Using your Ice

[Waystin]

I started looking online for a way to make my ice melt faster or slower depending on what I'm doing. (I like the ice release method. I haven't rigged anything up with a timer yet)

What I found is that if you put salt in your water before you freeze it, It will melt faster. I have not yet found anything you can add to make it melt slower though.

I did find out that you CAN make your ice melt slower by setting your freezer to the absolute coldest it can go and letting the ice freeze for longer than normal. The longer it can stay in deep freeze, the longer it will take to thaw out and melt when its exposed to temperatures above its melting point.

Another simple way to make your ice last longer is to point a fan at it. Apparently the fan keep the ice a little more dry, essentially "pushing" the water off of the ice as it melt, keeping it dry. The drier your ice, the slower it melts.

I anyone has any other tips about this (especially making your ice last longer because that is apparently hard to do) I'd appreciate the feedback!

[lj]

There have been a few discussions on ice releases.

The rate of thawing depends upon several things, but the rate of heating is what matters - this depends upon factors such as the surface-to-volume ratio, the difference between ambient and ice temperature and the thermal conductivity of the surrounding air.

So a big block of ice will thaw more slowly than a small one, and a spherical block will thaw slower than a rectangular one - surface-to-volume ratio - as the heat conducting air has a smaller surface to thaw compared to the ice volume. The fan helps by removing water by evaporation, which in itself requires heat (latent heat of evaporation) as it supplies a moving volume of dry air which includes warmth, so speeding up the thawing.

You can't do much about the thermal conductivity of air, except by increasing the humidity, which will reduce the rate of evaporation and slow the thawing by a different interaction!

You can also increase the thawing by subjecting the ice to pressure - so if you use a piece of cord frozen into the block, pulling on the cord will make the ice thaw, allowing the cord to slowly flow through the ice.

Putting ice into a deep-freeze won't make a substantial difference to the rate of thawing as ice is quite a good conductor of heat and will soon warm up to freezing point, it is the melting process that keeps it from thawing - latent heat required to change from solid to liquid.

The physics is quite complex, just use a bigger block of ice for a longer release time !!!

[boundsissy]

Perhaps insulating the ice will make it melt slower, getting your release out of a container or a cooler may add to your SB fun.

[Tenderfoot88]

Well, as far as modifying the melting point, you can only go one way. Pure water has the highest melting temperature - any impurities make the crystal form of the solid less perfect and therefore requires less energy to break apart. (This doesn't really do the physical processes justice, but it's somewhat analogous.) You can, of course, use something that has a higher melting temperature than water, but there aren't many options that fit between room temperature and freezing. (Lead is the next lowest melting temp I can think of, and it doesn't quite melt in boiling water, so...) So the purest water you can get your hands on, I guess, though the benefits you'll find from pure water (the expensive chem-lab grade stuff) vs tap water are pretty minimal.

The deep freeze will improve matters somewhat, but not immensely. It has a thermal conductivity approximately ten times that of air (and increases as you get colder - if you have one of those coolers for making dry ice, cooling your ice to -100 will result in a 56% increase over having it at 0 degrees), thus it will be able to transfer energy to the surfaces much more quickly than air takes it away. 2.2 W/mK really isn't that high of a thermal conductivity, but it's more than sufficient to efficiently warm ice in air. (Steel is somewhere around 50ish, which is why it feels cold to the touch - it effectively sucks the warmth out of you...or you're saliva, if you're one of the idiots who licks a flagpole in winter to prove it really does stick to your tongue.) You gain another 2 joules of energy per gram, per degree of cooling, (though again, this gets worse as you go on, by -100 it's only 1.4) - more energy exposed to the same conditions means more time for it to melt. As LJ stated, the big issue is the latent heat - the 334 Joules per gram it takes to get from solid to liquid.

Of course, that's assuming you've found a way to do an ice release that holds until the ice is entirely melted, which is virtually impossible. Cooling it that extra few degrees makes it just a bit more durable. It takes a bit longer for things to start melting, so it will maintain structural integrity a bit longer, which tacking another few mills on the end of the ice block won't do. But, like the water purity issue, you won't be getting enough benefit out of it to buy a liquid nitrogen freezer. Well, aside from the obvious awesomeness of being able to use liquid nitrogen whenever you want, but that's a bit off topic.

The fan idea produces two opposing effects, which depend on different conditions of the air. One is the air circulation - the air immediately around the ice is cooled as the ice warms (just as in a drink) - if this cooled air is moving about the room, it gets replaced by warmer air continuously, so you get a warming effect. The warmer the air is, the more pronounced this effect will be. The other is evaporative cooling. When water evaporates, it takes a lot of energy to go from liquid to gas (2260 Joules per gram) and thus it steals a lot of the energy available for melting ice. (This is exactly how sweat works.) This effect is slave to humidity - at low humidity, it is incredible (At zero humidity, 110 degrees Fahrenheit can be cooled down to about 60 degrees through evaporative cooling alone (If you're actually experiencing that, however, you're losing water like a squeezed sponge, so drink up). Sorry for using the barbaric version of temperature, that's what the only decent psychrometric chart I can find on the net is in) At 100% humidity, it doesn't do a damned thing. (With stagnant air, the humidity gets to 100% locally really fast - that's why breezes feel so good on hot summer days. So, unless you have really high humidity (so much so that a fan in your face doesn't cool you), the net result will probably be that it will cool you off.

The best way to make your ice last noticeably longer is to improve the design of your ice releases. I can't find enough information to be able to actually model an ideal structure for the ice, since it's normally considered unsuitable for construction due to melting issues (plus modeling how the ice melts to see how long it maintains integrity would be rather challenging, especially with the compression-heating issue LJ was talking about. (Ice palace construction crews probably have some ideas, but they don't seem to be publishing them). But basic principles suggest that having it thicker where you're pressing on it, and minimizing the number of holes in it and making the surfaces relatively even will give you the best strength and minimize stress concentrations.

[james_carmen]

depending on your release method, this may or may not be appropriate. To increase melting time wrap some tin foil around the ice? or perhaps a small foam cup? Essentially you want to insulate the ice so less thermal energy from your surroundings enter it to melt it. I would presume using less ice that melts longer is desirable to reduce the mess of melted water.....

[Tenderfoot88]

Way to make me feel like an idiot... Insulating it is a great idea. Though tin foil might not be such a good idea - aluminium (which has pretty much replaced tin in everything) has the highest thermal conductivity of anything on the list of thermal conductivities I'm using. Basically, it's the perfect material for a heat sink - the opposite effect. Looking at the list, though, it might be best to freeze the insulation, too - insulation materials consistently have a higher conductivity than air, they just counteract the convection effect (air circulates, insulation just changes temperature).

Admittedly, I could be looking at this entire thing wrong. I'm pretty sure my first year physics class (the only thing that got anywhere near this) just covered thermodynamics, which isn't concerned about trifling matters like time. (For instance, a thermodynamicist could make the argument that a pencil is more romantic a gift than a diamond because diamond decays into graphite, while graphite is thermodynamically stable. Indeed, my prof did make that argument. As a joke in a physics class.)

Regardless, insulation is a brilliant and simple idea for prolonging the ice.

[lj]

tin foil is fine in the insulating idea, as long as there is a good volume of trapped air around the ice - closed-cell plastic foam or bubble-wrap for example - then an overall cover of aluminium foil to reflect any heat coming from outside the package

but you still come back to the simple solution - a bigger ice block !

[Tenderfoot88]

Thing is, sometimes a bigger ice block won't work. If you're using one of those metal ice-lock contraptions (or one you made yourself) it only holds a set amount of water. And some of the ice block techniques can leave a sizable chunk of ice behind - if it just needs to melt the rope free. (The bag through a small ring, on the other hand, is great.)

[fatguy451]

to make a small icelock last a long time.. research 'Pykrete'. basically just water mixed with a little sawdust. Dramatically increases strength and melt time.

[bound_jenny]

to make a small icelock last a long time.. research 'Pykrete'. basically just water mixed with a little sawdust. Dramatically increases strength and melt time.

You can even build a boat with it!

Jenny.

[ponylady]

to make a small icelock last a long time.. research 'Pykrete'. basically just water mixed with a little sawdust. Dramatically increases strength and melt time.

You can even build a boat with it!

Jenny.

just what the doctor ordered.

a mobile release.

that would make for possible fun scenarios.

[bound_jenny]

that would make for possible fun scenarios.

Of course! Can you get loose before your craft melts away and you're underwater and helpless to avoid drowning? An added incentive to escape!



Disclaimer: that was entirely Jenny-nonsense and not a scenario suggestion at all. Do not try this at home (or anywhere else, for that matter). Caution: contents may be hot. This side up. Do not ingest. No user serviceable parts inside. Shock hazard if immersed in water.

Jenny.