Memory Effects in the Friction of Hydrogel Particles Público

Abstract

Hydrogels are cross-linked polymer networks that can absorb and retain a large fraction of water, often up to 90 percent by weight. They are widely used in many engineering applications as well as agriculture industries due to their ultralow friction, biocompatibility, and chemical transport capabilities. Previous research in our lab has identified and characterized three distinct regimes of friction in polyacrylamide (PAAm), polyacrylic acid (PAA), and agarose. Among other things, they have shown that around a critical velocity, the friction coefficient of hydrogels is reduced by an order of magnitude and displayed relaxation behavior over multiple timescales. Now, following upon that, we looked closer at the time dependent behavior of PAA gel. We have shown that this friction regime exhibits a nearly exponential decay sensitive to the sliding velocity and precedent shearing. In general, a higher sliding velocity will lead to a faster decay. We found multiple time scales in the friction decays. We have also found the recovery time of the PAA and agarose gels, which refers to the time it takes for the polymer network of the hydrogel to recover back to its initial unsheared state. Finally, we showed that the final state of the hydrogel is only determined by the external shear stress applied, no matter what the initial state of the hydrogel is. Our findings imply that we can tell from the current frictional behavior of hydrogel what kinds of shear the hydrogel has experienced. In other words, the hydrogel remembers, or has a "memory" of, the shear history it has been through.

Table of Contents

1 Introduction 4

1.1 What is a Hydrogel? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2 Industrial Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3 Classical Friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.4 Friction of Hydrogel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.4.1 Slow Velocity Regime . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.4.2 Time-dependent Regime . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.4.3 High Velocity Regime . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.5 Memory Effects in Friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2 Methods 11

2.1 Tribometer Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2 Preparation of Hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.2.1 Fabrication of Polyacrylamide Gels . . . . . . . . . . . . . . . . . . . 14

2.2.2 Fabrication of Agarose Gels . . . . . . . . . . . . . . . . . . . . . . . 14

2.2.3 Preparation of Polyacrylic acid Gels . . . . . . . . . . . . . . . . . . . 15

3 Results and Discussion 16

3.1 Effective d of PAA Gel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.2 Where is the Transition Regime? . . . . . . . . . . . . . . . . . . . . . . . . 18

3.3 Velocity Sensitive Decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.3.1 Experimental Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.3.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.4 Recovery of Hydrogel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.4.1 Experimental Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.4.2 Result . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.5 Shear Determines the State of PAA Gel . . . . . . . . . . . . . . . . . . . . . 26

3.5.1 Experimental Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.5.2 Result . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.6 Proposed Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4 Conclusion 31

5 Future Directions 32

About this Honors Thesis

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School	Emory College
Department	Physics
Degree	B.S.
Submission	Honors Thesis
Language	English
Research Field	Physics, Condensed Matter
Palabra Clave	Friction Memory Hydrogel
Committee Chair / Thesis Advisor	Justin C. Burton, Emory University
Committee Members	Eric Weeks, Emory University Khalid Salaita, Emory University

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