MORPHOLOGY SELECTION AND THE CONCENTRATION BOUNDARY-LAYER IN ELECTROCHEMICAL DEPOSITION

The concentration boundary layer encompassing a branched aggregate during growth by electrochemical deposition from unstirred solution is imaged by shadowgraph and Schlieren methods. It is shown that the solution depleted by the deposition process lies within a sharply defined envelope encompassing the branches of the aggregate. The depleted solution within this envelope is separated from the bulk solution by a thin boundary layer. The structure of the boundary layer reveals patterning of two distinct types, cellular and viscous-finger like, each corresponding to a particular morphology of the aggregate. This observation suggests a coupling between morphology selection in electrochemical deposition and instability at an interface between depleted and bulk electrolyte.