The induction of a lateral flow (orthogonal to axial flow) inside of the separation channel generates vortices. Lateral mixing can be achieved passively through the dedicated geometric designs, or more efficiently, using an active approach that requires additional power input, such as electroosmotic flow and acoustic wave. The latter is commonly established by matching the channel width with the applied acoustic wavelength (λw) to satisfy the resonance condition (characterization dimension =n*λw/2). Consequently, the channel width restricts to ~40μm (not applicable for chromatographic purposes) due to limitations in the commercially available equipment. In this project, hence, a novel strategy was proposed and validated, where the depth of the separation channel was matched with the λw to induce the vortices and the channel width was freely tuned to the chromatographically relevant dimensions. To perform the methodology successfully, it was necessary to consider different fabrication and operation aspects. First, the impact of curvature of the channel bottom was modeled in a simulation study. The results indicated that a curvature radius ranging from 0 to 2.0µm allows to achieving appropriate lateral flow. In practice, SEM images of the separation channel demonstrate the curvature radius in the range of 0.6-2µm. Second, the optimal aspect ratio (AR) of the separation channel (depth/width) was determined by consideration of sample dispersion, sample volume loadability, and availability of actuators. The AR of 7.5 (75/10) represents a practically optimal value, allowing a channel with a depth of 75.0μm to be acoustically matched with a commercially available 10.0 MHz ceramic piezoelectric (PZT) actuator. Third, the separation channel was coupled to the PZT with minimum damping effect and maximized acoustic energy transmission. Frequencies of 10.0 and 9.7 MHz were found to induce effective lateral flow at different potential amplitudes (0.5–2.0Vpp). However, a lower sample dispersion is achieved using the PZT actuating at a frequency of 10.0MHz and a potential of 2.0Vpp. By considering the critical operating aspects and maintaining optimal conditions, the acoustically vortex-mediated channel enables to reduce the sample dispersion up to ten-fold (see Fig.1).
The enhanced chromatographic performance of the methodology was demonstrated by separation of bovine serum albumin (BSA) and dextran (10kDa) as two large molecules in reverse-phase LC mode. Comparison of the obtained results using the same separation channel in the absence and the present of the acoustic-based vortices revealed that high dispersion of BSA leads to a faint peak in the absence of lateral flow (conventional LC), while vortex LC provides a nearly baseline separated chromatogram in half of the analysis time (see Fig.2).
