OrthoChirp  – A Fast Spectro-mechanical Probe for Monitoring Transient Microstructural Evolution During Shear

Abstract: Shear-induced microstructural changes in complex fluids as diverse as food gels, consumer products and fracking fluids are governed by fast internal time scales that are of relevance to material processing operations and often impact the final material properties. To gain insights into these time scales, traditional mechanical spectroscopic techniques such as Small Amplitude Oscillatory Shear (SAOS), and more recently MAOS/LAOS, have been proven to be useful.  However, probing such spectra and their transient evolution during shear is challenging: firstly, time-evolving microstructures complicate the measurement of discrete mechanical spectra, since the material evolves or “mutates” on time-scales comparable to the time scale of the measurement. Secondly, if both modes of deformation are parallel to each other, cross-coupling of the steady- and oscillatory flow fields can lead to unexpected changes in the material measurements (such as apparently negative values of the elastic modulus).

To address both of these challenges, in the present work, fast exponential ‘chirp' signals are superimposed orthogonally onto materials undergoing steady shearing deformation. The utilization of multispectral chirp signals reduces the measurement time required to acquire a frequency spectrum (as compared to traditional discrete frequency sweeps), whilst the orthogonal driving decouples the steady flow field from the oscillatory shear field.  We demonstrate the utility of this orthogonal chirping using physically-associated alginate solutions, for which we demonstrate how the linear-viscoelastic spectrum changes under shear, how the shear-rate–dependent orthogonal moduli can be modeled using continuous, rate-dependent relaxation spectra of the Lodge/Yamamoto type, and how the observed changes can be physically interpreted. The capabilities of this combined technique (the ‘OrthoChirp’) pave the way towards better understanding of shear-rate–dependent viscoelastic properties, in particular for weakly-associated physical gels undergoing (relatively) fast transient evolution.

Bio: Christian Clasen is a Full Professor (gewoon hooglerar) in the Department of Chemical Engineering of KU Leuven, Belgium. He obtained 2001 his PhD in chemistry from Hamburg University, Germany and, after a 2-year postdoctoral stay at MIT, his Habilitation in Technical and Macromolecular Chemistry in 2008.  Joining KU Leuven Belgium in 2006, Christian Clasen leads the Soft Matter, Rheology and Technology (SMaRT) division with 5 faculty members and 30 researchers. His research focuses on the development of novel experimental tools and high-resolution observation techniques in the area of rheology and the flow of complex fluids, as well as in the dimensional analysis and analytical model development. This includes sophisticated rheology (e.g. mechanical rheometry in combination with birefringence, dichroism, and superposition flows), and novel in-house developed measuring devices for the in-situ and time-resolved investigation of a fluid’s microstructure. Recent topics include the rheology of confined thin films, the stability of liquid filaments and jets and ultra-short relaxation processes in elongated polymer solutions, electrospinning and spraying of (bio)polymer solutions, the self-assembly and propulsion of nano-particles in flow fields, and instrument development to study the effects of confinement on the flow behavior of complex fluids. Christian Clasen obtained one of the first ERC grants (NANOFIB), is so far the only recipient of Publication Awards from both the Journal of Rheology (in 2007) and of Rheologica Acta (in 2015), and received the Award of the British Society of Rheology in 2015 for ‘outstanding contributions to experimental rheology’.

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