, the presence of wall slip reverses the migration direction as compared to the no-slip case. Hence, for sufficiently high slip coefficients, all the particles migrate toward the channel midplane in shear flow and toward the channel centerline in Poiseuille flow, regardless of their initial position through the channel.

In this work a design tool to control cell-opening in gas foaming of thermoplastic polymers is developed. The sequence of events following bubble nucleation, namely, bubble growth and impingement, is modeled to gain a comprehensive, perspective view on the mechanisms of bubble wall rupture and on the conditions for achieving a fully open-cell morphology. In particular, unlike the previously published literature, the polymer elastic recovery is recognized as an important factor for wall retraction, which is typically considered as solely driven by surface tension. The new approach is experimentally validated on poly(e-caprolactone) (PCL), foamed with CO 2 , as a model polymer/gas system.

In this work, we numerically investigate the dynamics of the growth and impingement of two gas bubbles in a Newtonian liquid in the presence of a rigid spherical particle. The computational analysis is carried out through 3D Arbitrary Lagrangian Eulerian (ALE) Finite Element Method (FEM) simulations. During their growth, as the bubbles start to ‘feel’ each other, they lose their spherical shape, with the side facing the other bubble becoming almost flat. In the liquid layer between the gas inclusions, an essentially biaxial extensional flow takes place. Depending on its initial position with respect to the bubbles, the solid particle can be ‘captured’ by the coalescing bubbles or ‘escape’ them. The effects of the physical and geometrical parameters of the system on such phenomenon are studied.

We study the dynamics of a slippery spherical particle suspended in an inertialess Newtonian or viscoelastic shear-thinning fluid, under shear or Poiseuille flow, by means of 3D direct numerical simulations. In particular, we investigate on the effect of particle slip on the cross-stream migration induced by fluid viscoelasticity. The governing equations are solved through the finite element method, by adopting an Arbitrary Lagrangian-Eulerian (ALE) formulation to handle the particle motion.In shear flow, the migration dynamics is qualitatively unchanged as compared to the no-slip case, i.e. the particle always moves towards the nearest wall regardless of the initial position. For increasing slip, the migration velocity first reaches a maximum, and then decreases to values lower than the no-slip one. Thus, a pronounced particle slip slows down the migration phenomenon.In Poiseuille flow, at variance with the no-slip case for a shear-thinning viscoelastic fluid, the tube wall becomes a `hydrodynamic repulsor' for high slip values, and all the particles migrate towards the channel centerline (`attractor'). In this sense, slippery particles are more easily aligned along the channel centerline than no-slip particles.

Strings of equally spaced particles (particle train) are tremendously important in a variety of microflu- idic applications. By using inertial microfluidics, particle trains can be formed near the channel walls. However, the high particle rotation and large local shear gradient near the microchannel walls can lead to blurred images and cell damage, thus negatively affecting applications related to flow cytometry. To address this challenge, we demonstrate that adding a tiny amount of hyaluronic acid biopolymer to an aqueous suspension drives self-assembly of a particle train on the centerline of a square-shaped straight microchannel, with a throughput up to approximately 2400 particles/s. The fraction of equally spaced par- ticles increases by increasing the volumetric flow rate and the distance from the channel inlet. Numerical simulations corroborate the experimental observations and, together with a simple qualitative argument on the particle train stability, shed insights on the underlying mechanism leading to particle ordering.

The effect of fluid rheology on particle migration induced by fluid viscoelasticity in a square-shaped microchannel is reported. Three water polymer solutions of PolyEthylene Oxyde at different concentrations, corresponding to different elasticity and degree of shear thinning, are prepared and rheologically characterized. Experiments are carried out for a wide range of flow rates, and the particle distributions over the channel cross section are reconstructed by combining particle tracking measurements and numerical simulations of the fluid velocity profile. The particle distributions show that the migration direction strongly depends on the fluid rheology. Specifically, when particles explore the constant viscosity region of the suspending liquids, they are focused around the channel centerline. Such an effect is more and more pronounced as the flow rate increases. On the other hand, for particles suspended in a shear-thinning fluid, a different scenario appears: At low flow rates, i.e., in the constant viscosity region, particles still migrate toward the channel centerline, while at high flow rates, i.e., in the shear thinning region, the migration reverts direction and the particles are driven toward the corners of the channel cross section. Those experimental observations elucidate the relevant and competing role of elasticity and shear thinning, with obvious implications in designing microfluidic devices for particle manipulation. Finally, our results highlight the weak effect of inertia on particle migration as compared to viscoelastic effects, even for low elastic suspending liquids.

Systems made by particles in viscoelastic liquids are ubiquitous in a variety of industrial and biological applications. Much work has been done in the last half-century in understanding the effect of non-Newtonian properties on the dynamics of the suspended particles. Theoretical predictions, experimental observations and numerical simulations highlighted peculiar phenomena induced by fluid elasticity that dramatically affect the particle motion and patterning. In this review, the existing literature on the dynamics of non-Brownian particles in viscoelastic fluids is discussed. The main part is focused on the dynamics of rigid particles passively transported in flowing viscoelastic liquids. The available results are classified by increasing level of complexity in terms of hydrodynamic interactions (single-particle problems, binary interactions, multi-body systems) and according to the flow field. Recent results on soft and active viscoelastic suspensions are also discussed.

The dynamics of a stick-slip “Janus” spherical particle suspended in a Newtonian fluid flowing in a cylindrical microchannel is studied by direct numerical simulations. Partial slip is imposed on half of the particle surface, whereas the no-slip boundary condition is present on the other half. The finite element method is used to solve the balance equations under creeping flow conditions. The translational and rotational velocities of the particle are evaluated at several orientations and distances from the tube centerline. The trajectories are then reconstructed by solving the kinematic equations where the velocities are taken by interpolating the simulation data. The particle dynamics is investigated by varying the initial position and orientation, the slip parameter, and the confinement ratio. The results, presented in terms of particle trajectories and phase portraits, highlight the existence of two relevant regimes: a periodic oscillation or a migration toward the tube axis for particle positions sufficiently far from or near the centerline, respectively. The basin of attraction of the tube axis grows with particle confinement and slip coefficient although the dynamics is qualitatively unaffected.The dynamics of a stick-slip “Janus” spherical particle suspended in a Newtonian fluid flowing in a cylindrical microchannel is studied by direct numerical simulations. Partial slip is imposed on half of the particle surface, whereas the no-slip boundary condition is present on the other half. The finite element method is used to solve the balance equations under creeping flow conditions. The translational and rotational velocities of the particle are evaluated at several orientations and distances from the tube centerline. The trajectories are then reconstructed by solving the kinematic equations where the velocities are taken by interpolating the simulation data. The particle dynamics is investigated by varying the initial position and orientation, the slip parameter, and the confinement ratio. The results, presented in terms of particle trajectories and phase portraits, highlight the existence of two relevant regimes: a periodic oscillation or a migration toward the tube axis for particle positions suf...

Migration and chaining of noncolloidal spheres in a worm-like micellar, viscoelastic solution under shear flow have been studied both experimentally and by numerical simulations. The microstructure dynamics have been experimentally investigated in the flow-gradient and in the flow-vorticity planes. 20 simulations in the flow-gradient plane have been performed for the same geometry, and with a proper selection for the constitutive equation of the suspending liquid. Experimental results show the formation of particle chains in the bulk, along with migration of a considerable fraction of spheres to the walls. At long times, chains in the bulk are stable, and cross-flow migration of individual spheres is suppressed. Numerical simulations with a standard viscoelastic constitutive equation (Giesekus fluid) reproduce the same phenomena observed experimentally, both in terms of fast particle migration to the wall and bulk chain stability. No alignment is, instead, found in simulations with a constant-viscosity, elastic fluid (Oldroyd-B model), in agreement with previous experimental results with Boger fluids. (C) 2013 Elsevier B.V. All rights reserved.

The dynamics of pairs and triplets of particles suspended in a viscoelastic fluid flowing along the centerline of a cylindrical channel is studied by numerical simulations. The governing equations are solved by the finite element method by employing an ALE formulation to handle the particle motion. For a pair of particles, at variance with the Newtonian case, the viscoelastic nature of the suspending medium alters the interparticle distance during the flow. For low and moderate Deborah numbers, the particles can approach or separate depending on the initial distance. For high Deborah numbers, the approaching dynamics disappears. Different fluid rheology and confinement ratio only quantitatively alter such a scenario. The three-particle dynamics is more complex. In a Newtonian liquid, the leftmost particle of the triplet approaches and slows down the middle one. Consequently, the rightmost particle separates, giving rise to a pair and an isolated particle. A similar scenario occurs for a viscoelastic liquid. In this case, however, depending on the initial configuration and the Deborah number, the particles forming the pair can subsequently approach or separate. In the latter case, the final configuration is the formation of three isolated particles.

The rising of spherical bodies in structured fluids with yield stress is studied. The system is a suspension of hydrogenated castor oil colloidal fibers in a surfactant micellar solution. The fiber network confers to the fluid a viscoelastic behavior, with a well-defined yield stress, which increases with increasing fiber concentration. Various fluids with different fiber contents are prepared and rheologically characterized. A home-made time-lapse photography setup is used to monitor the time evolution position of the spherical particles, and the rising motion of both hollow spheres and air bubbles, in the diameter range 65–550 μm, is measured. The experiments last as long as several weeks, corresponding to significantly low measured velocities. Finite element simulations are performed to support the experimental data, assuming both interfacial slip and no slip conditions. The fluid dynamic phenomenon is studied and discussed in terms of dimensionless numbers, such as yield ratio, Bingham number, and Stokes drag coefficient. The results are novel for the system (suspending medium and hollow spheres) and for the covered Bingham number range, which is extended over three orders of magnitude in comparison with already available literature results. Our values provide quantitative data of the mechanical properties (i.e., yield stress value) at very low shear rates, in a prohibitive range for a traditional rheometer, and agree with the macroscopic rheological response. Moreover, the important role of the power law index n of the Herschel-Bulkley model, used to fit the data, has been highlighted. Our results, based on a Bingham-like fluid, are compared with the experimental data already available with Carbopol, treated as a Herschel Bulkley fluid with n = 0.5. The results could have important implications in the fabric and personal care detergency, a technological area where many fluids have composition and show rheological properties similar to those considered in the current work.

The viscoelasticity-induced migration of a sphere in pressure-driven flow in a square-shaped microchannel is investigated under inertialess conditions. The effects of fluid rheology, i.e. of shear thinning and normal stresses, is studied by means of 3D finite element simulations. Two constitutive models are selected, in order to highlight differences due to rheological properties. A strong influence of the suspending fluid rheology on the migration phenomenon is shown, by particle trajectory analysis. When the second normal stress difference is negligible and, as a consequence, no secondary flows appear, the particle migrates towards the channel centerline or the closest corner, depending on its initial position. As shear thinning is increased, the center-attractive region is reduced, and the migration rate is faster. On the other hand, the existence of secondary flows, linked to the existence of a second normal stress difference, alters the migration scenario. The competition between the particle-wall hydrodynamic interactions, promoting the migration mechanism, and the secondary flow velocity components gives rise to further ‘equilibrium’ positions within the channel cross-section. Particles driven towards such positions trace out a spiral trajectory, following the vortex structure of the secondary flows. However, as the particle dimension is increased or the Deborah number is reduced, the cross-streamline migration velocity overcomes the secondary flow velocity. In this case, most of the particles are driven towards the channel centerline, i.e. a strong flow-focusing effect results.