Studying Large Amplitude Oscillatory Shear Response of Soft Materials

Summary

We present a detailed protocol outlining how to perform nonlinear oscillatory shear rheology on soft materials, and how to run the SPP-LAOS analysis to understand the responses as a sequence of physical processes.

Abstract

We investigate the sequence of physical processes exhibited during large amplitude oscillatory shearing (LAOS) of polyethylene oxide (PEO) in dimethyl sulfoxide (DMSO) and xanthan gum in water — two concentrated polymer solutions used as viscosifiers in foods, enhanced oil recovery, and soil remediation. Understanding the nonlinear rheological behavior of soft materials is important in the design and controlled manufacturing of many consumer products. It is shown how the response to LAOS of these polymer solutions can be interpreted in terms of a clear transition from linear viscoelasticity to viscoplastic deformation and back again during a period. The LAOS results are analyzed via the fully quantitative Sequence of Physical Processes (SPP) technique, using free MATLAB-based software. A detailed protocol of performing a LAOS measurement with a commercial rheometer, analyzing nonlinear stress responses with the freeware, and interpreting physical processes under LAOS is presented. It is further shown that, within the SPP framework, a LAOS response contains information regarding the linear viscoelasticity, the transient flow curves, and the critical strain responsible for the onset of nonlinearity.

Introduction

Concentrated polymeric solutions are used in a variety of industrial applications primarily to increase viscosity, including in foods¹ and other consumer products², enhanced oil recovery³, and soil remediation⁴. During their processing and use, they are necessarily subjected to large deformations over a range of timescales. Under such processes, they demonstrate rich and complex nonlinear rheological behaviors that depend on the flow or deformation conditions¹. Understanding these complex nonlinear rheological behaviors is essential for successfully ....

Protocol

1. Rheometer Setup

1. With the rheometer configured in the SMT mode (see note), attach the upper and lower drive geometries. To maintain as close to a homogeneous shear field as possible, use a 50 mm plate (PP50) as the lower fixture, and a 2-degree cone (CP50-2) for the upper fixture.
   Note: The rheometer we use (see the Table of Materials) can be configured in either a combined motor-transducer (CMT) or separate motor transducer (SMT) mode. With only a single motor integrated in the rheo.......

Discussion

We have demonstrated how to correctly perform large amplitude oscillatory shear rheometry tests using a commercial rheometer, and to run the SPP analysis freeware to interpret and understand the nonlinear stress responses of two distinct polymer solutions. The SPP framework, which has previously been shown to correlate with structural changes and facilitate understandings of numerous colloidal systems, can be equally applied to polymer systems. The responses of two concentrated polymeric solutions to LAOS have been inves.......

Materials

Name	Company	Catalog Number	Comments
SPP analysis software	Simon Rogers Group (UIUC)	SPPplus_v1p1	Attached as supplementary files
MATLAB	Mathwork
Rheometer	Anton Paar	MCR 702 TwinDrive
50mm 2-degree cone	Anton Paar	CP50-2	Upper measuring system
50mm plate	Anton Paar	PP50	Lower measuring system
Xanthan gum (XG)	Sigma-Aldrich	11138-66-2
Polyethylene oxide (PEO)	Sigma-Aldrich	25322-68-3	Mv=1,000,000
Dimethyl sulfoxide (DMSO)	Sigma-Aldrich	67-68-5