Porous anodic aluminium oxide (PAOX) has different practical applications (e.g. filters with uniform pore sizes, adsorbers, porous templates for functional nanomaterials), but the formation mechanism is still poorly understood. Equal-sized hexagonally ordered pores are formed during anodic oxidation of aluminium in water solutions of some acids at certain concentrations and temperatures, and comparatively high electrode potentials. Today, a limited range of pore diameters and the degree of hexagonal ordering are reached with empirically found conditions. Here, a theoretical model explaining the appearance of honeycomb structure in porous anodic alumina is presented. The proposed mechanism is based on a dissipative self-organization process, but not on the earlier accepted fieldassisted dissolution of pre-formed dense alumina. Our analysis rests on the concept that electrolyte currents near aluminium anode are organized in the same way as well-known Rayleigh-B enard convection currents. A simple yet effective way to predict pore formation in unexplored electrolytes is suggested. The validity of theoretical considerations is experimentally confirmed by the growth of hexagonally arranged porous alumina in a new electrolyte-aqueous formic acid solution.

The lack of a reliable method for theoretical prediction of nanoporous anodic alumina films obtained from non-familiar electrolytes prompted the search of a viable solution to this problem. The theory explaining the self-assembly mechanism was described in our preceding work. Here, the results of an extensive validation test are presented.

A comparative study of self-ordering behaviour of anodic alumina films fabricated in a series of diluted (down to 0.05 M) oxalic acid electrolytes allowed developing a relationship between the supporting electrolyte concentration and self-ordering voltages for the formation of porous oxide materials. Besides its practical importance, this work elucidates some fundamental principles of porous alumina formation, e.g. it suggests that the cell patterning arises from the electrohydrodynamic (EHD) convection process rather than the interfacial tension gradients near the anode surface (Marangoni-type instability).

Nanoporous anodic alumina films with long-range hexagonal order have been obtained from a series of highly diluted sulfuric acid electrolytes. A simple linear relationship was established between the selfordering voltages and acid concentrations (28, 29, and 30 V for 0.2, 0.1, and 0.05 M electrolytes, respectively). Besides establishing new self-ordering regimes, our experimental work sheds new light on some fundamental principles of honeycomb anodic alumina formation. It suggests that the spontaneous self-organization of a stable nanoscale structure originates from the electrohydrodynamic (EHD) convection rather than from Marangoni-type instability at the anode surface. Theoretical analysis displays a decreasing exponential functional relationship between electrolyte concentration and the critical values of the earlier found electrochemical analogue of Rayleigh number, which can be used for prediction of hexagonal cell pattern in currently unexplored anodizing electrolytes.