This paper describes experiments for characterizing mass transfer
at the hydrophilic surface of the substrate in a chemistrode. The chemistrode
uses microfluidic plugs to deliver pulses of chemicals to a substrate with
high temporal resolution, which requires efficient mass transfer between the
wetting layer and the hydrophilic surface of the substrate. Here, total internal
reflection fluorescence microscopy (TIRFM) was used to image the hydrophilic
surface of the substrate as plugs were made to flow over it. The surface of
the substrate was rapidly saturated with a fluorescent dye as the fluroesecent
plugs passed over the substrate, confirming effective mass transfer between the
wetting layer and the surface of the substrate. The dynamics of saturation are
consistent from cycle to cycle, indicating that the chemistrode can stimulate
surfaces with high reproducibility. The number of plugs required to reach
90% saturation of the hydrophilic surface of the substrate, φ(90%), only
weakly depended on experimental conditions (the Péclet number or the capillary
number). Furthermore, over a wide range of operating conditions, φ(90%) was
less than 4. These results are useful for improving the chemistrode and for
understanding other phenomena that involve diffusional transfer in multiphase
or recirculating flows near surfaces