240 research outputs found
Simulating structured fluids with tensorial viscoelasticity
We consider an immersed elastic body that is actively driven through a
structured fluid by a motor or an external force. The behavior of such a system
generally cannot be solved analytically, necessitating the use of numerical
methods. However, current numerical methods omit important details of the
microscopic structure and dynamics of the fluid, which can modulate the
magnitudes and directions of viscoelastic restoring forces. To address this
issue, we develop a simulation platform for modeling viscoelastic media with
tensorial elasticity. We build on the lattice Boltzmann algorithm and
incorporate viscoelastic forces, elastic immersed objects, a microscopic
orientation field, and coupling between viscoelasticity and the orientation
field. We demonstrate our method by characterizing how the viscoelastic
restoring force on a driven immersed object depends on various key parameters
as well as the tensorial character of the elastic response. We find that the
restoring force depends non-monotonically on the rate of diffusion of the
stress and the size of the object. We further show how the restoring force
depends on the relative orientation of the microscopic structure and the
pulling direction. These results imply that accounting for previously neglected
physical features, such as stress diffusion and the microscopic orientation
field, can improve the realism of viscoelastic simulations. We discuss possible
applications and extensions to the method.Comment: 17 pages, 11 figure
Fluid Vesicles in Flow
We review the dynamical behavior of giant fluid vesicles in various types of
external hydrodynamic flow. The interplay between stresses arising from
membrane elasticity, hydrodynamic flows, and the ever present thermal
fluctuations leads to a rich phenomenology. In linear flows with both
rotational and elongational components, the properties of the tank-treading and
tumbling motions are now well described by theoretical and numerical models. At
the transition between these two regimes, strong shape deformations and
amplification of thermal fluctuations generate a new regime called trembling.
In this regime, the vesicle orientation oscillates quasi-periodically around
the flow direction while asymmetric deformations occur. For strong enough
flows, small-wavelength deformations like wrinkles are observed, similar to
what happens in a suddenly reversed elongational flow. In steady elongational
flow, vesicles with large excess areas deform into dumbbells at large flow
rates and pearling occurs for even stronger flows. In capillary flows with
parabolic flow profile, single vesicles migrate towards the center of the
channel, where they adopt symmetric shapes, for two reasons. First, walls exert
a hydrodynamic lift force which pushes them away. Second, shear stresses are
minimal at the tip of the flow. However, symmetry is broken for vesicles with
large excess areas, which flow off-center and deform asymmetrically. In
suspensions, hydrodynamic interactions between vesicles add up to these two
effects, making it challenging to deduce rheological properties from the
dynamics of individual vesicles. Further investigations of vesicles and similar
objects and their suspensions in steady or time-dependent flow will shed light
on phenomena such as blood flow.Comment: 13 pages, 13 figures. Adv. Colloid Interface Sci., 201
Modeling and Simulation of Shape Changes of Red Blood Cells in Shear Flow
A description of the biomechanical character of red blood cells is given, along with an introduction to current computational schemes which use deformable capsules to simulate red blood cell shape change. A comprehensive two- and three-dimensional framework for the fluid-structure interaction between a deformable capsule and an ambient flow is provided. This framework is based on the immersed boundary method, using lattice Boltzmann and finite element methods for the fluid and structure, respectively. The characteristic response and recovery times of viscoelastic circular and spherical capsules are compared, and their dependence on simulation parameters is shown. The shape recovery of biconcave capsules in two and three dimensions is also considered, focusing on the role of simulation parameters and steady-state behaviour in two dimensions, while studying the capsule characteristics which lead to shape recovery and shape memory in three dimensions. Finally, the notion of interpreting membrane viscosity as an additional fluid viscosity is studied and a computational scheme based on power law fluids is described
Suspensions of viscoelastic capsules: effect of membrane viscosity on transient dynamics
Membrane viscosity is known to play a central role in the transient dynamics
of isolated viscoelastic capsules by decreasing their deformation, inducing
shape oscillations and reducing the loading time, that is, the time required to
reach the steady-state deformation. However, for dense suspensions of capsules,
our understanding of the influence of the membrane viscosity is minimal. In
this work, we perform a systematic numerical investigation based on coupled
immersed boundary -- lattice Boltzmann (IB-LB) simulations of viscoelastic
spherical capsule suspensions in the non-inertial regime. We show the effect of
the membrane viscosity on the transient dynamics as a function of volume
fraction and capillary number. Our results indicate that the influence of
membrane viscosity on both deformation and loading time strongly depends on the
volume fraction in a non-trivial manner: dense suspensions with large surface
viscosity are more resistant to deformation but attain loading times that are
characteristic of capsules with no surface viscosity, thus opening the
possibility to obtain richer combinations of mechanical features
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