A review of nonlinear oscillatory shear tests: Analysis and application of large amplitude oscillatory shear (LAOS)

Abstract

Dynamic oscillatory shear tests are common in rheology and have been used to investigate a wide range of soft matter and complex fluids including polymer melts and solutions, block copolymers, biological macromolecules, polyelectrolytes, surfactants, suspensions, emulsions and beyond. More specifically, Small Amplitude Oscillatory Shear (SAOS) tests have become the canonical method for probing the linear viscoelastic properties of these complex fluids because of the firm theoretical background [1][2][3][4] and the ease of implementing suitable test protocols. However, in most processing operations the deformations can be large and rapid: it is therefore the nonlinear material properties that control the system response. A full sample characterization thus requires well-defined nonlinear test protocols. Consequently there has been a recent renewal of interest in exploiting Large Amplitude Oscillatory Shear (LAOS) tests to investigate and quantify the nonlinear viscoelastic behavior of complex fluids. In terms of the experimental input, both LAOS and SAOS require the user to select appropriate ranges of strain amplitude (γ 0 ) and frequency (ω). However, there is a distinct difference in the analysis of experimental output, i.e. the material response. At sufficiently large strain amplitude, the material response will become nonlinear in LAOS tests and the familiar material functions used to quantify the linear behavior in SAOS tests are no longer sufficient. For example, the definitions of the linear viscoelastic moduli G΄(ω) and G˝(ω) are based inherently on the assumption that the stress response is purely sinusoidal (linear). However, a nonlinear stress response is not a perfect sinusoid and therefore the viscoelastic moduli are not uniquely defined; other methods are needed for quantifying the nonlinear material response under LAOS deformation. In the present review article, we first summarize the typical nonlinear responses observed with complex fluids under LAOS deformations. We then introduce and critically compare several methods that quantify the nonlinear oscillatory stress response. We illustrate the utility and sensitivity of these protocols by investigating the nonlinear response of various complex fluids over a wide range of frequency and amplitude of deformation, and show that LAOS characterization is a rigorous test for rheological models and 3 advanced quality control.

A novel method for visualising and quantifying through-plane skin layer deformations

“…For example, a direct control on the anticipated boundary conditions (no slip) is available from the imaging. Moreover, our method can be used to directly measure large strains, i.e., skin mechanics in the non-linear viscoelastic strain regime (40.5%; Geerligs et al, 2011a;Holt et al, 2008), in which moduli are strain dependent and the analysis and interpretation of conventional rheometer measurements is complicated (Hyun et al, 2011). In this pilot study, a global engineering strain of 10% was defined a priori to investigate the mechanical response of full-thickness skin at ''moderately large'' strain conditions, in which skin layers cannot be studied separately without disregarding interactions between the tissue layers and loosing information on the global mechanical skin behaviour.…”

Linear viscoelasticity of complex coacervates

“…SAOS probes the linear viscoelastic properties of complex fluids and can provide information on material functions such as viscosity, modulus, and yield stress, along with more dynamic information, such as the characteristic timescales for molecular motion within a material [117][118][119][120][121]. These types of analyses can also be used to unambiguously identify the nature of solid-to-liquid transitions; for instance, distinguishing gelation as compared to a glass transition [122,123].…”

A Novel and Straightforward Methodology to Analyze Materials Under Constant-Structure Oscillatory Motion (Saos and Ql-Laos)

“…However, a few macroscopic material properties that may be obtained through rheological experiments provide indirect and qualitative information about microscopic structural state (33). Other techniques such as light scattering (SANS, SALS and SAXS) and microscopy are necessary in order to obtain direct information about microscopic properties (22).…”

“…In colloidal science, the usual parameter that compares the externally applied stress σ with the Brownian stress k B T /a 3 is the Péclet number (22):…”

“…The rate of variation of the breakdown rate is also associated with the material's microscopic structure. Thus, complex materials with similar structures in the linear viscoelastic regime may present distinct structures and properties in the nonlinear regime (22).…”