Life? Really?

Life? Really?
04 February 2013 - Making It Move XI

In entry XI of Making it Move Pallaci et al. treat us to "living crystals" (DOI). What are they? And alive by what definition?
The crystals in question are hematite cubes coated to some extent with polymerized 3-methacryloxypropyl trimethoxysilane. They are suspended in water containing a surfactant and hydrogen peroxide where they display regular Brownian motion. Under the influence of a bright blue light, hematite on exposed parts of the cube catalyses the decomposition of the hydrogen peroxide, creating an osmotic gradient and a surfing opportunity. From then on things get complicated.
Added colloid particles made of silica and acting as tracers can be seen to migrate towards the hematite cubes when illuminated in a process called phoretic migration which travels in the opposite direction as the osmotic migration of a regular surface. Phoretic migration is also the reason why the cubes themselves migrate to the bottom of the sample cell, attracted by the silica surface and shielding the exposed hematite layer. Due to imperfections in their geometry cubes can still take off again by peroxide induced self-propulsion.
The nett effect on display is a process of random assembly and disassembly of clusters of cubes as long as the light is on and complete disorder when the light is switched off. The similarity is with swarm behaviour as seen in flocks of birds and that is where the "living" in "living crystals" comes from. But was exactly is life? Do not bother to check Wikipedia for an answer. In the current life article, life is any object that displays biological processes but in the biological process article, a biological process is a process of a living organism. Movie!