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- Wrinkling Labyrinth Patterns on Elastomeric Janus ParticlesPublication . Trindade, A. C.; Canejo, João; Pinto, L. F. V.; Patricio, Pedro; Brogueira, Pedro; Teixeira, Paulo; Godinho, Maria HelenaWe describe a novel, low-cost and low-tech method for the fabrication of elastomeric Janus particles with diameters ranging from micrometers to millimeters. This consists of UV-irradiating soft urethane/urea elastomer spheres, which are then extracted in toluene and dried. The spheres are thus composed of a single material: no coating or film deposition steps are required. Furthermore, the whole procedure is carried out at ambient temperature and pressure. Long, labyrinthine corrugations ("wrinkles") appear on the irradiated portions of the particles' surfaces, the spatial periodicity of which can be controlled by varying the sizes of particles. The asymmetric morphology of the resulting Janus particles has been confirmed by scanning electron microscopy, atomic force microscopy, and optical microscopy. We have also established that the spheres behave elastically by performing bouncing tests with dried and swollen spheres. Results can be interpreted by assuming that each sphere consists of a thin, stiff surface layer ("skin") lying atop a thicker, softer substrate ("bulk"). The skin's higher stiffness is hypothesized to result from the more extensive cross-linking of the polymer chains located near the surface by the UV radiation. Textures then arise from competition between the effects of bending the skin and compressing the bulk, as the solvent evaporates and the sphere shrinks.
- Hierarchical wrinkling on elastometric Janus spheresPublication . Trindade, Ana C.; Canejo, João; Patricio, Pedro; Brogueira, Pedro; Teixeira, Paulo; Godinho, Maria HelenaHierarchical wrinkling on elastomeric Janus spheres is permanently imprinted by swelling, for different lengths of time, followed by drying the particles in an appropriate solvent. First-order buckling with a spatial periodicity (lambda(11)) of the order of a few microns and hierarchical structures comprising of 2nd order buckling with a spatial periodicity (lambda(12)) of the order of hundreds of nanometers have been obtained. The 2nd order buckling features result from a Grinfeld surface instability due to the diffusion of the solvent and the presence of sol molecules.
- Light shutters from nanocrystalline cellulose rods in a nematic liquid crystalPublication . Geng, Yong; Brogueira, Pedro; Figueirinhas, João; Godinho, Maria Helena; Almeida, Pedro L.This work reports a recently developed electro-optical (EO) device that can potentially be used as a light shutter or a privacy window. By using nanocrystalline cellulose rods, we were able to improve some of the most relevant parameters characterising the EO behaviour. A brief description of the proposed working mechanism for these devices is presented, and numerical simulations based on this mechanism of both the optical transmission and the cells' electrical capacitance are compared with the obtained results, validating the underlying working model considered.
- Soft janus, wrinkles and allPublication . Trindade, A. C.; Patricio, Pedro; Teixeira, Paulo; Brogueira, Pedro; Godinho, Maria HelenaRight now you are probably sitting on a comfy cushion. This is most likely filled with polyurethane (PU) foam. PUs are very long molecules made up of many repeating units. If the repeating units are prepolymers - intermediate-mass building blocks - with more than two reactive end groups, a three-dimensional network will form - a rubber, or elastomer, which can behave elastically depending on the degree of network cross-linking.
- Structural Color and Iridescence in Transparent Sheared Cellulosic FilmsPublication . Fernandes, Susete N.; Geng, Yong; Vignolini, Silvia; Glover, Beverley J.; Trindade, Ana C.; Canejo, João P.; Almeida, Pedro L.; Brogueira, Pedro; Godinho, Maria HelenaShear transparent cellulose free-standing thin films can develop iridescence similar to that found in petals of the tulip Queen of the Night. The iridescence of the film arises from the modulation of the surface into bands periodically spread perpendicular to the shear direction. Small amounts of nanocrystalline cellulose (NCC) rods in the precursor liquid-crystalline solutions do not disturb the optical properties of the solutions but enhance the mechanical characteristics of the films and affects their iridescence. Smaller bands periodicity, not affected by the NCC rods, slightly deviated from the shear direction is also observed. NCCs are crucial to tune and understand the film's surface features formation. Our findings could lead to new materials for application in soft reflective screens and devices.