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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

Spring 2014

First Advisor

Ryan C. Hayward

Second Advisor

Alfred J. Crosby

Third Advisor

Narayanan Menon

Subject Categories

Applied Mechanics | Biomechanical Engineering | Polymer and Organic Materials | Polymer Chemistry | Statistical, Nonlinear, and Soft Matter Physics

Abstract

CREASING INSTABILITY OF HYDROGELS AND ELASTOMERS

MAY 2014

DAYONG CHEN, B.S., TIANJIN UNIVERISTY

M.S., TIANJIN UNIVERSITY

M.S., UNIVERSITY OF MASSACHUSETTS AMHERST

Ph.D., UNIVERSITY OF MASSACHUSETTS AMHERST

Directed by: Professor Ryan C. Hayward

Soft polymers placed under compressive stress can undergo an elastic creasing instability in which sharp folds spontaneously form on the free surfaces. This process may play an important role in contexts as diverse as brain morphogenesis, failure of tires, and electrical breakdown of soft polymer actuators. While the creasing instability has been used for collotype printing since as early as the 1850s, the scientific appreciation of this instability has become popular only recently and our understanding of this instability is still quite limited.

In chapter 2, we describe a simple experimental system to study creasing of thin elastomer films under uniaxial compression. The equilibrium depths, spacings and shapes of creases are characterized and found to show excellent agreements with numerical results. Further, we use this system to explore the important roles played by surface energy. Creases have been found to form in a nucleation and growth fashion, with surface energy providing a barrier in both processes.

While this process may play an important role in a variety of materials failures, it can also be harnessed to fabricate dynamic chemical patterns and as a new method for lithography. To understand the role of creasing in materials failures or to engineer it for applications, the study of hysteresis in creasing is of vital importance. In Chapter 3, we review that different degrees of hysteresis have been observed in different systems. By changing the interface energy, we for the first time show that it is the self-adhesion at the folding region rather than plastic deformation that gives rise to hysteresis. We design a soft elastic bilayer that can snap between the flat and creased states repeatedly, with hysteresis. The strains at which the creases form and disappear are highly reproducible, and are tunable over a large range, through variations in the level of pre-compression applied to the substrate and the relative thickness of the film. The introduction of bistable flat and creased states and hysteretic switching is an important step to enable applications of this type of instability.

In chapter 4, we design experiments to show that creases can also form on the interface of two soft hydrogels. In comparison with surface creases, which form self-contact, interfacial creases take on a singular non-self-contacting "V" shape. Interfacial creases form at higher strain than surface creases, but always form prior to interface wrinkles.

In chapter 5, we show how the morphology and onset of creases depend on materials properties, geometry, loading history, as well as stress states. While several results are promising, we also propose better experimental setups to facilitate future studies and better control crease morphology.

In chapter 6, we introduce an application of the creasing instability, where we utilize creased hydrogels as a dynamic platform to apply tensile strain on cells. We have demonstrated that using temperature as a stimulus, cultured muscle cells can be mechanically deformed with different strain states and amplitudes. This experiment also, for the first time, achieves local actuation of creasing instability with pneumatic/hydraulic pressure. Creases actuated by microfluidics offer potential for realization of high-throughput cell stretching devices on single cell level, through which different strain states, amplitudes, as well as loading rate and frequency could be modulated to mimic the mechanical environment cells experience in vivo.

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