@article {62, title = {How Euglena gracilis swims: Flow field reconstruction and analysis.}, journal = {Physical Review E}, volume = {103}, year = {2021}, pages = {023102}, abstract = {

Euglena gracilis is a unicellular organism that swims by beating a single anterior flagellum. We study the nonplanar waveforms spanned by the flagellum during a swimming stroke and the three-dimensional flows that they generate in the surrounding fluid. Starting from a small set of time-indexed images obtained by optical microscopy on a swimming Euglena cell, we construct a numerical interpolation of the stroke. We define an optimal interpolation (which we call synthetic stroke) by minimizing the discrepancy between experimentally measured velocities (of the swimmer) and those computed by solving numerically the equations of motion of the swimmer driven by the trial interpolated stroke. The good match we obtain between experimentally measured and numerically computed trajectories provides a first validation of our synthetic stroke. We further validate the procedure by studying the flow velocities induced in the surrounding fluid. We compare the experimentally measured flow fields with the corresponding quantities computed by solving numerically the Stokes equations for the fluid flow, in which the forcing is provided by the synthetic stroke, and find good matching. Finally, we use the synthetic stroke to derive a coarse-grained model of the flow field resolved in terms of a few dominant singularities. The far field is well approximated by a time-varying Stresslet, and we show that the average behavior of Euglena during one stroke is that of an off-axis puller. The reconstruction of the flow field closer to the swimmer body requires a more complex system of singularities. A system of two Stokeslets and one Rotlet, that can be loosely associated with the force exerted by the flagellum, the drag of the body, and a torque to guarantee rotational equilibrium, provides a good approximation.\ 

}, keywords = {BEM, flow reconstruction, general defocusing particle tracking, Micro-swimmers, non-planar flagellar wave forms, particle tracking velocimetry, Stokes singularities}, doi = {10.1103/PhysRevE.103.023102}, author = {Nicola Giuliani and Massimiliano Rossi and Giovanni Noselli and Antonio DeSimone} } @article {40, title = {Hydraulic fracture and toughening of a brittle layer bonded to a hydrogel}, journal = {Physical Review Letters}, volume = {115}, year = {2015}, pages = {188105}, abstract = {

Brittle materials propagate opening cracks under tension. When stress increases beyond a critical magnitude, then quasistatic crack propagation becomes unstable. In the presence of several precracks, a brittle material always propagates only the weakest crack, leading to catastrophic failure. Here, we show that all these features of brittle fracture are fundamentally modified when the material susceptible to cracking is bonded to a hydrogel, a common situation in biological tissues. In the presence of the hydrogel, the brittle material can fracture in compression and can hydraulically resist cracking in tension. Furthermore, the poroelastic coupling regularizes the crack dynamics and enhances material toughness by promoting multiple cracking.

}, keywords = {hydraulic fracture, multiple-cracking, toughening}, doi = {10.1103/PhysRevLett.115.188105}, author = {Alessandro Lucantonio and Giovanni Noselli and Xavier Trepat and Marino Arroyo and Antonio DeSimone} }