@article {579,
	title = {Computing the linear viscoelastic properties of soft gels using an optimally windowed chirp protocol},
	journal = {Journal of Rheology},
	volume = {62},
	year = {2018},
	month = {Jul-2018},
	pages = {1037 - 1050},
	abstract = {<p>We use molecular dynamics simulations to investigate the linear viscoelastic response of a model three-dimensional particulate gel. The numerical simulations are combined with a novel test protocol (the optimally windowed chirp or OWCh), in which a continuous exponentially varying frequency sweep windowed by a tapered cosine function is applied. The mechanical response of the gel is then analyzed in the Fourier domain. We show that (i) OWCh leads to an accurate computation of the full frequency spectrum at a rate significantly faster than with the traditional discrete frequency sweeps, and with a reasonably high signal-to-noise ratio, and (ii) the bulk viscoelastic response of the microscopic model can be described in terms of a simple mesoscopic constitutive model. The simulated gel response is in fact well described by a mechanical model corresponding to a fractional Kelvin-Voigt model with a single Scott-Blair (or springpot) element and a spring in parallel. By varying the viscous damping and the particle mass used in the microscopic simulations over a wide range of values, we demonstrate the existence of a single master curve for the frequency dependence of the viscoelastic response of the gel that is fully predicted by the constitutive model. By developing a fast and robust protocol for evaluating the linear viscoelastic spectrum of these soft solids, we open the path toward novel multiscale insight into the rheological response for such complex materials. (C) 2018 The Society of Rheology.</p>
},
	issn = {0148-6055},
	doi = {10.1122/1.5018715},
	url = {http://sor.scitation.org/doi/10.1122/1.5018715},
	author = {Bouzid, Mehdi and Keshavarz, Bavand and Geri, Michela and Divoux, Thibaut and Emanuela Del Gado and McKinley, Gareth H.}
}
@article {629,
	title = {Time-Resolved Mechanical Spectroscopy of Soft Materials via Optimally Windowed Chirps},
	journal = {Physical Review X},
	volume = {8},
	year = {2018},
	month = {Dec-06-2018},
	pages = {Article Number 041042 },
	abstract = {<p>The ability to measure the bulk dynamic behavior of soft materials with combined time and frequency resolution is instrumental for improving our fundamental understanding of connections between the microstructural dynamics and the macroscopic mechanical response. Current state-of-the-art techniques are often limited by a compromise between resolution in the time and frequency domains, mainly due to the use of elementary input signals that have not been designed for fast time-evolving systems such as materials undergoing gelation, curing, or self-healing. In this work, we develop an optimized and robust excitation signal for time-resolved mechanical spectroscopy through the introduction of joint frequency- and amplitude-modulated exponential chirps. Inspired by the biosonar signals of bats and dolphins, we optimize the signal profile to maximize the signal-to-noise ratio while minimizing spectral leakage with a carefully designed modulation of the envelope of the chirp, obtained using a cosine-tapered window function. A combined experimental and numerical investigation reveals that there exists an optimal range of window profiles (around 10\% of the total signal length) that minimizes the error with respect to standard single-frequency sweep techniques. The minimum error is set by the noise floor of the instrument, suggesting that the accuracy of an optimally windowed-chirp (OWCh) sequence is directly comparable to that achievable with a standard frequency sweep, while the acquisition time can be reduced by up to 2 orders of magnitude, for comparable spectral content. Finally, we demonstrate the ability of this optimized signal to provide time- and frequency-resolved rheometric data by studying the fast gelation process of an acid-induced protein gel using repeated OWCh pulse sequences. The use of optimally windowed chirps enables a robust time-resolved rheological characterization of a wide range of soft materials undergoing rapid mutation and has the potential to become an invaluable rheometric tool for researchers across different disciplines.</p>
},
	doi = {10.1103/PhysRevX.8.041042},
	url = {https://link.aps.org/doi/10.1103/PhysRevX.8.041042},
	author = {Geri, Michela and Keshavarz, Bavand and Divoux, Thibaut and Clasen, Christian and Curtis, Daniel J. and McKinley, Gareth H.}
}
