Fluorescein focusing and fluorescein–iodide quenching simulations and experiments in the fused silica chip region of the mixer. (a) Normalized fluorescence intensity fields at the y = 0 plane from simulations (left column) and confocal laser scanning experiments (right column). Shown is hydrodynamic focusing of a fluorescent sample by pure buffer (top row) and buffer with a quencher (bottom row). (b) Sample FWHM wsa versus axial position x as predicted by simulations and experimentally measured. (c) Quenched intensity ratio versus x predicted by simulations and experimentally measured. We hypothesize the fluctuations in the quenched intensity ratio are due to nonuniform illumination and detection within the focusing region of the chip. Note that those fluctuations appear for all flow conditions and are discussed in Appendix C. Data in (b) and (c) are for varied Qsa and fixed Qsh = 1 ml min−1.