Optimal conditions for pre-shearing thixotropic or aging soft materials
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ORIGINAL CONTRIBUTION
Optimal conditions for pre-shearing thixotropic or aging soft materials Jiho Choi 1 & Simon A. Rogers 1 Received: 9 March 2020 / Revised: 9 August 2020 / Accepted: 12 October 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Pre-shearing is widely recognized as a necessary step to guarantee repeatability in rheological studies of thixotropic or aging soft materials. When one-directional pre-shear protocols are used, unrecovered elastic strain which leads to biased material states that are not always relaxed because of the build-up of structure during the relaxation process. We propose a way of guaranteeing unbiased material states by incorporating recovery steps, consisting of steps of strain opposing the initial direction of shearing, into any pre-shear protocol. Using such a multi-step pre-shear protocol, we show that it is possible to produce identical results from shearing in the positive and negative directions for the same magnitude of rate after pre-shearing. We further show how this idea of unbiased material states can be used to obtain unbiased results for other fundamental rheological experiments such as flow curves and frequency sweeps. By performing the new pre-shear protocol for every single measurement point of a flow curve or frequency sweep, it is possible to obtain data which is not affected by previous data collection, which leads to material responses with simple and clear shear histories. Keywords Pre-shear . Thixotropy . Hysteresis . Recoverable strain . Aging
Background and criteria of optimal pre-shear The rheology of many materials exhibits long time transience that is typically classified as being the result of thixotropy or aging. The slowly changing rheology is a reflection of structural evolution that takes place across a range of length and time scales. Prevalent structure-kinetics models, for instance, describe the degree of structure based on scalar structure parameter (λ) that evolves according to rules dictated by the shear history and Brownian motion. Aging glassy systems are described by energy landscape in a phenomenological model (Bouchaud 1992) and the soft glassy rheology model (Sollich et al. 1997; Fielding et al. 2000; Radhakrishnan and Fielding 2018). Thixotropic and aging materials are differentiated from other systems by the ability to undo or reverse such changes. In this paper, the aging we discuss indicates only “physical aging” (Cipelletti and Ramos 2005; Mewis and Wagner 2012). To accurately study these behaviors,
* Simon A. Rogers [email protected] 1
Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
researchers require an accurate way of “resetting” the time, which is typically achieved by either shear or thermal protocols that typically fluidize the material to erase the shear history. Thermal protocols have been termed “quenches” and have been used to erase the shear history imparted at the microscopic structural scale by providing sufficie
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