An evaluation of volumetric velocimetry (3D-3C) techniques for use in microscale flows using a stereo microscope

Measuring volumetric velocity (3D-3C) in micro-PIV remains a challenge. Volumetric techniques have matured for measuring the velocities in non-microscale flows (tomographic PIV, Shake-the-Box: 3D particle tracking), but usually four or more cameras are used. This study examines whether the same volumetric techniques can be adapted to a standard stereo-microscope (using only two cameras). This would allow for volumetric measurements to be carried out using any existing stereo micro-PIV system. However, in contrast to stereo micro-PIV, the complete depth of the measurement volume needs to be in focus, and this has to be considered when adjusting magnification and aperture of the microscope. The flow in a micro-channel (channel height 100 μm) with a backward facing step (step height 50 μm) is investigated with two high-repetition rate cameras and a corresponding laser. “Two-camera” tomographic PIV and Shake-the-Box are used to get instantaneous and average flow fields. It is found that the thickness of the channel (in voxels) is too small to allow the detection of the parabolic flow profile in the out of plane direction with tomographic PIV. However, using Shake-the-Box and PTV-based binning, the velocity profile can be resolved and the average pressure field can be calculated.