Tag: 2019GOFM

Mixing Leidenfrost Drops
When placed on a very hot, patterned surface, droplets will self-propel on a layer of their own vapor. Here, researchers use this to drive droplets to coalesce so that they can observe how well they mix. After their head-on collision, the merged droplets have two major forces fighting in them: surface tension, which tries to minimize the overall surface area; and gravity, which tries to flatten the large droplet. Together, these forces drive the large oscillations we see in the merged drop, and those oscillations help mix the liquid from the two original drops together. (Image, video, and research credit: Y. Chiu and C. Sun)
April 23, 2020
The Birth of a Liquor
A water droplet immersed in a mixture of anise oil and ethanol displays some pretty complicated dynamics. Its behavior is driven, in part, by the variable miscibility of the three liquids. Water and ethanol are fully miscible, anise oil and ethanol are only partially miscible, and anise oil and water are completely immiscible. These varying levels of miscibility set up a lot of variations in surface tension along and around the droplet, which drives its stretching and eventual jump.
Once detached, the droplet takes on a flattened, lens-like shape that continues to spread. That spreading is driven by the mixing of ethanol and water, which generates heat and, thus, convection around the drop. This not only spreads the droplet, it causes turbulent behavior along the drop’s interface. (Image and video credit: S. Yamanidouzisorkhabi et al.)
April 3, 2020
Collapsing Inside a Soap Film
There’s a common demonstration of surface tension where a loop of string is placed in a soap film and then the film inside the loop is popped, making it suddenly form a perfect circle when the outer soap film’s surface tension pulls the string equally from every direction. In this video, researchers study a similar situation but with a few wrinkles.
Here the loop of string is replaced with an elastic ring, which has more internal stiffness and starts out entirely round within the soap film. Then the researchers pop the outer film. That burst instantly creates a stronger surface tension inside the ring, which causes it collapse inward. As the researchers note, this is the equivalent situation to applying an external pressure on the outside of the ring. The form of the buckling ring and film depends on just how large this “pressurization” is.
Thickening the elastic from a ring to a band alters the collapse, too. The thicker the elastic band, the harder it is to buckle in the plane of the soap film. So instead it wrinkles as the film collapses, which creates wrinkles in the soap film, too! (Image, video, and research credit: F. Box et al.; see also F. Box et al. on arXiv)

February 17, 2020
Coalescence in Heavy Metal Droplets
When a drop of water falls into a pool, it doesn’t always coalesce immediately. Instead, it can go through a coalescence cascade in which the drop partially coalesces, a daughter drop bounces off the surface, settles, and itself partially coalesces. We’ve seen this many times before, but today’s video shows something a little different: here the drop and pool in question are made of a gallium alloy immersed in a background of sodium hydroxide. This means that the drop has very high surface tension (and density) but does not form an oxidation layer on its surface that could inhibit coalescence. And just like the water droplet, the gallium alloy undergoes a series of partial coalescences.
There’s one key difference, though. Did you notice that the water droplets bounce higher as the drops get smaller, but the gallium droplets do the opposite? Previous research suggested that the droplet rebound height is driven by capillary forces, but the high surface tension of both of these liquids means that capillary forces should be large for both of them. Perhaps there’s much more viscous drag in the gallium and sodium hydroxide case? (Image, video, and research credit: R. McGuan et al.)
February 3, 2020
Superwalking Droplets
Vibrate a pool of silicone oil and you can generate walking droplets. Drive the vibration at two simultaneous frequencies and you can support much larger droplets, known as superwalkers. These superwalkers have their own intriguing dynamics, a few of which are featured in this video.
Superwalkers can create promenading pairs, chase one another, orbit, and even form ordered and disordered crystals. They can even generate stop-and-go traffic patterns. As with regular walkers, these complex behaviors come from the interaction of bouncing droplets with their ripples and those of their neighbors. (Image, video, and research credit: R. Valani et al.)
January 29, 2020
Captured by Waves
Acoustic levitation and optical tweezers both use waves — of sound and light, respectively — to trap and control particles. Water waves also have the power to move and capture objects, as shown in this award-winning poster from the 2019 Gallery of Fluid Motion. The central image shows a submerged disk, its position controlled by the arc-shaped wavemaker at work on the water’s surface. The complicated pattern of reflection and refraction of the waves we see on the surface draws the disk to a focal point and holds it there.
On the bottom right, a composite image shows the same effect in action on a submerged triangular disk driven by a straight wavemaker. As the waves pass over the object, they’re refracted, and that change in wave motion creates a flow that pulls the object along until it settles at the wave’s focus. (Image and research credit: A. Sherif and L. Ristroph)
January 23, 2020
Bouncing Off Defects
The splash of a drop impacting a surface depends on many factors — among them droplet speed and size, air pressure, and surface characteristics. In this award-winning video from the 2019 Gallery of Fluid Motion, we see how the geometry of a superhydrophobic surface can alter a splash.
When a drop falls on a protruding superhydrophobic surface, like the apex of a cone, it can be pierced from the inside, completely changing how the droplet rebounds and breaks up. The variations the video walks us through are all relatively simple, but resulting splashes may surprise you nevertheless. (Image and video credit: The Lutetium Project)
January 21, 2020
Swinging Jets
In the tiny realm of microfluidics, flows are, in general, completely laminar. That makes mixing a challenge. But it turns out that pumping water steadily into multiple inlets can spontaneously generate oscillations between the jets, allowing dramatic mixing even at low Reynolds numbers. Two inlets in a parallel channel (first image) oscillate steadily over a small range of conditions, but widening the channels (second image) allows the jets to switch back and forth over a larger range. And adding additional inlets (third image) can create even more complex fluid oscillators! (Image, video, and research credit: A. Bertsch et al.)
January 14, 2020
Flowery Splashes
Plunge a disk into water and you’ll get a dome-like splash that closes back on itself. But what happens when that disk has a patterned surface? In this video, researchers added a wedge-like surface pattern to the disk, creating a splash with petals like a flower. Just as the surface of disk is about to submerge completely, a jet of the remaining air spurts out the trough of each wedge. This air jet breaks up the tip of the triangular splashes focused by the wedge. (Image, research, and video credit: H. Kim et al.)

January 10, 2020
“Viscoelasticity Gives You Wings!”
What happens when you drop a hydrogel bead on a water droplet? Because of the hydrogel’s viscoelasticity and its hydrophilic nature, the rebounding bead carries the droplet with it. As seen in the video above, when the impact energy is small enough, the droplet forms a reverse crown during lift-off, kind of like giving the hydrogel bead a skirt. The key feature for lift-off is the bead’s deformation on impact. Because the hydrogel widens at its base, it is sometimes able to push the entire droplet off its initial footprint and detach it from the surface. (Image, research, and video credit: R. Rabbi et al.)
January 7, 2020