Refractive-index-matched hydrogel materials for measuring flow-structure interactions
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RESEARCH ARTICLE
Refractive-index-matched hydrogel materials for measuring flow-structure interactions Margaret L. Byron • Evan A. Variano
Received: 28 September 2012 / Revised: 14 December 2012 / Accepted: 2 January 2013 / Published online: 20 January 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract In imaging-based studies of flow around solid objects, it is useful to have materials that are refractiveindex-matched to the surrounding fluid. However, materials currently in use are usually rigid and matched to liquids that are either expensive or highly viscous. This does not allow for measurements at high Reynolds number, nor accurate modeling of flexible structures. This work explores the use of two hydrogels (agarose and polyacrylamide) as refractive-index-matched models in water. These hydrogels are inexpensive, can be cast into desired shapes, and have flexibility that can be tuned to match biological materials. The use of water as the fluid phase allows this method to be implemented immediately in many experimental facilities and permits investigation of high-Reynolds-number phenomena. We explain fabrication methods and present a summary of the physical and optical properties of both gels, and then show measurements demonstrating the use of hydrogel models in quantitative imaging.
1 Introduction Particle image velocimetry (PIV) has been an important tool for measurements in fluid mechanics (Raffel et al. 2007). However, this technique is not easily extended to the specific case of flow around solid objects. This is because PIV requires the use of a laser light sheet to illuminate the region of interest. When measuring flow around
M. L. Byron (&) E. A. Variano Department of Civil and Environmental Engineering, University of California, 202 O’Brien Hall, Berkeley, CA 94720-1710, USA e-mail: [email protected]
an opaque or translucent object, the object itself will interfere with the illumination by casting shadows and/or scattering light. Of specific interest to our research is the case of ‘‘macroparticles’’ (regular and irregular threedimensional shapes, of lengthscale &1 cm) suspended in a flow. When these macroparticles cast shadows, we often cannot see their wakes, nor can we see into the interior of a dense suspension. These problems are also encountered when using PIV around models (e.g., of organisms or turbomachinery): shadows eliminate large portions of the image area, preventing a complete analysis of the flow field (Schiacchitano et al. 2012). To resolve this issue, refractive index matching (RIM) has been employed with great success, as reviewed by Budwig (1994), Wiederseiner et al. (2010), and Dijksman et al. (2012), among others. This permits the use of optical techniques such as laser Doppler velocimetry and PIV (an example of the latter is given in Hassan and DominguezOntiveros 2008). In RIM–PIV, the test objects (in our case, macroparticles) are made of a material that is transparent and refractively matched to the surrounding fluid. This avoids blockage or distortion of the lase
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