Optical Film Thickness Measurements Using a Reflectance Mode Swept-Field Confocal Microscope

Multiphase flow in microchannels is frequently encountered in microdevices. Predicting the behavior of the multiphase flow has been difficult. The dynamics of liquid films inside of capillaries is important to microfluidic flows. At high flow rates with gas-liquid flows, the dynamics of liquid films on the interior surfaces of the microchannels can have a global affect on device operation. Measurement via optical microscopy of velocities in these liquid films and interface dynamics has been hampered by the inability to correctly image near dyanamic interfaces due to optical reflection and very short time scales of the flow. A novel technique for optically measuring liquid film thicknesses within circular microchannels has been developed. A swept-field confocal microscopy unit has been modified to capture the reflected light by replacing the dichroic mirror, commonly used in fluorescence confocal microscopy, with a 50/50 beam splitter. “Optical slicing” in the x-y plane, coupled with precision z-axis stepping allows for the detection of the gas-liquid interface by blocking out-of-focus light.