Exploring the potential of using hydrogel for refractive index matched measurements around aerodynamic objects

Mathieu Lempen

Semester project

Aim

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.

Hydrogel fabrication

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.

Viscoelastic effects
  • σf and E increase with strain rate
  • no definite trend for εf due to insufficient sampling 

  • Overall, relatively weak mechanical properties, but reinforcing with for instance fibers or particles may be considered

    Molding

    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.

    Conclusions
  • Excellent refractive-index-matching with water is achieved using PAAm-based hydrogels
  • Their weak mechanical properties is still a challenge, but it is expected that the addition of fibers or particles will reinforce them
  • After molding, we obtain a uniform and smooth hydrogel layer covering a wing structure
  • The refractive-index-matching with water and moldability of these hydrogels are encouraging for subsequent PIV measurements in a water channel