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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.

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    $\begingroup$ You can "make gravity stronger" by spinning the liquid. Ultracentrifuges can make up to 2,000,000g... although I would not advise to drop anything into one while it's spinning... Why would your job depend on this??? In any case, if you want real advice then you need to tell us what you really want to do rather then being cryptic about it. Usually when people want to do something in one specific way it turns out that they have just caught themselves in a mental cul-de-sac. Why are you dropping things in liquids and why is it important that the surface be smooth??? $\endgroup$
    – CuriousOne
    Commented Dec 27, 2015 at 1:56
  • $\begingroup$ It seems to me that you would get way better responses by asking "How do I do X" in engineering than going trough a list of unsuitable ideas in physics. Just my two cents. As for the physics of your problem, what you really need is to decrease the viscosity of your liquid. $\endgroup$
    – CuriousOne
    Commented Dec 27, 2015 at 2:03
  • $\begingroup$ Yes using a centrifuge was another idea I was thinking of. I should have mentioned it earlier but I'm not able to use a centrifuge as it would separate the particles in the liquid I am using and is a bit complicated for our application. It is for 3d printing. We use UV curable resins and a UV projector which shines images on top of the liquid to form shapes. $\endgroup$
    – user88720
    Commented Dec 27, 2015 at 2:18
  • $\begingroup$ Our printers launch in a couple of months and my role is to solve the issue we are having with the viscosity of the liquid and models with large flat surfaces. When we lower our models with large surface areas lower into the resin as we project images on the surface, it creates impressions/voids on the surface which messes up prints. The only solution we have is to print much slower, which isn't really an option for us. $\endgroup$
    – user88720
    Commented Dec 27, 2015 at 2:19
  • $\begingroup$ We can't use anything that touches the surface of the liquid, such as a glass or clear plastic plate/sheet as the resin will cure to it, and it also creates large suction forces as the build plate lowers, which can break the models. A wiper is an option in standard printing, where the model is dipped in the resin, then raised to the top again and exposed to another image. But ours uses a continuous method where we lower the build plate continuously and display an animation. $\endgroup$
    – user88720
    Commented Dec 27, 2015 at 2:19

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You say you are currently use heaters. May I suggest the following modification (not sure it will be practical, but who knows...). So you dropped something into your tub of oil, you can then close your tub with a metal cover and excite microwave field under the cover, as in a microwave oven. If your oil has appropriate conductivity (or absorption at the microwave frequency), the upper layer of an appropriate thickness will be heated, acquire lower viscosity, and oil will start to fill the depressions faster. You will need to make sure the microwave field does not leak through the walls of the tub. Other methods of radiative heating of the surface can be worth considering as well.

EDIT (12/27/2015): (based on answers to the OP's comments).

  1. The advantage of microwaves could be that you can choose the thickness of the heated layer (if you can modify, say, the conductivity of the resin). You heat exactly the layer that you need to be less viscous, rather than the entire volume (you just want to fill the depression on the surface). That means that you can heat it faster (and consuming less energy), and faster is what you want.

  2. You can additionally pour a light and stable liquid (or even heavier-than-air gas, like CO2 or argon) on the surface, so there will be no access of oxygen to your resin, so you won't need to care about flash point.

  3. Thermal curing is not necessarily a show-stopper, as it can take significantly more time than filling the depressions. This is where the microwave approach can be useful as well: you only heat a relatively thin layer, which would cool down much faster when microwave heating is turned off.

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  • $\begingroup$ I appreciate the suggestion, however at the moment we heat to just below the flash point of the resin. Any further heating may make it quite volatile. $\endgroup$
    – user88720
    Commented Dec 27, 2015 at 7:48
  • $\begingroup$ @user88720: You do not have to heat it more with microwaves. The advantage of microwaves could be that you can choose the thickness of the heated layer (if you can modify, say, the conductivity of the resin). $\endgroup$
    – akhmeteli
    Commented Dec 27, 2015 at 9:33
  • $\begingroup$ Yeah we can change the conductivity that's fine, I'm just not sure what the advantages would be for making the top layer more viscous than the bottom layers, when compared with making the whole volume less viscous that's all. $\endgroup$
    – user88720
    Commented Dec 27, 2015 at 12:12
  • $\begingroup$ @user88720: The advantage can be that you heat exactly the layer that you need to be less viscous, rather than the entire volume (you just want to fill the depression on the surface). That means that you can heat it faster (and consuming less energy), and faster is what you want, if I understand correctly. $\endgroup$
    – akhmeteli
    Commented Dec 27, 2015 at 13:26
  • $\begingroup$ Yeah I understand what you mean. Although energy use isn't a huge concern at present. Our resins are already heated to just below their flash point so the viscosity is pretty much as low as it can be, but we are still getting voids. $\endgroup$
    – user88720
    Commented Dec 27, 2015 at 13:40

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