178 J. Yang and C. Franck Fig. 30.1 Experimental setup and schematic of inertial microcavitation rheometry. (a) A single 6 ns, Q-switched 532 nm Nd:YAG laser pulse of 1–10 mJ passes through a beam expander to fill the back aperture of an objective mounted into an inverted TI-Eclipse microscope, and (inset, star) converges into a cylindrical hydrogel sample. Bright-field illumination is supplied by a condensed halogen lamp. (b) Bubble growth, collapse, and subsequent oscillation are imaged using a Phantom v2511 high speed camera (Vision Research, Wayne, NJ). Image size 512×128 pixels, filmed at 270,000 fps. Scale bar, 200 μm(Image courtesy of Estrada [7]) Fig. 30.2 IMR characterizes stiff & soft polyacrylamide hydrogels viscoelastic properties using neo-Hookean Kelvin-Voigt model and Fung Kelvin-Voigt model. (a) Bubble radius R-t curve of IMR cavitation experiments inside stiff & soft polyacrylamide hydrogels and water (reference). (b) Characterized viscoelastic properties of the tested stiff & soft polyacrylamide hydrogels using two types of nonlinear material models (neoHookean Kelvin-Voigt & Fung Kelvin-Voigt) Experımental Setup In this study, the experimental setup is the same as in Estrada et al [7] where single bubble inertial cavitation is generated via single pulses of a frequency-doubled Q-switched Nd:YAG 532 nm laser as shown in Fig. 30.1a. We use a Phantom v2511 CCD high speed camera (Vision Research, Wayne, NJ) to take a sequence of images (exposure time of single frame is set to be2μs) of bubble growth, collapse and subsequent oscillations as shown in Fig. 30.1b. Bubble images were then fit for their centroid and radius R(t) using a circle fit algorithm (see Figs. 30.1b and 30.2a). Here we tested cavitation experiments using two types (stiff & soft) of polyacrylamide hydrogels, whose quasi-static shear moduli have also been characterized using quasi-static compression indentations [7].
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