RESUMO
The equilibrium configuration of compound sessile drops has been calculated previously in the absence of gravity. Using the Laplace equations, we establish seven dimensionless parameters describing the axisymmetric configuration in the presence of gravity. The equilibrium axisymmetric configuration can be either stable or unstable depending on the fluid properties. A stability criterion is established by calculating forces on a perturbed Laplacian shape. In the zero Bond number limit, the stability criterion depends on the density ratio, two ratios of interfacial tensions, the volume ratio of the two drops, and the contact angle. We use Surface Evolver to examine the stability of compound sessile drops at small and large Bond numbers and compare with the zero Bond number approximation. Experimentally, we realize a stable axisymmetric compound sessile drop in air, where the buoyancy force exerted by the air is negligible. Finally, using a pair of fluids in which the density ratio can be tuned nearly independently of the interfacial tensions, the stability transition is verified for the axisymmetric configuration. Even though the perturbations are different for the theory, simulations and experiments, both simulations and experiments agree closely with the zero Bond number approximation, exhibiting a small discrepancy at large Bond number.
RESUMO
Nanometer-thick films at interfaces and surfaces exist in various materials and can substantially influence their properties. Whether these films are an equilibrium or transient state is debated. To address this question, we equilibrated 1.2-nanometer-thick films at gold-sapphire interfaces in the presence of anorthite glass and measured the solid-solid interface energy. The equilibrated film significantly reduced the interfacial energy and could be described by the Gibbs adsorption isotherm expanded to include structure in addition to chemical excess. Unlike artificially made conventional thin films, these films do not break up during equilibration and offer an alternative design criterion for thin-film technology. These results demonstrate that nanometer-thick films at interfaces and surfaces can be an equilibrium state and included in phase diagrams with dedicated tie-lines.
RESUMO
The III-V nanowire structure (zinc blende or wurtzite) grown by the vapor-liquid-solid process is shown to be highly dependent on the parameters which shape the droplet at the top of the nanowire. Under conditions that the droplet volume does not exceed a certain value, it is demonstrated that when the nucleation of the solid starts at the solid-liquid-vapor triple line, a relatively large droplet volume and low wetting angle favor the formation of the wurtzite structure. We show that the effective V/III flux ratio is the primary parameter controlling the structure.
RESUMO
The wetting of lead on silicon wafers with regularly patterned holes, and covered by native silica, has been investigated at 610 K under ultrahigh vacuum conditions. The advancing and receding macroscopic contact angles have been measured by slowly compressing and stretching a liquid lead bridge between two identically patterned substrates. These angles are shown to depend on the distribution of the holes in the wafers and the continuity of the triple line.
RESUMO
The shapes and energies of drops on substrates patterned with either holes or posts are computed using Surface Evolver software. The holes and posts are cylindrical in shape and distributed in a 6-fold symmetric pattern. The wetting conditions are such that the liquid does not fill the holes and the interface between the drop and the substrate is composite, i.e., partly solid/liquid and partly liquid/vapor. The sequence of stable drop configurations with increasing volume is analyzed and provides, in part, an explanation for superhydrophobic drop spreading.
