Figure 2.

(a) Cross-sectional scans perpendicular to the microchannel axis with droplets containing 20 mM Ca²⁺, reporting variations in XRF (X-ray fluorescence) intensity for Si and Ca. The energy is maintained at 4050 eV. The variation in Si XRF (red) indicates the position of the channel walls, whereas Ca XRF (blue) reports droplets centered in the middle of the channel. The Ca XRF increases at the channel walls due to Si pileup overlapping with the Ca K-α line. The inset shows an image of the microfluidic device, with the corresponding cross section marked as a dotted white line. The scale bar is 100 μm. (b) Comparison of Ca K-edge XANES spectrum obtained from a bulk solution (CaCl₂ 10 mM) within the flow cell (black) and from picoliter-volume droplets (CaCl₂ 60 mM) (purple). Spectra have been vertically shifted to aid visualization.