The effect of viscoelasticity in an airway closure model.

The closure of a human lung airway is modeled as a pipe coated internally with a liquid that takes into account the viscoelastic properties of mucus. For a thick enough coating, the Plateau-Rayleigh instability blocks the airway by the creation of a liquid plug, and the pre-closure phase is dominated by the Newtonian behavior of the liquid. Our previous study with a Newtonian-liquid model demonstrated that the bifrontal plug growth consequent to airway closure induces a high level of stress and stress gradients on the airway wall, which is large enough to damage the epithelial cells, causing sub-lethal or lethal responses. In this study, we explore the effect of the viscoelastic properties of mucus by means of the Oldroyd-B and FENE-CR model. Viscoelasticity is shown to be very relevant in the post-coalescence process, introducing a second peak of the wall shear stresses. This second peak is related to an elastic instability due to the presence of the polymeric extra stresses. For high-enough Weissenberg and Laplace numbers, this second shear stress peak is as severe as the first one. Consequently, a second lethal or sub-lethal response of the epithelial cells is induced.

Effects of surfactant on propagation and rupture of a liquid plug in a tube.

Surfactant-laden liquid plug propagation and rupture occurring in lower lung airways are studied computationally using a front-tracking method. The plug is driven by an applied constant pressure in a rigid axisymmetric tube whose inner surface is coated by a thin liquid film. The evolution equations of the interfacial and bulk surfactant concentrations coupled with the incompressible Navier-Stokes equations are solved in the front-tracking framework. The numerical method is first validated for a surfactant-free case and the results are found to be in good agreement with the earlier simulations of Fujioka et al. (2008) and Hassan et al. (2011). Then extensive simulations are performed to investigate the effects of surfactant on the mechanical stresses that could be injurious to epithelial cells such as pressure and shear stress. It is found that the liquid plug ruptures violently to induce large pressure and shear stress on airway walls and even a tiny amount of surfactant significantly reduces the pressure and shear stress and thus improves cell survivability. However, addition of surfactant also delays the plug rupture and thus airway reopening.

Liquid plug formation in an airway closure model.

The closure of a human lung airway is modeled as an instability of a two-phase flow in a pipe coated internally with a Newtonian liquid. For a thick enough coating, the Plateau-Rayleigh instability creates a liquid plug which blocks the airway, halting distal gas exchange. Owing to a bi-frontal plug growth, this airway closure flow induces high stress levels on the wall, which is the location of airway epithelial cells. A parametric numerical study is carried out simulating relevant conditions for human lungs, either in ordinary or pathological situations. Our simulations can represent the physical process from pre- to post-coalescence phases. Previous studies have been limited to pre-coalescence only. The topological change during coalescence induces a high level of stress and stress gradients on the epithelial cells, which are large enough to damage them, causing sub-lethal or lethal responses. We find that post-coalescence wall stresses can be in the range of 300% to 600% greater than pre-coalescence values, so introduce a new important source of mechanical perturbation to the cells.

Prospects of Wannier functions in investigating photonic crystal all-optical devices for signal processing.

Wannier functions derived from Bloch functions have been identified as an efficient means of analyzing the properties of photonic crystals in which localized functions have now opened the door for 2D and 3D structures containing defects to be investigated. In this paper, based on the Maxwell equations in diagonalized form and utilizing Bloch waves we have obtained an equivalent system of algebraic equations in eigenform. By establishing and exploiting several distinct properties of the resulting eigenpairs, we demonstrate an ability to construct Wannier functions associated with the simplest one-dimensional photonic structure. More importantly, the numerical investigation of the inner- and intra-band orthonormality conditions as well as Hilbert space partitioning features shows a capability for multi-resolution analysis that will make all-optical signal processing devices with photonic crystal structures feasible.

Reversible photothermal tuning of a salty water microdroplet.

A fully reversible photothermal tuning of an inorganic salt (NaCl)-water microdroplet standing on a superhydrophobic surface is demonstrated. The size change of the microdroplet is caused by a focused infrared laser beam in a humidity-controlled chamber and a fully reversible large spectral tuning up to approximately 40 nm is achieved. The evaporation and growth of the microdroplet are modeled using a lumped system formulation of mass and energy conservations and a good agreement is observed between the experimental and theoretical results.

Large spectral tuning of a water-glycerol microdroplet by a focused laser: characterization and modeling.

Large spectral tuning of a water-glycerol microdroplet standing on a superhydrophobic surface by local heating with a focused infrared laser is studied both experimentally by optical spectroscopy and computationally using a lumped system formulation of the mass and heat transfer between the microdroplet and the chamber. The effects of optical scattering force, chamber humidity, size of microdroplet and laser power on the tuning mechanism are examined. The reversibility of the tuning mechanism is also studied. In spite of its negligibly small volatility compared to that of water, irreversibility is found to be mainly caused by evaporation of glycerol. It is also found that reversibility increases dramatically with the relative water and glycerol humidities, and spectral tuning can be made almost fully reversible when the chamber is saturated with glycerol vapor and the relative water humidity approaches unity. Some hysteresis effects are observed, especially in large microdroplets, and this behavior is attributed to the whispering-gallery mode resonances in laser absorption. The time response of the tuning mechanism is also analyzed both experimentally and computationally. The technique presented can find applications in optical communication systems, and can be used in fundamental studies in cavity quantum electrodynamics and in characterizing liquid aerosols on a surface.