Numerical design of a T-shaped microfluidic device for deformability-based separation of elastic capsules and soft beads

\u3cp\u3eWe propose a square cross-section microfluidic channel with an orthogonal side branch (asymmetric T-shaped bifurcation) for the separation of elastic capsules and soft beads suspended in a Newtonian liquid on the basis of their mechanical properties. The design is performed through three-dimensional direct numerical simulations. When suspended objects start near the inflow channel centerline and the carrier fluid is equally partitioned between the two outflow branches, particle separation can be achieved based on their deformability, with the stiffer ones going straight and the softer ones being deviated to the side branch. The effects of the geometrical and physical parameters of the system on the phenomenon are investigated. Since cell deformability can be significantly modified by pathology, we give a proof of concept on the possibility of separating diseased cells from healthy ones, thus leading to illness diagnosis.\u3c/p\u3

Numerical simulations of deformable particle lateral migration in tube flow of Newtonian and viscoelastic media

The deformation and cross-streamline migration of an elastic particle in pressure-driven flows of Newtonian and viscoelastic (Oldroyd-B, Giesekus) fluids in a cylindrical tube are studied through 3D finite element method numerical simulations. The dependences of particle deformation and migration on geometric confinement, flow strength, and fluid rheology are investigated. If the particle is initially not at the channel axis, it attains an asymmetric shape and migrates. In a Newtonian liquid, the migration is always directed towards the tube axis. A project equation is proposed for the design of a microfluidic cylindrical device aimed at focusing elastic particles on the cylinder centerline. In a viscoelastic liquid, the migration direction and velocity depend on the competition among particle deformability, fluid elasticity, and fluid viscosity shear thinning (if any). In a certain range of parameters, an unstable radial position appears, which separates the region where the migration is directed towards the axis from the region where it is directed towards the wall

Dynamics of prolate spheroidal elastic particles in confined shear flow

We investigate through numerical simulations the dynamics of a neo-Hookean elastic prolate spheroid suspended in a Newtonian fluid under shear flow. Both initial orientations of the particle within and outside the shear plane and both unbounded and confined flow geometries are considered. In unbounded flow, when the particle starts on the shear plane, two stable regimes of motion are found, i.e., trembling (TR), where the particle shape periodically elongates and compresses in the shear plane and the angle between its major semiaxis and the flow direction oscillates around a positive mean value, and tumbling (TU), where the particle shape periodically changes and its major axis performs complete revolutions around the vorticity axis. When the particle is initially oriented out of the shear plane, more complex dynamics arise. Geometric confinement of the particle between the moving walls also influences its deformation and regime of motion. In addition, when the particle is initially located in an asymmetric position with respect to the moving walls, particle lateral migration is detected. The effects on the particle dynamics of the geometric and physical parameters that rule the system are investigated

Bubble impingement in the presence of a solid particle:a computational study

\u3cp\u3eIn 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.\u3c/p\u3

Particle motion in square channel flow of a viscoelastic liquid : migration vs. secondary flows

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 hydrodynamics interaction, 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

Modeling and simulation of viscoelastic film retraction

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.\u3cbr/\u3eWe 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.\u3cbr/\u3eWhen inertia is relevant, as far as the opening hole does not interact with\u3cbr/\u3ethe wall bounding the film, the influence of liquid elasticity is very moderate,\u3cbr/\u3eand the retraction dynamics tends to the one of Newtonian sheets; when\u3cbr/\u3ethe hole starts to interact with the solid wall, hole radius/opening velocity\u3cbr/\u3eoscillations are detected. Such oscillations enhance at increasing elasticity.\u3cbr/\u3eFrom the morphological point of view, the formation of a rim at the edge of\u3cbr/\u3ethe retracting film is observed. If inertial forces become less relevant with\u3cbr/\u3erespect to viscous forces, R-oscillations disappear, the hole opening velocity\u3cbr/\u3egoes through a maximum and then monotonically decays to zero, and no\u3cbr/\u3erim forms during the film retraction. Geometrical changes have the effect of\u3cbr/\u3eenlarging or reducing the portion of the retraction dynamics not influenced\u3cbr/\u3eby the presence of the solid wall with respect to the one governed by the\u3cbr/\u3ehole-wall interactions

Overview of the JET results

Since the installation of an ITER-like wall, the JET programme has focused on the consolidation of ITER design choices and the preparation for ITER operation, with a specific emphasis given to the bulk tungsten melt experiment, which has been crucial for the final decision on the material choice for the day-one tungsten divertor in ITER. Integrated scenarios have been progressed with the re-establishment of long-pulse, high-confinement H-modes by optimizing the magnetic configuration and the use of ICRH to avoid tungsten impurity accumulation. Stationary discharges with detached divertor conditions and small edge localized modes have been demonstrated by nitrogen seeding. The differences in confinement and pedestal behaviour before and after the ITER-like wall installation have been better characterized towards the development of high fusion yield scenarios in DT. Post-mortem analyses of the plasma-facing components have confirmed the previously reported low fuel retention obtained by gas balance and shown that the pattern of deposition within the divertor has changed significantly with respect to the JET carbon wall campaigns due to the absence of thermally activated chemical erosion of beryllium in contrast to carbon. Transport to remote areas is almost absent and two orders of magnitude less material is found in the divertor