Mike Murphy's Blog

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….

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…

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.

Exactly the same process occurs during laser treatment of tattoos. The ink particles are enclosed within macrophages and fibroblasts in the dermis, with encapsulated tissue water.

As the ink particles heat up, the surrounding tissue water forms steam and causes the cells to rupture. In addition, some of the ink particles fly out of the skin at very high speeds.

I spotted this some years ago and published a paper with my observations – click here for that paper. I calculated that some of those ink particles could be leaving the skin at around 700 metres/second!!

I’m currently working on the physics of all this stuff – it’s very interesting. I’ll let you know once I’m done…

Ciao for now,

Mike.