A novel seeding method, permitting high out-of-plane resolution and instantaneous (time-resolved) velocity field measurements using a standard Microscale Particle Image Velocimetry (micro-PIV) setup, is presented. The method relies on selective seeding of a thin fluid layer within an otherwise particle-free flow. The generated particle sheet defines the depth and position of the measurement plane, independently of the details of the optical setup. Therefore, for low magnification objectives in particular, considerable improvement in the measurement depth is possible. Selectively seeded micro-PIV (SeS-PIV) is applied to a microchannel flow, and the measured instantaneous velocity fields are in excellent agreement with the theoretical solution for the flowfield. The currently presented measurements have a depth-wise resolution 20% below the estimated optical measurement depth of the micro-PIV system. In principle, a measurement depth corresponding to the diameter of the tracer particles may be achieved.

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