Strong through to weak 'sheared' gels

The gelation of an agar biopolymer solution under quiescent conditions enables the formation of a strong network structure capable of sustaining mechanical deformation up to a critical strain or stress above which it fractures. Gelation of the same solution under shear conditions results in a weak gel which is capable of sustaining mechanical deformation at small strain or stresses but which flows if subjected to higher deformations. The purpose of this study is to investigate how shear can alter so drastically the rheological properties of biopolymer gels. We achieve this by investigating the nonisothermal gelation behaviour of agar biopolymer gels prepared under both oscillatory and steady shear conditions. A dynamic phase diagram summarises the effects of strain amplitude and frequency on the rheological properties of biopolymer gels formed at a constant cooling rate. We show that certain combinations of strain amplitude and frequency produce a new class of physical gel, a 'soft' gel. Such gels present both yielding and cohesive characteristics. It is proposed that the imposition of shear changes the pathway through which structures are developed by promoting short range interactions beyond those possible through temperature change alone. Understanding the strength of such interactions and the time scales over which they occur will ultimately allow tuning of the structure development during cooling. Crown Copyright (C) 2004 Published by Elsevier B.V. All rights reserved