Date of Award

Spring 2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical & Aerospace Engineering

Committee Director

Julie Zhili Hao

Committee Member

Colin Britcher

Committee Member

Helmet Baumgart

Committee Member

Onur Bilgen

Abstract

This dissertation presents a novel experimental technique, namely concurrent spatial mapping (CSM), for measuring the viscoelastic behavior of heterogeneous soft materials via a polymer-based microfluidic device. Comprised of a compliant polymer microstructure and an array of electrolyte-enabled distributed resistive transducers, the microfluidic device detects both static and dynamic distributed loads. Distributed loads deform the polymer microstructure and are recorded as resistance changes at the locations of the transducers.

The CSM technique identifies the elastic modulus of soft materials by applying a precisely controlled indentation depth using a rigid probe to a sample placed on the device. The spatially-varying elastic modulus of the sample translates to a non-uniform load, causing a non-uniform deformation of the microstructure and variations in the recorded resistance changes. The CSM technique measures the loss modulus of soft materials through a dynamic measurement by applying varying sinusoidal loads to a sample placed on the device. The spatially-varying loss modulus of the sample causes the microstructure to respond with corresponding time delay. Consequently, the phase shift between the sinusoidal load and deflection of the sample along its length are captured by the distributed transducers.

As the first step of the experimental protocol, control experiments are implemented on the device to determine its static performance and system-level dynamic parameters. Next, the CSM technique is applied to both homogeneous and heterogeneous synthetic soft materials to measure their elastic moduli by applying a precisely controlled indentation depth through a probe, and the recorded load and device deflection are the output. The data are processed to obtain the overall load and the deflection of the sample at each transducer location and are further used to extract the elastic modulus distribution of the sample. The CSM technique is then applied to measure the loss modulus of soft materials. The measurable sinusoidal loads are the input, and the sinusoidal deflections of the device are the output. By applying the Fast Fourier Transform (FFT) algorithm and the nonlinear regression method, the data are processed to obtain the phase shift between the applied load and the device response along its microchannel length as well as the system-level parameters, namely stiffness (K), damping coefficient (D), and mass (M). In conjunction with the system-level parameters of the system with the device, obtained from the control experiment, the stiffness and the damping coefficient of a sample are calculated, and the sample’s loss modulus distribution is estimated accordingly. This CSM technique successfully measures the spatially-varying elastic modulus and loss modulus of soft materials. As compared with the nanoindentation-based technique, the CSM technique demonstrates its efficiency in spatially mapping the viscoelastic behavior of a sample without excluding interactions among neighboring compositions in a sample.

ISBN

9781339860121

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