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Using Liquid Nitrogen to Freeze Water

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This is not technically a computer game-programming question, but I thought maybe a physics person could help me out. I'm building and programming a robot to navigate an obstacle course for a mechatronics class I'm taking this quarter (cool stuff, see me118.stanford.edu for final projects from other years). Some of the obstacles include small lakes fill with water. It occurred to me that I wouldn't need to bother trying to navigate around these lakes if I could use liquid nitrogen to freeze them so that my robot could simply roll over them. These lakes are about 2 feet long and 8 inches wide and 3 inches deep. I estimate it's about three liters of water. I was wondering if anyone could give me an idea of how much liquid nitrogen would be required in practice to freeze this amount of water so that a 5 pound robot could roll over it. I've been poking around online and all I've been able to find is that the specific heat of LN2 seems to be a quarter that of H2O, so I would need more than 12 liters of LN2 to freeze one lake (since there is also the latent heat of fusion to consider). This is a lot of LN2. I was wondering if anyone here on the physics forum has practical experience working with LN2 (I've only played with it myself) and if so, could they comment on whether this plan is likely to work. Other issues I've considered is the amount of LN2 my robot can reasonably carry, since the robot can't be larger than one foot in all dimensions and most vacuum flasks I've seen are larger than this. I'd also be worried about the condensation messing with my electronics. Is there a better freezing agent that I could use? I settled on LN2 because it doesn't explode like liquid hydrogen might and it's much colder than dry ice.

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[lol] That idea made me smile. Here is something for you to think about though. Let's say you do end up being able to freeze the little pond. I know you would need a decent amount of it so it becomes really solid, but the issue becomes how are you going to roll over it? You will need some sort of tracks or something with 'cleats' so the robot can move across the ice, which has a really low coefficient of friciton. Otherwise your robot will just spin its wheels when it tries to do so.

Just for kicks, imagine if you did the opposite and "steamed" the ponds. I'm talking about using therminte. Now that would be fun [smile] except for the fact that your robot will probabally be in the radius and melt itself...oh well it was worth a try though.

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Quote:
Original post by Fragmo
Instead of carrying 12 liters of LN2, why not carry a 3 liter jug and vaccum out the water. perhaps you could name your robot 'moses' then :)


Why bother carrying a jug to suck it into? why not just suck it up and spray it out behind the robot?

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Pumping it out would be a great solution except that the robot is not allowed to touch the bottom of the lake. We also have been toying with the idea of filling it with sand or something like that. I do like the thermite idea, though. How much thermite would be required to boil 3 liters of water? I know that stuff is crazy hot, but I bet it's a lot.

Simplier ideas like going *around* the lake is what most groups are doing. We want to do something a little bit more impressive since we are engineering hotshots who feel the need to show off.

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Extendable bridge? Have it come out the top of the Robot, then drop it, drive off. Or, you could have it come out the bottom, and have it drive across on tracks, and then un-extend it(that way, it keeps the ladder). Just throwing out ideas.

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why carry 12 liters of liquid nitrogen if you can use one small stick of dinamite to blow any obstacle? [grin][lol]

seriously though, how to compute how many liters of liquid nitrogen you need to to freeze one liter of water:


heat_capacity_of_water*themperature_of_water_in_C + heat_of_freezing_water
---------------------------------------------------------------------------------------------------------
density_of_liquid_nitrogen*(heat_of_evaporation_of_liquid_hitrogen+heat_capacity_of_GASEOUS_nitrogen*196)


and seems you computed something different and wrong.

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Original post by oliii
buoyancy anyone?


nice idea but it's not allowed to tuch the bottom of the lake. There's no possible way a 5 pound robot could displace enough water to float since entire "lake" is only about 6 pounds and 3 inches deep (bloody kg vs pounds). It would have to be almost as big as the lake itself. In real life, bouyancy would be the only real option of course (that or crawling across the bottom or going around).

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At construction sites people use some kind of foam which hardens after a while. They use it to fill the gaps between windows and walls, but maybe it could be used to make a bridge. I believe that the foam floats, so maybe it could work with deeper water too.

You could also try to find information about bridge-laying tanks. They are pretty impressive.

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On more serious ice nine - related note, add some chemical to water that turns it into strong enough gel... or use some hard glue that float.

(or better yet, make pathfinding software as you'll anyway have other obstacles to avoid)

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I wonder how strong of a blower you'd need to make a hovercraft/skirt style lift system. Havign this could replace basically anything else you have for steering,wheels and such too. Might end up being lighter.
You could still steer by fan, small obstacles wouldnt be much of a problem, steering would just take practice, water and such would be easy.

You'd have to paint it like the purple car from F-Zero though, only then would it be cool.

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I've never tried it, but my guess is that if your robot poured liquid nitrogen into the water, the liquid nitrogen would explode and empty the lake. Then there would be no ice to cross over.

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Original post by JohnBolton
I've never tried it, but my guess is that if your robot poured liquid nitrogen into the water, the liquid nitrogen would explode and empty the lake. Then there would be no ice to cross over.

Why would it explode?

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Quote:
Original post by Raduprv
Quote:
Original post by JohnBolton
I've never tried it, but my guess is that if your robot poured liquid nitrogen into the water, the liquid nitrogen would explode and empty the lake. Then there would be no ice to cross over.

Why would it explode?


I don;t know but it could be that the liquid nitrogen evaporates immediately. When going from a solid or liquid to a gas, the expansion in volume is quite big. If this happens at a fast rate, you'll get an explosion.

hmm, I've been told there will be some smoke, no really cool effects though, and probably very little ice unless you pour a lot of liquid nitrogen.

[Edited by - Airo on February 15, 2005 4:27:15 AM]

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you plan to build a kickass robot and you come here to ask a question like this?

id have thought any engineer undertaking such a project could calculate the effectiveness of this approach in a couple of seconds.

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Original post by Eelco
you plan to build a kickass robot and you come here to ask a question like this?

id have thought any engineer undertaking such a project could calculate the effectiveness of this approach in a couple of seconds.

i also thought about it, but then i realized that at uni they almost don't design anything.

[Edited by - Dmytry on February 15, 2005 5:26:17 AM]

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Original post by Dmytry
Quote:
Original post by Eelco
you plan to build a kickass robot and you come here to ask a question like this?

id have thought any engineer undertaking such a project could calculate the effectiveness of this approach in a couple of seconds.

i also thought about it, but then i realized that at uni they almost don't design anything.

then that would mean he would atleast be able to answer his own theoretical questions, no?

what specificly do you mean be designing btw? i think i do that about 40% of my time on uni.

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