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.

VL - 62 UR - http://sor.scitation.org/doi/10.1122/1.5018715 IS - 4 JO - Journal of Rheology ER - TY - JOUR T1 - Time-Resolved Mechanical Spectroscopy of Soft Materials via Optimally Windowed Chirps JF - Physical Review X Y1 - 2018 A1 - Geri, Michela A1 - Keshavarz, Bavand A1 - Divoux, Thibaut A1 - Clasen, Christian A1 - Curtis, Daniel J. A1 - McKinley, Gareth H. AB -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.

VL - 8 UR - https://link.aps.org/doi/10.1103/PhysRevX.8.041042 IS - 4 JO - Phys. Rev. X ER - TY - JOUR T1 - Nonlinear Viscoelasticity and Generalized Failure Criterion for Polymer Gels JF - ACS Macro Letters Y1 - 2017 A1 - Keshavarz, Bavand A1 - Divoux, Thibaut A1 - Manneville, Sébastien A1 - McKinley, Gareth H. AB -Polymer gels behave as soft viscoelastic solids and exhibit a generic nonlinear mechanical response characterized by pronounced stiffening prior to irreversible failure, most often through macroscopic fractures. Here, we describe this scenario for a model protein gel using an integral constitutive equation built upon the linear and the nonlinear viscoelastic properties of the gel. We show that this formalism predicts quantitatively the gel mechanical response in shear start-up experiments, up to the onset of macroscopic failure. Moreover, we couple the computed stress response with Bailey's durability criterion for brittle solids in order to predict the critical values of the stress sigma(c) and strain gamma(c) at failure. The excellent agreement between theory and experiments suggests that failure in this soft viscoelastic gel is a Markovian process and that Bailey's failure criterion extends beyond hard materials such as metals, glasses, or minerals.

VL - 6 UR - http://pubs.acs.org/doi/10.1021/acsmacrolett.7b00213 IS - 7 JO - ACS Macro Lett. ER - TY - JOUR T1 - Simultaneous Rheoelectric Measurements of Strongly Conductive Complex Fluids JF - Physical Review Applied Y1 - 2016 A1 - Helal, Ahmed A1 - Divoux, Thibaut A1 - McKinley, Gareth H. AB -We introduce an modular fixture designed for stress-controlled rheometers to perform simultaneous rheological and electrical measurements on strongly conductive complex fluids under shear. By means of a nontoxic liquid metal at room temperature, the electrical connection to the rotating shaft is completed with minimal additional mechanical friction, allowing for simultaneous stress measurements at values as low as 1 Pa. Motivated by applications such as flow batteries, we use the capabilities of this design to perform an extensive set of rheoelectric experiments on gels formulated from attractive carbon-black particles, at concentrations ranging from 4 to 15 wt %. First, experiments on gels at rest prepared with different shear histories show a robust power-law scaling between the elastic modulus G(0)' and the conductivity sigma(0) of the gels-i.e., G(0)' similar to sigma(alpha)(0), with alpha = 1.65 +/- 0.04, regardless of the gel concentration. Second, we report conductivity measurements performed simultaneously with creep experiments. Changes in conductivity in the early stage of the experiments, also known as the Andrade-creep regime, reveal for the first time that plastic events take place in the bulk, while the shear rate gamma decreases as a weak power law of time. The subsequent evolution of the conductivity and the shear rate allows us to propose a local yielding scenario that is in agreement with previous velocimetry measurements. Finally, to establish a set of benchmark data, we determine the constitutive rheological and electrical behavior of carbon-black gels. Corrections first introduced for mechanical measurements regarding shear inhomogeneity and wall slip are carefully extended to electrical measurements to accurately distinguish between bulk and surface contributions to the conductivity. As an illustrative example, we examine the constitutive rheoelectric properties of five different grades of carbon-black gels and we demonstrate the relevance of this rheoelectric apparatus as a versatile characterization tool for strongly conductive complex fluids and their applications.

VL - 6 UR - https://link.aps.org/doi/10.1103/PhysRevApplied.6.064004 IS - 6 JO - Phys. Rev. Applied ER -