Bubble nucleation is a phenomenon ubiquitous in physics, with
applications ranging from the geometry of tree crowns, the structure
of porous media and of sphere packing. Bubbles also find applications
in cosmology such as the characterization of cosmic voids in the large
scale structure and the signatures of cosmological phase
transitions. In a new preprint, **Pierre**
proposes a new method to determine the fractal properties of the
so-called Random Apollonian Packing.

Random Apollonian Packing (RAP) is inspired by the better-known Apollonian Gasket. In mathematics, an Apollonian gasket is a fractal generated by starting with a triple of circles, each tangent to the other two, and successively filling in more circles, each tangent to another three.

In Ref. [1], Pierre examines a related mechanism in which \(d\)-dimensional spheres are randomly seeded in space, one at a time, within a finite-sized volume, and with the largest possible radius that avoids overlap.

The interest of the RAP mechanism is that it is thought to share universal properties with more general dynamic mechanisms, such as the ABK mechanism (named after Andrienko, Brilliantov and Krapivsky) in which bubbles grow linearly with time.

Pierre has built a model to explain at which rate Random Apollonian Packings grow after the insertion of one sphere after the other. The modelâ€™s prediction for the fractal properties of RAP are consistent with numerical simulations made in two, three and four dimensions.

If you want to explore a RAP in depth, feel free to zoom in that
**picture**, it
contains more than a million spheres.