Shapes, contact angles, and line tensions of droplets on cylinders

Using an interface displacement model we calculate the shapes of nanometer-size liquid droplets on homogeneous cylindrical surfaces. We determine effective contact angles and line tensions, the latter defined as excess free energies per unit length associated with the two contact lines at the ends of the droplet. The dependences of these quantities on the cylinder radius and on the volume of the droplets are analyzed.Comment: 26 pages, RevTeX, 10 Figure

Thermodynamics of heterogeneous crystal nucleation in contact and immersion modes

One of most intriguing problems of heterogeneous crystal nucleation in droplets is its strong enhancement in the contact mode (when the foreign particle is presumably in some kind of contact with the droplet surface) compared to the immersion mode (particle immersed in the droplet). Many heterogeneous centers have different nucleation thresholds when they act in contact or immersion modes, indicating that the mechanisms may be actually different for the different modes. Underlying physical reasons for this enhancement have remained largely unclear. In this paper we present a model for the thermodynamic enhancement of heterogeneous crystal nucleation in the contact mode compared to the immersion one. To determine if and how the surface of a liquid droplet can thermodynamically stimulate its heterogeneous crystallization, we examine crystal nucleation in the immersion and contact modes by deriving and comparing with each other the reversible works of formation of crystal nuclei in these cases. As a numerical illustration, the proposed model is applied to the heterogeneous nucleation of Ih crystals on generic macroscopic foreign particles in water droplets at T=253 K. Our results show that the droplet surface does thermodynamically favor the contact mode over the immersion one. Surprisingly, our numerical evaluations suggest that the line tension contribution to this enhancement from the contact of three water phases (vapor-liquid-crystal) may be of the same order of magnitude as or even larger than the surface tension contribution

Conservação e restauro de uma urna em vidro do século I d.C., encontrada em Mértola (Portugal)

International audienceThe evolution of capillary forces during evap-oration and the corresponding changes in the geometrical characteristics of liquid (water) bridges between two glass spheres with constant separation are examined experimen-tally. For comparison, the liquid bridges were also tested for mechanical extension (at constant volume). The obtained results reveal substantial differences between the evolution of capillary force due to evaporation and the evolution due to extension of the liquid bridges. During both evaporation and extension, the change of interparticle capillary forces consists in a force decrease to zero either gradually or via rupture of the bridge. At small separations between the grains (short & wide bridges) during evaporation and at large volumes during extension, there is a slight initial increase of force. During evaporation, the capillary force decreases slowly at the begin-ning of the process and quickly at the end of the process; during extension, the capillary force decreases quickly at the beginning and slowly at the end of the process. Rup-ture during evaporation of the bridges occurs most abruptly for bridges with wider separations (tall and thin), sometimes occurring after only 25 % of the water volume was evapo-rated. The evolution (pinning/depinning) of two geometri-cal characteristics of the bridge, the diameter of the three-phase contact line and the "apparent" contact angle at the solid/liquid/gas interface, seem to control the capillary force evolution. The findings are of relevance to the mechanics of unsaturated granular media in the final phase of drying