The FCC and HCP lattices are 3D concepts, but you're on the right track. They're using a square lattice, but you could get a higher density using a hexagonal on

The FCC and HCP lattices are 3D concepts, but you're on the right track. They're using a square lattice, but you could get a higher density using a hexagonal one. What's really interesting is that they're modulating drop size to change the color intensity through white space and the coffee-ring effect!