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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

Winter 2015

First Advisor

Alfred J. Crosby

Second Advisor

Ryan C. Hayward

Third Advisor

Jonathan P. Rothstein

Subject Categories

Polymer and Organic Materials

Abstract

Gels and other soft elastic networks are a ubiquitous and important class of materials whose unique properties enable special behavior, but generally elude characterization due to the inherent difficulty in manipulating them. This work focuses on understanding and utilizing large, local deformation and failure in soft solids for characterization of the mechanical properties of otherwise inaccessible or nonmanipulable materials. Cavitation Rheology (CR), a pioneering mechanical measurement technique in this regard, where local material failure is imposed and monitored at the tip of a pressurized needle, serves a central role in this thesis. First, CR is used to make in vivo biomechanical measurements. Following, is a detailed characterization of the mechanics of needle insertion into soft solids, inspired by CR measurements. The resulting analysis not only contributes to the fundamental understanding of soft material failure, but also offers a simple and precise method for making mechanical measurements of soft materials. Finally, this analysis is applied to understanding the effect the stress state surrounding an embedded needle on CR measurements.

First, we demonstrate the use of CR to make mechanical measurement of the skin of rats and mice. Building on extensive proof of concept as a method for local mechanical measurement in a variety of synthetic and biological materials, this work represents two advancements in the application of CR. First, in vivo measurements in the skin of radiation-treated mice represent the first use of CR in living tissue. Second, measurement of healed incisional wounds relative to unwounded skin demonstrates the ability of CR to quantify differences in the mechanical properties of afflicted tissues. These two studies represent important milestones towards the clinical use of CR.

Next, we address an important question relevant to the use of CR, that is, how does a high-aspect-ratio-indenter (needle) puncture a soft solid? To answer this, we investigate the mechanical response of model soft solids of varying composition to deep indentation by indenters of different radii and tip geometries. Further, we examine the criteria for puncture failure in these systems over a wide range of size scales, identifying a transition length separating two distinct failure regimes. The resulting analysis yields a simple and useful protocol for the measurement of multiple mechanical properties of soft solids, as well as provides new insight into the failure of soft materials. Mechanical properties extracted from puncture experiments conducted in a space covering two orders of magnitude in fracture energy and three orders of magnitude in modulus are corroborated with independent measurement techniques, confirming the robustness of the analysis.

Finally, the forces acting on a needle embedded in a soft material are quantified and related to the mechanical properties of the material and the geometry of the needle. These findings provide necessary understanding of the residual force on an embedded needle at an arbitrary position of the insertion and withdrawal process. Residual forces on needles inserted to various depths are recorded and related to the depth dependence of Cavitation Rheology measurements.

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