Bioaerosols–tiny airborne fluid droplets generated by coughing or sneezing–are a major concern for the spread of contagions like influenza. It may be possible, however, to mitigate some of these effects by manipulating biological fluid properties. The video above shows an experimental model of a cough, complete with the generation of bioaerosols from some fake human lung mucus. Contrast this with a cough where the model’s mucus has been treated to increase its viscoelasticity. The treated mucus generates substantially fewer droplets during a cough. The results suggest that drugs that increase viscoselasticity of biofluids may help stem the spread of disease. (Video credit: K. Argue et al.; research credit: M. D. A. Hasan et al.)
Category: Research
Hanging Liquids
A horizontal filament of viscous liquid hanging between two plates stretches under gravity. The photo above is a composite showing the stretching of a single thread over several time steps. The fluid forms a catenary, the same shape as a hanging chain or cable when supported only at its ends. This behavior is confined to viscous filaments of sufficient length and diameter. Short and thin filaments instead form a U-shape with a thin horizontal filament joined to two thicker vertical threads. This difference in shape occurs due to the drainage of the liquid along the filament’s length. If the viscous thread begins to fall before surface tension drains the fluid from the center toward the ends, then a catenary of essentially uniform diameter forms. If instead the liquid drains before falling, the non-uniform U-shape is observed. (Photo credit: M. Le Merrer et al.)
Stretching to Break
Have you ever wondered what happens inside a jet of fluid as it breaks into droplets? Such events are not commonly or readily measured. This video uses a double emulsion–in which immiscible fluids are encapsulated into a multi-layer droplet–to demonstrate interior fluid flow during the Plateau-Rayleigh instability. The innermost drops and the fluid encapsulating them have a low surface tension between them, thanks to the addition of a surfactant to the inner drops. As a result, the inner drops are easily deformed by motion in the fluid surrounding them. Flow on the left side of the jet is clearly parabolic, similar to pipe flow. Closer to the pinch-off, the inner droplets shift to vertical lines, indicating that the interior flow’s velocity is constant across the jet. After pinch-off, the inner droplets return to a spherical shape because they are no longer being deformed by fluid movement around them. The coiling of the inner drops inside the bigger one is due to the electrical charges in the surfactant used. (Video credit: L. L. A. Adams and D. A. Weitz)
Meeting the Wall
Even something as simple as a falling sphere meeting a wall is composed of beautiful fluid motion. In Figure 1 above, we see side-view images of a sphere at low Reynolds number falling toward a wall over several time. Initially an axisymmetric vortex ring is visible in the sphere’s wake; when the sphere touches the wall, secondary vortices form and the wake vortex moves down and out along the wall in an axisymmetric fashion (Figure 2, top view). At higher Reynolds numbers, like those in Figure 3, this axisymmetric spreading of the vortex ring develops an instability and ultimately breaks down. (Photo credit: T. Leweke et al.)
Droplets Within Droplets
This video shows a multi-layered droplet, in which several droplets are formed one inside the other as an initial drop falls through a layer of oil sitting atop another liquid. When the drop falls, its potential energy gets transformed into interface energy, creating a fascinating interplay of surface tension, deformation, and miscibility between the fluids. Such self-contained multi-layered droplets, similar to multiple emulsions, could be helpful in pharmaceutical development. (Video credit: E. Lorenceau and S. Dorbolo 2004)
Turbulence and Magnetic Field Lines
During a solar flare, magnetic field lines on the sun are often visible due to the flow of plasma–charged particles–along the lines. According to theory, these magnetic lines should remain intact, but they are sometimes observed breaking and reconnecting with other lines. An interdisciplinary team of researchers suggests that turbulence may be the missing link. In their magnetohydrodynamic simulation, they found that the presence of chaotic turbulent motions made the magnetic line motion entirely unpredictable, whereas laminar flows behaved according to conventional flux-freezing theory. (Photo credit: NASA SDO; Research credit: G. Eyink et al.; via SpaceRef; submitted by jshoer)
Dendritic Designs
Imagine a thin layer of viscous liquid sandwiched between two horizontal glass plates. Then pull those plates apart at a constant velocity. What you see in the image above is the shape the viscous fluid takes for different speeds, with velocity increasing from left to right and from top to bottom. For lower velocities, the fluid forms tree-like fingers as air comes in from the edges. At higher velocities, though, there’s a transition from the finger-like pattern to a cell-like one. The cells are actually caused by cavitation within the fluid. When the plates are pulled apart fast enough, the local low pressure in the fluid causes cavitation bubbles to form just before the force required to remove the plate reaches its peak. (Photo credit: S. Poivet et al.)
Electrowetting
The electrowetting effect can change the shape of a liquid droplet on a surface by applying a voltage across the surface and droplet. Surface tension is a kind of measure of the energy required to maintain a certain drop shape, and that energy can be both chemical and electrical. In the video above, the droplet maintains a small contact area naturally (with no voltage). It expands and flattens under an electrical charge. Varying the voltage will change the degree to which the droplet flattens, but only to a point. Electrowetting is used to control variable lenses and some types of electronic displays. The technology may be used to replace current generation LCDs. (Video credit: V. Arya/Duke University)
Droplet Impact Visualized
When a drop falls from a moderate height into a shallow pool, its impact creates a complicated pattern. The photo above is a composite image showing a top-down view 100 ms after such an impact. On the left side, the flow is visualized using dye whereas the right shows a schlieren photograph, in which contrast indicates variations in density. Both methods show the same general structure – an inner vortex ring generated at the edge of the impact crater and formed mostly of drop fluid and an outer vortex ring, consisting primarily of pool fluid, formed by the spreading wave. Both regions show signs of instability and breakdown. (Photo credit: A. Wilkens et al.)
The Silence of Owls
Owls are nearly silent hunters, able to swoop down on their prey without the rush of air over their wings giving away their approach, thanks to several key features of their feathers. The trailing edge of their feathers–or any lifting body, like an airplane wing–are a particular source of acoustic noise due to the interaction of turbulence near the surface with the edge. Since owls are especially good at eliminating self-produced noise in a frequency range that overlaps human hearing, investigators want to learn what works for owls and apply to it aircraft. A recent theoretical analysis uses a simplified model of the feather as a porous, elastic plate. The researchers found that the combination of porosity with the elasticity of the trailing edge significantly reduced noise relative to a rigid edge. (Photo credit: N. Jewell; research credit: J. Jaworski and N. Peake)
