Stretching the Printability Metric in Direct-ink Writing with Highly Extensible Yield-Stress Fluids

TL;DR

This study investigates how extensional rheological properties, especially strain-to-break, influence printability in direct-ink writing with yield-stress fluids, leading to a framework for designing high-performance printable inks.

Contribution

It introduces a novel focus on extensional properties in rheology-printability correlation and establishes a property-manufacturing relationship for ink design.

Findings

Extensional failure strain correlates strongly with drawability.

High yield stress and extensibility can be achieved simultaneously.

A bijective relationship links shear yield stress to buildability and extensional strain-to-break to drawability.

Abstract

Direct-ink writing leverages the rheological complexity of yield-stress fluids to construct complex geometries, particularly those with large gaps across internal structures. However, extensional rheological properties have rarely been considered in work that studies rheology-printability correlations. Here, we test our hypothesis that extensional properties correlate with drawability, a key indicator of printability that signifies speed robustness, printing resolution, and gap-spanning performance. We formulated cementitious suspensions using hydroxyapatite (HAp) particles, independently tuning them for yield stress and extensibility, two crucial rheological properties, and test-printed. To enhance extensibility, we incorporated hydroxypropyl methylcellulose as a polymeric modifier, but this enhancement may decrease as yield stress increases, presenting a challenge in materials design. We modulated particle interactions to achieve a wide range of yield stress and extensibility, allowing for rigorous testing of our hypothesis. This approach created inks with high extensibility and high yield stress, generally considered mutually exclusive properties. We evaluated correlations between drawability and key rheological properties, finding the strongest positive correlation with extensional failure strains (strain-to-break) rather than yield stress. We establish a bijective property-manufacturing relationship (one-on-one mapping of shear yield stress to buildability and extensional strain-to-break to drawability) by combining our findings on drawability with previous studies on buildability. This relationship provides a comprehensive framework for designing high-performance inks that can be self-supporting, capable of high-speed printing, and allow gap-spanning features.