Michael E. McConney

Voxelated liquid crystal elastomers

Making small actuators more effective Liquid-crystal molecules orient locally in response to external fields. When long-chain liquid-crystalline molecules are crosslinked together, changes in local orientation can lead to significant volume changes. Ware et al. made efficient microactuators that can change their shape from flat to three-dimensional structures (see the Perspective by Verduzco). By patterning volume elements so that each has a different preferred alignment for the liquid-crystalline molecules, they could fine-tune the volume changes. Science, this issue p. 982; see also p. 949 Liquid crystal elastomers are spatially patterned to create microactuators with controlled local volume changes. [Also see Perspective by Verduzco] Dynamic control of shape can bring multifunctionality to devices. Soft materials capable of programmable shape change require localized control of the magnitude and directionality of a mechanical response. We report the preparation of soft, ordered materials referred to as liquid crystal elastomers. The direction of molecular order, known as the director, is written within local volume elements (voxels) as small as 0.0005 cubic millimeters. Locally, the director controls the inherent mechanical response (55% strain) within the material. In monoliths with spatially patterned director, thermal or chemical stimuli transform flat sheets into three-dimensional objects through controlled bending and stretching. The programmable mechanical response of these materials could yield monolithic multifunctional devices or serve as reconfigurable substrates for flexible devices in aerospace, medicine, or consumer goods.

Probing Soft Matter with the Atomic Force Microscopies: Imaging and Force Spectroscopy

The development of atomic force microscopy has evolved into a wide variety of microscopy and characterization techniques well beyond conventional imaging. The focus of this review is on characterization methods based on the scanning probe and their application in characterizing physical properties of soft materials. This consideration is broken into three major categories focusing on mechanical, thermal, and electrical/magnetic properties in addition to a brief review of high-resolution imaging. Surface spectroscopy is discussed to great extent and consideration includes procedural information, common pitfalls, capabilities, and their practical application in characterizing soft matter. Key examples of the method are presented to communicate the capabilities and impact that probe-based characterization techniques have had on the mechanical, thermal, and electrical characterization of soft materials.

Dynamic color in stimuli-responsive cholesteric liquid crystals

ROY G. BIV is the acronym used around the English-speaking world to aid children in the memorization of the traditional colors of the rainbow (red, orange, yellow, green, blue, indigo, and violet). Color surrounds us and the ability to change color by external stimuli (heat, force, light exposure, magnetic or electric field) continues to be leveraged for many present day applications. This review focuses on the state of the art in the use of cholesteric liquid crystals (CLCs) as color changing optical materials. After a brief summary of thermal and electrically induced color changes, the bulk of the article describes recent efforts in photoresponsive CLCs, materials in which light is used to control the color output.

Enhancing UV photoconductivity of ZnO nanobelt by polyacrylonitrile functionalization

In this work, we present a bilayer polymer/ZnO photoconductor based on ZnO NBs and plasma polymerized acrylonitrile PP-AN nanoscale surface coating. By taking advantage of a rectangular cross section of NBs, uniform bilayered PP-AN/ZnO NBs were fabricated by exploiting plasma enhanced chemical vapor deposition PECVD .W e demonstrated that under identical UV illumination conditions, the photocurrent of ZnO NBs was increased by a factor of 750 after coating with PP-AN. The suggested mechanism includes a consequence of the efficient exciton dissociation under UV illumination due to enhanced electron transfer from valence band of ZnO NB to the photon-excited PP-AN rich on double and triple carbon-nitrogen bonds and then back to the conduction band of ZnO. The present study on the PP-AN-functionalized NBs presents a simple and costeffective method for improving the performance of oxide NW/NB-based devices, possibly leading to a new generation of potential photodetector for applications such as imaging, photosensing, and intrachip optical interconnects.

Topography from Topology: Photoinduced Surface Features Generated in Liquid Crystal Polymer Networks

Films subsumed with topological defects are transformed into complex, topographical surface features with light irradiation of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs). Using a specially designed optical setup and photoalignment materials, azo-LCN films containing either singular or multiple defects with strengths ranging from |½| to as much as |10| are examined. The local order of an azo-LCN material for a given defect strength dictates a complex, mechanical response observed as topographical surface features.

Continuous ultra-thin MoS2 films grown by low-temperature physical vapor deposition

Uniform growth of pristine two dimensional (2D) materials over large areas at lower temperatures without sacrifice of their unique physical properties is a critical pre-requisite for seamless integration of next-generation van der Waals heterostructures into functional devices. This Letter describes a vapor phase growth technique for precisely controlled synthesis of continuous, uniform molecular layers of MoS2 on silicon dioxide and highly oriented pyrolitic graphite substrates of over several square centimeters at 350 °C. Synthesis of few-layer MoS2 in this ultra-high vacuum physical vapor deposition process yields materials with key optical and electronic properties identical to exfoliated layers. The films are composed of nano-scale domains with strong chemical binding between domain boundaries, allowing lift-off from the substrate and electronic transport measurements from contacts with separation on the order of centimeters.

Biologically inspired design of hydrogel-capped hair sensors for enhanced underwater flow detection

Using a precision drop-casting method, a bioinspired hydrogel-capped hair sensory system was created, which enhanced the performance of flow detection by about two orders of magnitude and endowed the sensors with threshold sensitivities that rival those of fish.

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.

Thermally Induced, Multicolored Hyper-Reflective Cholesteric Liquid Crystals

where n s and n o are the extraordinary and ordinary refractive indices, respectively. The maximum refl ection of unpolarized light is 50% because a planar aligned cell only refl ects circularly polarized light of the same handedness as the helical pitch. Methodologies to overcome this limitation include the stacking of two opposite-handed CLC fi lms or stacking two same-handed CLC fi lms separated by a half waveplate. [ 1–8 ] Here a methodology is reported to obtain near 100% refl ectivity in a single CLC fi lm (so called hyper-refl ectivity) in which surface-bound polymer stabilization is used to spatially segregate disparate regions of opposite handedness in a single cell. Through the use of a thermally tunable CLC mixture, the high contrast condition can be induced with temperature. The versatility of the approach is further demonstrated by creating a cell with two static refl ection notches at different wavelengths and thermally inducing high contrast at each notch by varying the temperature. High-contrast, selectively refl ecting materials have a variety of prospective applications in displays and photonics, allowing for on/off control of a portion of the spectrum while passing all other wavelengths. A simple method to attain high contrast refl ectors has utilized chiral polymeric materials mixed with opposite-handed CLC mixtures to yield hyper-refl ective CLCs (defi ned here as single-cell CLCs with a refl ection greater

Spontaneous Self-Folding in Confined Ultrathin Polymer Gels

Adv. Mater. 2010, 22, 1263–1268 2010 WILEY-VCH Verlag G T IO N Mechanical instabilities are frequently observed in all length scales in a wide variety of material systems. Buckling is a popular example of stress (thermal, mechanical, osmotic) induced de novo pattern formation in a thin rigid material (glassy polymer films, metal layers, 1D nanostructures) bound to a thick compliant substrate. Recently, it has been demonstrated that the buckling of the weak structural elements (struts) of periodic porous polymer structures results in dramatic pattern transformation (from square lattice to diamond plate structure) in these structures applicable for tunable photonic and phononic structures. Buckling phenomenon has been employed as a novel metrology technique for measuring elastic moduli of nanoscale polymeric films, composite nanomembranes, and 1D and 2D nanostructures, for which conventional mechanical testing approaches cannot be readily applied. Buckling instabilities have also been demonstrated to be valuable in controlling adhesion, enabling flexible electronics, fabricating microfluidic structures, providing means for microand nanopatterning and optical microdevices based upon microgratings. Responsive synthetic systems in the form of gel films and other cross-linked and functionalized surface structures, which are sensitive to environmental conditions (pH, temperature, solvent, stress) can be utilized to mimic the biological environment and serve as prospective media for biological and chemical sensing. Moreover, water-swollen gel structures with extremely large swelling rates are exploited as biomaterials for cell support, muscle-inspired actuation, and sensing. Gels, swelling uniaxially under external stimuli exhibit smooth buckles and cusp-shaped singularities which were explained by linear elastic buckling. Various aspects of the buckling patterns in these structures have been extensively studied to reveal the temporal evolution and mechanistic aspects of the buckling phenomenon. It has been demonstrated that buckling and folding are not independent phenomena and wrinkles appear as a first-order linear response. Further folding and crumpling result in localization of the strain energy into a network of ridges and sharp corners. While mechanical instabilities have been extensively explored in thin rigid skins on soft underlayers, they remain relatively underexplored and elusive in ultrathin, nanoscale gels firmly tethered to rigid surfaces under strong restrictive boundary conditions. Herein, we demonstrate that de novo pattern formation from ultrathin hydrogel films caused by interfacial mechanical stresses can form discrete high-aspect folded structures with uniform feature sizes and scale-invariance. Ultrathin (20–100 nm) hydrogel films exhibited unusual spontaneous and regular self-folding and release behavior. We suggest that the anchoring of the gel to a stiff substrate constrains swelling and frustrates the buckling deformation, thus leading to localized pinched folds and eventually to anisotropically folded sheets at wide-scale lengths. The folding patterns observed here are strikingly different compared to the smooth sinusoidal buckling and cusp-shaped singularities reported earlier in relatively thick (a few tens of micrometers to a few millimeters) and stiffer films. To the best of our knowledge, this is the first report of the folding instability and linear scaling of dimensions of the folds with thickness in ultrathin polymer gels. We employed poly-2-vinylpyridine (P2VP), a weak cationic polyelectrolyte that exhibits globule–coil transformation due to the protonation of the pyridine group below pH 4.0. The cross-linked P2VP gel film grafted onto a silicon wafer with a native oxide layer of 1 nm shows uniform and smooth surface morphology (see Fig. S1, Supporting Information (SI)). P2VP exhibits strong interactions with the silicon surface owing to the hydrogen bonding between the hydroxyl groups of silica and nitrogen on the pyridine ring. In the protonated state (below pH 4.0), the electrostatic interaction between the positively charged pyridine units and negatively charged silicon oxide surface result in strong anchoring to the surface. Exposure of the cross-linked gel films to a pH 2 solution resulted in the transformation of the initially smooth morphology into a network of anisotropic structures, the dimensions of which are governed by the thickness of the film (Fig. 1A–B and Fig. 3A). Careful inspection revealed that the observed anisotropic structures are indeed folds in the film (see the contour of the fold highlighted in Fig. 1C). Each fold is made up of two distinct regions; the hinge and the limb, with slightly different thickness as depicted by the cross-section of the atomic force microscopy (AFM) image (see Fig. S2 (SI) for cross-section). The hinge side is straight, and the limb side comprises either one or two vertices connected by straight ridges (Fig. 1D). Each fold starts and ends in a sharp corner with an angle of 308, which in most of the cases overlaps with an adjacent fold. The overlapping sharp corner of the double fold aligns with the vertex of the other fold’s limb, indicating that the fold collision resulted in an inhibitory effect on the pinched buckle growth. The folded film exhibits a trimodal