Application of Micro-PIV for the Investigation of Evaporation-Driven Flow Inside a Microdroplet

Evaporation-driven flow inside sessile microdroplets governs transport, deposition, and drying in inkjet printing, coating, biochemical analysis, and microfluidic handling. Micro-particle image velocimetry (Micro-PIV) can measure these flows, but conventional side view measurements are constrained by optical distortion near the curved droplet interface. Ray tracing correction can map visible vectors back to physical coordinates, yet peripheral regions and low contact angle stages often suffer from particle image loss or severe blur. Here, an extended Micro-PIV framework combines side view reference measurements with a bottom view two-dimensional, three-component (2D-3C) reconstruction method. The bottom view method keeps the robust in-plane velocity measurement of cross-correlation PIV and estimates the out-of-plane velocity component from particle image appearance changes. Representative bottom view measurements assemble xy velocity, signed z velocity, and 2D-3C vectors on single measurement planes and across stacked substrate referenced depth slices. In the central droplet region, the reconstructed circulation is qualitatively consistent with the side view reference trend: recirculating flow weakens during evaporation and the apparent vortex region shifts downward. In the edge region, where side view Micro-PIV becomes unreliable, the bottom view reconstruction provides access to previously obscured flow features and indicates an additional upper circulation with weak upward velocity components near the lower edge. Together, the central and edge measurements extend Micro-PIV toward optically difficult regions of evaporating microdroplets and identify particle-wise depth estimation as a feasible approach for bottom view 2D-3C droplet flow measurement.