Evidence for electrohydrodynamic convection as a source of spontaneous self-ordering in porous anodic alumina films

Abstract

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).

Conceptual Progress for Explaining and Predicting Self-Organization on Anodized Aluminum Surfaces

“…4)). Adapted with permissions from [65] and [68]. Copyright 2016 The Royal Society of Chemistry and American Chemical Society.…”

“…By introducing an abstract concept of entropy production from a single growing PAA cell, Heinschke and Schneider aimed to establish a theoretical connection between the macroscopic parameters of bulk electrolyte and the evolution of pores. The authors have also proposed an alternative expression for the criterion P that should better reflect its non-constant character, which was earlier established by experimental means [65,68]:…”

“…More specifically, they can be also grouped into the following three main categories: Organic carboxylic acids. As a rule, these are polyprotic acids [ 1 , 9 , 17 , 60 , 61 , 62 , 63 , 64 , 65 ], but the applicability of monoprotic acids has been also demonstrated on the example of formic acid HCOOH [ 16 ]; Oxoacids with a chalcogen central atom (sulfuric or selenic) [ 2 , 66 , 67 , 68 , 69 ]. The derivatives with a substituted oxo-group (e.g., thiosulfuric) or peroxoacids are excluded because they are instable either thermally, or in diluted water solutions, or can be even explosive; Oxoacids containing phosphorus (such as phosphoric or phosphonic) [ 8 , 50 , 70 ].…”

“…Organic carboxylic acids. As a rule, these are polyprotic acids [ 1 , 9 , 17 , 60 , 61 , 62 , 63 , 64 , 65 ], but the applicability of monoprotic acids has been also demonstrated on the example of formic acid HCOOH [ 16 ];…”

“…Thus, the criterion P is expressed in Equation (4) in terms of measurable macroscopic electrolyte solution parameters. The numerical value of P is non-constant for different electrolyte compositions and strongly depends, among other factors, on the concentration of charge carriers (or concentration and strength of a supporting electrolyte) [ 65 , 68 ]. In this regard, self-organized structures on anodized aluminum behave similarly to other known cellular patterns that arise due to electroconvection.…”