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- Ordering of oblate hard particles between symmetric penetrable wallsPublication . Teixeira, Paulo; Anquetil-Deck, Candy; Cleaver, Douglas J.We find the structure of a model discotic liquid crystal (DLC) confined between symmetric walls of controllable penetrability. The model consists of oblate hard Gaussian overlap (HGO) particles. Particle-substrate interactions are modelled as follows: each substrate sees a particle as a disc of zero thickness and diameterless than or equal to that of the actual particle,, embedded inside the particle and located halfway along, and perpendicular to, its minor axis. This allows us to control the anchoring properties of the substrates, from planar (edge-on) forto homeotropic (face-on) for. This system is investigated using both Monte Carlo simulation and density-functional theory, the latter implemented at the level of Onsager's second-virial approximation with Parsons-Lee rescaling. We find that the agreement between theory and simulation is substantially less good than for prolate HGOs; in particular, the crossover from edge-on to face-on alignment is predicted by theory to occur at, but simulation finds it for. These discrepancies are likely a consequence of the fact that Onsager's theory is less accurate for discs than for rods. We quantify this by computing the bulk isotropic-nematic phase diagram of oblate HGOs.
- The shape of two-dimensional liquid bridgesPublication . Teixeira, Paulo; Teixeira, MiguelWe have studied a single vertical, two-dimensional liquid bridge spanning the gap between two flat, horizontal solid substrates of given wettabilities. For this simple geometry, the Young–Laplace equation can be solved (quasi-)analytically to yield the equilibrium bridge shape under gravity. We establish the range of gap widths (as described by a Bond number Bo) for which the liquid bridge can exist, for given contact angles at the top and bottom substrates (θt c and θb c, respectively). In particular, we find that the absolute maximum span of a liquid bridge is four capillary lengths, for θb c = 180◦ and θt c = 0◦; whereas for θb c = 0◦ and θt c = 180◦ no bridge can form, for any substrate separation. We also obtain the minimum value of the cross-sectional area of such a liquid bridge, as well as the conditions for the existence and positions of any necks or bulges and inflection points on its surface. This generalises our earlier work in which the gap was assumed to be spanned by a liquid film of zero thickness connecting two menisci at the bottom and top substrates.
- Dynamics of liquid bridges between patterned surfacesPublication . Rodrigues, Margarida S.; Coelho, Rodrigo; Teixeira, PauloWe have simulated the motion of a single vertical, two-dimensional liquid bridge spanning the gap between two flat, horizontal solid substrates consisting of alternating hydrophilic and hydrophobic stripes, using a multicomponent pseudopotential lattice Boltzmann method. This extends our earlier work where the substrates were uniformly hydrophilic or hydrophobic. In steady-state conditions, we calculate the following, as functions of pattern wavelength: (i) the velocity fields of moving bridges, in particular their (time-averaged) terminal velocities; (ii) the deformation of moving bridges, as measured by the deviation of bridge contact angles from their equilibrium values; (iii) the minimum applied force that breaks a moving bridge. In addition, we found that a bridge moving between patterned substrates cannot be mapped onto a bridge moving between uniform substrates endowed with some effective contact angle, even in the limit of very small pattern wavelength compared to bridge width.
- Física: uma introduçãoPublication . Silvestre, António Jorge; Teixeira, PauloO presente livro destina-se ao ensino de uma primeira disciplina de Física de cursos de ciências ou de engenharia, compreendendo o cânone da Física Clássica - Mecânica, Electromagnetismo e Termodinâmica — que um futuro cientista ou engenheiro deve dominar, e que poderá, eventualmente, aprofundar em disciplinas mais avançadas. Tem a sua génese na unidade curricular de “Física” que temos vindo a leccionar há vários anos no segundo semestre do primeiro ano da Licenciatura em Matemática Aplicada à Tecnologia e à Empresa do Instituto Superior de Engenharia de Lisboa - ISEL. É, portanto, um livro marcadamente pedagógico, em que procurámos, sem prejuízo do rigor, que a exposição teórica fosse sempre acompanhada de exemplos que permitam uma mais fácil assimilação das diferentes matérias.
- Building up DNA, bit by bit: a simple description of chain assemblyPublication . Foffi, Riccardo; SCIORTINO, Francesco; Tavares, Jose; Teixeira, PauloWe simulate the assembly of DNA copolymers from two types of short duplexes (short double strands with a single-stranded overhang at each end), as described by the oxDNA model. We find that the statistics of chain lengths can be well reproduced by a simple theory that treats the association of particles into ideal (i.e., non-interacting) clusters as a reversible chemical reaction. The reaction constants can be predicted either from SantaLucia's theory or from Wertheim's thermodynamic perturbation theory of association for spherical patchy particles. Our results suggest that theories incorporating very limited molecular detail may be useful for predicting the broad equilibrium features of copolymerisation.
- Dynamics of two-dimensional liquid bridgesPublication . Coelho, Rodrigo; Cordeiro, Luis A. R. G.; Gazola, Rodrigo B.; Teixeira, PauloWe have simulated the motion of a single vertical, two-dimensional liquid bridge spanning the gap between two flat, horizontal solid substrates of given wettabilities, using a multicomponent pseudopotential lattice Boltzmann method. For this simple geometry, the Young-Laplace equation can be solved (quasi-)analytically to yield the equilibrium bridge shape under gravity, which provides a check on the validity of the numerical method. In steady-state conditions, we calculate the drag force exerted by the moving bridge on the confining substrates as a function of its velocity, for different contact angles and Bond numbers. We also study how the bridge deforms as it moves, as parametrized by the changes in the advancing and receding contact angles at the substrates relative to their equilibrium values. Finally, starting from a bridge within the range of contact angles and Bond numbers in which it can exist at equilibrium, we investigate how fast it must move in order to break up.
- Remnants of the disappearing critical point(s) in patchy fluids with distinct interaction patchesPublication . Tavares, Jose; Teixeira, PauloWe investigate the disappearance of the critical points of a model consisting of particles decorated with two patches of type A and a variable number (n) of patches of type B (2AnB patchy particles), in which only AA and AB bonds can form. This has been shown to exhibit a very rich phase behavior including one, two, or no liquid-vapor critical points, depending on two parameters: the ratio of the volumes available to each type of bond and the ratio of the bond strengths. We apply Wertheim's theory in the limit of strong AA bonds to a lattice version of the model [Almarza et al., J. Chem. Phys. 137, 244902 (2012)] and show that the critical point does not always vanish at zero density and temperature, in contrast with results for particles decorated with only one type of patch. We uncover two remnants of the critical points-the lines of maximum and ideal compressibility-that survive even when no critical points are present.
- Ordering of oblate hard particles between hybrid penetrable wallsPublication . Anquetil-Deck, Candy; Cleaver, Douglas J.; Teixeira, PauloWe report a Monte Carlo (MC) simulation study of a model discotic liquid crystal (DLC) confined between hybrid walls with controllable penetrability. The model consists of oblate hard Gaussian overlap (HGO) particles. Particle-substrate interactions are modeled as follows: each substrate sees a particle as a disc of zero thickness and diameter D less than or equal to that of the actual particle, sigma(0), embedded inside the particle and located halfway along, and perpendicular to, its minor axis. This allows us to control the anchoring properties of the substrates, from planar (edge-on) for D approximate to 0 to homeotropic (face-on) for D approximate to sigma(0), which can be done independently at either substrate. Depending on the values of D-s = D/sigma(0) at the top (D-s(t)) and bottom (D-s(b)) substrates, we find domains in (D-s(b), D-s(t)) space in which particle alignment is uniform planar (UP), is uniform homeotropic (UH), or varies linearly from planar at one substrate to homeotropic at the other (Lin). These domains are separated by regions of bistability (P-Lin and H-Lin), which appear to be wider than for prolate HGOs, and there may be also a small tristable (P-H-Lin) region. Results are compared with the predictions of density functional theory, implemented at the level of Onsager's second-virial approximation with Parsons-Lee rescaling. As in the case of symmetric confinement studied previously, the agreement between theory and simulation is substantially less good than for prolate HGOs: in particular, for the investigated substrate separation L = 6 sigma(0), the Lin configuration is never predicted. These discrepancies are likely a consequence of the fact that Onsager's theory is less accurate for discs than for rods.