Water entry and fall of hydrophobic and superhydrophobic Teflon spheres.

Hydrophobic and superhydrophobic solid Teflon spheres have been observed while settling in water under the action of gravity, starting from different initial conditions, and have been followed until the steady-state is reached. The superhydrophobic sphere features a nano/microtextured surface and advancing and receding water contact angles equal to, respectively, [Formula: see text] and [Formula: see text]. When impacting water from air, both spheres can entrap a conspicuous amount of air deriving from the sealing of a macro-sized air cavity formed upon impact (air cavity trapping) and standing at the rear part of the settling sphere. It is shown that this air amount, like a spindle, reduces the force coefficient exerted on the sphere, basically acting on the pressure drag. However, the air cavity trapping occurs above a critical impact velocity which for the superhydrophobic spheres is significantly lower than that pertaining to the hydrophobic one; thus a certain range of impact velocities exists at which the superhydrophobic sphere experiences a lower pressure drag and a higher mean velocity. As soon as the air cavity vanishes, the dynamics of the superhydrophobic sphere becomes indistinguishable from that of the hydrophobic one, in spite of the persistence of air within the surface micro-texture.

Predicting different adhesive regimens of circulating particles at blood capillary walls.

A fundamental step in the rational design of vascular targeted particles is the firm adhesion at the blood vessel walls. Here, a combined lattice Boltzmann-immersed boundary model is presented for predicting the near-wall dynamics of circulating particles. A moving least squares algorithm is used to reconstruct the forcing term accounting for the immersed particle, whereas ligand-receptor binding at the particle-wall interface is described via forward and reverse probability distributions. First, it is demonstrated that the model predicts with good accuracy the rolling velocity of tumor cells over an endothelial layer in a microfluidic channel. Then, particle-wall interactions are systematically analyzed in terms of particle geometries (circular, elliptical with aspect ratios 2 and 3), surface ligand densities (0.3, 0.5, 0.7 and 0.9), ligand-receptor bond strengths (1 and 2) and Reynolds numbers (Re = 0.01, 0.1 and 1.0). Depending on these conditions, four different particle-wall interaction regimens are identified, namely not adhering, rolling, sliding and firmly adhering particles. The proposed computational strategy can be efficiently used for predicting the near-wall dynamics of particles with arbitrary geometries and surface properties and represents a fundamental tool in the rational design of particles for the specific delivery of therapeutic and imaging agents.

Evaluation of prosthetic-valved devices by means of numerical simulations.

The in vivo evaluation of prosthetic device performance is often difficult, if not impossible. In particular, in order to deal with potential problems such as thrombosis, haemolysis, etc., which could arise when a patient undergoes heart valve replacement, a thorough understanding of the blood flow dynamics inside the devices interacting with natural or composite tissues is required. Numerical simulation, combining both computational fluid and structure dynamics, could provide detailed information on such complex problems. In this work, a numerical investigation of the mechanics of two composite aortic prostheses during a cardiac cycle is presented. The numerical tool presented is able to reproduce accurately the flow and structure dynamics of the prostheses. The analysis shows that the vortical structures forming inside the two different grafts do not influence the kinematics of a bileaflet valve or the main coronary flow, whereas major differences are present for the stress status near the suture line of the coronaries to the prostheses. The results are in agreement with in vitro and in vivo observations found in literature.

Fluid-structure interaction of deformable aortic prostheses with a bileaflet mechanical valve.

Two different aortic prostheses can be used for performing the Bentall procedure: a standard straight graft and the Valsalva graft that better reproduces the aortic root anatomy. The aim of the present work is to study the effect of the graft geometry on the blood flow when a bileaflet mechanical heart valve is used, as well as to evaluate the stress concentration near the suture line where the coronary arteries are connected to graft. An accurate three-dimensional numerical method is proposed, based on the immersed boundary technique. The method accounts for the interactions between the flow and the motion of the rigid leaflets and of the deformable aortic root, under physiological pulsatile conditions. The results show that the graft geometry only slightly influences the leaflets dynamics, while using the Valsalva graft the stress level near the coronary-root anastomoses is about half that obtained using the standard straight graft.