Reversible Cavitation-Induced Junctional Opening in an Artificial Endothelial Layer.

Targeting pharmaceuticals through the endothelial barrier is crucial for drug delivery. In this context, cavitation-assisted permeation shows promise for effective and reversible opening of intercellular junctions. A vessel-on-a-chip is exploited to investigate and quantify the effect of ultrasound-excited microbubbles-stable cavitation-on endothelial integrity. In the vessel-on-a-chip, the endothelial cells form a complete lumen under physiological shear stress, resulting in intercellular junctions that exhibit barrier functionality. Immunofluorescence microscopy is exploited to monitor vascular integrity following vascular endothelial cadherin staining. It is shown that microbubbles amplify the ultrasound effect, leading to the formation of interendothelial gaps that cause barrier permeabilization. The total gap area significantly increases with pressure amplitude compared to the control. Gap opening is fully reversible with gap area distribution returning to the control levels 45 min after insonication. The proposed integrated platform allows for precise and repeatable in vitro measurements of cavitation-enhanced endothelium permeability and shows potential for validating irradiation protocols for in vivo applications.

Drag reduction by a linear viscosity profile.

Drag reduction by polymers in turbulent flows raises an apparent contradiction: the stretching of the polymers must increase the viscosity, so why is the drag reduced? A recent theory proposed that drag reduction, in agreement with experiments, is consistent with the effective viscosity growing linearly with the distance from the wall. With this self-consistent solution the reduction in the Reynolds stress overwhelms the increase in viscous drag. In this Rapid Communication we show, using direct numerical simulations, that a linear viscosity profile indeed reduces the drag in agreement with the theory and in close correspondence with direct simulations of the FENE-P model at the same flow conditions.

Drag reduction by polymers in turbulent channel flows: Energy redistribution between invariant empirical modes.

We address the phenomenon of drag reduction by a dilute polymeric additive to turbulent flows, using direct numerical simulations (DNS) of the FENE-P model of viscoelastic flows. It had been amply demonstrated that these model equations reproduce the phenomenon, but the results of DNS were not analyzed so far with the goal of interpreting the phenomenon. In order to construct a useful framework for the understanding of drag reduction we initiate in this paper an investigation of the most important modes that are sustained in the viscoelastic and Newtonian turbulent flows, respectively. The modes are obtained empirically using the Karhunen-Loéve decomposition, allowing us to compare the most energetic modes in the viscoelastic and Newtonian flows. The main finding of the present study is that the spatial profile of the most energetic modes is hardly changed between the two flows. What changes is the energy associated with these modes, and their relative ordering in the decreasing order from the most energetic to the least. Modes that are highly excited in one flow can be strongly suppressed in the other, and vice versa. This dramatic energy redistribution is an important clue to the mechanism of drag reduction as is proposed in this paper. In particular, there is an enhancement of the energy containing modes in the viscoelastic flow compared to the Newtonian one; drag reduction is seen in the energy containing modes rather than the dissipative modes, as proposed in some previous theories.