A critical review on applications of the Avrami equation beyond materials science.

The Johnson-Mehl-Avrami-Kolmogorov (JMAK) formalization, often referred to as the Avrami equation, was originally developed to describe the progress of phase transformations in material systems. Many other transformations in the life, physical and social sciences follow a similar pattern of nucleation and growth. The Avrami equation has been applied widely to modelling such phenomena, including COVID-19, regardless of whether they have a formal thermodynamic basis. We present here an analytical overview of such applications of the Avrami equation outside its conventional use, emphasizing examples from the life sciences. We discuss the similarities that at least partially justify the extended application of the model to such cases. We point out the limitations of such adoption; some are inherent to the model itself, and some are associated with the extended contexts. We also propose a reasoned justification for why the model performs well in many of these non-thermodynamic applications, even when some of its fundamental assumptions are not satisfied. In particular, we explore connections between the relatively accessible verbal and mathematical language of everyday nucleation- and growth-based phase transformations, represented by the Avrami equation, and the more challenging language of the classic SIR (susceptible-infected-removed) model in epidemiology.

Large negative thermal expansion of a polymer driven by a submolecular conformational change.

Mechanoresponsive polymers hold great technological potential in drug delivery, 'smart' optical systems and microelectromechanical systems. However, hysteresis and fatigue (associated with large-scale polymer chain rearrangement) are often problematic. Here, we describe a polyarylamide film that contains s-dibenzocyclooctadiene (DBCOD), which can generate unconventional and completely reversible thermal contraction under low-energy stimulation. The films exhibit a giant negative thermal expansion coefficient of approximately -1,200 ppm K(-1) at ambient or near-ambient temperatures, much higher than any known negative-thermal-expansion materials under similar operating conditions. Mechanical characterization, calorimetry, spectroscopic analysis and density-functional theory calculations all point to the conformational change of the DBCOD moiety, from the thermodynamic global energy minimum (twist-boat) to a local minimum (chair), as the origin of this abnormal thermal shrinkage. This newly identified, low-energy-driven, thermally agile molecular subunit opens a new pathway to creating near-infrared-based macromolecular switches and motors, and for ambient thermal energy storage and conversion.

Sample selection, preparation methods, and the apparent tensile properties of silkworm (B. mori) cocoon silk.

Reported literature values of the tensile properties of natural silk cover a wide range. While much of this inconsistency is the result of variability that is intrinsic to silk, some is also a consequence of differences in the way that silk is prepared for tensile tests. Here we explore how measured mechanical properties of Bombyx mori cocoon silk are affected by two intrinsic factors (the location from which the silk is collected within the cocoon, and the color of the silk), and two extrinsic factors (the storage conditions prior to testing, and different styles of reeling the fiber). We find that extrinsic and therefore controllable factors can affect the properties more than the intrinsic ones studied. Our results suggest that enhanced inter-laboratory collaborations, that lead to standardized sample collection, handling, and storage protocols prior to mechanical testing, would help to decrease unnecessary (and complicating) variation in reported tensile properties.

Biomimicry as a route to new materials: what kinds of lessons are useful?

We consider the attributes of a successful engineered material, acknowledging the contributions of composition and processing to properties and performance. We recognize the potential for relevant lessons to be learned from nature, at the same time conceding both the limitations of such lessons and our need to be selective. We then give some detailed attention to the molecular biomimicry of filamentous phage, the process biomimicry of silk and the structure biomimicry of hippopotamus 'sweat', in each case noting that the type of lesson now being learned is not the same as the potential lesson that originally motivated the study.

Fibre science: supercontraction stress in wet spider dragline.

Unrestrained spider dragline 'super-contracts' when it is wetted, causing its length to shrink by about half and its diameter to almost double. Here we measure the supercontraction stresses generated upon initial exposure of spider dragline to moisture and find that they are transient, as well as being greater than previously estimated. Our findings cast doubt on suggestions that supercontraction may help to maintain tension in wet webs and could limit the potential load-bearing applications of silk and its analogues.