arXiv subjects

J. J. de Pablo

Publications and source records attributed to J. J. de Pablo.

5 recordsLinked to original sources

arXiv2026

Over what length scale does an inorganic substrate perturb the structure of a glassy organic semiconductor?

While the bulk structure of vapor-deposited glasses has been extensively studied, structure at buried interfaces has received little attention, despite being important for organic electronic applications. To learn about glass structure at buried interfaces, we study the structure of vapor-deposited glasses of the organic semiconductor DSA-Ph (1,4-di-[4-(N,N-diphenyl)amino]styryl-benzene) as a function of film thickness; structure is probed with grazing incidence X-ray scattering. We deposit on silicon and gold substrates and span a film thickness range of 10-600 nm. Our experiments demonstrate that interfacial molecular packing in vapor-deposited glasses of DSA-Ph is more disordered compared to the bulk. At a deposition temperature near room temperature, we estimate ~ 8 nm near the substrate can have modified molecular packing. Molecular dynamics simulations of a coarse-grained representation of DSA-Ph reveal a similar length scale. In both the simulations and the experiments, deposition temperature controls glass structure beyond this interfacial layer of a few nanometers.

cond-mat.mtrl-sci↗

arXiv2018

Observation of the pressure effect in simulations of droplets splashing on a dry surface

At atmospheric pressure, a drop of ethanol impacting on a solid surface produces a splash. Reducing the ambient pressure below its atmospheric value suppresses this splash. The origin of this so-called pressure effect is not well understood and this is the first study to present an in-depth comparison between various theoretical models that aim to predict splashing and simulations. In this work the pressure effect is explored numerically by resolving the Navier-Stokes equations at a 3-nm resolution. In addition to reproducing numerous experimental observations, it is found that different models all provide elements of what is observed in the simulations. The skating droplet model correctly predicts the existence and scaling of a gas film under the droplet, the lamella formation theory is able to correctly predict the scaling of the lamella ejection velocity as function of the impact velocity for liquids with different viscosity, and lastly, the dewetting theory's hypothesis of a lift force acting on the liquid sheet after ejection is consistent with our results.

physics.flu-dyn↗

arXiv2016

Structural Transitions in Fibers of Bent-Core Liquid Crystals from Field-Theory Monte Carlo Simulations

Fibers of bent-core liquid crystals present an internal structure of a rolled smectic layer and can be used as optical waveguides. We used a field-theoretical Monte Carlo simulation to analyze the internal configuration of such fibers as a function of the radial coordinate and to study their equilibrium sates. In contrast to previous studies, we analyzed the fully nonlinear model proposed in [Bailey et al., Phys. Rev. E, 2007, 75, 031701] and revised in [Pérez-Ortiz et al., Phys. Rev. E, 2011, 84, 011701]. We found that, due to the non-differentiable character of such model, the Euler-Lagrange equations are not able to find all equilibrium states. Our Monte Carlo procedure identified both differentiable and non-differentiable equilibria and first order transitions between them. In all cases, the equilibrium states show inhomogenous configurations that display a boundary layer. This methodology can by applied to other models of liquid crystals that have more degrees of freedom, including those with non-differentiable minima. The equilibrium structures found can be used as inputs to model the transmission of light along the liquid crystal fibers.

cond-mat.soft↗

arXiv2016

Generalized Navier Boundary Condition for a Volume Of Fluid approach using a Finite-Volume method

In this work, an analytical Volume Of Fluid (VOF) implementation of the Generalized Navier Boundary Condition is presented based on the Brackbill surface tension model. The model is validated by simulations of droplets on a smooth surface in a planar geometry. Looking at the static behavior of the droplets, it is found that there is a good match between the droplet shape resolved in the simulations and the theoretically predicted shape for various values of the Young's angle. Evaluating the spreading of a droplet on a completely wetting surface, the Voinov-Tanner-Cox law ($θ\propto \text{Ca}^{1/3}$) can be observed. At later times scaling follows $r \propto t^{1/2}$, suggesting spreading is limited by inertia. These observations are made without any fitting parameters except the slip length.

physics.flu-dyn↗

arXiv2002

Local Elastic Constants in Thin Films of an FCC Crystal

In this work we present a formalism for the calculation of the local elastic constants in inhomogeneous systems based on a method of planes. Unlike previous work, this formalism does not require the partitioning of the system into a set of finite volumes over which average elastic constants are calculated. Results for the calculation of the local elastic constants of a nearest neighbor Lennard-Jones fcc crystal in the bulk and in a thin film are presented. The local constants are calculated at exact planes of the (001) face of the crystal. The average elastic constants of the bulk system are also computed and are consistent with the local constants. Additionally we present the local stress profiles in the thin film when a small uniaxial strain is applied. The resulting stress profile compares favorably with the stress profile predicted via the local elastic constants. The surface melting of a model for argon for which experimental and simulation data are available is also studied within the framework of this formalism.

cond-mat.soft↗