M. Ravi Shankar

Contactless, photoinitiated snap-through in azobenzene-functionalized polymers

Significance Photomechanical effects in polymers are distinguished by the ease with which actinic light can be regulated to contactlessly trigger the magnitude and directionality of mechanical adaptivity with spatio-temporal control. The materials examined to date have not demonstrated power densities or actuation speeds necessary for applications seeking to exploit the promise of wirelessly triggered actuation. Using mechanical design, we employ two classes of azobenzene-functionalized polymers and demonstrate contactless snap-through of bistable arches realizing orders-of-magnitude enhancement in the actuation rates (∼102 mm/s) and powers (∼1 kW/m3) under moderate irradiation intensities (<<100 mW/cm2). The experimental characterization of the snap-through is supported with modeling that elucidates the effect of geometry, mechanical properties, and photogenerated strain on the actuation rate and energy output. Photomechanical effects in polymeric materials and composites transduce light into mechanical work. The ability to control the intensity, polarization, placement, and duration of light irradiation is a distinctive and potentially useful tool to tailor the location, magnitude, and directionality of photogenerated mechanical work. Unfortunately, the work generated from photoresponsive materials is often slow and yields very small power densities, which diminish their potential use in applications. Here, we investigate photoinitiated snap-through in bistable arches formed from samples composed of azobenzene-functionalized polymers (both amorphous polyimides and liquid crystal polymer networks) and report orders-of-magnitude enhancement in actuation rates (approaching 102 mm/s) and powers (as much as 1 kW/m3). The contactless, ultra-fast actuation is observed at irradiation intensities <<100 mW/cm2. Due to the bistability and symmetry of the snap-through, reversible and bidirectional actuation is demonstrated. A model is developed to elucidate the underlying mechanics of the snap-through, specifically focusing on isolating the role of sample geometry, mechanical properties of the materials, and photomechanical strain. Using light to trigger contactless, ultrafast actuation in an otherwise passive structure is a potentially versatile tool to use in mechanical design at the micro-, meso-, and millimeter scales as actuators, as well as switches that can be triggered from large standoff distances, impulse generators for microvehicles, microfluidic valves and mixers in laboratory-on-chip devices, and adaptive optical elements.

Photomotility of polymers

Light is distinguished as a contactless energy source for microscale devices as it can be directed from remote distances, rapidly turned on or off, spatially modulated across length scales, polarized, or varied in intensity. Motivated in part by these nascent properties of light, transducing photonic stimuli into macroscopic deformation of materials systems has been examined in the last half-century. Here we report photoinduced motion (photomotility) in monolithic polymer films prepared from azobenzene-functionalized liquid crystalline polymer networks (azo-LCNs). Leveraging the twisted-nematic orientation, irradiation with broad spectrum ultraviolet–visible light (320–500 nm) transforms the films from flat sheets to spiral ribbons, which subsequently translate large distances with continuous irradiation on an arbitrary surface. The motion results from a complex interplay of photochemistry and mechanics. We demonstrate directional control, as well as climbing.

How surface stresses lead to size-dependent mechanics of tensile deformation in nanowires

It has been proposed that surface and interface stresses can modify the elastic behavior in nanomaterials such as nanowires. The authors show that surface stresses modify the tensile response of nanowires only when nonlinear elastic effects become important leading to cross terms between the applied stress and the surface stress. These effects are only significant when the radius of the nanowire is of the order of a few nanometers. The resulting alteration of tensile stiffness, though effected in part by the nonlinear elastic modulus, is particularly wrought by a modification of the stress state in the deformed nanowire.

Crystallographic Textures Resulting from Severe Shear Deformation in Machining

Evolution of textures on surfaces created using plane strain machining (PSM) under machining-relevant thermomechanical conditions is studied and compared against that in the chips. By analyzing orientation distribution functions, it is shown that the texture on the surface is comprised of prominent and distinct fibers. By analyzing the pole figures of chips and the surface, it is shown that the two textures have features distinct from one other, even though the scale of the microstructure on the surface and the chips are traditionally considered to be comparable. In situ characterization using high speed imaging of PSM is coupled with a visco-plastic self-consistent (VPSC) model and is used to predict the pole figures in the chip and the surface. A finite element model of PSM is generated and coupled with the VPSC model to create a fully computational route for predicting textures from machining.

Deformation Mechanics Associated with Formation of Ultra-Fine Grained Chips in Machining

The deformation field associated with chip formation in plane strain (2-D) machining has been simulated using the finite element method (FEM), with the objective of developing 2-D machining as an experimental technique for studying very large strain deformation phenomena. The principal machining parameters are the tool rake angle, cutting velocity and the friction at the toolchip interface while the deformation field parameters are strain, strain rate and temperature. The relation between rake angle and the shear strain in the deformation zone is studied for the low-speed cutting of lead. This correspondence is validated by comparison with measurements of the deformation parameters made by applying a Particle Image Velocimetry (PIV) technique to highspeed photographic image sequences of the deformation. It is shown that plastic strains in the range of 1-15 can be realized in a controlled manner by appropriate choice of the rake angle. The unique capabilities offered by 2-D machining for studying micro- and nano- mechanics of large strain deformation, and the creation of ultra-fine grained materials are highlighted in the context of these results.

Mechanics of Intermittent Plasticity Punctuated by Fracture During Shear Deformation of Mg Alloys at Near-Ambient Temperatures

Microstructure evolution of basal-textured Mg alloy AZ31B (Mg: Al: Zn; 96: 3: 1 wt pct) during simple shear deformation at near-ambient temperatures was studied by plane-strain machining. Using Schmid factor calculations in conjunction with quantitative electron microscopy, it was found that plastic deformation in AZ31B in the primary deformation zone of machining commences by extension twinning followed by basal slip. Characteristics of twinning in individual grains were described by correlating the direction of twinning with the principal stress state. The implications of these deformation mechanics for the microstructure inherited by the freshly generated surfaces in shear-based material removal processes are examined. These include the identification of extensive surface texture reorientation at machined surfaces via extension twins, limits on surface integrities wrought by fracture events that punctuate plastic deformation, and their relationship to the cutting tool geometry.

Statistical characterization of nanostructured materials from severe plastic deformation in machining

Endowing conventional microcrystalline materials with nanometer-scale grains at the surfaces can offer enhanced mechanical properties, including improved wear, fatigue, and friction properties, while simultaneously enabling useful functionalizations with regard to biocompatibility, osseointegration, electrochemical performance, etc. To inherit such multifunctional properties from the surface nanograined state, existing approaches often use coatings that are created through an array of secondary processing techniques (e.g., physical or chemical vapor deposition, surface mechanical attrition treatment, etc.). Obviating the need for such surface processing, recent empirical evidence has demonstrated the introduction of integral surface nanograin structures on bulk materials as a result of severe plastic deformation during machining-based processes. Building on these observations, if empirically driven, process–structure mappings can be developed, it may be possible to engineer enhanced nanoscale surface microstructures directly using machining processes while simultaneously incorporating them within existing computer-numeric-controlled manufacturing systems. Toward this end, this article provides a statistical characterization of nanograined metals created by severe plastic deformation in machining-based processes that maps machining conditions to the resulting microstructure, namely, the mean grain size. A specialized designed experiments approach is used to hypothesize and test a linear mixed-effects model of two important machining parameters. Unlike standard analysis approaches, the statistical dependence between subsets of experimental grain size observations is accounted for and it is shown that ignoring this inherent dependence can yield misleading results for the mean response function. The statistical model is applied to pure copper specimens to identify the factors that most significantly contribute to variability in the mean grain size and is shown to accurately predict the mean grain size under a few scenarios.

Interaction Between Dislocations in a Couple Stress Medium

Taylors theory of crystal plasticity is reformulated for dislocations in a couple stress medium. The divergence between Taylors approach and an approach that includes the effects of couple stresses on dislocation interactions is demonstrated. It is shown that dislocations separated by a distance that is comparable to a characteristic material length scale, have mutual interaction somewhat weaker than that predicted by classical elasticity.

Surface steps lead to heterogeneous contact mechanics and facilitate dislocation nucleation in nanoindentation

In this letter the author examines the consequences of a very small surface step on the mechanics of contact in indentation using linear continuum elasticity. Surface steps are shown to lead to a loss of contact in the vicinity of the surface steps as well as a singular contact pressure distribution. This singularity exists for any finite surface step height and leads to shear stress concentration in the bulk material that declines in proportion to the inverse of distance from the surface step. This concentration facilitates the nucleation of dislocations and lowers the threshold for onset of plasticity in nanoindentation.

Multiplicity of shape selection in functionally graded liquid crystalline polymers

The synergy of through-thickness gradation in the orientation of the molecular director and the extent of polymerization is shown to offer a framework for controlling shape selection in integral polymer films. Native curvatures are realized under ambient conditions in splayed liquid crystalline polymers that are photopolymerized on anchoring surfaces, while being exposed to the atmosphere. Residual multiaxial polymerization strains drive the spontaneous assembly of a range of geometries (tape springs, helical coils and arches) in strips that are excised from the as-prepared films. Gradients in the director orientation and the cross linking through the thickness enable a temperature dependent structural evolution. Following a moderate temperature rise ( 100 °C), which generates curvature orthogonal to that in the native state. This multiplicity in shape selection, which spontaneously emerges without requiring any mechanical training offers a useful framework for actuation and morphing. In a prototypical demonstration, a suitably excised sample is shown to spontaneously jump when placed on a hot-plate as a result of the eversion. Also, when confined in ring-like geometries, hinge-like structures are generated due to the interplay of the imposed bending strains with that existing in the native state. The evolution of the curvatures as a function of temperature offers control over the active hinge/fold and expands the multiplicity of shapes that can be realized.