The low-temperature dynamics of ultraviscous liquids hold the key to understanding the nature of glass transition and relaxation phenomena, including the potential existence of an ideal thermodynamic glass transition. Unfortunately, existing viscosity models, such as the Vogel-Fulcher-Tammann (VFT) and Avramov-Milchev (AM) equations, exhibit systematic error when extrapolating to low temperatures. We present a model offering an improved description of the viscosity-temperature relationship for both inorganic and organic liquids using the same number of parameters as VFT and AM. The model has a clear physical foundation based on the temperature dependence of configurational entropy, and it offers an accurate prediction of low-temperature isokoms without any singularity at finite temperature. Our results cast doubt on the existence of a Kauzmann entropy catastrophe and associated ideal glass transition.modeling ͉ supercooled liquids ͉ configurational entropy ͉ relaxation P erhaps the most intriguing feature of a supercooled liquid is its dramatic rise in viscosity as it is cooled toward the glass transition. This sharp, super-Arrhenius increase is accompanied by very little change in the structural features observable by typical diffraction experiments. Several basic questions remain unanswered:1. Is the behavior universal (i.e., is the viscosity of all liquids described by the same underlying model)?2. Does the viscosity diverge at some finite temperature below the glass transition (i.e., is there a dynamic singularity)?3. Is the existence of a thermodynamic singularity the cause of the dramatic viscous slowdown?Answers to these questions are critical for understanding the behavior of deeply supercooled liquids. Unfortunately, equilibriumviscosity measurements cannot be carried out at temperatures much below the glass transition owing to the long structural relaxation time. It thus becomes critical to find a model that best describes the temperature dependence of viscosity by using the fewest possible number of fitting parameters (1, 2). Because two parameters are needed for a simple Arrhenius description, modeling of super-Arrhenius behavior requires a minimum of three parameters. We focus on three-parameter models only, with the goal of describing the universal physics of supercooled liquid viscosity in the most economical form possible.The most popular viscosity model is the Vogel-FulcherTammann (VFT) equation (3) log 10 ͑T, x͒ ϭ log 10 ϱ ͑x͒ ϩ A͑x͒where T is temperature, x is composition, and the three VFT parameters ( ϱ , A, and T 0 ) are obtained by fitting Eq. 1 to experimentally measured viscosity data. In the polymer science community, Eq. 1 is also known as the Williams-Landel-Ferry (WLF) equation (4). Although VFT has met with notable success for a variety of liquids, there is some indication that it breaks down at low temperatures (3, 5). Another successful three-parameter viscosity model is the Avramov-Milchev (AM) equation (6), derived based on an atomic hopping approach:where ϱ , , and ␣ are fittin...

Borosilicate glasses display a rich complexity of chemical behavior depending on the details of their composition and thermal history. Noted for their high chemical durability and thermal shock resistance, borosilicate glasses have found a variety of important uses from common household and laboratory glassware to high-tech applications such as liquid crystal displays. In this paper, we investigate the topological principles of borosilicate glass chemistry covering the extremes from pure borate to pure silicate end members. Based on NMR measurements, we present a two-state statistical mechanical model of boron speciation in which addition of network modifiers leads to a competition between the formation of nonbridging oxygen and the conversion of boron from trigonal to tetrahedral configuration. Using this model, we derive a detailed topological representation of alkali-alkaline earth-borosilicate glasses that enables the accurate prediction of properties such as glass transition temperature, liquid fragility, and hardness. The modeling approach enables an understanding of the microscopic mechanisms governing macroscopic properties. The implications of the glass topology are discussed in terms of both the temperature and thermal history dependence of the atomic bond constraints and the influence on relaxation behavior. We also observe a nonlinear evolution of the jump in isobaric heat capacity at the glass transition when substituting SiO(2) for B(2)O(3), which can be accurately predicted using a combined topological and thermodynamic modeling approach.

We have discovered and clarified the ultrahigh glass-forming ability of the metal-organic frameworks—ZIF-62 [Zn(Im2−xbImx)].

We present a topological model for the composition dependence of glass transition temperature and fragility. Whereas previous topological models are derived for zero temperature conditions, our approach incorporates the concept of temperature-dependent constraints that freeze in as the system is cooled from high temperature. Combining this notion of temperature-dependent constraints with the Adam-Gibbs model of viscosity, we derive an analytical expression for the scaling of glass transition temperature and fragility in the binary Ge(x)Se(1-x) system. In the range of 0

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This article presents theoretical analysis and experimental data for the use of resonant waveguide grating (RWG) biosensors to characterize stimulation-mediated cell responses including signaling. The biosensor is capable of detecting redistribution of cellular contents in both directions that are perpendicular and parallel to the sensor surface. This capability relies on online monitoring cell responses with multiple optical output parameters, including the changes in incident angle and the shape of the resonant peaks. Although the changes in peak shape are mainly contributed to stimulation-modulated inhomogeneous redistribution of cellular contents parallel to the sensor surface, the shift in incident angle primarily reflects the stimulation-triggered dynamic mass redistribution (DMR) perpendicular to the sensor surface. The optical signatures are obtained and used to characterize several cellular processes including cell adhesion and spreading, detachment and signaling by trypsinization, and signaling through either epidermal growth factor receptor or bradykinin B2 receptor. A mathematical model is developed to link the bradykinin-mediated DMR signals to the dynamic relocation of intracellular proteins and the receptor internalization during B2 receptor signaling cycle. This model takes the form of a set of nonlinear, ordinary differential equations that describe the changes in four different states of B2 receptors, diffusion of proteins and receptor-protein complexes, and the DMR responses. Classical analysis shows that the system converges to a unique optical signature, whose dynamics (amplitudes, transition time, and kinetics) is dependent on the bradykinin signal input, and consistent with those observed using the RWG biosensors. This study provides fundamentals for probing living cells with the RWG biosensors, in general, optical biosensors.

Glass transition temperature and fragility are two important properties derived from the temperature dependence of the shear viscosity of glass-forming melts. While direct calculation of these properties from atomistic simulations is currently infeasible, we have developed a new topological modeling approach that enables accurate prediction of the scaling of both glass transition temperature and fragility with composition. A key feature of our approach is the incorporation of temperature-dependent constraints that become rigid as a liquid is cooled. Using this approach, we derive analytical expressions for the composition (x) dependence of glass transition temperature, T(g)(x), and fragility, m(x), in binary alkali borate systems. Results for sodium borate and lithium borate systems are in agreement with published values of T(g)(x) and m(x). Our modeling approach reveals a natural explanation for the presence of the constant T(g) regime observed in alkali borate systems.

The development of new glassy materials is key for addressing major global challenges in energy, medicine, and advanced communications systems. For example, thin, flexible, and large-area glass substrates will play an enabling role in the development of flexible displays, roll-to-roll processing of solar cells, next-generation touch-screen devices, and encapsulation of organic semiconductors. The main drawback of glass and its limitation for these applications is its brittle fracture behavior, especially in the presence of surface flaws, which can significantly reduce the practical strength of a glass product. Hence, the design of new ultrastrong glassy materials and strengthening techniques is of crucial importance. The main issues regarding glass strength are discussed, with an emphasis on the underlying microscopic mechanisms that are responsible for mechanical properties. The relationship among elastic properties and fracture behavior is also addressed, focusing on both oxide and metallic glasses. From a theoretical perspective, atomistic modeling of mechanical properties of glassy materials is considered. The topological origin of these properties is also discussed, including its relation to structural and chemical heterogeneities. Finally, comments are given on several toughening strategies for increasing the damage resistance of glass products.