Interfacial transport driven by redox active surfactants

Fluid mechanics and mass transfer characteristics of surface tension driven flows have been investigated in an effort to develop a novel mixing enhancement scheme. Through electrochemical methods, redox-active surfactants can be easily switched between oxidized state and reduced state, which have different surface tensions. Surface tension gradients will drive Marangoni flows and enhance the mixing in the solutions. Marangoni flows will also affect the reactants mass transfer at the region close to the electrode and affect the current at the electrode, so the effects of Marangoni flows on the voltammetries are also studied using numerical simulations. Asymmetric cyclic voltammograms, which possess higher and sharper anodic peak than the catholic peak, were observed when Marangoni convection is included in the simulations.; Numerical simulations showed that a small velocity gradient could have a big effect on the currents of potential step voltammetry. The apparent diffusion coefficients under the influence of Marangoni flows may be order of magnitude different from the true diffusion coefficients. Through simulations, mixing enhancement in sub millimeter size channels was also studied by using redox-active surfactants. Under different potential control schemes, different flow patterns stretch and fold the fluid elements and generate chaotic mixing. Big incoherence between the periods of voltage waveforms leads to faster mixing.; By monitoring Marangoni flows driven by surface active species electrochemically generated at the electrode, surfactant desorption kinetics can be quantified. Using lubrication theory, the relation between surface tension gradients and surface velocities was derived. Under the assumption of low surfactant surface coverage at flow leading edge, model predicts that leading edge velocity will follow an exponential decay with time, and the decay rate is equal to the desorption rate constant. Experimental data verified this exponential behavior and the rate constant is close to values found for similar surfactants using conventional methods.