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- Very low surface tension liquid–vapour interfaces of patchy colloidsPublication . Teixeira, PauloThe liquid–vapour interface of a model of one-component patchy colloids is re-visited. The model consistsof hard spheres decorated with short-ranged attractive sites (‘patches’) of different types on their surfaces.In an earlier paper [A. Oleksy and P.I.C. Teixeira, Phys. Rev. E 91 (2015) 012301], the density-functionaltheory (DFT) form of Wertheim’s first-order perturbation theory (TPT1) was used to study a realisationof this model with two patches of type A and nine patches of type B (2A9B colloids), which exhibit re-entrant liquid–vapour coexistence curves and very low-density liquid phases. Here, it is shown that thenon-monotonic temperature dependence of the surface tension and interface thickness in this modelcan be qualitatively reproduced by a local DFT, where the spatial non-uniformity is introduced throughthe square gradient of the density multiplied by a density-dependent prefactor. This simpler theory isthen applied to colloids decorated with two patches of type A and ten patches of type B (2A10B colloids),and interactions chosen so as to exhibit a closed-loop phase diagram when only AA and AB bonds arepresent [N.G. Almarza et al., J. Chem. Phys. 137 (2012) 244902]. In this case, the surface tension goesto zero at both upper and lower critical points, with a maximum at some intermediate temperature. AsBB attractions are gradually switched on, the lower critical point disappears and the surface tension vstemperature curve acquires a very deep minimum at a temperature lower than that of the maximum. Onfurther increasing the strength of the BB attractions, both the minimum and the maximum disappear andthe surface tension becomes a monotonically decreasing function of the temperature, as in atomic fluids.The very low surface tension at the minimum for weak BB attractions, combined with the ability to formphysical gels, suggests that 2AnB colloids might be exploited as temperature-controlled surfactants andfoam stabilizers.
- 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.
- Nematicliquid crystal order reconstruction in ultraconfinement, from density-functional theoryPublication . Teixeira, PauloI use the Onsager approximation of density-functional theory with a simple Parsons-Lee re-scaling to study a hybrid-aligned liquid crystal under very strong confinement. The system is modelled as a film of hard Gaussian overlap particles in a slit geometry, where anchoring is planar at one wall and homeotropic at the other. As the film density is increased from the isotropic phase, a planar-disordered layer grows from the planar-anchoring wall, and a homeotropic layer from the homeotropic-anchoring wall. For film thicknesses between 2 and 7 particle lengths, the degree of overall nematic order as measured by the spatial average of TrQ(2) (where Q is the nematic order parameter tensor) is a continuously increasing function of the density, with a pronounced rise around the bulk isotropic-nematic transition. This rise is steeper the thicker the film, and is preceded by a large amount of biaxiality growing from the planar-anchoring wall. Deeper into the nematic phase, most of the film is either planar or homeotropic, but some biaxiality persists in a region of otherwise reduced overall nematic order whose position varies with film thickness.
- Criticality of colloids with three distinct interaction patches: As simple as A, B, C?Publication . Tavares, Jose; Teixeira, PauloWe systematically study the phase behavior of a simple model of associating fluids which consists of hard spherical particles with three short-ranged attractive sites on their surfaces (sticky spots or patches), of types A, B, and C, that can form bonds with energy ij (i,j = A,B,C). We consider realizations of the model with one, two, or three nonzero ij. Using Wertheim’s first order perturbation theory of association, we establish the minimum requirements on the bond energies for the model to exhibit a liquid-vapor critical point, and investigate the nature of criticality in each case. As a preliminary, we rigorously show that, within this theory, particles with M identical sites do not condense if M < 3, a result that was previously conjectured, but never proved.
- 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.
- When is a surface foam-phobic?Publication . Teixeira, Miguel; Arscott, Steve; Cox, Simon; Teixeira, Paulo• In confined foams there exist wall Plateau borders (PBs), or menisci, where the films meet the confining substrates. • What is the shape of a PB of a given size on a surface of a given wettability (i.e., contact angle θc)? Can that surface support a foam? • This is important for firefighting foams, containers for foamy foodstuffs, etc • We solve the Young-Laplace equation with gravity for a planar film spanning a gap between two horizontal, flat substrates, to predict the shape of the PBs. • Validate results by comparing with Surface Evolver and experimental data
- Phase behaviour of pure and mixed patchy colloids - theory and simulationPublication . Teixeira, Paulo; Tavares, JoseWe review the phase behaviour of pure and mixed patchy colloids, as revealed (mostly) by theory and computer simulation. These experimentally-realisable systems are excellent models for investigating the general problem of the interplay between (equilibrium) phase transitions and self-assembly in soft condensed matter. We focus on how liquid vapour condensation can be pre-empted by the formation of different types of aggregates, in particular rings, which we argue is relevant to the criticality of empty fluids and network fluids, and possibly also of dipolar fluids. In this connection we also discuss percolation and gelation in pure and mixed patchy colloids. Finally, we describe the rich phase behaviour of (mostly binary) patchy colloid mixtures.
- Effective isotropic potential for dipolar hard spheresPublication . Teixeira, PauloA new effective isotropic potential is proposed for the dipolar hard-sphere fluid, on the basis of recent results by others for its angle-averaged radial distribution function. The new effective potential is shown to exhibit oscillations even for moderately high densities and moderately strong dipole moments, which are absent from earlier effective isotropic potentials. The validity and significance of this result are briefly discussed.
- Liquid-vapor interfaces of patchy colloidsPublication . Oleksy, Anna; Teixeira, PauloWe investigate the liquid-vapor interface of a model of patchy colloids. This model consists of hard spheres decorated with short-ranged attractive sites ("patches") of different types on their surfaces. We focus on a one-component fluid with two patches of type A and nine patches of type B (2A9B colloids), which has been found to exhibit reentrant liquid-vapor coexistence curves and very low-density liquid phases. We have used the density-functional theory form of Wertheim's first-order perturbation theory of association, as implemented by Yu and Wu [J. Chem. Phys. 116, 7094 (2002)], to calculate the surface tension, and the density and degree of association profiles, at the liquid-vapor interface of our model. In reentrant systems, where AB bonds dominate, an unusual thickening of the interface is observed at low temperatures. Furthermore, the surface tension versus temperature curve reaches a maximum, in agreement with Bernardino and Telo da Gama's mesoscopic Landau-Safran theory [Phys. Rev. Lett. 109, 116103 (2012)]. If BB attractions are also present, competition between AB and BB bonds gradually restores the monotonic temperature dependence of the surface tension. Lastly, the interface is "hairy," i.e., it contains a region where the average chain length is close to that in the bulk liquid, but where the density is that of the vapor. Sufficiently strong BB attractions remove these features, and the system reverts to the behavior seen in atomic fluids.