Relationships among structure, memory, and flow in sheared disordered materials

A fundamental challenge for disordered solids is predicting macroscopic yield from the microscopic arrangements of constituent particles. Yield is accompanied by a sudden and large increase in energy dissipation due to the onset of plastic rearrangements. This suggests that one path to understanding bulk rheology is to map particle configurations to their mode of deformation. Here, we perform laboratory experiments and numerical simulations that are designed to do just that: 2D dense colloidal systems are subjected to oscillatory shear, and particle trajectories and bulk rheology are measured. We quantify particle microstructure using excess entropy. Results reveal a direct relation between excess entropy and energy dissipation, that is insensitive to the nature of interactions among particles. We use this relation to build a physically-informed model that connects rheology to microstructure. Our findings suggest a framework for tailoring the rheological response of disordered materials by tuning microstructural properties.

Designing Phononic Band Gaps With Sticky Potentials

Spectral gaps in the vibrational modes of disordered solids are key design elements in the synthesis and control of phononic meta-materials that exhibit a plethora of novel elastic and mechanical properties. However, reliably producing these gaps often require a high degree of network specificity through complex control optimization procedures. In this work, we present as an additional tool to the existing repertoire, a numerical scheme that rapidly generates sizeable spectral gaps in absence of any fine tuning of the network structure or elastic parameters. These gaps occur even in disordered polydisperse systems consisting of relatively few particles (N ~ 102 − 103). Our proposed procedure exploits sticky potentials that have recently been shown to suppress the formation of soft modes, thus effectively recovering the linear elastic regime where band structures appear, at much shorter length scales than in conventional models of disordered solids. Our approach is relevant to design and realization of gapped spectra in a variety of physical setups ranging from colloidal suspensions to 3D-printed elastic networks.

EXPLORING RELATIONSHIP BETWEEN LOCAL DYNAMICS AND RELAXATION PROCESSES ON THE TERAHERTZ FREQUENCIES IN POLYMERS WITH HYDROGEN BONDS

На терагерцовых частотах либрационно-колебательное движение связано с диэлектрической релаксацией в неупорядоченных твердых телах с водородными связями. Взаимодействие между этими процессами ещё мало изучено, особенно при температурах ниже температуры стеклования, что особенно существенно для молекулярной подвижности в полимерах. Изучены полимеры с водородными связями (полиамид-6 и поливинилхлорид) при температурах от 90 до 4000К в диапазоне 0,25 - 4 ТГц с использованием дальней ИК-спектроскопии. Три общих особенностей наблюдались в спектре диэлектрических потерь, Ɛ"(ν):(а) при температурах значительно ниже стеклования (T) эти потери представлены низкочастотным крылом пика поглощения, обусловленного либрацией мономерных звеньев полимеров. (б) При 0.7 T < T < T наблюдаются дополнительные температурно-зависимые потери, которые могут быть связаны с проявлением вторичных релаксационных процессов. (с) При температурах выше T преобладающим вкладом в терагерцовые потери становятся первичные процессы α-релаксации. Полученные результаты показывают, что эволюция терагерцовых потерь с температурой вызвана изменением структуры водородных связей, которое, по-видимому, является универсальным для систем с подобными межмолекулярными взаимодействиями At terahertz frequencies, torsional-vibrational motion is associated with dielectric relaxation in disordered solids with hydrogen bonds. The interaction between these processes has not been studied much, especially at temperatures below the glass transition temperature, which is especially important for molecular mobility in polymers. Polymers with hydrogen bonds (polyamide-6 and polyvinyl chloride) were studied at temperatures from 90 to 4000 K in the range 0.25 - 4 THz using far-infrared spectroscopy. Three common features were observed in the spectrum of dielectric losses, Ɛ"(ν):(А)at temperatures well below glass transition (T), these losses are represented by the low-frequency wing of the absorption peak due to libration of the monomer units of the polymers. (B) At 0.7 T < T < T , additional temperature-dependent losses are observed, which may be associated with the manifestation of secondary relaxation processes. (C) At temperatures above T, the primary relaxation processes become the dominant contribution to terahertz losses. The results show that the evolution of terahertz losses with temperature is caused by a change in the structure of hydrogen bonds, which, apparently, is universal for systems with similar intermolecular interactions.

A structural approach to vibrational properties ranging from crystals to disordered systems

Many scientists generally attribute the vibrational anomalies of disordered solids to the structural disorder, which, however, is still under intense debate.