Oobleck

[Blaze]

Hey! im doing a project on oobleck. i just ran into a bit of trouble. i would highly appreciate some help.

 

I am dropping weights upto 200 grams into the oobleck to find the point at which the impulse(F*dt = mv) is just enough for the weight to rest on the surface for a significant amount of time(say 200 miliseconds).

My result is that if 32 gramms weight is dropped from a hieght of 2m then it is just enough impulse(.002) or 1N.

 

I would like to know whether the depth of the oobleck matters for the resistance it provides. also does the temperature matter?

Please help me as i need to finish this project asap.

 

Thanks!

 

sorry. the impulse was 0.200352 kgm2s-1

[Schrödinger's hat]

Hey! im doing a project on oobleck. i just ran into a bit of trouble. i would highly appreciate some help.

 

I am dropping weights upto 200 grams into the oobleck to find the point at which the impulse(F*dt = mv) is just enough for the weight to rest on the surface for a significant amount of time(say 200 miliseconds).

My result is that if 32 gramms weight is dropped from a hieght of 2m then it is just enough impulse(.002) or 1N.

 

I would like to know whether the depth of the oobleck matters for the resistance it provides. also does the temperature matter?

Please help me as i need to finish this project asap.

 

Thanks!

 

sorry. the impulse was 0.200352 kgm2s-1

 

 

Hi Blaze,

Just missed you in IRC it seems. This is great reasoning for a highschool project, but oobleck is tricky stuff. The depth will matter, although it is difficult to say how much. Temperature is important from my experiences with oobleck as well.

Hypervalent Iodine tells me you've controlled for surface area, which is great (adjusting pressure rather than impulse, which is more important) but there are time domain things to take into account as well.

Assuming you are using a rigid object, the only thing that will give will be the oobleck, so that will help control things to some extent. Although -- without spending hours with the equations, and not being an expert in fluid dynamics -- I would think that energy would be slightly more important.

You could test this distinction by using a light object at higher velocity, and a heavy one at low velocity, I would be surprised if the thickening was constant at constant impulse. (But by all means, surprise me if I am wrong )

 

 

By far the most important factor would be the ratio of corn starch to water (if this is the type of non-linear fluid you are using). This needs to be kept within as precise bounds as possible as changes of a few percent can have a large effect. This includes settling (the corn starch will settle to the bottom of the mixture over time).

 

Back to your question of depth -- I apologise for the stream of consciousness -- this will tend to be a factor. In a shallow dish the oobleck will need to shear (slide against itself) a lot more than in a deep dish to get out of the way, and so will more easily thicken (Have you noticed that you have to move very slowly to push your finger into the drops and dribbles that you spill near your work area?). The easiest way to rule this out as a factor is to increase (and decrease) the size of your bowl repeatedly until you find the domain in which the amount does not change. Anecdotally (I have done no precise measurements) I would think that once you get over 50mm deep it would not matter much for a small object.

[Blaze]

hi!

I cant tell u how much i appreciate this! thank you so much!

sorry but a few more questions.

how exactly do u propose i change the temperature of the oobleck? i heated my oobleck on the stove and it hardened almost instantly.

 

as for depth, my oobleck was 6 cm deep in a bucket and i changed it to 4cm and it did not seem to make too much of a difference...

 

the thought of energy did cross my mind although i have no idea why i dint act upon it... so to get things clear, you think energy and not impulse is the factor concerning hardness? so if i just use 1/2 mv2 = Kinetic Energy i should be ok? since right now im using mv for impulse or momentum.

 

i will make momentum/ impulse the same by adjusting the velocities of the light and heavy weight and see what happens.

 

thank you!

[Schrödinger's hat]

hi!

I cant tell u how much i appreciate this! thank you so much!

sorry but a few more questions.

how exactly do u propose i change the temperature of the oobleck? i heated my oobleck on the stove and it hardened almost instantly.

 

as for depth, my oobleck was 6 cm deep in a bucket and i changed it to 4cm and it did not seem to make too much of a difference...

 

the thought of energy did cross my mind although i have no idea why i dint act upon it... so to get things clear, you think energy and not impulse is the factor concerning hardness? so if i just use 1/2 mv2 = Kinetic Energy i should be ok? since right now im using mv for impulse or momentum.

 

i will make momentum/ impulse the same by adjusting the velocities of the light and heavy weight and see what happens.

 

thank you!

 

 

I would alter the temperature by making your oobleck with cold water, or refrigerating it for a time. Heating it up too much will change it chemically. (Also be sure to control the amount of water present as precisely as possible between measurements. I would recommend stirring frequently, don't handle it with your hands too much as they absorb water and try and keep time scales short to avoid evaporation. To give you an idea of how quickly the viscosity can change, I have had blobs that are barely thick enough to be picked up, even with very fast motions go hard in 2 or 3 minutes)

Being able to change the depth without the required impact changing is a good sign that this is not an important factor (although I would try a slightly wider range of depths to be sure, maybe go up to 8 or 10cm and down to 2 or 3 if possible. Bear in mind that having too narrow a container could have a similar effect)

 

I was saying I'd be more inclined to think energy was the determining factor over impulse, but it's more likely somewhere in between constant energy and constant impulse (or possibly some m^x*v^y where x and y could be anything). In my experience the thickening is related to the initial shear rate or directly proportional to the velocity of the in coming object which would lead one to think of momentum. However, I have only played with objects that have a force behind them (such as my hand or foot, or a rod).

 

We can do a thought experiment:

Consider a small object coming in at high velocity, this will tend to bounce (although not quite well enough to play handball on a tub of oobleck, we tried)

Where as a very large object moving slowly will sink.

These can easily have the same momentum. So momentum alone is not a factor.

 

 

You have controlled reasonably well for this effect as your objects are all moving quite fast. So they will be imparting their momentum in a time span of roughly how long it takes a sound wave to travel through your oobleck (providing they stop, and the oobleck does not deform too much). So you are effectively comparing different (average) forces applied for a set time.

 

The one other factor to take into account is distance (how far does the oobleck deform in this time). The further the object goes before it stops, the more the oobleck shears. If we continue with the assumption of roughly constant time then distance travelled over constant time gives us a shear rate (shear/time, but time is const so only shear).

Shear rate is related to viscosity in a dilatant, which is what will give us resistance force

 

The more I think about this, the less sure I am that it will be either constant momentum or constant energy. I am thinking there may be limits for both of those, as well as just velocity.

 

Consider a very massive object such that it has more of both momentum and energy than the objects you dropped, but it is dropped from very close so it is moving slowly. The oobleck will not thicken and it will not stop (or even be resisted).

 

So first we need to find the shear rate at which we would consider the oobleck solid. Only once that is exceeded can any assumptions about near constant time be validated and impulse or energy be examined.

 

Only one thing can save us now...

 

SCIENCE!!!

 

 

Do you know how viscosity is usually tested?

http://en.wikipedia.org/wiki/Viscometer