Viscosity Effects in Thin-Layer Electrodeposition

We present experimental results and a theoretical macroscopic model on the effects of viscosity in thin-layer electrochemical growth. The viscosity was changed through glycerol additions; simultaneous use was made of optical and schlieren techniques for tracking concentration and convective fronts, while pH indicators were used for migratory fronts. The theoretical model describes diffusive, migratory, and convective ion transport in a fluid subject to an electric field. The equations are written in terms of dimensionless quantities, in particular, the Migration, Peclet, Poisson, Reynolds, and electrical Grashof numbers, which are found to depend on viscosity. Experiments reveal that with increasing viscosity, convection decreases, concentration profiles are less pronounced, while electric resistance and voltage increase. Concentration and convective fronts slow down with viscosity, but their time scaling follows the same law as for solutions without glycerol, only differing by a constant. Moreover, under constant electrical current, an increase in viscosity yields slower deposit front velocities, a more uniform deposit with smaller separation between branches, i.e., a change in morphology from more separated compact trees to a more dense, fractal-like structure. © 2001 The Electrochemical Society. All rights reserved.