Page 221 - Blog Posts v1

E10 - Experiments with India ink and a Q-switched Nd:YAG laser
What happens when India ink absorbs laser energy?
I irradiated some dry India ink trapped between two glass slides here (the ink was allowed to dry overnight). I used 5 J/cm^2 with a 1064 nm wavelength. As you can see, the laser energy ‘vapourises’ the ink and leaves voids behind.
But, very little ink leaves the slides....
https://youtu.be/myynQwTmlV0
The ink absorbs the laser energy and is vaporised. However, since there is very little water in the ink, there is virtually no steam formation. Hence the ink remains in situ.
When I tried the same experiment with wet ink, the results were quite different...
https://youtu.be/Oj_6nwUGtmQ
Here the ink is suspended in a colloid in water. The ink absorbs the laser energy and heats up rapidly. As it does so, some thermal energy conducts to the surrounding water raising its temperature significantly. This generates steam.
Steam occupies around 2000 times the volume of the same quantity of water. Its expansion is very fast. Hence, the water boils and rapidly expands its volume throwing ink out of the enclosed environment.
The ink 'splatters' the paper, quite clearly. This is entirely due to the creation of the steam bubbles. When the ink is dry (as in the top video) no such water expansion occurs.
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