The effect of pressure on the ‘crystal-like’ ordering of monodisperse polystyrene spheres in deionized aqueous suspension

Abstract

‘Crystal-like’ structures, face-centred cubic (f.c.c.) and body-centred cubic (b.c.c.) lattices of monodisperse polystyrene spheres (85–269 nm in diameter) have been studied at high pressures (1–2000 bar) using reflection spectrum measurements. Enhancement of b.c.c. lattice formation and/or transformation from f.c.c. to b.c.c. is observed under elevated pressures. This tendency is more significant for larger spheres. At very high pressures, the melting of the b.c.c. structures and/or the transformation from b.c.c. to f.c.c. occurs especially for small spheres. The influence of pressure on the ‘crystal-like’ structures is explained by the increase of the effective charge number of the spheres and the decrease in the Debye screening length as the pressure increases. Furthermore, a delicate balance between change in the effective sizes of spheres including the Debye length and the compression of the suspensions is suggested. These results support the idea that the electrostatic interparticle repulsion and the elongated Debye length around the spheres are essential for the ‘crystal-like’ ordering, transformation and melting phenomena under high pressures.