Dancing Water Drops In Earth Orbit

An astronaut once told me that fellow space flier Don Pettit could fix anything with a paper clip. Indeed, Pettit has nicknames like Mr. Wizard and Mr. Fixit, and he is well-known for his Saturday Morning Science videos during his first stay on the International Space Station and his “Zero G Coffee Cup” from a space shuttle mission he was on in 2008. Now in his second long-duration stint on the ISS, Pettit has a new video series called “Science off the Sphere” and the first video is above. Pettit uses “knittin” needles (watch the video to hear Pettit’s pronunciation) and water droplets to demonstrate physics in space, and shows what fun astronauts can have with water in zero-G with his ‘dancing’ water droplets.

This new video series is partnership between NASA and the American Physical Society. But there’s more than just videos, as at the end of each video Pettit poses a challenge question. Submit your answers at the Science Off the Sphere website for a chance to have your name read from space and receive a snazzy t-shirt from Earth.

Here’s this week’s Challenge Question:

In the end of the video, Dr. Pettit put a nylon knitting needle near the syringe used to squirt water droplets onto the Teflon knitting needle. Why did Dr. Pettit need a nylon needle near the injected drops and why must the other needle be Teflon?

A winner will be randomly chosen from all correct answers submitted. You have one week from the date the challenge is posted. The winner will receive a “Science off the Sphere” t-shirt and have their name and answer read by Dr. Pettit from the ISS. Winners and answers are posted one week after the challenge question is posed.

12 Replies to “Dancing Water Drops In Earth Orbit”

  1. Hi, I don’t have an answer to the question, but I do have a question. Do these water drops dance around the knitting needle in the same kind of pattern as the Trojan asteroid travels in Earth’s orbit around the Sun?

    1. In my understanding, (if we are talking about the same thing) the Lagrange point that the Trojan centres on is a point, not a linear attractor like the knittin’ needle. So even though the moonlet is traveling along in orbit, it acts more like a normal orbital that just happens to be traveling along much in the same way we orbit the sun as it orbits the centre of our galaxy.

    2. Are you referring to the trapping of the droplets to the charged (rubbed) piece of knitting needle? It is analogous to a magnetic mirror, which can happen as a magnetic field bunches up at the magnetic poles. The increased density of line bundles forces the migration along the field lines of charged particles to slow down and reverse direction.

      For the needle, the electrostatic mirror would be at the two edges of the charged region where the field lines would tend to bunch up slightly I think.

  2. An accelerating charge will radiate energy, leading to spirals when you have tiny particles in a cloud chamber. Is this effect too small to make a difference with big drops doing to accelerating?

    1. It seems to me that this effect must be negligibly small. The spiralling process is probably facilitated by dissipation of tidal forces; as the water drop dances around the needle, it will be deformed slightly due to the attractive forces of the needle and starts vibrating a bit, absorbing a fraction of the rotational energy. Due to internal friction in the water, the vibrational energy is converted to heat and eventually lost. In this way, vibrations in the drop serve as a channel that transfers orbital energy from the drop.

    2. I tend to agree with Maarten’s statements. I might add another source of this inspiral. A force that is F ~ -1/r^2 and F ~ -r^2 derives orbits or paths which close on themselves. Since the attracting charge is on a line we might think of the force per unit length of the knitting needle as f = ?/(r + l)^2, and the force requires integrating over the length of the needle. The result is a 1/r type of force. As a result the one dimensional geometry of these orbits do not necessarily close upon themselves into ellipses or circles.


  3. Clearly the nyon and teflon acquire opposite charges when rubbed with the “cat skin,” or what ever is used to do the rubbing. How that happens for the two materials I am not sure of.

    There is one further departure from standard gravity or electrostatics from a point charge. The charge is distributed on a line (nearly a line or thick line). The potential function is then computed by Gauss’ law. The force, evaluated by the potential F = -?? evaluated on an imaginary cylinder around the knitting needle (line) is ?Fd?dl for the angular variable evaluated from 0 to 2? and the length from 0 to L. The Gauss law tells us that

    ?Fd?dl = ???Fdrd?dl = 2?qL

    which evaluated the divergence of the force in the volume contained in the cylinder, integrated out to the radius of the cylinder. As a result the force is F ~ q/r instead of F ~ q/r^2.


    1. Opposite charges should be the explanation, the nylon would be positively charged and draw electrons from the droplets so they acquire they same charge. The teflon would be opposite charge, and we would see the dynamics of the video.

      As for how that happens, I think it is straight forward.

      – A hydrocarbon would tend to give electrons by having covalently bounded hydrogen loose them, acting like an alkaline.

      – A fluorocarbon would tend to take electrons by having covalently bounded fluorine taking them, being a halogen.

  4. …or in laymans terms…
    (Google ‘triboelectric’)
    Nylon will develop a positive static charge. The nylon needle, which has been charged, induces a positive charge in the droplet as it is squeezed out.
    Teflon will develop a negative static charge.
    This makes the difference between the droplet and the teflon needle larger and, therefor, the effects are more pronounced.
    The downside of this ‘more pronounced’ effect is that the charge on the teflon needle will be dissipated more rapidly because the charged droplet will ‘balance the differential’ more directly.

  5. When rubbed, the Teflon knitting needle produces a negative charge, while the nylon knitting needle produces a positive charge. The nylon needle gives its positive charge to the injected water drops, which are then attracted to the Teflon needle.

  6. How can electromagnetic force affect objects at a distance? Well, taking into account the constant of Planck, the uncertainty of Heisenberg, and the relativity of Einstein, we still don’t know. But seriously, does anybody know what is really going on with electromagnetic force? Obviously I am no physicist, but this demonstration makes me wonder if electromagnetic force warps space as gravity seems to do. And because electromagnetic force is a “strong force” while gravity is a “weak force” the drops are orbiting very fast and close to the knitting needle. I think without the air resistance, the drops would orbit for a long time. Maybe Pettit could repeat the experiment outside the space station.

  7. Nylon is above paper in the turboelectric series, and teflon is (very far) below. so when you rub them with paper, Nylon becomes positively charged and teflon becomes negatively charged. water is usually electrically neutral, so the nylon rod near the water syringe attracts any free-moving negative charge. this leaves the water droplets with a net positive charge once they get near the negatively charged teflon, making them attracted to the teflon and spiral inwards. if the nylon rod was not there, the water droplets would have no net charge; there would still be a net attractive force due to the water’s dipole moments, but the force would be much weaker.

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