When droplets on a superhydrophobic surface coalesce with one another, they jump. Individually, each drop has a surface energy that depends on its size. When two smaller droplets coalesce into a larger drop, the final drop’s surface energy is smaller than the sum of the parent droplets. Energy has to be conserved, though, so that excess surface energy gets converted to kinetic energy, causing the new droplet to leap up. Smaller droplets have higher jumping velocities. For more, see the original video. (Image credit: J. Boreyko and C. Chen, source video)
Search results for: “droplet”
Levitating Droplets with Motion
There are many ways to levitate a droplet – heating, vibration, and acoustic levitation all come to mind – but this video demonstrates a simpler method: a moving wall. Depositing a drop on a moving wall keeps it aloft with a thin, constantly replenished layer of air. The thickness of this lubricating air film is directly measurable from interference fringes created by light reflecting off the surface of the drop. Incredibly, the air layer is only a few microns thick, but the resulting pressure in the air film is high enough to levitate millimeter-sized droplets! (Video credit: M. Saito et al.; via @AlvaroGuM)
Shooting Droplets with Lasers
Last week we saw what happens when a solid projectile hits a water droplet; today’s video shows the impact of a laser pulse on a droplet. Several things happen here, but at very different speeds. When the laser impacts, it vaporizes part of the droplet within nanoseconds. A shock wave spreads from the point of impact and a cloud of mist sprays out. This also generates pressure on the impact face of the droplet, but it takes milliseconds–millions of nanoseconds–for the droplet to start moving and deforming. The subsequent explosion of the drop depends both on the laser energy and focus, which determine the size of the impulse imparted to the droplet. The motivation for the work is extreme ultraviolet lithography–a technique used for manufacturing next-generation semiconductor integrated circuits–which uses lasers to vaporize microscopic droplets during the manufacturing process. (Video credit: A. Klein et al.)
Shooting Droplets
This animation shows high-speed video of a polystyrene particle striking a falling water droplet. Under the right conditions, the particle rips through the droplet, stretching the water into a bell-shaped lamella extending from a thicker rim. When the particle detaches, surface tension rapidly collapses the lamella into a ring which destabilizes. Thin ligaments and droplets fly off the crown-like ring as momentum overcomes surface tension’s ability to hold the droplet together. Be sure to check out the full video on YouTube or later next month at the APS Division of Fluid Dynamics meeting. (Yes, I will be there!) (Image credit: V. Sechenyh et al., source video)
Bead-Infused Droplet
A Leidenfrost droplet impregnated with hydrophilic beads hovers on a thin film of its own vapor. The Leidenfrost effect occurs when a liquid touches a solid surface much, much hotter than its boiling point. Instead of boiling entirely away, part of the liquid vaporizes and the remaining liquid survives for extended periods while the vapor layer insulates it from the hot surface. Hydrophilic beads inserted into Leidenfrost water droplets initially sink and are completely enveloped by the liquid. But, as the drop evaporates, the beads self-organize, forming a monolayer that coats the surface of the drop. The outer surface of the beads drys out, trapping the beads and causing the evaporation rate to slow because less liquid is exposed. (Photo credit: L. Maquet et al.; research paper – pdf)
Droplets Surfing
The Leidenfrost effect can make water droplets skitter across a hot griddle or briefly protect a hand dunked in liquid nitrogen. When a liquid is exposed to a solid surface much, much hotter than its boiling point, the contact vaporizes part of the liquid, and, in the case of a droplet, forms a thin lubricating layer of vapor that the liquid drop can skate around on. Researchers have found that releasing these Leidenfrost droplets on textured surfaces creates self-propelling drops by directing the flow of vapor. In this video, one team demonstrates some of the neat tracks they’ve built for their drops. (Video credit: D. Soto et al.)
Self-Propelled Droplets
Leidenfrost drops hover and move above hot surfaces on a thin layer of their own vapor. Over a flat surface, this vapor flows radially out from under the droplet, but creating rachets in the surface forces the vapor to flow in a single direction. The vapor then acts like exhaust, generating propulsion in the droplet and making it roll. How quickly the drop moves depends both on the droplet’s size and the rachets’ aspect ratio. For a given length, deeper rachets propel a drop faster than their shallower counterparts. The droplet’s size also affects the thrust with different scalings depending on the drop’s initial size. Like all of this week’s videos, this video is an entry in the 2013 Gallery of Fluid Motion. (Video credit: A. G. Marin et al.)
Shaping and Levitating Droplets
Opposing ultrasonic speakers can be used to trap and levitate droplets against gravity using acoustic pressure. Changes to field strength can do things like bring separate objects together or flatten droplets. The squished shape of the droplet is the result of a balance between acoustic pressure trying to flatten the drop and surface tension, which tries to pull the drop into a sphere. If the acoustic field strength changes with a frequency that is a harmonic of the drop’s resonant frequency, the drop will oscillate in a star-like shape dependent on the harmonic. The video above demonstrates this for many harmonic frequencies. It also shows how alterations to the drop’s surface tension (by adding water at 2:19) can trigger the instability. Finally, if the field strength is increased even further, the drop’s behavior becomes chaotic as the acoustic pressure overwhelms surface tension’s ability to hold the drop together. Like all of this week’s videos, this video is a submission to the 2103 Gallery of Fluid Motion. (Video credit: W. Ran and S. Fredericks)
Droplet Collisions
When droplets collide, there are three basic outcomes: they bounce off one another; they coalesce into one big drop; or they coalesce and then separate. Which outcome we observe depends on the relative importance of the droplets’ inertia compared to their surface tension. This is expressed through the dimensionless Weber number, made up of density, velocity, droplet diameter, and surface tension. For a low Weber number droplet, surface tension is still significant, so colliding droplets bounce off one another. At a moderate Weber number, the droplets coalesce. But when the fluid inertia is too high, as in the high Weber number example, the drops will coalesce but still have too much momentum and ultimately separate. (Video credit: G. Oldenziel)
Maze-Solving Droplets
The Leidenfrost effect occurs when liquids come in contact with a substrate much, much hotter than their boiling temperature. Rather than immediately boiling away, a thin layer of the liquid vaporizes and insulates the bulk of the liquid from the heat. This essentially turns droplets into tiny hovercrafts that skate over the surface. If you use a rough surface with rachets, the Leidenfrost drops will self-propel toward the steepest part of the rachet. The vapor underneath the drop is constantly trying to flow away, and the rachets in the surface prevent the vapor from escaping in the steeper direction. The vapor instead flows out the shallower side and–thanks to Newton’s third law–creates thrust that pushes the droplet the opposite direction. Here students from the University of Bath have used these effects to build a maze through which the droplets fly. (Video credit: C. Cheng et al.; via Flow Visualization FB page and several submissions)
For readers at Texas A&M University, I will be giving a talk Wednesday, October 2nd entitled “The Beauty of the Flow” as part of the Applied Mathematics Undergraduate Seminar series at 17:45 in BLOC 164.