Nonadditivity of van der Waals forces on liquid surfaces

Disjoining pressure analysis of the lubricant nanofilm stability of liquid-infused surface upon lubricant depletion

HYPOTHESIS

The structure of the slippery layer and the evolution of functional properties of a lubricant infused substrate (LIS) is determined by the isotherm of disjoining pressure in the lubricant film.

METHODS

The macroscopic theory of van der Waals forces was applied to the layered system used to model the structure and properties of LIS. For a lubricant layer sandwiched between the flat substrate and air or water, the isotherms of disjoining pressure were calculated and their analysis was used to conclude about stability of LIS.

FINDINGS

The results obtained for silicone oil and perfluorodecalin on smooth and porous hydrophilic and hydrophobic solids allow selecting the LIS components corresponding to stability of lubricant films in air and water. It was found that for hydrophilic substrates in conditions of lubricant depletion, silicone oil and perfluorodecalin show lubricant film stability in both air and water. On flat or post microtexture hydrophobic substrate with flat tops, the perfluorodecalin lubricating layer is typically stable in air and unstable in water. In contrast, silicone oil lubricating layer demonstrates the stability in a wide range of lubricant film thicknesses for the hydrophobic substrate with flat-top textures in water, however, it can be unstable in air.

Optical and surface energy probe of Hamaker constant in copper oxide thin films for NEMS and MEMS stiction control applications

Copper oxide films hold substantial promise as anti-stiction coatings in micro-electromechanical (MEMS) devices and with shrinking dimensions on the nanometre scale on nano electromechanical (NEMS) devices. The Hamaker constant will play a very significant role in understanding stiction and tribology in these devices. We used an approximate but sufficiently accurate form of the Lifshitz theory using the multiple oscillator model to calculate the Hamakers constant of symmetric copper oxide thin films based on experimentally obtained dielectric data in the wavelength range 190-850 nm using spectroscopic ellipsometry. We also used the Tabor–Winterton approximation (TWA) and Surface energy measurements to determine the Hamaker constant. There was better agreement in the Hamaker constant values obtained by the limited Lifshitz theory and TWA approach than with the Surface energy approach. The difference is explained through the influence of surface roughness on the surface energy using extensions of the stochastic KPZ growth model and the Family-Vicsek scaling relation and rigorous treatment of the Cassie-Baxter and Wenzel models as optimisations of a surface free energy functional linking roughness and surface tension. The dominance of the Cu₂O phase in the films and of the London dispersion force on the surface of the films was previously confirmed by FTIR Cu(I)–O vibrational mode observation and XPS Cu 2p_3/2 binding energy peak and its fitted satellites. The use of the limited Lifshitz theory and ellipsometry data would seem to provide a suitable best first approximation for determining the Hamaker constant of predominantly dispersive anti-stiction coatings in technologically important MEMS/NEMS devices.

Behavior of the van der Waals force between a plate and a single-walled carbon nanotube under uniform hydrostatic pressure: a theoretical study

We study the behaviour of the non-retarded van der Waals force between a planar substrate and a single-walled carbon nanotube, assuming that the system is immersed in a liquid medium which exerts hydrostatic pressure on the tube's surface, thereby altering its cross section profile. The shape of the latter is described as a continual structure characterized by its symmetry indexn. Two principle mutual positions of the tube with respect to the substrate are studied: when one keeps constant the minimal separation between the surfaces of the interacting objects; when the distance from the tube's axis to the substrates bounding surface is fixed. Within these conditions, using the technique of the surface integration approach, we derive in integral form the expressions which give the dependance of the commented force on the applied pressure.