Visualization and numerical modelling of microfluidic on-chip injection processes.

Sample injection processes accomplished using a microfluidic-cross chip are investigated experimentally and numerically. Fluorescent dye is employed to differentiate the sample solution from the pure buffer. Different sample geometries are achieved using different applied electric fields and dyes with different electrophoretic mobilities. Of particular interest here are concentration-dense samples with large axial extent (extending beyond the intersection). The ability to load and subsequently dispense these large axial extent samples is predicted numerically and verified experimentally by direct visualization. Containing more mass, larger samples exhibited lower concentration gradients, making them less sensitive to diffusion and well-suited to transport once dispensed. In the loading process, however, larger samples were found to be more sensitive to pressure effects than more focused samples. This was investigated by imaging sample geometries under various applied fields in the presence of a constant pressure gradient. Laplace pressure originating from differential meniscus curvatures in the reservoirs was found to be the most significant source of such pressure disturbances in these geometries.