Constriction in the flow passage of the physiological circulatory system is central to the occurrence of several diseased conditions such as thrombosis and is also pivotal towards the understanding of several regulatory processes in the human microvasculature. It is, therefore, imperative to advance a mechanistic insight into the dynamics of lipid vesicles, cell mimicking, fluid-filled compartments, through a physiologically relevant microconfinement, with particular focus on deciphering the role of its mechanophysical properties. Here we bring out the role of membrane bending rigidity and the initial deflation (deviation in shape from sphericity) on the transient shape evolution of a lipid vesicle as it migrates through a microfluidic constriction, a paradigm that is unexplored thus far. Based on our experimental observations as well as theoretical insights, we construct a regime map to elucidate the range of the key dimensionless parameters orchestrating this dynamical transition. Furthermore, our observations on the vesicle's stretching dynamics emerging from selective mapping with viscosity contrast between the encapsulated and the suspending fluid medium offer potential physiologically relevant cues on the impact of cell aging on its deformability across a constricted path. Such mechanistic insights may help in establishing quantitative correlations between the dynamical transition of a lipid vesicle and its membrane mechanics, which may in turn have decisive implications in health and disease while circulating across microvascular fluidic pathways.

In this paper, a computational model for the natural state of an inextensible capsule has been successfully combined with a spring model of the capsule membrane to simulate the motion of the capsule in two-dimensional shear flow. Besides the viscosity ratio of the internal fluid and external fluid of the capsule, the natural state also plays a role for having the transition between two well known motions, tumbling and tank-treading (TT) with the long axis oscillates about a fixed inclination angle (a swinging mode), when varying the shear rate. Between tumbling and tank-treading, the intermittent behavior has been obtained for the capsule with a biconcave rest shape. The estimated critical value of the swelling ratio for having the intermittent transition behavior is less than 0.7, i.e., the capsules with rest shape closer to a full disk do not have the intermittent behavior in shear flow. The intermittent dynamics of the capsule in the transition region is a mixture of tumbling and TT with a swinging mode. Just like the motion of TT with a swing mode, which can be viewed as a tank-treading with an incomplete tumbling, the membrane tank-treads backward and forward within a small range during the tumbling motion.

A modified direct-forcing immersed-boundary (IB) pressure correction method is developed to simulate the flows of a falling ellipse. The pressure correct method is used to solve the solutions of the two dimensional Navier-Stokes equations and a direct-forcing IB method is used to handle the interaction between the flow and falling ellipse. For a fixed aspect ratio of an ellipse, the types of the behavior of the falling ellipse can be classified as three pure motions: Steady falling, fluttering, tumbling, and two transition motions: Chaos, transition between steady falling and fluttering. Based on two dimensionless parameters, Reynolds number and the dimensionless moment of inertia, a Reynolds number-inertia moment phase diagram is established. The behaviors and characters of five falling regimes are described in detailed.

In this paper, we investigate the effects of kinematic transports on the nematic liquid crystal system numerically and theoretically. The model we used is a “1+2” elastic continuum model simplified from the Ericksen-Leslie system. The numerical experiments are carried out by using a Legendre-Galerkin spectral method which can preserve the energy law in the discrete form. Based on this highly accurate numerical approach we find some interesting and important relationships between the kinematic transports and the characteristics of the flow. We make some analysis to explain these results. Several significant scaling properties are also verified by our simulations.

Of the nearly 3900 near-Earth asteroids (NEAs) known as of June 2006, 325 have estimated rotation periods, with most of those determined by lightcurve analysis led by a few dedicated programs. NEAs with diameters down to 10 meters have been sampled. Observed spin distribution shows a major changing point around diameter of 200 meters. Larger NEAs show a barrier against spins faster than 11 d−1 (period about 2.2 h) that shifts to slower rates (longer periods) with increasing lightcurve amplitude (i.e., with increasing equatorial elongation). The spin barrier is interpreted as a critical spin rate for bodies in a gravity regime; NEAs larger than 200 meters are predominantly bodies with tensile strength too low to withstand a centrifugal acceleration for rotation faster than the critical spin rate. The cohesionless spin barrier disappears at sizes less than 200 meters where most objects rotate too fast to be held together by self-gravitation only, so a cohesion is implied in the smaller NEAs.

The distribution of NEA spin rates in the cohesionless size range (D0.2 km) is highly non-Maxwellian, suggesting that mechanisms other than just collisions have been at work. There is a pile up just in front of the barrier, at periods 2–3 h. It may be related to a spin up mechanism crowding asteroids to the barrier. An excess of slow rotators is observed at periods longer than 30 hours. A spin-down mechanism has no obvious lower limit on spin rate; periods as long as tens of days have been observed.

Most NEAs appear to be in their basic spin states with rotation around principal axis with maximum moment of inertia. Tumbling objects (i.e., bodies in excited, non-principal axis rotation) are present and actually predominate among slow rotators with estimated damping timescales longer than the age of the solar system. A few tumblers observed among fast rotating coherent objects appear to be either more rigid or younger than the larger (cohesionless) tumblers.

An abundant population of binary systems has been found among NEAs. The fraction of binaries among NEAs larger than 0.3 km has been estimated to be 15 ± 4%. Primaries of binary systems concentrate at fast spin rates (periods 2–3 h) and low amplitudes, i.e., they lie just below the cohesionless spin barrier. The total angular momentum content in binary systems suggests that they formed from parent bodies spinning at the critical rate. The fact that a very similar population of binaries has been found among small main belt asteroids suggests a binary formation mechanism that may not be related to close encounters with the terrestrial planets.