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- First curl, then wrinklePublication . Trindade, Ana C.; Canejo, João; Teixeira, Paulo; Patricio, Pedro; Godinho, Maria HelenaThe excellent properties of elastomers are exploited to trigger wrinkling instabilities in curved shells. Micro‐ and nano‐fibres are produced by electrospinning and UV irradiated: each fibre consists of a soft core and a stiff outer half‐shell. Upon solvent de‐swelling, the fibres curl because the shell and the core have different natural lengths. Wrinkling only starts after the fibre has attained a well‐defined helical shape. A simple analytical model is proposed to find the curling curvature and wrinkle wavelength, as well as the transition between the “curling” and “wrinkling” regimes. This new instability resembles that found in the tendrils of climbing plants as they dry and lignify.
- 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.
- Longitudinal versus polar wrinkling of core-shell fibers with anisotropic size mismatchesPublication . Patricio, Pedro; Teixeira, Paulo; Trindade, A. C.; Godinho, Maria HelenaWe consider a fiber made of a soft elastic material, encased in a stiff elastic shell (core-shell geometry). If the core and shell dimensions are mismatched, e.g., because the core shrinks while the shell does not, but the two remain attached, then an elastic instability is triggered whereby wrinkles may appear on the shell. The wrinkle orientation may be longitudinal (along the fiber axis), polar (along the fiber perimeter), or a mixture of both, depending on the fiber's geometrical and material parameters. Here we investigate under what conditions longitudinal or polar wrinkling will occur.
- 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.
- From wrinkly elastomers to Janus particles to Janus fibresPublication . Trindade, A. C.; Canejo, J. P.; Patricio, Pedro; Brogueira, P.; Teixeira, Paulo; Godinho, Maria HelenaAim is to fabricate Janus objects possessing two distinct sides, through control and manipulation of uneable sub-micron scale long-lived textures in urethane/urea elastomer films, spheres and fibres. Textures are obtained by UV-irradiation of the elastomers followed by mechanical deformation or swelling in a solvent. Standard chemistry, no complex deposition techniques required. Resulting textures investigated using AFM, SEM, SALS and POM. Interpreted by assuming that each film, sphere or fibre consists of a thin, stiff surface layer (“skin”) lying atop a thicker, softer substrate (“bulk”).
- First Curl, Then WrinklePublication . Trindade, Ana C.; Canejo, João; Teixeira, Paulo; Patricio, Pedro; Godinho, Maria HelenaThe excellent properties of elastomers are exploited to trigger wrinkling instabilities in curved shells. Micro- and nano-fibres are produced by electrospinning and UV irradiated: each fibre consists of a soft core and a stiff outer half-shell. Upon solvent de-swelling, the fibres curl because the shell and the core have different natural lengths. Wrinkling only starts after the fibre has attained a well-defined helical shape. A simple analytical model is proposed to find the curling curvature and wrinkle wavelength, as well as the transition between the “curling” and “wrinkling” regimes. This new instability resembles that found in the tendrils of climbing plants as they dry and lignify.