@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 = {

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.

}, 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 {580, title = {Improving the practicality and safety of artificial corneas: Pre-assembly and gamma-rays sterilization of the Boston Keratoprosthesis}, journal = {The Ocular Surface}, volume = {16}, year = {2018}, month = {Jul-2018}, pages = {322 - 330}, abstract = {

Purpose: To make the Boston keratoprosthesis (B-KPro), together with its carrier corneal graft, more easily procured, transported and stored, as well as less expensive, easier for the surgeon to implant and safer for the patient, it is proposed that the B-KPro-graft combination be pre-assembled by an expert technician, followed by sterilization with gamma ray irradiation (GI) allowing long-term storage at room temperature. For this to be possible, it must be shown that the B-KPro itself (not only the graft) remains unharmed by the irradiation.

Methods: Polymethyl methacrylate (PMMA) discs and B-KPros were submitted to either ethylene oxide sterilization or different doses of GI. Cell biocompatibility, mechanical strength and optical quality were evaluated. The feasibility of assembling the B-KPro to a corneal graft, and gamma-radiate afterwards, was also assessed.

Results: There were no differences in cell biocompatibility between the samples. The optical evaluation showed high levels of transparency for all the groups. The absorbance of ultraviolet was higher for the groups treated with GI. The mechanical evaluation by nanoindentation showed no alterations of the PMMA discs after GI. The flexure test revealed a similar mechanical behavior. Technically, pre-assembly and GI of the B-KPro revealed no problems.

Conclusions: Sterilization of B-KPro using GI has no detrimental influence on the device. The pre-assembly of B-KPro to a donor cornea, followed by gamma sterilization, emerges as an efficient and safe procedure. (C) 2018 Elsevier Inc. All rights reserved.

}, keywords = {Artificial corneas; Boston keratoprosthesis; Gamma radiation; Gamma rays; Preassembly; Sterilization; Polymethyl methacrylate}, issn = {15420124}, doi = {10.1016/j.jtos.2018.04.002}, url = {https://linkinghub.elsevier.com/retrieve/pii/S1542012418300442}, author = {Gonzalez-Andrades, Miguel and Sharifi, Roholah and Islam, Mohammad Mirazul and Divoux, Thibaut and Haist, Michael and Paschalis, Eleftherios I. and Gelfand, Larisa and Mamodaly, Shamina and Di Cecilia, Luca and Cruzat, Andrea and Franz-Josef Ulm and Chodosh, James and Delori, Francois and Dohlman, Claes 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 = {

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.

}, 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.} } @article {595, title = {Heat-induced aging of agar solutions: Impact on the structural and mechanical properties of agar gels}, journal = {Food Hydrocolloids}, volume = {64}, year = {2017}, month = {Mar-2017}, pages = {59 - 69}, abstract = {

Thermoreversible gels are commonly prepared by cooling down to ambient temperature, aqueous polymer solutions first brought to a boil for a short time. Here, we study the effect of the latter duration on the subsequent gelation of an agar solution. Increasing the incubation time at high temperature (T= 80 degrees C) from hours to a few days leads to the hydrolysis and the oxidation of the polysaccharides, and results in the decrease of the viscosity and the pH of the solution. Samples withdrew at different incubation times are cooled down to form gels whose structure and mechanical properties are systematically determined. Cryoelectron microscopy and X-ray diffraction experiments reveal that agar gels formed from solutions of increasing incubation times, display a coarser microstructure composed of micron-sized foils which result from the condensation of the polysaccharides, and contrast with the fibrous-like microstructure of gels prepared from a fresh agar solution. Along with structural changes, a prolonged incubation time of the agar solution at T = 80 degrees C leads to weaker gels than those made from fresher solutions, and extensive macro-indentation experiments coupled to direct visualization show that the gel rupture scenario turns from brittle to ductile-like as the incubation time increases. Our study suggests that the incubation time of agar solutions at high temperature could be used as an external control parameter to tune the mechanical properties of agar-based gels. (C) 2016 Elsevier Ltd. All rights reserved.

}, keywords = {Agar; Aging; Gelation; Debonding; Indentation; Fracture}, issn = {0268005X}, doi = {10.1016/j.foodhyd.2016.10.020}, url = {https://linkinghub.elsevier.com/retrieve/pii/S0268005X16305793}, author = {Mao, Bosi and Bentaleb, Ahmed and Louerat, Fr{\'e}d{\'e}ric and Divoux, Thibaut and Snabre, Patrick} } @article {584, title = {Impact of saccharides on the drying kinetics of agarose gels measured by in-situ interferometry}, journal = {Scientific Reports}, volume = {7}, year = {2017}, month = {Jan-23-2017}, pages = { Article Number: 41185 }, abstract = {

Agarose gels are viscoelastic soft solids that display a porous microstructure filled with water at 90\% w/w or more. Despite an extensive use in food industry and microbiology, little is known about the drying kinetics of such squishy solids, which suffers from a lack of time-resolved local measurements. Moreover, only scattered empirical observations are available on the role of the gel composition on the drying kinetics. Here we study by in-situ interferometry the drying of agarose gels of various compositions cast in Petri dishes. The gel thinning is associated with the displacement of interference fringes that are analyzed using an efficient spatiotemporal filtering method, which allows us to assess local thinning rates as low as 10 nm/s with high accuracy. The gel thinning rate measured at the center of the dish appears as a robust observable to quantify the role of additives on the gel drying kinetics and compare the drying speed of agarose gels loaded with various non-gelling saccharides of increasing molecular weights. Our work shows that saccharides systematically decrease the agarose gel thinning rate up to a factor two, and exemplifies interferometry as a powerful tool to quantify the impact of additives on the drying kinetics of polymer gels.

}, doi = {10.1038/srep41185}, url = {http://www.nature.com/articles/srep41185}, author = {Mao, Bosi and Divoux, Thibaut and Snabre, Patrick} } @article {582, title = {Nonlinear Viscoelasticity and Generalized Failure Criterion for Polymer Gels}, journal = {ACS Macro Letters}, volume = {6}, year = {2017}, month = {Jun-12--2017}, pages = {663 - 667}, abstract = {

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\&$\#$39;s durability criterion for brittle solids in order to predict the critical values of the stress sigma(c) and strain gamma(c) at failure. The excellent agreement between theory and experiments suggests that failure in this soft viscoelastic gel is a Markovian process and that Bailey\&$\#$39;s failure criterion extends beyond hard materials such as metals, glasses, or minerals.

}, issn = {2161-1653}, doi = {10.1021/acsmacrolett.7b00213}, url = {http://pubs.acs.org/doi/10.1021/acsmacrolett.7b00213}, author = {Keshavarz, Bavand and Divoux, Thibaut and Manneville, S{\'e}bastien and McKinley, Gareth H.} } @article {583, title = {Understanding rheological hysteresis in soft glassy materials}, journal = {Soft Matter}, volume = {13}, year = {2017}, month = {Mar-07-2017}, pages = {1834-1852}, abstract = {

Motivated by recent experimental studies of rheological hysteresis in soft glassy materials, we study numerically strain rate sweeps in simple yield stress fluids and viscosity bifurcating yield stress fluids. Our simulations of downward followed by upward strain rate sweeps, performed within fluidity models and the soft glassy rheology model, successfully capture the experimentally observed monotonic decrease of the area of the rheological hysteresis loop with sweep time in simple yield stress fluids, and the bell shaped dependence of hysteresis loop area on sweep time in viscosity bifurcating fluids. We provide arguments explaining these two different functional forms in terms of differing tendencies of simple and viscosity bifurcating fluids to form shear bands during the sweeps, and show that the banding behaviour captured by our simulations indeed agrees with that reported experimentally. We also discuss the difference in hysteresis behaviour between inelastic and viscoelastic fluids. Our simulations qualitatively agree with the experimental data discussed here for four different soft glassy materials.

}, issn = {1744-683X}, doi = {10.1039/C6SM02581A}, url = {http://xlink.rsc.org/?DOI=C6SM02581A}, author = {Radhakrishnan, Rangarajan and Divoux, Thibaut and Manneville, S{\'e}bastien and Fielding, Suzanne M.} } @article {581, title = {Yield stress materials in soft condensed matter}, journal = {Reviews of Modern Physics}, volume = {89}, year = {2017}, month = {Aug-21-2017}, pages = { Article Number: 035005 }, abstract = {

A comprehensive review is presented of the physical behavior of yield stress materials in soft condensed matter, which encompasses a broad range of materials from colloidal assemblies and gels to emulsions and non-Brownian suspensions. All these disordered materials display a nonlinear flow behavior in response to external mechanical forces due to the existence of a finite force threshold for flow to occur: the yield stress. Both the physical origin and rheological consequences associated with this nonlinear behavior are discussed and an overview is given of experimental techniques available to measure the yield stress. Recent progress is discussed concerning a microscopic theoretical description of the flow dynamics of yield stress materials, emphasizing, in particular, the role played by relaxation time scales, the interplay between shear flow and aging behavior, the existence of inhomogeneous shear flows and shear bands, wall slip, and nonlocal effects in confined geometries.

}, issn = {0034-6861}, doi = {10.1103/RevModPhys.89.035005}, url = {https://link.aps.org/doi/10.1103/RevModPhys.89.035005}, author = {Bonn, Daniel and Denn, Morton M. and Berthier, Ludovic and Divoux, Thibaut and Manneville, S{\'e}bastien} } @article {585, title = {Simultaneous Rheoelectric Measurements of Strongly Conductive Complex Fluids}, journal = {Physical Review Applied}, volume = {6}, year = {2016}, month = {Dec-09-2016}, pages = { Article Number: 064004 }, abstract = {

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)\&$\#$39; and the conductivity sigma(0) of the gels-i.e., G(0)\&$\#$39; 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.

}, doi = {10.1103/PhysRevApplied.6.064004}, url = {https://link.aps.org/doi/10.1103/PhysRevApplied.6.064004}, author = {Helal, Ahmed and Divoux, Thibaut and McKinley, Gareth H.} }