Line tension and contact angle of heterogeneous nucleation of binary fluids

The role of seashell wastes in TiO2/Seashell composites: Photocatalytic degradation of methylene blue dye under sunlight

This paper proposes a sustainable and facile approach for the synthesis of photocatalysts in which shell waste is used as support material. The synthesized photocatalysts exhibited a significant performance in the mineralization of organic substances under solar irradiation or artificial lighting. Calcined abalone shell with a TiO₂ loading of 23.4% led to a significant improvement in optical absorption: the degradation efficiencies of methylene blue (MB) after 140 min under UV light, vis light, UV-vis light, and natural sunlight were 93%, 96%, 100%, and 100%, respectively. Notably, the byproducts obtained after the degradation by commercial P25 TiO₂ disappeared with the utilization of shell waste as support material. The Na, Sr, S present in the calcined abalone shell were doped into the substitutional sites of TiO₂ and were indispensable to achieve the desired band-gap narrowing and photocatalytic performance; moreover, the Ti and Zn oxides in the calcined abalone shell acted as semiconductors and improved the charge separation efficiency of TiO₂. Above all, this paper describes a green synthesis based on the use of waste seashell. This material acts as an excellent photocatalyst support for environmental pollution treatments, leading to the 'control of waste by waste' and opening up new possibilities for shell waste reutilization and sustainable chemistry.

Prediction of Contact Angles and Density Profiles of Sessile Droplets Using Classical Density Functional Theory Based on the PCP-SAFT Equation of State

This study demonstrates the capability of the density functional theory (DFT) formalism to predict contact angles and density profiles of model fluids and of real substances in good quantitative agreement with molecular simulations and experimental data. The DFT problem is written in cylindrical coordinates, and the solid-fluid interactions are defined as external potentials toward the fluid phase. Monte Carlo (MC) molecular simulations are conducted in order to assess the density profiles resulting from the Helmholtz energy functional used in the DFT formalism. Good quantitative agreement between DFT predictions and MC results for Lennard-Jones and ethane nanodroplets is observed, both for density profiles and for contact angles. That comparison suggests, first, that the Helmholtz energy functional proposed in a previous study [ Sauer , E. ; Gross , J. Ind. Eng. Chem. Res. 56 , 2017 , 4119 - 4135 ] is suitable for three-phase contact lines and, second, that Lagrange multipliers can be used to constrain the number of molecules, similar to a canonical ensemble. Experiments of sessile droplets on solid surfaces are performed to assess whether a real solid with its microscopic roughness can be described through a simple model potential. Comparison of DFT results to experimental data is done for a Teflon surface because Teflon can be regarded as a substrate exhibiting only attractive interactions of van der Waals type. It is shown that the real solid can be described as a perfectly planar solid with effective solvent-to-solid interactions, defined through a single adjustable parameter for the solid. Subsequent predictions for the contact angle of eight solvents, including polar components such as water, are found in very good agreement to experimental data using simple Berthelot-Lorentz combining rules. For the eight investigated solvents, we find mean absolute deviations of 3.77°.

Do the contact angle and line tension of surface-attached droplets depend on the radius of curvature?

Results from Monte Carlo simulations of wall-attached droplets in the three-dimensional Ising lattice gas model and in a symmetric binary Lennard-Jones fluid, confined by antisymmetric walls, are analyzed, with the aim to estimate the dependence of the contact angle [Formula: see text] on the droplet radius [Formula: see text] of curvature. Sphere-cap shape of the wall-attached droplets is assumed throughout. An approach, based purely on 'thermodynamic' observables, e.g. chemical potential, excess density due to the droplet, etc, is used, to avoid ambiguities in the decision which particles belong (or do not belong, respectively) to the droplet. It is found that the results are compatible with a variation [Formula: see text], [Formula: see text] being the contact angle in the thermodynamic limit ([Formula: see text]). The possibility to use such results to estimate the excess free energy related to the contact line of the droplet, namely the line tension, at the wall, is discussed. Various problems that hamper this approach and were not fully recognized in previous attempts to extract the line tension are identified. It is also found that the dependence of wall tensions on the difference of chemical potential of the droplet from that at the bulk coexistence provides effectively a change of the contact angle of similar magnitude. The simulation approach yields precise estimates for the excess density due to wall-attached droplets and the corresponding free energy excess, relative to a system without a droplet at the same chemical potential. It is shown that this information suffices to estimate nucleation barriers, not affected by ambiguities on droplet shape, contact angle and line tension.

Evaluation of macroscale wetting equations on a microrough surface

The wettability of critical droplets on microscale geometric rough surfaces has been investigated using a density functional theory approach. In order to analyze the effect of roughness on nucleation free-energy barriers, the local density fluctuations at liquid-solid interfaces induced by the multi-interactions of a corner substrate are presented to interpret the interfacial free-energy variations, and the vapor-liquid-solid contact line tensions are derived from the contact angles of nuclei to account for the three-phase contact energies. The corresponding wetting diagrams have been constructed in Cassie, Wenzel, and impregnation regions. It is shown that, under the same condition, modest deviations between the microscale and the macroscale models can be observed within the Cassie region, whereas these deviations have been enlarged in the Wenzel and impregnation regions as well as the Cassie-Wenzel transition region. These deviations are also correlated to the roughness of the surface. The reason can be attributed to the cooperative effect of the liquid-solid interfacial free energy and line tension. This study offers a fundamental understanding of wettability of ultrasmall droplets on a microscale geometric rough surface.

Influence of contact-line curvature on the evaporation of nanodroplets from solid substrates

The effect of the three-phase contact-line curvature on the evaporation mechanism of nanoscopic droplets from smooth and chemically homogenous substrates is studied by molecular dynamics simulations. Spherical droplets, whose three-phase contact line is curved, and cylindrical droplets, whose contact radius is infinite, are compared. It is found that the evaporation of cylindrical droplets takes place at constant contact angle, while spherical droplets evaporate by simultaneous reduction of their contact area and their contact angle. This is independent of the substrate-liquid interaction strength. The dependence of the evaporation mechanism on the contact-line curvature can be rationalized with the help of the concept of a contact-line tension, and the evaporation simulations of the spherical droplets are used to extract the line tension on each surface. The corresponding values for the Lennard-Jones systems studied here are of the order of 10(-11)N, which is in a good agreement with previous theoretical and experimental estimates. With this order of magnitude, the line tension is expected to have an effect on the contact angle of spherical droplets only, when their diameter is less than about 100 nm. The observed difference in evaporation mechanism is interpreted as a manifestation of the line tension whose existence has been controversial.