Chronoamperometry has been undertaken at insonated electrodes of both micro and macro dimensions, for a range of simple, well-defined redox couples in water (298 K), DMF (298 and 218 K), and ammonia (218 K) as solvents. These are analyzed to assess the relative contributions of acoustic streaming and cavitational activity to the observed currents: both contribute significantly under the usual conditions adopted for sonovoltammetry. Differential pulse voltammetry (DPV) was then used to explore the nature of the diffusion layer prevailing under steady-state electrolysis of insonated macroelectrodes. Simulations showed that pure convection within a diffusion layer enhances the DPV currents for simple redox systems as compared to silent conditions. The experimentally observed decrease was attributed to cavitational disruption of the diffusion layer leading to a physical model of an insonated electrode which may be described as a steady diffusion layer a few microns thick brought about by acoustic streaming which is occasionally and randomly punctuated by a cavitational event. The frequency and violence of the event is dependent on the solvent and ultrasound power, except at very short electrode-to-horn separation where the cavitational contribution becomes substantial.
Microfluidic technologies are highly adept at generating controllable compositional gradients in fluids, a feature that has accelerated the understanding of the importance of chemical gradients in biological processes. That said, the development of versatile methods to generate controllable compositional gradients in the solid‐state has been far more elusive. The ability to produce such gradients would provide access to extensive compositional libraries, thus enabling the high‐throughput exploration of the parametric landscape of functional solids and devices in a resource‐, time‐, and cost‐efficient manner. Herein, the synergic integration of microfluidic technologies is reported with blade coating to enable the controlled formation of compositional lateral gradients in solution. Subsequently, the transformation of liquid‐based compositional gradients into solid‐state thin films using this method is demonstrated. To demonstrate efficacy of the approach, microfluidic‐assisted blade coating is used to optimize blending ratios in organic solar cells. Importantly, this novel technology can be easily extended to other solution processable systems that require the formation of solid‐state compositional lateral gradients.
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