Make a hydrogel that has an index of refraction very close to that of water (cf. Exp. Fluids (2014) 55:11862). Perform Particle Image Velocimetry (PIV) around aerodynamic objects made out of this particular hydrogel.
Free radical copolymerization
Optimized composition yields polymerization times of about 3 minutes.
Particle Image Velocimetry
Tracer particles scatter light when going through laser plane. Recorded by CCD cameras. Time between two laser pulses is known, so one can calculate the velocity field of the tracer particles. Advantage of using refractive-index-matched hydrogel as material for the object: no shadow casting or light diffraction at material interface, therefore accurate velocity field visualization is achieved even close to the object boundaries
Optical and mechanical properties
Excellent refractive-index-matching with deionized water, so hydrogel literally invisible when submerged.
Overall, relatively weak mechanical properties, but reinforcing with for instance fibers or particles may be considered
The wing structure is fixed into the open mold and the hydrogel is poured into the mold while it is still liquid. After polymerisation has occured, a smooth and uniform hydrogel layer covers nearly the entire wing area. In order to facilitate the pulling out of the wing out of the mold, we suggest to cover the inner walls of the mold with a less adherent material.