Residual Stresses in Glasses

The history dependence of the glasses formed from flow-melted steady states by a sudden cessation of the shear rate $\dot\gamma$ is studied in colloidal suspensions, by molecular dynamics simulations, and mode-coupling theory. In an ideal glass, stresses relax only partially, leaving behind a finite persistent residual stress. For intermediate times, relaxation curves scale as a function of $\dot\gamma t$, even though no flow is present. The macroscopic stress evolution is connected to a length scale of residual liquefaction displayed by microscopic mean-squared displacements. The theory describes this history dependence of glasses sharing the same thermodynamic state variables, but differing static properties.Comment: submitted to Physical Revie

Shear stresses of colloidal dispersions at the glass transition in equilibrium and in flow

We consider a model dense colloidal dispersion at the glass transition, and investigate the connection between equilibrium stress fluctuations, seen in linear shear moduli, and the shear stresses under strong flow conditions far from equilibrium, viz. flow curves for finite shear rates. To this purpose thermosensitive core-shell particles consisting of a polystyrene core and a crosslinked poly(N-isopropylacrylamide)(PNIPAM) shell were synthesized. Data over an extended range in shear rates and frequencies are compared to theoretical results from integrations through transients and mode coupling approaches. The connection between non-linear rheology and glass transition is clarified. While the theoretical models semi-quantitatively fit the data taken in fluid states and the predominant elastic response of glass, a yet unaccounted dissipative mechanism is identified in glassy states.Comment: 17 pages, 11 figures; J. Chem. Phys. in print (2008

Nonlinear response of dense colloidal suspensions under oscillatory shear: Mode-coupling theory and FT-rheology experiments

Using a combination of theory, experiment and simulation we investigate the nonlinear response of dense colloidal suspensions to large amplitude oscillatory shear flow. The time-dependent stress response is calculated using a recently developed schematic mode-coupling-type theory describing colloidal suspensions under externally applied flow. For finite strain amplitudes the theory generates a nonlinear response, characterized by significant higher harmonic contributions. An important feature of the theory is the prediction of an ideal glass transition at sufficiently strong coupling, which is accompanied by the discontinuous appearance of a dynamic yield stress. For the oscillatory shear flow under consideration we find that the yield stress plays an important role in determining the non linearity of the time-dependent stress response. Our theoretical findings are strongly supported by both large amplitude oscillatory (LAOS) experiments (with FT-rheology analysis) on suspensions of thermosensitive core-shell particles dispersed in water and Brownian dynamics simulations performed on a two-dimensional binary hard-disc mixture. In particular, theory predicts nontrivial values of the exponents governing the final decay of the storage and loss moduli as a function of strain amplitude which are in excellent agreement with both simulation and experiment. A consistent set of parameters in the presented schematic model achieves to jointly describe linear moduli, nonlinear flow curves and large amplitude oscillatory spectroscopy

Divergence of the third harmonic stress response to oscillatory strain approaching the glass transition

The leading nonlinear stress response in a periodically strained concentrated colloidal dispersion is studied experimentally and by theory. A thermosensitive microgel dispersion serves as well-characterized glass-forming model, where the stress response at the first higher harmonic frequency (3 omega for strain at frequency omega) is investigated in the limit of small amplitude. The intrinsic nonlinearity at the third harmonic exhibits a scaling behavior which has a maximum in an intermediate frequency window and diverges when approaching the glass transition. It captures the (in-) stability of the transient elastic structure. Elastic stresses in-phase with the third power of the strain dominate the scaling. Our results qualitatively differ from previously derived scaling behavior in dielectric spectroscopy of supercooled molecular liquids. This might indicate a dependence of the nonlinear response on the symmetry of the external driving under time reversal

One-year follow-up-case report of secondary tension pneumothorax in a COVID-19 pneumonia patient

PURPOSE The Coronavirus Disease 2019 (COVID-19) may result not only in acute symptoms such as severe pneumonia, but also in persisting symptoms after months. Here we present a 1~year follow-up of a patient with a secondary tension pneumothorax due to COVID-19 pneumonia. CASE PRESENTATION In May 2020, a 47-year-old male was admitted to the emergency department with fever, dry cough, and sore throat as well as acute chest pain and shortness of breath. Sputum testing (polymerase chain reaction, PCR) and computed tomography (CT) confirmed infection with the severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2). Eleven days after discharge, the patient returned to the emergency department with pronounced dyspnoea after coughing. CT showed a right-sided tension pneumothorax, which was relieved by a chest drain (Buelau) via mini open thoracotomy. For a period of 3~months following resolution of the pneumothorax the patient complained of fatigue with mild joint pain and dyspnoea. After 1~year, the patient did not suffer from any persisting symptoms. The pulmonary function and blood parameters were normal, with the exception of slightly increased levels of D-Dimer. The CT scan revealed only discrete ground glass opacities (GGO) and subpleural linear opacities. CONCLUSION Tension pneumothorax is a rare, severe complication of a SARS-CoV-2 infection but may resolve after treatment without negative long-term sequelae. LEVEL OF EVIDENCE V

Core shell microgels as model colloids for rheological studies

We review recent work done on the rheology of thermosensitive suspensions. These systems consist of aqueous suspensions of core shell particles having a solid polystyrene core and a shell of thermosensitive crosslinked poly N isopropylacrylamide PNIPA . In cold water the thermosensitive PNIPA network is swollen leading to a high effective volume fraction of the particles in suspension. Approaching the volume transition at 32 C the network shrinks by expelling water. Hence, the effective volume fraction can be adjusted by the temperature. We demonstrate that these suspensions are a well characterized model system for the study of the flow behavior of concentrated suspensions. In particular, experimental work done on this system can be compared to the predictions of the mode coupling theory MCT of the fluid to glass transition. Excellent agreement is found demonstrating that MCT captures the essential features of the dynamics of flowing suspensions. In particular, MCT predicts a melting of the glass by shear which is fully corroborated by the experimental dat

Creep in Colloidal Glasses

We investigate the nonlinear response to shear stress of a colloidal hard-sphere glass, identifying several regimes depending on time, sample age, and the magnitude of applied stress. This emphasizes a connection between stress-imposed deformation of soft and hard matter, in particular, colloidal and metallic systems. A generalized Maxwell model rationalizes logarithmic creep for long times and low stresses. We identify diverging time scales approaching a critical yield stress. At intermediate times, strong aging effects are seen, which we link to a stress overshoot seen in stress-strain curves