Quantitative flow analysis of swimming dynamics with coherent Lagrangian vortices

Particle Shape Influences Settling and Sorting Behavior in Microfluidic Domains

We present a new numerical model to simulate settling trajectories of discretized individual or a mixture of particles of different geometrical shapes in a quiescent fluid and their flow trajectories in a flowing fluid. Simulations unveiled diverse particle settling trajectories as a function of their geometrical shape and density. The effects of the surface concavity of a boomerang particle and aspect ratio of a rectangular particle on the periodicity and amplitude of oscillations in their settling trajectories were numerically captured. Use of surrogate circular particles for settling or flowing of a mixture of non-circular particles were shown to miscalculate particle velocities by a factor of 0.9–2.2 and inaccurately determine the particles’ trajectories. In a microfluidic chamber with particles of different shapes and sizes, simulations showed that steady vortices do not necessarily always control particle entrapments, nor do larger particles get selectively and consistently entrapped in steady vortices. Strikingly, a change in the shape of large particles from circular to elliptical resulted in stronger entrapments of smaller circular particles, but enhanced outflows of larger particles, which could be an alternative microfluidics-based method for sorting and separation of particles of different sizes and shapes.

A critical comparison of Lagrangian methods for coherent structure detection

We review and test twelve different approaches to the detection of finite-time coherent material structures in two-dimensional, temporally aperiodic flows. We consider both mathematical methods and diagnostic scalar fields, comparing their performance on three benchmark examples: the quasiperiodically forced Bickley jet, a two-dimensional turbulence simulation, and an observational wind velocity field from Jupiter's atmosphere. A close inspection of the results reveals that the various methods often produce very different predictions for coherent structures, once they are evaluated beyond heuristic visual assessment. As we find by passive advection of the coherent set candidates, false positives and negatives can be produced even by some of the mathematically justified methods due to the ineffectiveness of their underlying coherence principles in certain flow configurations. We summarize the inferred strengths and weaknesses of each method, and make general recommendations for minimal self-consistency requirements that any Lagrangian coherence detection technique should satisfy.

Smart mechanosensing machineries enable migration of vascular smooth muscle cells in atherosclerosis-relevant 3D matrices

At the early stage of atherosclerosis, neointima is formed due to the migration of vascular smooth muscle cells (VSMCs) from the media to the intima. VSMCs are surrounded by highly adhesive 3D matrices. They take specific strategies to cross various 3D matrices in the media, including heterogeneous collagen and mechanically strong basement membrane. Migration of VSMCs is potentially caused by biomechanical mechanism. Most in vitro studies focus on cell migration on 2D substrates in response to biochemical factors. How the cells move through 3D matrices under the action of mechanosensing machineries remains unexplored. In this review, we propose that several interesting tension-dependent machineries act as "tractor"-posterior myosin II accumulation, and "wrecker"-anterior podosome maintaining, to power VSMCs ahead. VSMCs embedded in 3D matrices may accumulate a minor myosin II isoform, myosin IIB, at the cell rear. Anisotropic myosin IIB distribution creates cell rear, polarizes cell body, pushes the nucleus and reshapes the cell body, and cooperates with a uniformly distributed myosin IIA to propel the cell forward. On the other hand, matrix digestion by podosome further promote the migration when the matrix becomes denser. Actomyosin tension activates Src to induce podosome in soft 3D matrices and retain the podosome integrity to steadily digest the matrix.

Global variational approach to elliptic transport barriers in three dimensions

We introduce an approach to identify elliptic transport barriers in three-dimensional, time-aperiodic flows. Obtained as Lagrangian Coherent Structures (LCSs), the barriers are tubular non-filamenting surfaces that form and bound coherent material vortices. This extends a previous theory of elliptic LCSs as uniformly stretching material surfaces from two-dimensional to three-dimensional flows. Specifically, we obtain explicit expressions for the normals of pointwise (near-) uniformly stretching material surfaces over a finite time interval. We use this approach to visualize elliptic LCSs in steady and time-aperiodic ABC-type flows.