Inkjet printing involves firing microscopic droplets of ink, usually water-based, as small as one picolitre, or a 10-12 litre, onto the surface of the print media. More technically sophisticated inkjet printers are able to lay down droplets of different sizes to improve the definition and tonality of prints. Inkjet printer ink is not just coloured water and photographic paper bears little relation to the sheets that go into a copier machine. An inkjet printer lays down millions of these microscopic droplets of ink to build the smooth and rich tonal qualities that make a photo-realistic print. The way that those drops of ink behave when they land on the surface of the paper has a big influence on the colour, contrast, tonality and sharpness of the final print.
Ideally, each droplet of ink should create a well-defined spot of the correct size. Drops that overlap should not bleed into each other, corrupting colour in the process. The chemistry of the ink means that the colouring agent, which can be either a dissolved dye or a pigment in suspension, is just one of a team of ingredients that affect the surface tension of the liquid, its viscosity, its resistant to oxidation, how well it keeps pigment particles from crashing out of suspension, and much more.
The ink needs to be resistant to chemical reaction with the surface of the paper, as well as resistant to fading, both by visible light and UV, and chemical agents in the air, such as ozone, for example. Batch to batch consistency is essential too. Add to all that the growing expectation of tolerance to water exposure of a finished print and you begin to realise just how complex the engineering of inkjet printer ink is.