The brand name bathroom cleaner Scrubbing Bubbles has a mascot, that is a bubble with a rotating bush on the bottom.

Please view this video to see an example of the motion.

I would like to know what this kind of propulsion is called, and or what it would take for a real life device to move around like shown in the video.

  • $\begingroup$ Welcome - it would be helpful to users if you could describe in more detail the object in question rather than asking users to follow a link to watch a video. $\endgroup$ – lux Jan 3 '20 at 18:33
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    $\begingroup$ Are you really asking for the physics behind an unrealistic animation which has nothing to do with reality? $\endgroup$ – user243267 Jan 3 '20 at 18:48
  • $\begingroup$ @FakeMod: what exactly is wrong about asking whether an animation is physically plausible? Understanding if models (ie animations in this case) are physically plausible is one way people learn about physics. $\endgroup$ – tfb Jan 3 '20 at 18:59
  • $\begingroup$ @tfb One should not apply physics where it is not meant to be applied. However I saw your answer and it is an okay one, and since now I have seen this question, so I am also getting curious so please answer my comment to your answer. Hopefully we can extract some good physics out of a, in my opinion, not so good question. $\endgroup$ – user243267 Jan 3 '20 at 19:03
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    $\begingroup$ @tfb See the help center. This question does not qualify as on-topic. Questions about physics of fictional worlds which are not sufficiently grounded in real physics are off topic here, but they may be on topic at Science Fiction & Fantasy. $\endgroup$ – user243267 Jan 3 '20 at 19:05

I would say that it is just a cartoon example of motion via Metachronal Waves in Cilia Arrays on a thin (and bubbling) liquid layer self-created by the cartoon character on the floor. This kind of motion, notwithstanding the apparently fancy question, downvotes and a few comments, is a hot topic within sound contemporary Physics. If one doesn't know about it, it is just an occasion to learn some significant physics about interactions between cilia motion and hydrodynamic modes. See, for further reference this paper on PNAS.


Such a thing could exist, unless it's doing a trick (below) only in a very idealised situation, for two reasons.

First of all, in the real world there will be friction between the brushes and the surface. This means that the blue smiley thing will have to exert some torque on the brushes. That in turn means that the blue smiley thing will experience a change in angular momentum over time: in other words it will spin, and it will spin increasingly fast. It's not doing this in the animation.

For that to be the case you'd require two things:

  • there must be no friction between the brushes and the surface;
  • either the whole brush assembly must have zero mass, or the rate at which the brushes rotate could never change.

Neither of these is physically plausible for a real system.

So the answer is that the animation is physically implausible.

One possible exception (ruling out various trickery) would be a counter-rotating set of brushes inside the set you see, so the angular momentum & friction gets balanced out. Doing something like that would probably be plausible, especially if you were willing to have a very active control system which could adjust rotation rates and how hard the brushes press on the surface.

  • $\begingroup$ In the video, the scrub is magically generating foam and bubbles and moving happily and smoothly. How would you make the scrub move like that? $\endgroup$ – user243267 Jan 3 '20 at 19:11

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