What kinda heatsink? How much LN2 (as Liquid Nitrogen is abbreviated in industry)? Portable of fixed use? How cold do you have to keep things?
If your talking a flashlight, forget it - first you'd have to "pre-cool" the heatsink, or else the LN2 would boil off in a few seconds if not faster, and you are going to boil of the LN2 fairly fast - it's heat of vaporization is lower than say, water - you'd be better off using water, and boiling that - the heatsink doesn't go over 100c till the water is gone (assuming perfect thermal transfer, which never happens)
On top of that, if you don't know what you are doing, it's incredibly easy to get minor frostbite with LN2, and not too difficult to get majore burns/frostbite and worse.
I used to be the "LN2" guy at the electronics company I worked for - and yeah, we used to cool some of our stuff with LN2 - then again, the electronics were being thermal cycle tested, and the LN2 was just to increase the cooling speed of the test chamber (picture a 4x4x4 box cooled with dual 15 HP compressors (r-502 cooling the R-13 loop) with 2 3HP fans blowing the air around, plus a 1/2" spray nozzle for LN2, plus a 1/2 LN2 line into the mechanical fixtures for conduction cooling! Of course my "small" LN2 reservoir was a 3000 gallon tank sitting out my window, and it was filled EVERY 3 days, no matter if that 3rd day was a Sunday, Holiday, or whatever! Our test cycle was 40 hours, where we would blow through about 1200 gallons, and then we typically had 24 hours off to setup the next run
(the fun time was when one of the 1/2 lines jammed open on a Sunday, and we vented all the LN2 into the test room. Came in on Monday AM, went to go to check on things, and the alarms were going off - "Low Oxygen" (was down to 0%) and the room was full of fog - I had to get on the SCBA with a buddy rescue setup, and go in to vent the room
Oh that reminds me of another problem with working with LN2 - when it boils off, it does a real good job of displacing any air in the room, and the nasty part is, the human body does NOT sense a lack of oxygen - it actually senses a buildup of CO2. You keep breathing, don't feel out of breath, and you fall over and pass out - and never feel light headed (as I found out one day when I didn't wait quite long enough for the fans to vent the chamber - I was only out about 1-2 seconds or so - long enough to fall, hit my head, but NOT make it all the way to the floor - I fell OUT of the chamber, and regained conciousness about 1/2 way to the ground!). We got a lot more serious about safety setups after that one - if the chamber had been run or closed withing the previous 24 hours, the door was opened, the fans run for 5 minutes, then an O2 sensor put in the chamber before you reached in to get the electronics out
