Discontinuous rate-stiffening in a granular composite modeled after cornstarch and water

David Z Chen,Dong Wang,Hu Zheng,Robert P Behringer

doi:10.1038/s41467-019-09300-z

David Z Chen, Dong Wang + Show 2 more

Open Access

https://doi.org/10.1038/s41467-019-09300-z

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Abstract

Cornstarch in water exhibits impact-activated solidification (IAS) and strong discontinuous shear thickening, with “shear jamming”. However, these phenomena are absent in cornstarch in ethanol. Here we show that cornstarch granules swell under ambient conditions. We postulate that this granule swelling is linked to an interparticle force scale that introduces a discontinuous rate-dependence to the generation of stable contacts between granules. We studied this force scale by coating sand with ~ 2 μm-thick polydimethysiloxane, creating a material that exhibits a similar IAS and discontinuous deformation rate-stiffening despite being a granular composite, not a suspension. This result suggests rate-dependence can be tuned by coating granular materials, introducing an interparticle force scale from rate-dependent properties present in the coating material. Our work provides insights into the unique behavior of cornstarch in water, bridges our understanding of suspensions and dry granular materials, and introduces a method to make discontinuous rate-dependent materials without suspending particles.

Highlights

Cornstarch in water exhibits impact-activated solidification (IAS) and strong discontinuous shear thickening, with “shear jamming”
Cornstarch in water shows non-Newtonian behavior: impact-activated solidification (IAS), where a high-speed intruder is stopped by the rapid solidification of the underlying suspension[1], and discontinuous shear thickening (DST), where increasing shear rates leads to a sudden increase in viscosity that spans several orders of magnitude[2]
Interparticle contacts are essential for the emergence of DST in simulations[22], but smooth hard particles are repelled from contact by the lubrication singularity

Summary

Introduction

Cornstarch in water exhibits impact-activated solidification (IAS) and strong discontinuous shear thickening, with “shear jamming”. Despite a number of studies on the properties of cornstarch in water, the underlying mechanism for its unusually strong response under shear compared to typical shear thickening suspensions[6,7] and “shear jamming” response[8,9] has been elusive: hydrodynamic forces[10,11], dilatancy[12,13], and shear jamming[14,15] have all been suggested to play a role in DST This lack of understanding of the underlying mechanisms involved makes it prohibitive to explore the material design space in order to make new materials with strongly discontinuous rate-dependent properties. We show that IAS and DST in aqueous cornstarch suspensions are strongly influenced by a previously unexplored phenomenon, namely that cornstarch swells appreciably in water under ambient conditions, which alters its mechanical properties We postulate that this swelling introduces a granule-level force scale that (1) allows for strong frictional contacts to form, and (2) leads to a discontinuous deformation rate-dependence on stiffness/viscosity via a granule-level force scale. Our finding gives insight into IAS and DST in suspensions, suggests similarities between suspensions and dry granular materials, and provides a blueprint for creating strongly deformation rate-dependent materials with no suspending fluid

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