Measuring Viscoelastic Properties Using Compliant Systems
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Measuring Viscoelastic Properties Using Compliant Systems
Materials Project 2), with support from Advantage West Midlands (AWM) and partly funded by the. European Regional Development Fund (ERDF). 1.
Measuring Viscoelastic Properties Using Compliant Systems James W Andrews†, David Cheneler‡, James Bowen† † School of Chemical Engineering, University of Birmingham, UK; ‡ Department of Engineering, Lancaster University, UK
1. Introduction Spherical indentation is used to measure the transient mechanical properties of materials, including viscoelasticity, using stress relaxation. Example instruments include atomic force microscopes and optical tweezers. When modelling the acquired data, common sources of error include: (i) relaxation processes during the loading ramp are neglected; (ii) the compliance of the measuring device is not considered. In this work, a model has been developed which incorporates both features.
Rectangular AFM cantilever with spherical SiO2 colloid probe attached at the apex
2. Modelling and theory Instrument designs generally contain three common elements: (i) a fixed end, actuated piezoelectrically, mechanically or magnetically; (ii) a compliant element or spring, which is the load measuring element; (iii) a free end, a probe of a specific geometry which contacts the sample. Key assumptions for modelling stress relaxation in viscoelastic materials: • Indenter is rigid compared to the material, a flat, planar half-space. • Viscoelastic material described by a generalised Prony series model. • Probe approaches normal to the surface. • Contact is frictionless and there is no adhesion between the materials.
3. Results • Optimised experimental sensitivity and accuracy, ensuring relaxation spectrum is captured as completely as possible. • Optimal indentation velocities for specified values of spring constant and indenter radius. • Ideal ramp duration equal to one-eighth of the relaxation time. • Materials exhibiting multiple relaxation times can be studied. • Applicable to characterisation of poroelastic materials.
4. Conclusions
• Construct maps showing permissible measurement conditions. • Sub-optimal measurement conditions compromises the resolution of the relaxation region, giving the false impression of greater elasticity. • Characterisation using short ramp durations and/or high spring constants is improved using this model. • Ideal ramp durations for materials relaxation elements can be calculated.
Acknowledgements
This poster is a summary of the paper published in Soft Matter:
This work was supported by the European Union under the FP7 programme (NANOBIOTOUCH Project: FP7-NMP-228844). The authors also acknowledge the equipment funded by Birmingham Science City: Innovative Uses for Advanced Materials in the Modern World (West Midlands Centre for Advanced Materials Project 2), with support from Advantage West Midlands (AWM) and partly funded by the European Regional Development Fund (ERDF).
