- Complex fluids and materials
- Active condensed matter
- Liquid crystal physics
- Nonlinear elasticity

- Fluid-structure interaction
- Soft robotics
- Biological flows and biomedicine
- High performance computing

1.
Soft colloids

Microscopic soft particles are commonly found in nature and engineeringapplications. Examples include red blood cells, fluid vesicles and microgel particles. When placed in a liquid, soft particles can readily undergo large deformations to accommodate the hydrodynamic forces, which in turn has a significant impact on the macroscopic rheological properties of the mixture.

Microscopic soft particles are commonly found in nature and engineeringapplications. Examples include red blood cells, fluid vesicles and microgel particles. When placed in a liquid, soft particles can readily undergo large deformations to accommodate the hydrodynamic forces, which in turn has a significant impact on the macroscopic rheological properties of the mixture.

* Shear
flow We consider a suspension of
elastic solid particles in a viscous liquid. The particles are assumed
to be neo-Hookean and
can undergo finite deformation. When placed in a shear flow, three
types of motion - steady-state,
trembling and tumbling - are found. The rheological properties
generally exhibit shear-thinning
behavior, and can even show negative intrinsic viscosity for
sufficiently soft particles.

* Extensional
flow We investigate the dynamics
and rheology of neo-Hookean elastic particles in a viscous extensional
flow under Stokes flow condition. When subjected to an extensional
field, an initially ellipsoidal particle stretches and rotates
simultaneously, tending to deform into another stable ellipsoidal
shape.
However, the steady-state solutions may not exist when the particle
stiffness is very lower. The rheology study shows that the suspension
exhibits
strain-thickenning effect,
similar to some polymer blends.

* Electrohydrodynamics of soft
particle We study the dynamics
of a long elastic particle
undergoing electrophoresis. The particle is elliptical in shape and is
initially aligned with its
major axis perpendicular to the direction of a uniformly applied
electric field. The particle tends
to curl up at its ends and arches in the middle. After a transient
deformation, the particle migrates
at Helmholtz-Smoluchowski velocity.

2. Active condensed matter

As a new branch of complex fluids, active matter is composed of self-driven constitutes with emergence of nonequilibrium physics. Despite the difference in composition, all these active systems orchestrate cooperative actions across various length and time scales, accompanying energy conversion from one form (e.g., chemical fuel) to another (e.g., mechanical work). Typical systems include cytoskeletal networks, synthetic microswimmers, bacterial suspensions, etc.

As a new branch of complex fluids, active matter is composed of self-driven constitutes with emergence of nonequilibrium physics. Despite the difference in composition, all these active systems orchestrate cooperative actions across various length and time scales, accompanying energy conversion from one form (e.g., chemical fuel) to another (e.g., mechanical work). Typical systems include cytoskeletal networks, synthetic microswimmers, bacterial suspensions, etc.

* Bacteria and algae
Active suspensions of swimming microorganisms, such as bacteria or algae, can exhibit fascinating collective behaviors that feature large-scale coherent structures, enhanced mixing, ordering transition, and anomalous diffusion. Even in the limit of vanishing Reynolds numbers, densely packed self-driven or swimming micro-particles effectively exert stresses upon the ambient liquid to act as a coupling medium for the generation of active flows via instability concatenations to amplify the disturbances due to particle motions and local (e.g., steric) interactions. We build a computation model, including the high-fidelity particle simulator and bottom-up continuum models, to study the non-equilibrium physics of suspensions of rear- and front-actuated microswimmers, or respectively the so-called “pusher” and “puller” particles. .

* Active cellular matter
Microtubules and
motor-proteins are the building blocks of
self-organized subcellular structures such as the mitotic spindle and
the centrosomal microtubule array. They are ingredients in new
"bioactive" liquid-crystalline fluids that are powered by ATP, and
driven out of equilibrium by motor-protein activity to display
complex flows and defect dynamics. We develop a multiscale
theory for such systems. Brownian dynamics simulations of polar
microtubule ensembles, driven by active crosslinks, are used to
study microscopic organization and the stresses created by
microtubule interactions. This identifies two polar-specific sources
of active destabilizing stress: polarity-sorting and crosslink
relaxation. We develop a Doi-Onsager theory that captures polarity
sorting, and the hydrodynamic flows generated by polar-specific
active stresses. In simulating experiments of active flows on
immersed surfaces, the model exhibits turbulent dynamics and
continuous generation and annihilation of disclination defects.
Analysis shows that the dynamics follows from two linear
instabilities, and gives characteristic length- and time-scales.

* Geometric control and manipulation
To effectively control the collective dynamics in various internally-driven systems, it is critical to manipulate the emergent coherent structures. One way of doing this is to tune the suspension concentration and the amount of chemical fuels. Alternatively, we can take advantage of the particle interactions, either individually or collectively, with obstacles and geometric boundaries to manipulate the system more directly. By trapping active suspensions (such as Pusher swimmers or Quincke rollers) within the straight and curved boundaries, stable flow patterns, such as unidirectional circulations, traveling waves, density shocks, and rotating vortices, have already been constructed. More interestingly, active nematic flows under soft confinement by surface tension are able to generate internal flows to break symmetry and drive the whole-body movement.

3.
Numerical methods and theoretical models

Modeling and simulation of fluid-structure interaction problems in complex fluids environement is very challenging, especially when the objects are moving or deforming. Furthermore, it becomes more and more important to develop and integrate hybrid algorithms that combine computational fluid dynamics techniques and stochastic methods to perform large-scale simulations, together with coarse-grained modeling, to reveal the multiscale physics in novel soft matter systems.

Modeling and simulation of fluid-structure interaction problems in complex fluids environement is very challenging, especially when the objects are moving or deforming. Furthermore, it becomes more and more important to develop and integrate hybrid algorithms that combine computational fluid dynamics techniques and stochastic methods to perform large-scale simulations, together with coarse-grained modeling, to reveal the multiscale physics in novel soft matter systems.

* Arbitrary Lagrangian-Eulerian
finite element We
develop a new
finite element method with moving mesh technique to
solve the dynamics of elastic particles deforming in a viscous shear
flow.
In comparison with the previous treatments, we solve the unknown
variables in both fluid and solid phase are the
velocity, pressure and stress simultaneously. In this method,
consistent time integration schemes and
discretizing methods can be employed for all physical variables,
eventually leading to a linear system which can be solved by efficient
iterative schemes with appropriate preconditioners.

* Fictitious domain method We have been developing a fictitious domain (FD)/active muscle model that can handle active swimming of soft robots through periodic nonlinear elastic deformations that are actuated by distributing active stresses or strains on desired locations. The key idea of FD method is to virtually extend the exterior fluid field into the material domain where a distribution of pseudo body forces are employed to enforce the structural movement through Lagrange multipliers.

* Hybrid Cartesian/immersed
boundary method We
propose an
improved hybrid Cartesian/immersed boundary method based on ghost point
treatment.
A second-order Taylor series expansion is used to evaluate the values
at the ghost points, and an inverse
distance weighting method to interpolate the values due to its
properties of preserving local extrema and
smooth reconstruction. The present method effectively eliminates
numerical instabilities caused by matrix
inversion and flexibly adopts the interpolation in the vicinity of the
boundary.

* Continuum models for
active fluids For biological and
synthetic fluids, their physical properties are determined by many-body hydrodynamic
interactions between tens of thousands of suspended
microstructures. Although direct simulations are possible, it is much more convenient to model the complex fluids through continuum models. At small Reynolds number, we investigate suspensions of the rod-like microparticles that are of different shapes, material properties,
and activities. Their suspension mechanics is then investigated by using either (microscopic) Doi-Onsager models or coarse-grained (macroscopic) liquid crystal models.

Funding Support

**National Science Foundation****MSU's Strategic Partnership Grants (SPG)****MSU-HFH Partnership Grants**