The dynamics of two-dimensional ordering of micrometer-size polystyrene latex spheres on a horizontal glass substrate has been directly observed by means of optical microscopy. The ordering starts when the thickness of the water layer containing particles becomes approximately equal to the particle diameter. By variation of the electrolyte concentration, the charge of the particles, and their volume fraction, it is proven that neither the electrostatic repulsion nor the van der Waals attraction between the particles is responsible for the formation of two-dimensional crystals. The direct observations revealed the main factors governing the ordering – the convective transport of particles toward the ordered region and (only at the nucleus formation stage) the attractive capillary forces due to the menisci formed around the particles. The control of the water evaporation rate allows obtaining either well-ordered monolayers or well-ordered domains consisting of multilayers (bilayers, trilayers, etc.).

Microphotograph of the "live" process of 2D array formation. The latex microspheres are carried by the convective flux and are incorporated in the growing crystal. The worm-like traces on the top left side of the photograph are the actual trajectories of the particles moving during the exposure period. Particle diameter = 1  mm.

N.D. Denkov, O.D. Velev, P.A. Kralchevsky, I.B. Ivanov, H. Yoshimura and K. Nagayama, "Dynamics of Two-Dimensional Crystallization", Nature, 361, 26 (1993).

N.D. Denkov, O.D. Velev, P.A. Kralchevsky, I.B. Ivanov, H. Yoshimura and K. Nagayama, "Mechanism of Formation of Two-Dimensional Crystals from Latex Particles on Substrata", Langmuir, 8, 3183 (1992).

O.D. Velev, N.D. Denkov, P.A. Kralchevsky, I.B. Ivanov, H. Yoshimura and K. Nagayama, "Mechanism of Formation of Two-Dimensional Crystals from Latex Particles on Substrata", Progr. Colloid Polym. Sci, 93, 366 (1993).