Connecting Anomalous Elasticity and Sub-Arrhenius Structural Dynamics in a Cell-Based Model.

In order to understand the mechanisms for glassy dynamics in biological tissues and shed light on those in non-biological materials, we study the low-temperature disordered phase of 2D vertex-like models. Recently it has been noted that vertex models have quite unusual behavior in the zero-temperature limit, with rigidity transitions that are controlled by residual stresses and therefore exhibit very different scaling and phenomenology compared to particulate systems. Here we investigate the finite-temperature phase of two-dimensional Voronoi and Vertex models, and show that they have highly unusual, sub-Arrhenius scaling of dynamics with temperature. We connect the anomalous glassy dynamics to features of the potential energy landscape associated with zero-temperature inherent states.

One of the greatest challenges in contemporary condensed matter physics is to ascertain whether the formation of glasses from liquids is fundamentally thermodynamic or dynamic in origin. Although the thermodynamic paradigm has dominated theoretical research for decades, the purely kinetic perspective of the dynamical facilitation (DF) theory has attained prominence in recent times. In particular, recent experiments and simulations have highlighted the importance of facilitation using simple model systems composed of spherical particles. However, an overwhelming majority of liquids possess anisotropy in particle shape and interactions, and it is therefore imperative to examine facilitation in complex glass formers. Here, we apply the DF theory to systems with orientational degrees of freedom as well as anisotropic attractive interactions. By analyzing data from experiments on colloidal ellipsoids, we show that facilitation plays a pivotal role in translational as well as orientational relaxation. Furthermore, we demonstrate that the introduction of attractive interactions leads to spatial decoupling of translational and rotational facilitation, which subsequently results in the decoupling of dynamical heterogeneities. Most strikingly, the DF theory can predict the existence of reentrant glass transitions based on the statistics of localized dynamical events, called excitations, whose duration is substantially smaller than the structural relaxation time. Our findings pave the way for systematically testing the DF approach in complex glass formers and also establish the significance of facilitation in governing structural relaxation in supercooled liquids.

Broadband dielectric spectroscopy and aging of glass formers

Using a mean field approach and simulations, we study the non-linear mechanical response of the vertex model (VM) of biological tissue to compression and dilation. The VM is known to exhibit a transition between solid and fluid-like, or floppy, states driven by geometric incompatibility. Target perimeter and area set a target shape which may not be geometrically achievable, thereby engendering frustration. Previously, an asymmetry in the linear elastic response was identified at the rigidity transition between compression and dilation. Here we show that the asymmetry extends away from the transition point for finite strains. Under finite compression, an initially solid VM can completely relax perimeter tension, resulting in a drop discontinuity in the mechanical response. Conversely, an initially floppy VM under dilation can rigidify and have a higher response. These observations imply that re-scaling of cell area shifts the transition between rigid and floppy states. Based on this insight, we calculate the re-scaling of cell area engendered by intrinsic curvature and write a prediction for the rigidity transition in the presence of curvature. The shift of the rigidity transition in the presence of curvature for the VM provides a new metric for predicting tissue rigidity from image data of curved tissues in a manner analogous to the flat case.

Effect of temperature and volume on structural relaxation time: Interpretation in terms of decrease of configurational entropy

One of the challenging problems related to the liquid-glass transition phenomenon is establishing a link between the character of intermolecular interactions and the behavior of molecular dynamics. Introducing the density scaling concept, according to which dynamic quantities, e.g., viscosity or structural relaxation time (τ_{α}) measured at different thermodynamic conditions are expressed as a single universal curve if plotted against ρ^{γ}/T, led to significant progress in solving this problem since the scaling exponent γ defines the steepness of the repulsive part of the intermolecular potential. Herein, we found that relaxation dynamics of van der Waals and H-bonding glass formers, for which the Kirkwood factor (g_{K}) is an isomorph-invariant quantity, satisfy an alternative scaling, logτ_{α} vs T(Δϵ_{s}T)^{-γ}. As a result, the exponent γ is determined from the temperature and pressure evolutions of τ_{α} and dielectric relaxation strength Δϵ-both obtained in a single dielectric experiment, which makes the γ coefficient to be accessed in the future for an extensive database of glass-forming liquids.

The nature of the glass transition is one of the frontier questions in Statistical Physics and Materials Science. Highly cooperative structural processes develop in glass-forming melts exhibiting relaxational dynamics which is spread out over many decades in time. While considerable progress has been made in recent decades towards understanding dynamical slowing-down in quiescent systems, the interplay of glassy dynamics with external fields reveals a wealth of novel phenomena yet to be explored. This special issue focuses on recent results obtained by the Research Unit FOR 1394 ‘Nonlinear response to probe vitrification’ which was funded by the German Science Foundation (DFG). In the projects of the research unit, strong external fields were used in order to gain insights into the complex structural and transport phenomena at the glass transition under far-from-equilibrium conditions. This aimed inter alia to test theories of the glass transition developed for quiescent systems by pushing them beyond their original regime. Combining experimental, simulational, and theoretical efforts, the eight projects within the FOR 1394 measured and determined aspects of the nonlinear response of supercooled metallic, polymeric, and silica melts, of colloidal dispersions, and of ionic liquids. Applied fields included electric and mechanic fields, and forced active probing (‘micro-rheology’), where a single probe is forced through the glass-forming host. Nonlinear stress-strain and force-velocity relations as well as nonlinear dielectric susceptibilities and conductivities were observed. While the physical manipulation of melts and glasses is interesting in its own right, especially technologically, the investigations performed by the FOR 1394 suggest to use the response to strong homogeneous and inhomogeneous fields as technique to explore on the microscopic level the cooperative mechanisms in dense melts of strongly interacting constituents. Questions considered concern the (de-)coupling of different dynamical degrees of freedom in an external field, and the ensuing state diagrams. What forces are required to detach a localized probe particle from its initial environment in a supercooled liquid, in a glassy or granular system? Do metallic and colloidal glasses yield homogeneously or by strain localization under differently applied stresses? Which mechanisms determine field-dependent susceptibilities in dielectric and ionically conducting glass formers?

In the advanced glass melter glass batch materials are entrained in gas and then rapidly heated to glass formation temperature by combusting the gas. The glass batch separates on a target where glass formation takes place. A numerical model has been developed to simulate this glass formation process, which differs from the conventional glass formation process because the batch materials are mixed at glass formation temperatures and because the calcination of Na2CO3, CaCO3, and MgCO3 occurs on the surface of the glass layer, not in the bulk material. The numerical model is based on diffusion‐controlled dissolution of sand particles in the liquid glass. Preliminary experimental data support the rapid glass formation rates predicted by the numerical model.

A new approach is theoretically proposed to study the glass transition of active pharmaceutical ingredients and a glass-forming anisotropic molecular liquid at high pressures. We describe amorphous materials as a fluid of hard spheres. Effects of nearest neighbor interactions and cooperative motions of particles on glassy dynamics are quantified through a local and collective elastic barrier calculated using the elastically collective nonlinear Langevin equation theory. Inserting two barriers into Kramer's theory gives the structural relaxation time. Then, we formulate a new mapping based on the thermal expansion process under pressure to intercorrelate particle density, temperature, and pressure. This analysis allows us to determine the pressure and temperature dependence of α relaxation. From this, we estimate the effective elastic modulus of amorphous materials and capture the effects of conformation on the relaxation process. Remarkably, our theoretical results agree well with experiments.

The routine transformation of a liquid, as it is rapidly cooled, resulting in glass formation, is remarkably complex. A theoretical explanation of the dynamics associated with this process has remained one of the major unsolved problems in condensed matter physics. The random first order transition (RFOT) theory, which was proposed over 25 years ago, provides a theoretical basis for explaining much of the phenomena associated with glass forming materials. It links or relates multiple metastable states, slow or glassy dynamics, dynamic heterogeneity, and both a dynamical and an ideal glass transition. Remarkably, the major concepts in the RFOT theory can also be profitably used to understand many spectacular phenomena in biology and condensed matter physics, as illustrated here. The presence of a large number of metastable states and the dynamics in such complex landscapes in biological systems from molecular to cellular scale and beyond leads to behavior, which is amenable to descriptions based on the RFOT theory. Somewhat surprisingly even intratumor heterogeneity arising from variations in cancer metastasis in different cells is hauntingly similar to glassy systems. There are also deep connections between glass physics and electronically disordered systems undergoing a metal-insulator transition, aging effects in which quantum effects play a role, and the physics of superglasses (a phase that is simultaneously a superfluid and a frozen amorphous structure). It is argued that the common aspect in all these diverse phenomena is that multiple symmetry unrelated states governing both the equilibrium and dynamical behavior---a lynchpin in the RFOT theory---controls the behavior observed in these unrelated systems.

Precise cutting of biological tissue is possible with the Er:YAG laser because of the strong absorption of radiation exhibited by water containing media at 2.94 micrometers wavelength. To achieve control over the thermal damage caused to the tissue and over the extent of the coagulation zone, a thorough knowledge of the local temperature distribution arising near the impact zone is necessary. Calculations are possible in some simple cases, whereas in others, where liquified tissue material acts as a secondary heat source long after the pulse, a time resolved direct measurement of the temperature distributions with microscopical spatial resolution would be desirable. We have developed a method for measuring two-dimensional temperature distributions in optically transparent media with a high time resolution (up to 4 ns) and with microscopical spatial resolution by imaging the temperature dependent fluorescence distribution of 2 micrometers thin films positioned inside the target. With this method we have measured the temperature distributions at different times after the impact of single pulses from an Er:YAG laser at various fluences in gelatin targets, which we use as model for biological tissue. The results are compared with the thermal damage inflicted in vitro to different types of animal tissue. A strong dependence of the temperature distributions and their dynamical behavior on pulse fluence and water content of the target is observed, in congruence with the coagulation zones observed biological tissue.

Graphene exhibits many unusual elastic properties, making it an intriguing material for mechanical measurement and actuation at the quantum limit. We theoretically examine the viability of graphene for cavity optomechanics from near-infrared to terahertz wavelengths, fully taking into account its large optical absorption and dispersion. A large optomechanical coupling coefficient, on the same order of that observed in state-of-the-art optomechanical materials, can be realized in the mid-infrared spectrum with highly doped graphene, a high optical quality factor, and optimal positioning of graphene. Around 100 THz, the dispersive coupling coefficient reaches 180 MHz/nm and 500 MHz/nm in the resolved and unresolved sideband regimes, respectively. We find that predominantly dispersive coupling requires a high graphene Fermi level and mid-infrared excitation, while predominantly dissipative coupling favors a moderate graphene Fermi level and near-infrared excitation.

Despite decades of studies on the grand problem of the glass transition the question of well-defined universal patterns, including the key problem of the previtreous behavior of the primary (structural) relaxation time, remains elusive. This report shows the universal previtreous behavior of the apparent fragility, i.e. the steepness index mP (T > Tg) = d log10τ(T)/d( Tg/T). It is evidenced that mP(T) = 1(T − T*), for T → Tg and T*= Tg − Δ T*. Basing on this finding, the new 3-parameter dependence for portraying the previtreous behavior of the primary relaxation time has been derived: τ(T) = CΩ((T − T*)/T)−Ω × [exp((T − T*)/T)]Ω. The universality of obtained relations is evidenced for glass formers belonging to low molecular weight liquids, polymers (melt and solid), plastic crystals, liquid crystals, resins and relaxors. They exhibit clear preferences either for the VFT or for the critical-like descriptions, if recalled already used modeling. The novel relation can obey even above the dynamic crossover temperature, with the power exponent Ω ranging between ~17 (liquid crystals) to ~57 (glycerol), what may indicate the impact of symmetry on the previtreous effect. Finally, the emerging similarity to the behavior in the isotropic phase of nematic liquid crystals is recalled.

This paper presents new results on a novel coupled-physics modality called magneto-acousto-electrical tomography (MAET). With static magnetic field, a beam of ultrasound is applied to the sample to be imaged; the ultrasonic vibration can lead to the separation of the charges due to the Lorentz force and produces the current distributed in the object. In this paper, simulations were conducted on biological tissue model, and the electrical properties were reconstructed, which could reflect the state of physiological or pathological. We also conducted experiments to detect the voltage signal of microvolt level by the electrodes placed around the sample. The series of experiments were conducted on low-conductivity (0.2 S/m) animal gelatin phantom and real biological tissue, successively placed in a 260 mT magnetic field and sonicated with a plane ultrasonic transducer of central frequency 2.25 MHz. In this paper, we obtained axis resolution of 1 mm, much more significant improvement than the previous reports. More importantly, the experimental result does also demonstrate the feasibility of MAET to image the electrical properties of biological tissue.

The Cauchy-like relation M(infinity) = A + BG(infinity) has recently been found to hold for the high frequency limit values of the longitudinal modulus M(infinity) and transverse modulus G(infinity) of viscoelastic liquids, with B approximately 3 in all the investigated systems. The Brillouin scattering results here reported for curing epoxy systems and thermal glass formers give evidence for the validity of a Cauchy-like relation M(') = A + BG(') for the real part of the elastic moduli measured at finite frequencies. Our results suggest as well the validity of a pure Cauchy relation DeltaM = 3 DeltaG for the relaxation strengths of longitudinal and shear moduli in relaxing liquids.