# How to 'smooth/flatten' the surface of a disturbed liquid without contacting it?

I would like to essentially flatten or smooth the surface of a viscous liquid after dropping an object into it, without actually touching the liquid. For example, if I drop a rock into a tub of oil, I need the oil to move into the void and return to having a smooth surface as fast as possible.

It seems that I need to create some sort of force that acts against the liquid surface from above, or a force from the sides that acts as a kind of 'wiper' to smooth it out.

I have looked into using ferrofluids and electromagnets on the sides or bottom of the liquid vat/container, however many demonstrations I have seen result in a 'spiky' surfaces when magnets are brought near the liquid due to non-uniform field lines.

I have also looked into trying to suspend diamagnetic nanomaterials in the liquid which could be repelled by a magnetic field from above the vat, however I'm not sure about whether diamagnetic materials exist which would could be used like ferrofluids in the sense of staying suspended in the liquid.

My job depends on being able to do this, so if anyone has any ideas I'd really appreciate hearing them.

• – hazzey
Dec 27 '15 at 2:36
• Yes that was my question, however I'm now looking for alternative solutions to ferrofluids as it's apparent they won't work. Dec 27 '15 at 2:41
• Wow I answered that question, and rather than accepting my answer you opted to delete the question? The third paragraph of this question summarizes my answer lol. Dec 27 '15 at 13:07
• Sorry mate I only realised I didn't accept it until after deleting. I'll contact admin and try and have them reinstate it so I can accept it as an answer. I deleted as it seemed similar to this question but it is actually different, so I just wanted to try and remove what may appear to be a double post. Dec 27 '15 at 13:16
• Also, you provide no specifications. What are the restrictions? What is the viscosity? How much fluid? What is considered "flat"? (Why does your job depend on this??) For instance, you could put an air jet on an XY table and 'puff' at the surface, or you could build a centrifuge, or you could heat the fluid to lower its viscosity then let it cool, etc. No specs, open-ended, so every answer is speculative. Dec 27 '15 at 13:25

Some quick dimensional analysis for a Newtonian fluid suggests that the settling time scales as follows:

$$\tau = \sqrt{\frac{L}{g}} f\left(\frac{\nu}{\sqrt{g L^3}}\right)$$

where $\tau$ is the time it takes for the liquid surface to settle, $L$ is some characteristic length of the dropped object (say, the diameter of a ball), $\nu$ is the kinematic viscosity, and $g$ is the acceleration due to gravity. I expect the function $f$ to increase as the parameter inside it increases, which makes sense. The higher the viscosity, the slower the settling, and the stronger the gravity, the faster the settling.

So, influencing these parameters in some way could help. Unfortunately, you have no control over these parameters aside from the viscosity, which you might influence via heating.

An oscillation applied to the container could help smooth things out. The speed by which this occurs surely is strongly dependent on the viscosity for a Newtonian fluid. For a non-Newtonian fluid (say, peanut butter), I'm not sure this will necessarily work.

I also imagine an air jet applied parallel to the surface could help, depending on the viscosity, especially if its direction is varied. Targeted jets striking at an angle to the surface might help too, but this could require a complicated control system or manual human intervention to do the targeting.

Suction applied directly above the hole may help, again, depending on the viscosity.

(After writing this, I noticed Chuck said some similar things in the comments. He also recommended a centrifuge, which I didn't think of.)

• Thanks mate this helps a lot. I'm planning to set up an oscillating diaphram above the vat with a variable speed control to observe the effects it has on the resin surface. Failing that I though I could try a pump which circulated the resin from the bottom of the vat and over an overflow, along with a fan forcing air onto the surface. Dec 28 '15 at 9:20
• Glad you found this helpful. Recirculating the fluid is another interesting idea. Might be worth shooting a jet of the fluid directly into the hole to fill it in, though that would only help in the initial stages if the hole is large enough. Dec 28 '15 at 14:54
• The centrifuge idea seems neat. If your oil is rotating, your disturbance should be transported to the edge of your surface depending on how fast your rotating. You'd have to recirculate which could also help mitigate the disturbance if you're shooting the jet into the place of impact as @BenTrettel suggested. You'd have the same problem with a high viscosity liquid though. Dec 28 '15 at 16:45
• Excellent idea, @RossV. Combining a centrifuge and recirculation does seem to offer advantages. Dec 30 '15 at 14:19