Oscillatory nanoindentation of highly compliant hydrogels: A critical comparative analysis with rheometry
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Emily R. Draper and Dave J. Adams School of Chemistry, WESTChem, University of Glasgow, Glasgow G12 8QQ, U.K.
Jennifer Hay Nanomechanics Inc., Oak Ridge, Tennessee 37830, USA (Received 15 February 2018; accepted 8 March 2018)
We present a method for measuring the shear complex modulus of hydrogels by oscillatory nanoindentation, with unprecedented attention to procedure and uncertainty analysis. The method is verified by testing a typical low-molecular-weight gelator formed from the controlled hydrolysis of glucono-d-lactone. Nanoindentation results are compared with those obtained by rheometry using both vane-in-cup and parallel-plate fixtures. At 10 Hz, the properties measured by oscillatory nanoindentation were G9 5 38.1 6 2.8 kPa, tan d 5 0.22 6 0.02. At the same frequency, the properties measured by rheometry were G9 5 15.3 6 2.9 kPa, tan d 5 0.11 6 0.016 (vane-in-cup) and G9 5 7.9 6 1.1 kPa, tan d 5 0.05 6 0.004 (parallel-plate). The larger shear modulus measured by nanoindentation is due to the scale of testing. Whereas rheometry characterizes the bulk material response, nanoindentation probes the fibrous network of the gel. The procedure and analysis presented here are valuable for nanoindentation testing of other compliant materials such as hydrogels, soft biological tissue, and food products.
I. INTRODUCTION
Nanoindentation is a well-developed technique for measuring the mechanical properties of stiff materials. There is growing interest in using the technique for determining the properties of more compliant materials1 including biological tissues2–4 and hydrogels.5–7 The advantages of using a nanoindenter instead of more conventional techniques include the ability to probe small volumes of tissue and the ability to spatially resolve the properties of the material. The latter is particularly important, given that biological materials or hydrogels may not have homogeneous microstructures8,9 or may have been designed to have spatially varying properties.10 One of the key developments in nanoindentation is the ability to conduct oscillatory tests. This method superimposes an oscillating force and measures the resulting indenter oscillation amplitude and phase shift. The analysis to derive contact stiffness and damping is straightforward if the indentation system is well modeled as a simple-harmonic oscillator, both prior to, and during, sample contact.11–14 Such testing enhances instrumented indentation as a materials characterization tool. Of Contributing Editor: Erik G. Herbert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.62
relevance to the present work is the ability to measure the viscoelastic properties of a material at a series of specific frequencies. Many studies have compared oscillatory nanoindentation data with macroscale dynamic test methods, including dynamic mechanical analysis (DMA), dynamic mechanical thermal analysis (DMTA), and rheometry. These macroscale methods have a long history and thus serve to validate nanoindentation methods.
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