The physical properties of glassy polymer films change as they become confined. These changes are often attributed to increased average molecular mobility and reduction in entanglement density. Both are known to alter mechanical behavior, including the formation of strain localizations, e.g., crazing and shear deformation zones. Here, we determine how the entanglement density and surface mobility change the mechanical behavior of a glassy polymer film when it becomes confined. We utilize a custom-built uniaxial tensile tester for ultrathin films and dark-field optical microscopy to characterize the complete stress–strain response and the associated strain localizations for ultrathin polystyrene films of varying thickness (h F = 20–360 nm). These experiments provide direct measurement of the stress in a craze as well as the stresses involved through the transition from crazing to shear deformation zones. Most significantly, we observe a transition in strain localization from crazing to shear deformation zones as film thickness changes from 30 to 20 nm, providing new insights into how the surfaces alter the mechanical behavior in confined polymer films.

The mechanical properties of ultrathin polystyrene (PS) films have been shown to change as the thickness approaches the average size of a polymer molecule. Previous measurements of the uniaxial stress−strain relationship for ultrathin polymer films have required the use of liquid-support layers. However, the influence of the liquid support layer, specifically water, on the mechanical properties of PS films has remained an open question.Here, we introduce a method for directly measuring the complete stress−strain response of ultrathin freestanding polymer films. For freestanding PS thin films, we observe a constant elastic modulus and maximum stress with decreasing thickness for film thicknesses as thin as 30 nm, consistent with the liquid supported measurement. From the freestanding measurements, we identify that the liquid supporting layer leads to enhanced craze stability for ultrathin PS films. We compare these results to the previous liquid-supported measurements and provide insights into how the liquid surface interactions can alter polymer behavior in thin polymer films.

How do growing bacterial colonies get their shapes? While colony morphogenesis is well studied in two dimensions, many bacteria grow as large colonies in three-dimensional (3D) environments, such as gels and tissues in the body or subsurface soils and sediments. Here, we describe the morphodynamics of large colonies of bacteria growing in three dimensions. Using experiments in transparent 3D granular hydrogel matrices, we show that dense colonies of four different species of bacteria generically become morphologically unstable and roughen as they consume nutrients and grow beyond a critical size—eventually adopting a characteristic branched, broccoli-like morphology independent of variations in the cell type and environmental conditions. This behavior reflects a key difference between two-dimensional (2D) and 3D colonies; while a 2D colony may access the nutrients needed for growth from the third dimension, a 3D colony inevitably becomes nutrient limited in its interior, driving a transition to unstable growth at its surface. We elucidate the onset of the instability using linear stability analysis and numerical simulations of a continuum model that treats the colony as an “active fluid” whose dynamics are driven by nutrient-dependent cellular growth. We find that when all dimensions of the colony substantially exceed the nutrient penetration length, nutrient-limited growth drives a 3D morphological instability that recapitulates essential features of the experimental observations. Our work thus provides a framework to predict and control the organization of growing colonies—as well as other forms of growing active matter, such as tumors and engineered living materials—in 3D environments.

The petals of some flowers form hierarchical structures when nano‐scale cuticular ridges overlay bulged epidermal cells. These hierarchical structures can broaden the observable angles of iridescence. The resulting optical effect enhances the foraging efficiency of pollinators. Although efforts have been devoted to mimicking this unique broad‐angle structural color, the intrinsic tunability offered by natural systems to control such a broadened spectrum is still absent in synthetic models. A hierarchical system is developed that provides hierarchical wrinkle‐based structures that tune the observable angles for structural color. Laser diffraction measurements demonstrate that the observable angle of reflectance is broadened in proportion to the square root of the applied compressive strain. The morphology controls the diffraction pattern: the small wrinkles control the diffraction angles and the large wrinkles broaden the observable range. The development of a multi‐mode wrinkling system to produce this broad‐angle structural color only occurs within a limited range of conditions, which are experimentally discovered and theoretically modeled. Without diffractive small wrinkles, single wrinkling modes do not display structural colors. The control of wrinkling modes mimics the tunability of petals, which gives new insight into the natural system and provides a robust foundation for tunable structural color control.

The mechanical properties of polymer nanocomposites have been widely studied; however, very few studies have investigated how these properties change for films with thickness approaching the average size of a polymer molecule. Polymer nanocomposites can provide tunable mechanical properties that are essential for advanced applications that require ultrathin glassy polymer films, including filtration membranes and coatings. Herein, we directly measured the stress−strain response of ultrathin polymethylmethacrylate (PMMA)/silica nanocomposite films on a water surface as a function of film thickness and nanoparticle loading. We found that the elastic modulus was independent of nanoparticle loading and thickness for films as thin as 19 nm. The maximum stress prior to failure decreased as nanoparticle loading increased, and the rate of decrease with nanoparticle loading was dependent strongly upon film thickness. We hypothesize that as the volume fraction increases, the interparticle distance decreases, leading to chain confinement between the nanoparticles. This chain confinement limits the extent of interchain entanglement, which has deleterious effects on strength for this weakly interacting polymer−nanoparticle system. We relate these dimensionally controlled effects to the measured changes in the mechanical response of neat PMMA films with film thickness near the average configurational size scale. Our findings suggest that free surface−nanoparticle interactions, film thickness, and interparticle distance all can impact the mechanical response of ultrathin nanocomposites.