Yun-Ru Huang

Structure-property relationships from universal signatures of plasticity in disordered solids

Behavioral universality across size scales Glassy materials are characterized by a lack of long-range order, whether at the atomic level or at much larger length scales. But to what extent is their commonality in the behavior retained at these different scales? Cubuk et al. used experiments and simulations to show universality across seven orders of magnitude in length. Particle rearrangements in such systems are mediated by defects that are on the order of a few particle diameters. These rearrangements correlate with the materials softness and yielding behavior. Science, this issue p. 1033 A range of particle-based and glassy systems show universal features of the onset of plasticity and a universal yield strain. When deformed beyond their elastic limits, crystalline solids flow plastically via particle rearrangements localized around structural defects. Disordered solids also flow, but without obvious structural defects. We link structure to plasticity in disordered solids via a microscopic structural quantity, “softness,” designed by machine learning to be maximally predictive of rearrangements. Experimental results and computations enabled us to measure the spatial correlations and strain response of softness, as well as two measures of plasticity: the size of rearrangements and the yield strain. All four quantities maintained remarkable commonality in their values for disordered packings of objects ranging from atoms to grains, spanning seven orders of magnitude in diameter and 13 orders of magnitude in elastic modulus. These commonalities link the spatial correlations and strain response of softness to rearrangement size and yield strain, respectively.

Robust scaling of strength and elastic constants and universal cooperativity in disordered colloidal micropillars.

Significance The mechanical response of glassy materials is important in numerous technological and natural processes, yet the link between the embryonic stages of plastic deformation and macroscopic mechanical failure remains elusive. The incipient inelastic rearrangements are believed to be highly cooperative and characterized by a scaling of yield strength and elastic constants. Whether this behavior transcends the nature of bonding is still an open question. Here, we show that disordered colloidal micropillars spanning the spectrum of glassy packing also demonstrate a robust scaling of elastic and plastic properties. Our measured relationship and deduced cooperative rearrangement strains bear striking resemblance to other glassy systems with disparate bonding, implying a universal building block for macroscopic flow. We study the uniaxial compressive behavior of disordered colloidal free-standing micropillars composed of a bidisperse mixture of 3- and 6-μm polystyrene particles. Mechanical annealing of confined pillars enables variation of the packing fraction across the phase space of colloidal glasses. The measured normalized strengths and elastic moduli of the annealed freestanding micropillars span almost three orders of magnitude despite similar plastic morphology governed by shear banding. We measure a robust correlation between ultimate strengths and elastic constants that is invariant to relative humidity, implying a critical strain of ∼0.01 that is strikingly similar to that observed in metallic glasses (MGs) [Johnson WL, Samwer K (2005) Phys Rev Lett 95:195501] and suggestive of a universal mode of cooperative plastic deformation. We estimate the characteristic strain of the underlying cooperative plastic event by considering the energy necessary to create an Eshelby-like ellipsoidal inclusion in an elastic matrix. We find that the characteristic strain is similar to that found in experiments and simulations of other disordered solids with distinct bonding and particle sizes, suggesting a universal criterion for the elastic to plastic transition in glassy materials with the capacity for finite plastic flow.

Multifunctional all-TiO2 Bragg stacks based on blocking layer-assisted spin coating

Multifunctional all-TiO2 Bragg stacks displaying structural color are fabricated by sequentially spin coating two TiO2 nanoparticles of different shapes and sizes. We find that Bragg stacks generated by simple sequential deposition of the two nanoparticles have poor reflective properties compared to what would be expected based on the optical properties of individual TiO2 nanoparticle layers. This poor photonic property is due to the cross-contamination of neighboring layers during the deposition process; that is, small TiO2 nanoparticles infiltrate the pre-existing TiO2 nanoparticle layers, changing their optical properties. A sacrificial polymer-blocking layer is added between the low- and high-refractive-index layers to prevent their cross-contamination upon fabrication, which, in turn, significantly improves the overall photonic performance of these all-TiO2 Bragg stacks. All-TiO2 Bragg stacks exhibit excellent superhydrophilicity and self-cleaning properties, due to the porosity of the stacked structure and the photocatalytic nature of TiO2 nanoparticles, respectively, enabling patterning of wettability in these multilayer structures. Additionally, we demonstrate that the high quality photonic properties of these all-TiO2 multilayer structures render them ideal for use in enhancing the energy conversion efficiency of dye-sensitized solar cells (DSSCs).