  Based upon two overlapped, body-unfitted meshes, a type of unified-field
monolithic fictitious domain-finite element method (UFMFD-FEM) is developed in
this paper for moving interface problems of dynamic fluid-structure
interactions (FSI) accompanying with high-contrast physical coefficients across
the interface and contacting collisions between the structure and fluidic
channel wall when the structure is immersed in the fluid. In particular, the
proposed novel numerical method consists of a monolithic, stabilized mixed
finite element method within the frame of fictitious domain/immersed boundary
method (IBM) for generic fluid-structure-contact interaction (FSCI) problems in
the Eulerian-updated Lagrangian description, while involving the no-slip type
of interface conditions on the fluid-structure interface, and the repulsive
contact force on the structural surface when the immersed structure contacts
the fluidic channel wall. The developed UFMFD-FEM for FSI or FSCI problems can
deal with the structural motion with large rotational and translational
displacements and/or large deformation in an accurate and efficient fashion,
which are first validated by two benchmark FSI problems and one FSCI model
problem, then by experimental results of a realistic FSCI scenario -- the
microfluidic deterministic lateral displacement (DLD) problem that is applied
to isolate circulating tumor cells (CTCs) from blood cells in the blood fluid
through a cascaded filter DLD microchip in practice, where a particulate fluid
with the pillar obstacles effect in the fluidic channel, i.e., the effects of
fluid-structure interaction and structure collision, play significant roles to
sort particles (cells) of different sizes with tilted pillar arrays.
