When water droplets sit on a cold substrate, they freeze into a shape with a pointed tip. At first glance, this behavior seems very odd since surface tension usually acts to prevent such sharp protrusions. The shape is, however, a result of water’s expansion as it freezes. The droplet freezes from the substrate upward, with a concave shape to the solidification front. The angle of the point does not depend on the substrate temperature or the wetting angle between the water and surface. Instead, it turns out that this concave front shape and water’s expansion are the key factors that determine the pointed cusp’s angle, and that the final geometry of the cusp is essentially universal. (Video credit: M. Nauenberg; additional research credit: A. Marin et al.)
Tag: droplets
Water and Aerogel
Aerogel is an extremely light porous material formed when the liquid inside a gel is replaced with gas. When combined with water, aerogel powders can have some wild superhydrophobic effects. Here water condensed on a liquid nitrogen cooler has dripped onto a floor scattered with aerogel powder from the nitrogen’s shipping container. The result is that the water gets partially coated in aerogel powder and takes on some neat properties. Its contact angle with the surface increases – in other words, it beads up – which is typical of superhydrophobicity. When disturbed, the water breaks easily into droplets which do not immediately recombine upon contact. With sufficient distortion, they can rejoin. You can see some other neat examples of aerogel-coated water behaviors in this second video as well. (Video credit: ophilcial; submitted by Jason I.)
Air Pressure Affects Splashes
When a drop falls on a dry surface, our intuition tells us it will splash, breaking up into many smaller droplets. Yet this is not always the case. The splashing of a droplet depends on many factors, including surface roughness, viscosity, drop size, and–strangely enough–air pressure. It turns out there is a threshold air pressure below which splashing is suppressed. Instead, a drop will spread and flatten without breaking up, as shown in the video above. For contrast, here is the same fluid splashing at atmospheric pressure. This splash suppression at low pressures is observed for both low and high viscosity fluids. Although the mechanism by which gases affect splashing is still under investigation, measurements show that no significant air layer exists under the spreading droplet except near the very edges. This suggests that the splash mechanism depends on how the spreading liquid encroaches on the surrounding gas. (Video credit: S. Nagel et al.; research credit: M. Driscoll et al.)
Hydrodynamic Quantum Analogs
Over the past few years, researchers have been exploring the dynamics of droplets bouncing on a vibrating fluid. These systems display many behaviors associated with quantum mechanics, including wave-particle duality, single-slit and double-slit diffraction, and tunneling. A new paper examines the system mathematically, showing that the droplets obey many of the same mathematics as quantum systems. In fact, the droplet-wave system behaves as a macroscopic analog of 2D quantum behaviors. The implications are intriguing, especially for teaching. Now students of quantum mechanics can experiment with a simple apparatus to understand some of the non-intuitive aspects of quantum behavior. For more, see the paper on arxiv. (Image credit: D. Harris and J. Bush; research credit: R. Brady and R. Anderson)
The Inside of an Evaporating Drop
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Evaporating droplets may not look like much to the naked eye, but they contain complicated flow patterns. The type of pattern observed depends strongly on the contact line, the place where the liquid, solid, and air meet. When the contact line is pinned–kept unchanged–during evaporation, any particulates in the drop get pulled toward the edges as the drop evaporates. This is what leaves the classic coffee ring stain. It is also what is shown in the first clip in the video above. Contrast this with the second clip, in which the contact line is unpinned and varies irregularly as the drop evaporates. In the unpinned drop, particles are drawn inward during evaporation. The flow patterns are very different as well, complicated by swirling that is the result of force imbalances caused by the irregularly receding contact line. (Video credit: H. Kim)
Evaporating Drops
When still drops evaporate from a surface, they do so in several phases, as illustrated in the video above. Initially, the drop forms a spherical cap. At this point the velocity within the droplet is so small that it is difficult to resolve, but particles within the drop move outward toward the contact line. As the drop evaporates, they form a circle of sediment – the familiar coffee ring. As the drop flattens, radial velocity increases, drawing more and more particles to the coffee ring. Eventually the drop pulls away from the ring, leaving surface tension and evaporation to compete in driving the internal flow. During this phase, some parts of the contact line try to re-establish the flow pattern that made the first ring; this leaves behind circular segments broken up by the increasing instabilities in the contact line. In the final stage, surface tension smooths some of the irregularities and drives an inward velocity that leaves behind radial sediment segments. (Video credit: G. Hernandez-Cruz et al.)
Vibrating Droplets
When still, water drops sitting on a surface are roughly hemispherical, drawn into that shape by surface tension. But on a vibrating surface, the same water drop displays many different shapes, like those in the video above. Researchers have observed more than 30 different mode shapes by varying the driving frequency. The metal mesh placed beneath the glass on which the drops sit helps the researchers determine the drop’s shape. As the drop deforms, the mesh appears to distort due to the refraction of light through the changing shape of the drop’s water-air interface. The distortion allows observers to visualize (and in some experiments even reconstruct) the shape of the drop’s surface. Understanding this kind of droplet behavior is valuable for many applications, including ink-jet printing and microfluidic devices. (Video credit: C. Chang et al.; via Science)
Dublin’s Pitch-Drop Experiment
Readers may recall the University of Queensland’s pitch-drop experiment, recognized as the longest continuously running experiment in the world. Back in 1927, a professor started the experiment with the goal of measuring the extremely high viscosity of pitch. Since then, only eight drops have fallen. Queensland’s is not the only version of this experiment, though; Trinity College Dublin has a similar set-up and have just caught a falling pitch drop on camera for the first time ever. Take a look in the video above. Queensland is expecting a drop to fall sometime this year as well. (Video credit: Trinity College Dublin Physics; via SciAm)
Hydrophobia
Hydrophobic literally means water-fearing, and, once a surface is treated with a hydrophobic coating, the effect on water droplets is stark. The tendency of the non-polar hydrophobic molecules to repel the polar water molecules leads to high contact angles – which make the droplets almost spherical as they glide along the surface. The droplets dance across the surface, colliding and bouncing and coalescing. (Video and submission credit: M. Bell)
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)
